Showing papers in "Journal of Experimental Botany in 2009"

Elevated CO2 effects on plant carbon, nitrogen, and water relations: six important lessons from FACE

Abstract: Plant responses to the projected future levels of CO2 were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO2 were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO2 Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO2 on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO2 under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO2 stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO2 improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO2 stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO2 does not directly stimulate C4 photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO2 in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.

Class III peroxidases in plant defence reactions

Abstract: When plants are attacked by pathogens, they defend themselves with an arsenal of defence mechanisms, both passive and active. The active defence responses, which require de novo protein synthesis, are regulated through a complex and interconnected network of signalling pathways that mainly involve three molecules, salicylic acid (SA), jasmonic acid (JA), and ethylene (ET), and which results in the synthesis of pathogenesis-related (PR) proteins. Microbe or elicitor-induced signal transduction pathways lead to (i) the reinforcement of cell walls and lignification, (ii) the production of antimicrobial metabolites (phytoalexins), and (iii) the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Among the proteins induced during the host plant defence, class III plant peroxidases (EC 1.11.1.7; hydrogen donor: H(2)O(2) oxidoreductase, Prxs) are well known. They belong to a large multigene family, and participate in a broad range of physiological processes, such as lignin and suberin formation, cross-linking of cell wall components, and synthesis of phytoalexins, or participate in the metabolism of ROS and RNS, both switching on the hypersensitive response (HR), a form of programmed host cell death at the infection site associated with limited pathogen development. The present review focuses on these plant defence reactions in which Prxs are directly or indirectly involved, and ends with the signalling pathways, which regulate Prx gene expression during plant defence. How they are integrated within the complex network of defence responses of any host plant cell will be the cornerstone of future research.

Raising yield potential in wheat

Abstract: Recent advances in crop research have the potential to accelerate genetic gains in wheat, especially if co-ordinated with a breeding perspective. For example, improving photosynthesis by exploiting natural variation in Rubisco’s catalytic rate or adopting C4 metabolism could raise the baseline for yield potential by 50% or more. However, spike fertility must also be improved to permit full utilization of photosynthetic capacity throughout the crop life cycle and this has several components. While larger radiation use efficiency will increase the total assimilates available for spike growth, thereby increasing the potential for grain number, an optimized phenological pattern will permit the maximum partitioning of the available assimilates to the spikes. Evidence for underutilized photosynthetic capacity during grain filling in elite material suggests unnecessary floret abortion. Therefore, a better understanding of its physiological and genetic basis, including possible signalling in response to photoperiod or growth-limiting resources, may permit floret abortion to be minimized for a more optimal source:sink balance. However, trade-offs in terms of the partitioning of assimilates to competing sinks during spike growth, to improve root anchorage and stem strength, may be necessary to prevent yield losses as a result of lodging. Breeding technologies that can be used to complement conventional approaches include wide crossing with members of the Triticeae tribe to broaden the wheat genepool, and physiological and molecular breeding strategically to combine complementary traits and to identify elite progeny more efficiently.

Recent advances in ethylene research

Abstract: Ethylene regulates many aspects of the plant life cycle, including seed germination, root initiation, flower development, fruit ripening, senescence, and responses to biotic and abiotic stresses. It thus plays a key role in responses to the environment that have a direct bearing on a plant's fitness for adaptation and reproduction. In recent years, there have been major advances in our understanding of the molecular mechanisms regulating ethylene synthesis and action. Screening for mutants of the triple response phenotype of etiolated Arabidopsis seedlings, together with map-based cloning and candidate gene characterization of natural mutants from other plant species, has led to the identification of many new genes for ethylene biosynthesis, signal transduction, and response pathways. The simple chemical nature of ethylene contrasts with its regulatory complexity. This is illustrated by the multiplicity of genes encoding the key ethylene biosynthesis enzymes 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase, multiple ethylene receptors and signal transduction components, and the complexity of regulatory steps involving signalling relays and control of mRNA and protein synthesis and turnover. In addition, there are extensive interactions with other hormones. This review integrates knowledge from the model plant Arabidopsis and other plant species and focuses on key aspects of recent research on regulatory networks controlling ethylene synthesis and its role in flower development and fruit ripening.

Root-to-shoot Cd translocation via the xylem is the major process determining shoot and grain cadmium accumulation in rice

Abstract: Physiological properties involved in divergent cadmium (Cd) accumulation among rice genotypes were characterized using the indica cultivar ‘Habataki’ (high Cd in grains) and the japonica cultivar ‘Sasanishiki’ (low Cd in grains). Time-dependence and concentration-dependence of symplastic Cd absorption in roots were revealed not to be responsible for the different Cd accumulation between the two cultivars because root Cd uptake was not greater in the Cd-accumulating cultivar ‘Habataki’ compared with ‘Sasanishiki’. On the other hand, rapid and greater root-to-shoot Cd translocation was observed in ‘Habataki’, which could be mediated by higher abilities in xylem loading of Cd and transpiration rate as a driving force. To verify whether different abilities in xylem-mediated shoot-to-root translocation generally account for the genotypic variation in shoot Cd accumulation in rice, the world rice core collection, consisting of 69 accessions which covers the genetic diversity of almost 32 000 accessions of cultivated rice, was used. The results showed strong correlation between Cd levels in xylem sap and shoots and grains among the 69 rice accessions. Overall, the results presented in this study revealed that the root-to-shoot Cd translocation via the xylem is the major and common physiological process determining the Cd accumulation level in shoots and grains of rice plants.

Normalization of qRT-PCR data: the necessity of adopting a systematic, experimental conditions-specific, validation of references

Abstract: Quantitative RT-PCR (reverse transcription polymