Showing papers in "Plant Physiology in 1992"
TL;DR: The results demonstrate the role of both light intensity and Mg nutritional status on the regulation of O(2) (.-) and H( 2)O(3) scavenging enzymes in chloroplasts.
Abstract: The influence of varied Mg supply (10-1000 micromolar) and light intensity (100-580 microeinsteins per square meter per second) on the concentrations of ascorbate (AsA) and nonprotein SH-compounds and the activities of superoxide dismutase (SOD; EC 1.15.11) and the H2O2 scavenging enzymes, AsA peroxidase (EC 1.11.1.7), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2) were studied in bean (Phaseolus vulgaris L.) leaves over a 13-day period. The concentrations of AsA and SH-compounds and the activities of SOD and H2O2 scavenging enzymes increased with light intensity, in particular in Mg-deficient leaves. Over the 12-day period of growth for a given light intensity, the concentrations of AsA and SH-compounds and the activities of these enzymes remained more or less constant in Mg-sufficient leaves. In contrast, in Mg-deficient leaves, a progressive increase was recorded, particularly in concentrations of AsA and activities of AsA peroxidase and glutathione reductase, whereas the activities of guaiacol peroxidase and catalase were only slightly enhanced. Partial shading of Mg-deficient leaf blades for 4 days prevented chlorosis, and the activities of the O2.− and H2O2 scavenging enzymes remained at a low level. The results demonstrate the role of both light intensity and Mg nutritional status on the regulation of O2.− and H2O2 scavenging enzymes in chloroplasts.
1,520 citations
TL;DR: The Acid Growth Theory, which states that when exposed to auxin, susceptible cells excrete protons into the wall (apoplast) at an enhanced rate, resulting in a decrease in apoplastic pH, activates wall-loosening processes, the precise nature of which is unknown.
Abstract: Plant cells elongate irreversibly only when load-bearing bonds in the walls are cleaved. Auxin causes the elongation of stem and coleoptile cells by promoting wall loosening via cleavage of these bonds. This process may be coupled with the intercalation of new cell wall polymers. Because the primary site of auxin action appears to be the plasma membrane or some intracellular site, and wall loosening is extracellular, there must be communication between the protoplast and the wall. Some "wall-loosening factor" must be exported from auxin-impacted cells, which sets into motion the wall loosening events. About 20 years ago, it was suggested that the wall-loosening factor is hydrogen ions. This idea and subsequent supporting data gave rise to the Acid Growth Theory, which states that when exposed to auxin, susceptible cells excrete protons into the wall (apoplast) at an enhanced rate, resulting in a decrease in apoplastic pH. The lowered wall pH then activates wall-loosening processes, the precise nature of which is unknown. Because exogenous acid causes a transient (1-4 h) increase in growth rate, auxin must also mediate events in addition to wall acidification for growth to continue for an extended period of time. These events may include osmoregulation, cell wall synthesis, and maintenance of the capacity of walls to undergo acid-induced wall loosening. At present, we do not know if these phenomena are tightly coupled to wall acidification or if they are the products of multiple independent signal transduction pathways.
786 citations
TL;DR: It is concluded that both methods can be used to determine mesophyll conductance and each method has particular strengths and will prove useful in the future.
Abstract: The conductance for CO2 diffusion in the mesophyll of leaves can limit photosynthesis. We have studied two methods for determining the mesophyll conductance to CO2 diffusion in leaves. We generated an ideal set of photosynthesis rates over a range of partial pressures of CO2 in the stroma and studied the effect of altering the mesophyll diffusion conductance on the measured response of photosynthesis to intercellular CO2 partial pressure. We used the ideal data set to test the sensitivity of the two methods to small errors in the parameters used to determine mesophyll conductance. The two methods were also used to determine mesophyll conductance of several leaves using measured rather than ideal data sets. It is concluded that both methods can be used to determine mesophyll conductance and each method has particular strengths. We believe both methods will prove useful in the future.
772 citations
TL;DR: It appears that gibberellin normally plays a role in initiating flowering of Arabidopsis, although the ga1-3 mutant showed some delay in CL, while all the mutants flowered readily in CL.
Abstract: Mutants of Arabidopsis thaliana deficient in gibberellin synthesis (ga1-3 and ga1-6), and a gibberellin-insensitive mutant (gai) were compared to the wild-type (WT) Landsberg erecta line for flowering time and leaf number when grown in either short days (SD) or continuous light (CL). The ga1-3 mutant, which is severely defective in ent-kaurene synthesis because it lacks most of the GA1 gene, never flowered in SD unless treated with exogenous gibberellin. After a prolonged period of vegetative growth, this mutant eventually underwent senescence without having produced flower buds. The gai mutant and the “leaky” ga1-6 mutant did flower in SD, but took somewhat longer than WT. All the mutants flowered readily in CL, although the ga1-3 mutant showed some delay. Unlike WT and ga1-3, the gai mutant failed to respond to gibberellin treatment by accelerating flowering in SD. A cold treatment promoted flowering in the WT and gai, but failed to induce flowering in ga1-3. From these results, it appears that gibberellin normally plays a role in initiating flowering of Arabidopsis.
603 citations
TL;DR: It is concluded that copper tolerance in S. cucubalus does not depend on the production of phytochelatins but is related to the plant's ability to prevent glutathione depletion resulting from copper-induced phytOChelatin production, e.g. by restricting its copper uptake.
Abstract: The relation between loss of glutathione due to metal-induced phytochelatin synthesis and oxidative stress was studied in the roots of copper-sensitive and tolerant Silene cucubalus (L.) Wib., resistant to 1 and 40 micromolar Cu, respectively. The amount of nonprotein sulfhydryl compounds other than glutathione was taken as a measure of phytochelatins. At a supply of 20 micromolar Cu, which is toxic for sensitive plants only, phytochelatin synthesis and loss of total glutathione were observed only in sensitive plants within 6 h of exposure. When the plants were exposed to a range of copper concentrations for 3 d, a marked production of phytochelatins in sensitive plants was already observed at 0.5 micromolar Cu, whereas the production in tolerant plants was negligible at 40 micromolar or lower. The highest production in tolerant plants was only 40% of that in sensitive plants. In both varieties, the synthesis of phytochelatins was coupled to a loss of glutathione. Copper at toxic concentrations caused oxidative stress, as was evidenced by both the accumulation of lipid peroxidation products and a shift in the glutathione redox couple to a more oxidized state. Depletion of glutathione by pretreatment with buthionine sulfoximine significantly increased the oxidative damage by copper. At a comparably low glutathione level, cadmium had no effect on either lipid peroxidation or the glutathione redox couple in buthionine sulfoximine-treated plants. These results indicate that copper may specifically cause oxidative stress by depletion of the antioxidant glutathione due to phytochelatin synthesis. We conclude that copper tolerance in S. cucubalus does not depend on the production of phytochelatins but is related to the plant9s ability to prevent glutathione depletion resulting from copper-induced phytochelatin production, e.g. by restricting its copper uptake.
480 citations
TL;DR: Large early-wood vessels in oak were most vulnerable to embolism by freezing, small vessels in Populus and Betula were intermediate, and tracheids in conifers were most resistant; the same trend was found within a stem: embolisms by freeze-thawing occurred preferentially in wider conduits.
Abstract: Vulnerability to xylem embolism by freeze-thaw cycles and water stress was quantified in ring-porous ( Quercus gambelii Nutt.), diffuse-porous ( Populus tremuloides Michx., Betula occidentalis Hook.), and conifer species ( Abies lasiocarpa Nutt., Juniperus scopulorum Sarg.). Embolism was measured by its reduction of xylem hydraulic conductivity; it was induced by xylem tension (water-stress response) and by a tension plus a freeze-thaw cycle (freeze response). Conifers showed little ( Juniperus ) or no ( Abies ) freeze response even to repeated cycles. In contrast, Quercus embolized more than 90% by freezing at tensions below 0.2 MPa, whereas similar embolism without freezing required tensions above 4.5 MPa. Diffuse-porous trees ( Betula, Populus ) showed an intermediate freeze response. The magnitude of the freeze response was correlated with conduit volume but occurred at higher tensions than predicted from theory. Large early-wood vessels (2.8 × 10 −9 m 3 ) in oak were most vulnerable to embolism by freezing, small vessels in Populus and Betula were intermediate (approximately 7 × 10 −11 m 3 ), and tracheids in conifers (about 3 × 10 −13 m 3 ) were most resistant. The same trend was found within a stem: embolism by freeze-thawing occurred preferentially in wider conduits. The water-stress response was not correlated with conduit volume; previous work indicates it is a function of interconduit pit membrane structure. Native embolism levels during winter corroborated laboratory results on freezing: Quercus embolized 95% with the first fall freeze, Populus and Betula showed gradual increases to more than 90% embolism by winter9s end, and Abies remained below 30%.
457 citations
TL;DR: It is suggested that in response to wound signals generated by insect or pathogen attacks, linolenic acid is released into the cytoplasm from plant cell membrane lipids and is rapidly converted in cells to jasmonic acid, which serves as a signal to regulate the expression of proteinase inhibitor genes.
Abstract: Gel electrophoretic analysis of the proteinase inhibitor proteins induced in tomato leaves by airborne methyl jasmonate (EE Farmer, CA Ryan [1990] Proc Natl Acad Sci USA 87: 7713-7716) revealed the new appearance of inhibitors I and II and two other, higher molecular mass proteins (63.5 and 87 kilodaltons). Northern analysis of methyl jasmonate-induced inhibitors I and II mRNAs in tomato (Lycopersicon esculentum) leaves, and of alfalfa trypsin inhibitor (a Bowman-Birk family inhibitor) mRNA in alfalfa (Medicago sativa) leaves, indicated that nascent inhibitor mRNAs were regulated in a manner similar to wounding, that is, at the transcriptional level. In tobacco (Nicotiana tabacum), transformed with a fused gene composed of the 5′ and 3′ regions of a wound-inducible potato inhibitor II and a chloramphenicol acetyl transferase (CAT) gene coding region, CAT activity was induced in leaves by methyl jasmonate, consistent with a transcriptional regulation of the inhibitor II gene. In tomato leaves, inhibitor I and II mRNAs and proteins accumulated in leaves distal to those exposed to methyl jasmonate or jasmonic acid to similar levels as in exposed leaves. We suggest that in response to wound signals generated by insect or pathogen attacks, linolenic acid is released into the cytoplasm from plant cell membrane lipids and is rapidly converted in cells to jasmonic acid (or perhaps a closely related derivative such as methyl jasmonate), which serves as a signal to regulate the expression of proteinase inhibitor genes.
431 citations
TL;DR: Recent progress in elucidating the mechanism(s) determining tolerance versus intolerance to anaerobic environments in higher plants is discussed, drawing most heavily on experimental systems using seeds or seedlings.
Abstract: Exposure to oxygen deficits is more widespread in biological systems than is commonly believed. Until recently, the general perception of anaerobic metabolism was often limited to the induction of alcoholic or lactic acid fermentation as the sole biochemical response to hypoxia/anoxia. Developments in the physiology, biochemistry, and molecular biology of anaerobic responses in invertebrates, lower plants, and higher plants have demonstrated that, depending upon the species, anaerobic metabolism may encompass much more than simple glycolytic metabolism. Here, recent progress in elucidating the mechanism(s) determining tolerance versus intolerance to anaerobic environments in higher plants is discussed, drawing most heavily on experimental systems using seeds or seedlings.
426 citations
TL;DR: Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO(3) (-) induced shoot nitrate reduction more slowly than root nitrates reduction in all three genotypes.
Abstract: We examined nitrate assimilation and root gas fluxes in a wild-type barley (Hordeum vulgare L. cv Steptoe), a mutant (nar1a) deficient in NADH nitrate reductase, and a mutant (nar1a;nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO3− induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes. When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient—the ratio of carbon dioxide evolved to oxygen consumed—did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and nar1a mutant, but had negligible effects on root gas fluxes in the nar1a;nar7w mutant. These results indicate that, under NH4+ nutrition, 14% of root carbon catabolism is coupled to NH4+ absorption and assimilation and that, under NO3− nutrition, 5% of root carbon catabolism is coupled to NO3− absorption, 15% to NO3− assimilation, and 3% to NH4+ assimilation. The additional energy requirements of NO3− assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH4+ and NO3− absorption and assimilation constitute a significant portion of root respiration.
423 citations
TL;DR: The results of the present study demonstrate the possibility of the occurrence of end-product inhibition of photosynthesis in a large number of crop plants and show the inhibition was strongest in starch storers, but starch itself cannot be directly responsible, because the starchless mutant of N. sylvestris was also strongly inhibited.
Abstract: In the present study, leaves of different plant species were girdled by the hot wax collar method to prevent export of assimilates. Photosynthetic activity of girdled and control leaves was evaluated 3 to 7 days later by two methods: (a) carbon exchange rate (CER) of attached leaves was determined under ambient CO2 concentrations using a closed gas system, and (b) maximum photosynthetic capacity (Amax) was determined under 3% CO2 with a leaf disc O2 electrode. Starch, hexoses, and sucrose were determined enzymically. Typical starch storers like soybean (Glycine max L.) (up to 87.5 milligrams of starch per square decimeter in girdled leaves), cotton (Gossypium hirsutum L.), and cucumber (Cucumis sativus L.) responded to 7 days of girdling by increased (80-100%) stomatal resistance (rs) and decreased Amax (>50%). On the other hand, spinach (Spinacia oleracea L.), a typical sucrose storer (up to 160 milligrams of sucrose per square decimeter in girdled leaves), showed only a slight reduction in CER and almost no change in Amax. Intermediate plants like tomato (Lycopersicon esculentum Mill.), sunflower (Helianthus annuus L.), broad bean (Vicia faba L.), bean (Phaseolus vulgaris L.), and pea (Pisum sativum L.), which upon girdling store both starch and sucrose, responded to the girdle by a considerable reduction in CER but only moderate inhibition of Amax, indicating that the observed reduction in CER was primarily a stomatal response. Both the wild-type tobacco (Nicotiana sylvestris) (which upon girdling stored starch and hexoses) and the starchless mutant (which stored only hexoses, up to 90 milligrams per square decimeter) showed 90 to 100% inhibition of CER and approximately 50% inhibition of Amax. In general, excised leaves (6 days) behaved like girdled leaves of the respective species, showing 50% reduction of Amax in wild-type and starchless N. sylvestris but only slight decline of Amax in spinach. The results of the present study demonstrate the possibility of the occurrence of end-product inhibition of photosynthesis in a large number of crop plants. The long-term inhibition of photosynthesis in girdled leaves is not confined to stomatal responses since the Amax declined up to 50%. The inhibition of Amax by girdling was strongest in starch storers, but starch itself cannot be directly responsible, because the starchless mutant of N. sylvestris was also strongly inhibited. Similarly, the inhibition cannot be attributed to hexose sugars either, because soybean, cotton, and cucumber are among the plants most strongly inhibited although they do not maintain a large hexose pool. Spinach, a sucrose storer, showed the least inhibition in both girdled and excised leaf systems, which indicates that sucrose is probably not directly responsible for the end-product inhibition of photosynthesis. The occurrence of strong end-product inhibition appears to be correlated with high acid-invertase activity in fully expanded leaves. The inhibition may be related to the nature of soluble sugar metabolism in the extrachloroplastic compartment and may be caused by a metabolite that has different rates of accumulation and turnover in sucrose storers and other plants.
416 citations
TL;DR: The HATS for nitrate uptake by barley roots is essentially similar to those reported for Lemna and Zea mays by earlier workers, establishing that there are, nevertheless, distinct differences between barley and corn in their quantitative responses to external NO(3) (-).
Abstract: Transmembrane electrical potential differences (Δψ) of epidermal and cortical cells were measured in intact roots of barley (Hordeum vulgare L. cv Klondike). The effects of exogenous NO3− on Δψ (in the concentration range from 100 micromolar to 20 millimolar) were investigated to probe the mechanisms of nitrate uptake by the high-affinity (HATS) and low-affinity (LATS) transport systems for NO3− uptake. Both transport systems caused depolarization of Δψ, demonstrating that the LATS (like the HATS) for NO3− uptake is probably mediated by an electrogenic cation (H+?) cotransport system. Membrane depolarization by the HATS was “inducible” by NO3−, and saturable with respect to exogenous [NO3−]. By contrast, depolarization by the LATS was constitutive, and first-order in response to external [NO3−]. H+ fluxes, measured in 200 micromolar and in 5 millimolar Ca(NO3)2 solutions, failed to alkalinize external media as anticipated for a 2 H+:1 NO3− symport. However, switching from K2SO4 solutions (which were strongly acidifying) to KNO3 solutions at the same K+ concentration caused marked reductions in H+ efflux. These observations are consistent with NO3− uptake by the HATS and the LATS via 2 H+:1 NO3− symports. These observations establish that the HATS for nitrate uptake by barley roots is essentially similar to those reported for Lemna and Zea mays by earlier workers. There are, nevertheless, distinct differences between barley and corn in their quantitative responses to external NO3−.
TL;DR: The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO2 diffusion determined in unstressed plants at high light as mentioned in this paper.
Abstract: The resistance to diffusion of CO2 from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, 13C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO2 diffusion determined in unstressed plants at high light. The relatively low CO2 partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O2 sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO2 partial pressure upon switching to low O2. We found no correlation between mesophyll conductance and the ratio of internal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO2 and O2 partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO2-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Simmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO2 level.
TL;DR: The results indicate that in sycamore maple suspension-cultured cells, different types of Golgi cisternae contain different sets of glycosyl transferases, and that xyloglucan and polygalacturonic acid/rhamnogalacturonan-I can be synthesized concomitantly within the same Golgi stack.
Abstract: The Golgi apparatus of plant cells is engaged in both the processing of glycoproteins and the synthesis of complex polysaccharides. To investigate the compartmentalization of these functions within individual Golgi stacks, we have analyzed the ultrastructure and the immunolabeling patterns of high-pressure frozen and freeze-substituted suspension-cultured sycamore maple (Acer pseudoplatanus L.) cells. As a result of the improved structural preservation, three morphological types of Golgi cisternae, designated cis, medial, and trans, as well as the trans Golgi network, could be identified. The number of cis cisternae per Golgi stack was found to be fairly constant at approximately 1, whereas the number of medial and trans cisternae per stack was variable and accounted for the varying number of cisternae (3-10) among the many Golgi stacks examined. By using a battery of seven antibodies whose specific sugar epitopes on secreted polysaccharides and glycoproteins are known, we have been able to determine in which types of cisternae specific sugars are added to N-linked glycans, and to xyloglucan and polygalacturonic acid/rhamnogalacturonan-I, two complex polysaccharides. The findings are as follows. The β-1,4-linked d-glucosyl backbone of xyloglucan is synthesized in trans cisternae, and the terminal fucosyl residues on the trisaccharide side chains of xyloglucan are partly added in the trans cisternae, and partly in the trans Golgi network. In contrast, the polygalacturonic/rhamnogalacturonan-I backbone is assembled in cis and medial cisternae, methylesterification of the carboxyl groups of the galacturonic acid residues in the polygalacturonic acid domains occurs mostly in medial cisternae, and arabinose-containing side chains of the polygalacturonic acid domains are added to the nascent polygalacturonic acid/rhamnogalacturonan-I molecules in the trans cisternae. Double labeling experiments demonstrate that xyloglucan and polygalacturonic acid/rhamnogalacturonan-I can be synthesized concomitantly within the same Golgi stack. Finally, we show that the xylosyl residue-linked β-1,2 to the β-linked mannose of the core of N-linked glycans is added in medial cisternae. Taken together, our results indicate that in sycamore maple suspension-cultured cells, different types of Golgi cisternae contain different sets of glycosyl transferases, that the functional organization of the biosynthetic pathways of complex polysaccharides is consistent with these molecules being processed in a cis to trans direction like the N-linked glycans, and that the complex polysaccharide xyloglucan is assembled exclusively in trans Golgi cisternae and the trans Golgi network.
TL;DR: The influence of growth temperature on the free sugar and sucrose phosphate synthase content and activity of spinach leaf tissue was studied and increases in sucrose and its primary biosynthetic enzyme, sucrose phosphorus synthase, are discussed in relation to adjustment of metabolism to low nonfreezing temperature and freezing stress tolerance.
Abstract: The influence of growth temperature on the free sugar and sucrose phosphate synthase content and activity of spinach (Spinacia oleracea) leaf tissue was studied. When plants were grown at 25 degrees C for 3 weeks and then transferred to a constant 5 degrees C, sucrose, glucose, and fructose accumulated to high levels during a 14-d period. Predawn sugar levels increased from 14- to 20-fold over the levels present at the outset of the low-temperature treatment. Sucrose was the most abundant free sugar before, during, and after exposure to 5 degrees C. Leaf sucrose phosphate synthase activity was significantly increased by the low-temperature treatment, whereas sucrose synthase and invertases were not. Synthesis of the sucrose phosphate synthase subunit was increased during and after low-temperature exposure and paralleled an increase in the steady-state level of the subunit. The increases in sucrose and its primary biosynthetic enzyme, sucrose phosphate synthase, are discussed in relation to adjustment of metabolism to low nonfreezing temperature and freezing stress tolerance.
TL;DR: It is concluded that stachyose plays an important role in conferring desiccation tolerance in soybean, suggesting that slow drying serves to induce the accumulation of the raffinose series sugars at a point after galactinol biosynthesis.
Abstract: The desiccation-tolerant state in seeds is associated with high levels of certain sugars and maturation proteins. The aim of this work was to evaluate the contributions of these components to desiccation tolerance in soybean (Glycine max [L.] Merrill cv Chippewa 64). When axes of immature seeds (34 d after flowering) were excised and gradually dried (6 d), desiccation tolerance was induced. By contrast, seeds held at high relative humidity for the same period were destroyed by desiccation. Maturation proteins rapidly accumulated in the axes whether the seeds were slowly dried or maintained at high relative humidity. During slow drying, sucrose content increased to five times the level present in the axes of seeds held at high relative humidity (128 versus 25 μg/axis, respectively). Stachyose content increased dramatically from barely detectable levels upon excision to 483 μg/axis during slow drying but did not increase significantly when seeds were incubated at high relative humidity. Galactinol was the only saccharide that accumulated to higher levels in axes from seeds incubated at high relative humidity relative to axes from seeds that were slowly dried. This suggests that slow drying serves to induce the accumulation of the raffinose series sugars at a point after galactinol biosynthesis. We conclude that stachyose plays an important role in conferring desiccation tolerance.
TL;DR: The results demonstrate the existence of a marked antagonism between physicochemical stresses, with water stress enhancing the resistance of photosystem II to constraints (heat, strong light at high temperature) that are usually associated with drought in the field.
Abstract: The in vivo photochemical activity of photosystem II was inferred from modulated chlorophyll fluorescence and photoacoustic measurements in intact leaves of several plant species (Lycopersicon esculentum Mill., Solanum tuberosum L., Solanum nigrum L.) exposed to various environmental stresses (drought, heat, strong light) applied separately or in combination. Photosystem II was shown to be highly drought-resistant: even a drastic desiccation in air of detached leaf samples only marginally affected the quantum yield for photochemistry in photosystem II. However, water stress markedly modified the responses of photosystem II to superimposed constraints. The stability of photosystem II to heat was observed to increase strongly in leaves exposed to water stress conditions: heat treatments (e.g. 42°C in the dark), which caused a complete and irreversible inhibition of photosystem II in well-watered (tomato) leaves, resulted in a small and fully reversible reduction of the photochemical efficiency of photosystem II in drought-stressed leaves. In vivo photoacoustic data indicated that photosystem I was highly resistant to both heat and water stresses. When leaves were illuminated with intense white light at 25°C, photoinhibition damage of photosystem II was more pronounced in water-stressed leaves than in undesiccated controls. However, in nondehydrated leaves, photoinhibition of photosystem II was strongly temperature dependent, being drastically stimulated at high temperatures above 38 to 40°C. As a consequence, when exposed to strong light at high temperature, photosystem II photochemistry was significantly less inhibited in dehydrated leaves than in control well-hydrated leaves. Our results demonstrate the existence of a marked antagonism between physicochemical stresses, with water stress enhancing the resistance of photosystem II to constraints (heat, strong light at high temperature) that are usually associated with drought in the field.
TL;DR: Aspirin, atrade name for acetylsalicylic acid, was introduced by the BayerCompany in1898 andrapidly became one of world's best-selling drugs and salicylates, from theLatin wordSalix forwillow tree, was given to this active ingredient ofwillow bark byRaffaele Piria in1838.
Abstract: Centuries ago,theAmerican Indians andancient Greeks independently discovered thattheleaves andbarkofthe willow tree cured aches andfevers. Itwasnotuntil 1828that Johann Buchner, working inMunich, successfully isolated a tiny amountofsalicin, theglucoside ofsalicyl alcohol, which wasthemajorsalicylate inwillow bark(for review seeref. 23). ThenameSA2, fromtheLatin wordSalix forwillow tree, wasgiven tothis active ingredient ofwillow barkbyRaffaele Piria in1838. Thefirst commercial production ofsynthetic SAbeganinGermany in1874.Aspirin, atrade namefor acetylsalicylic acid, wasintroduced bytheBayerCompany in1898andrapidly became oneofworld's best-selling drugs. Inspite ofthefact thatthemodeofmedicinal action of salicylates isasubject ofcontinual debate, theyarebeing usedtotreat humandiseases ranging fromthecommoncold toheart attacks. Because eveninaqueous solutions aspirin undergoes spontaneous hydrolysis toSA,thetwocompounds havesimilar effects inplants andwill betreated together in this review. Salicylic orortho-hydroxybenzoic acid belongs toadiverse
TL;DR: The results indicate that the galactosyluronic acid units of grass pectic polysaccharides may be converted to other kinds of esters or form ester-like chemical interactions during expansion of the cell wall.
Abstract: Cell walls of grasses have two major polysaccharides that contain uronic acids, the hemicellulosic glucuronoarabinoxylans and the galactosyluronic acid-rich pectins. A technique whereby esterified uronic acid carboxyl groups are reduced selectively to yield their respective 6,6-dideuterio neutral sugars was used to determine the extent of esterification and changes in esterification of these two uronic acids during elongation of maize (Zea mays L.) coleoptiles. The glucosyluronic acids of glucuronoarabinoxylans did not appear to be esterified at any time during coleoptile elongation. The galactosyluronic acids of embryonal coleoptiles were about 65% esterified, but this proportion increased to nearly 80% during the rapid elongation phase before returning to about 60% at the end of elongation. Methyl esters accounted for about two-thirds of the total esterified galacturonic acid in cell walls of unexpanded coleoptiles. The proportion of methyl esters decreased throughout elongation and did not account for the increase in the proportion of esterified galactosyluronic acid units during growth. The results indicate that the galactosyluronic acid units of grass pectic polysaccharides may be converted to other kinds of esters or form ester-like chemical interactions during expansion of the cell wall. Accumulation of novel esters or ester-like interactions is coincident with covalent attachment of polymers containing galactosyluronic acid units to the cell wall.
TL;DR: The results indicate that cessation of elongation followed an increase in cell wall peroxidase activity, which occurred at comparable developmental stages in the two genotypes of tall fescue.
Abstract: Previous work suggested that cell wall peroxidase activity increased as cells were displaced through the elongation zone in leaf blades of tall fescue (Festuca arundinacea Schreb.). In this study, two genotypes that differ in length of the elongation zone were used to examine the relationship between peroxidase activity in apoplastic fluid of intact leaf blade segments and the spatial distribution of leaf growth. Apoplastic fluid was extracted by vacuum infiltration and centrifugation, and peroxidase activity was assayed spectrophotometrically. Isoelectric focusing was used to characterize the isoforms of apoplastic peroxidase within the region of elongation and in the region of secondary cell wall deposition, which is distal to the elongation zone. A striking correlation was found in each genotype between both the location and timing of increase in apoplastic peroxidase activity and the onset of growth deceleration. Only cationic isoforms of apoplastic peroxidase could be identified in the elongation zone, whereas additional anionic isoforms appeared in the region of secondary cell wall deposition. We conclude that cessation of elongation growth in tall fescue leaf blades is likely to be related to the secretion of cationic isoforms of peroxidase into the cell wall.
TL;DR: The observations suggest that epidermal water relations may act as a modulator of the responses of stomata to ABA, and it is argued that such changes must be taken into account in studies or modeling of plant responses to drought stress.
Abstract: We investigated, under laboratory and field conditions, the possibility that increasing abscisic acid (ABA) concentrations and decreasing water potentials can interact in their effects on stomata. One experiment was carried out with epidermal pieces of Commelina communis incubated in media with a variety of ABA and polyethylene glycol concentrations. In the media without ABA, incubation in solutions with water potentials between −0.3 and −1.5 megapascals had no significant effect on stomatal aperture. Conversely, the sensitivity of stomatal aperture to ABA was trebled in solutions at −1.5 megapascals compared with sensitivity at −0.3 megapascals. The effect of the change in sensitivity was more important than the absolute effect of ABA at the highest water potential. In a field experiment, sensitivity of maize stomatal conductance to the concentration of ABA in the xylem sap varied strongly with the time of the day. We consider that the most likely explanation for this is the influence of a change in leaf or epidermal water potential that accompanies an increase in irradiance and saturation deficit as the day progresses. These observations suggest that epidermal water relations may act as a modulator of the responses of stomata to ABA. We argue that such changes must be taken into account in studies or modeling of plant responses to drought stress.
TL;DR: The results indicate that the growth rate of deepwater rice internodes is determined by the ratio of an endogenous growth promoter (GA) and a growth inhibitor (ABA), which may be related to the semiaquatic growth habit of this plant.
Abstract: Submergence induces rapid elongation of rice coleoptiles (Oryza sativa L) and of deepwater rice internodes This adaptive feature helps rice to grow out of the water and to survive flooding Earlier, we found that the growth response of submerged deepwater rice plants is mediated by ethylene and gibberellin (GA) Ethylene promotes growth, at least in part, by increasing the responsiveness of the internodal tissue to GA In the present work, we examined the possibility that increased responsiveness to GA was based on a reduction in endogenous abscisic acid (ABA) levels Submergence and treatment with ethylene led, within 3 hours, to a 75% reduction in the level of ABA in the intercalary meristem and the growing zone of deepwater rice internodes The level of GA1 increased fourfold during the same time period An interaction between GA and ABA could also be shown by application of the hormones ABA inhibited growth of submerged internodes, and GA counteracted this inhibition Our results indicate that the growth rate of deepwater rice internodes is determined by the ratio of an endogenous growth promoter (GA) and a growth inhibitor (ABA) We also investigated whether ABA is involved in regulating the growth of rice coleoptiles Rice seedlings were grown on solutions containing fluridone, an inhibitor of carotenoid and, indirectly, of ABA biosynthesis Treatment with fluridone reduced the level of ABA in coleoptiles and first leaves by more than 75% and promoted coleoptile growth by more than 60% Little or no enhancement of growth by fluridone was observed in barley, oat, or wheat The involvement of ABA in determining the growth rate of rice coleoptiles and deepwater rice internodes may be related to the semiaquatic growth habit of this plant
TL;DR: Results in this study showed that loss of hydraulic conductivity occurred in stem segments pressurized in a pressure chamber while the xylem water was under positive pressure, consistent with the air-seeding hypothesis of how embolism are nucleated, but not with the nucleation of embolisms at hydrophobic cracks.
Abstract: Loss of hydraulic conductivity occurs in stems when the water in xylem conduits is subjected to sufficiently negative pressure. According to the air-seeding hypothesis, this loss of conductivity occurs when air bubbles are sucked into water-filled conduits through micropores adjacent to air spaces in the stem. Results in this study showed that loss of hydraulic conductivity occurred in stem segments pressurized in a pressure chamber while the xylem water was under positive pressure. Vulnerability curves can be defined as a plot of percentage loss of hydraulic conductivity versus the pressure difference between xylem water and the outside air inducing the loss of conductivity. Vulnerability curves were similar whether loss of conductivity was induced by lowering the xylem water pressure or by raising the external air pressure. These results are consistent with the air-seeding hypothesis of how embolisms are nucleated, but not with the nucleation of embolisms at hydrophobic cracks because the latter requires negative xylem water pressure. The results also call into question some basic underlying assumptions used in the determination of components of tissue water potential using "pressure-volume" analysis.
TL;DR: Results indicate that this mechanism of gene regulation in response to sulfur nutrition is conserved in petunia even though it is not used to regulate its own major seed storage proteins.
Abstract: The 7S seed storage protein (β-conglycinin) of soybean (Glycine max [L]. Merr.) has three major subunits; α, α′, and β. Accumulation of the β-subunit, but not the α- and α′-subunits, has been shown to be repressed by exogenously applied methionine to the immature cotyledon culture system (LP Holowach, JF Thompson, JT Madison [1984] Plant Physiol 74: 576-583) and to be enhanced under sulfate deficiency in soybean plants (KR Gayler, GE Sykes [1985] Plant Physiol 78: 582-585). Transgenic petunia (Petunia hybrida) harboring either the α′- or β-subunit gene were constructed to test whether the patterns of differential expression were retained in petunia. Petunia regulates these genes in a similar way as soybean in response to sulfur nutritional stimuli, i.e. (a) expression of the β-subunit gene is repressed by exogenous methionine in in vitro cultured seeds, whereas the α′-subunit gene expression is not affected; and (b) accumulation of the β-subunit is enhanced by sulfur deficiency. The pattern of accumulation of major seed storage protein of petunia was not affected by these treatments. These results indicate that this mechanism of gene regulation in response to sulfur nutrition is conserved in petunia even though it is not used to regulate its own major seed storage proteins.
TL;DR: The results indicate that although there may be a linkage between isoprene emission rate and photosynthesis, the temperature dependence of isopren emission is not determined solely by the rates of CO(2) assimilation or electron transport.
Abstract: Isoprene emissions from the leaves of velvet bean ( Mucuna pruriens L. var utilis ) plants exhibited temperature response patterns that were dependent on the plant9s growth temperature. Plants grown in a warm regimen (34/28°C, day/night) exhibited a temperature optimum for emissions of 45°C, whereas those grown in a cooler regimen (26/20°C, day/night) exhibited an optimum of 40°C. Several previous studies have provided evidence of a linkage between isoprene emissions and photosynthesis, and more recent studies have demonstrated that isoprene emissions are linked to the activity of isoprene synthase in plant leaves. To further explore this linkage within the context of the temperature dependence of isoprene emissions, we determined the relative temperature dependencies of photosynthetic electron transport, CO 2 assimilation, and isoprene synthase activity. When measured over a broad range of temperatures, the temperature dependence of isoprene emission rate was not closely correlated with either the electron transport rate or the CO 2 assimilation rate. The temperature optima for electron transport rate and CO 2 assimilation rate were 5 to 10°C lower than that for the isoprene emission rate. The dependence of isoprene emissions on photon flux density was also affected by measurement temperature in a pattern independent of those exhibited for electron transport rate and CO 2 assimilation rate. Thus, despite no change in the electron transport rate or CO 2 assimilation rate at 26 and 34°C, the isoprene emission rate changed markedly. The quantum yield of isoprene emissions was stimulated by a temperature increase from 26 to 34°C, whereas the quantum yield for CO 2 assimilation was inhibited. In greenhouse-grown aspen leaves ( Populus tremuloides Michaux.), the high temperature threshold for inhibition of isoprene emissions was closely correlated with the high temperature-induced decrease in the in vitro activity of isoprene synthase. When taken together, the results indicate that although there may be a linkage between isoprene emission rate and photosynthesis, the temperature dependence of isoprene emission is not determined solely by the rates of CO 2 assimilation or electron transport. Rather, we propose that regulation is accomplished primarily through the enzyme isoprene synthase.
TL;DR: Coupled cyclic electron transport is assigned a role in the protection of leaves against photoinhibition in addition to its role in ATP synthesis and avoidance of overreduction of the electron transport chain is a prerequisite for the efficient protection of the photosynthetic apparatus against photo inactivation.
Abstract: Coupled cyclic electron transport is assigned a role in the protection of leaves against photoinhibition in addition to its role in ATP synthesis. In leaves, as in reconstituted thylakoid systems, cyclic electron transport requires “poising,” i.e. availability of electrons at the reducing side of photosystem I (PSI) and the presence of some oxidized plastoquinone between photosystem II (PSII) and PSI. Under self-regulatory poising conditions that are established when carbon dioxide limits photosynthesis at high light intensities, and particularly when stomata are partially or fully closed as a result of water stress, coupled cyclic electron transport controls linear electron transport by helping to establish a proton gradient large enough to decrease PSII activity and electron flow to PSI. This brings electron donation by PSII, and electron consumption by available electron acceptors, into a balance in which PSI becomes more oxidized than it is during fast carbon assimilation. Avoidance of overreduction of the electron transport chain is a prerequisite for the efficient protection of the photosynthetic apparatus against photoinactivation.
TL;DR: The needle content and kinetic properties of GR, the increased activities of other enzymes, and the high substrate concentrations observed during the winter are consistent with the protective function this pathway may provide against photooxidative, winter injury.
Abstract: Antioxidant metabolites in eastern white pine (Pinus strobus L.) needles increased two- to fourfold from the summer to the winter season. Antioxidant enzymes in needle tissue increased between 2- and 122-fold during this same period. These seasonal changes were determined by monitoring ascorbate and glutathione concentrations and the activity of ascorbate peroxidase, glutathione reductase (GR), and superoxide dismutase. Levels of antioxidant metabolites and enzymes were observed always to be lowest during the summer, or active growing season, and highest during the winter, or dormant season. These data correlated well with the thermal kinetic window for purified GR obtained from summer needles. The minimum, apparent Km,NADPH for two isoforms of GR (GRA and GRB) occurred at 5 and 10°C, respectively. The upper limit of the thermal kinetic window (200% of the minimum Km) for GRA and GRB was 20 and 25°C, respectively, indicating that needle temperatures exceeding 25°C may result in impairment of antioxidant metabolism. The needle content and kinetic properties of GR, the increased activities of other enzymes, and the high substrate concentrations observed during the winter are consistent with the protective function this pathway may provide against photooxidative, winter injury.
TL;DR: The results demonstrate that near-ambient O(3) levels can induce pathogenesis-related proteins and may thereby alter the disposition of plants toward pathogen attack.
Abstract: A single pulse of O3 (0.15 microliter per liter, 5 hours) induced β-1,3-glucanase and chitinase activities in O3-sensitive and -tolerant tobacco (Nicotiana tabacum L.) cultivars. In the O3-sensitive cultivar Bel W3, the response was rapid (maximum after 5 to 10 hours) and was far more pronounced for β-1,3-glucanase (40- to 75-fold) than for chitinase (4-fold). In the O3-tolerant cultivar Bel B, β-1,3-glucanase was induced up to 30-fold and chitinase up to 3-fold under O3 concentrations that did not lead to visible damage. Northern blot hybridization showed a marked increase in β-1,3-glucanase mRNA in cultivar Bel W3 between 3 and 24 hours following O3 treatment, a transient induction in cultivar Bel B, and no change in control plants. The induction of β-1,3-glucanase and chitinase activities following O3 treatment occurred within the leaf cells and was not found in the intercellular wash fluids. In addition, O3 treatment increased the amount of the β-1,3-glucan callose, which accumulated predominantly around the necrotic spots in cultivar Bel W3. The results demonstrate that near-ambient O3 levels can induce pathogenesis-related proteins and may thereby alter the disposition of plants toward pathogen attack.
TL;DR: A role for cell-surface electrical potential in the rhizotoxicity of Al(3+, La(3+), H(+), and other toxic cations is suggested: negatively charged cell surfaces of the root accumulate the toxic cATIONS, and amelioration is effected by treatments that reduce the negativity of the cell- surface electrical potential by charge screening or cation binding.
Abstract: The rhizotoxicities of Al(3+) and of La(3+) to wheat (Triticum aestivum L.) were similarly ameliorated by cations in the following order of effectiveness: H(+) approximately C(3+) > C(2+) > C(1+). Among tested cations of a given charge, ameliorative effectiveness was similar except that Ca(2+) was slightly more effective than other divalent cations and H(+) was much more effective than other monovalent cations. H(+) rhizotoxicity was also ameliorated by cations in the order C(3+) > C(2+) > C(1+). These results suggest a role for cell-surface electrical potential in the rhizotoxicity of Al(3+), La(3+), H(+), and other toxic cations: negatively charged cell surfaces of the root accumulate the toxic cations, and amelioration is effected by treatments that reduce the negativity of the cell-surface electrical potential by charge screening or cation binding. Membrane-surface activities of free Al(3+) or La(3+) computed according to a Gouy-Chapman-Stern model correlated well with growth inhibition, which correlated only poorly with Al(3+) or La(3+) activities in the external medium. The similar responses of Al-intoxicated and La-intoxicated roots to ameliorative treatments provide evidence that Al(3+), rather than AlOH(2+) or Al(OH)(2) (+), is the principal toxic species of mononuclear Al. Comparisons of the responses of Al-sensitive and Al-tolerant wheats to Al(3+) and to La(3+) did not support the hypothesis that varietal sensitivity to Al(3+) is based upon differences in cell-surface electrical potential.
TL;DR: The inability of bean to activate fully certain Calvin cycle enzymes and sucrose-phosphate synthase, or to increase nonphotochemical quenching of chlorophyll fluorescence at 10 degrees C, may be factors contributing to its poor performance at low temperature.
Abstract: The aim of this study was to determine the response of photosynthetic carbon metabolism in spinach and bean to low temperature. (a) Exposure of warm-grown spinach and bean plants to 10°C for 10 days resulted in increases in the total activities of a number of enzymes, including ribulose 1,5-bisphosphate carboxylase (Rubisco), stromal fructose 1,6 bisphosphatase (Fru 1,6-P2ase), sedoheptulose 1,7-bisphosphatase (Sed 1,7-P2ase), and the cytosolic Fru 1,6-P2ase. In spinach, but not bean, there was an increase in the total activity of sucrose-phosphate synthase. (b) The CO2-saturated rates of photosynthesis for the cold-acclimated spinach plants were 68% greater at 10°C than those for warm-acclimated plants, whereas in bean, rates of photosynthesis at 10°C were very low after exposure to low temperature. (c) When spinach leaf discs were transferred from 27 to 10°C, the stromal Fru 1,6-P2ase and NADP-malate dehydrogenase were almost fully activated within 8 minutes, and Rubisco reached 90% of full activation within 15 minutes of transfer. An initial restriction of Calvin cycle fluxes was evident as an increase in the amounts of ribulose 1,5-bisphosphate, glycerate-3-phosphate, Fru 1,6-P2, and Sed 1,7-P2. In bean, activation of stromal Fru 1,6-P2ase was weak, whereas the activation state of Rubisco decreased during the first few minutes after transfer to low temperature. However, NADP-malate dehydrogenase became almost fully activated, showing that no loss of the capacity for reductive activation occurred. (d) Temperature compensation in spinach evidently involves increases in the capacities of a range of enzymes, achieved in the short term by an increase in activation state, whereas long-term acclimation is achieved by an increase in the maximum activities of enzymes. The inability of bean to activate fully certain Calvin cycle enzymes and sucrose-phosphate synthase, or to increase nonphotochemical quenching of chlorophyll fluorescence at 10°C, may be factors contributing to its poor performance at low temperature.
TL;DR: The results indicate that gamma-TIP may be expressed primarily at the time when the large central vacuoles are being formed during cell enlargement.
Abstract: The vacuolar membrane (tonoplast) contains an abundant intrinsic protein with six membrane-spanning domains that is encoded by a small gene family. Different isoforms of tonoplast intrinsic protein (TIP) are expressed in different tissues or as a result of specific signals. Using promoter-β-glucuronidase (GUS) fusions and in situ hybridization, we have examined the expression of γ-TIP in Arabidopsis thaliana. GUS staining of plants transformed with promoter-GUS fusions showed that γ-TIP gene expression is high in recently formed tissues of young roots. In the shoot, γ-TIP gene expression was highest in the vascular bundles of stems and petioles, as well as in the stipules and in the receptacle of the flower. No GUS activity was detected in root or shoot meristems or in older tissues, suggesting temporal control of γ-TIP gene expression associated with cell elongation and/or differentiation. In situ hybridization carried out with whole seedlings confirmed that in root tips, γ-TIP mRNA was present only in the zone of cell elongation just behind the apical meristem. In seedling shoots, mRNA abundance was also found to be correlated with cell expansion. These results indicate that γ-TIP may be expressed primarily at the time when the large central vacuoles are being formed during cell enlargement.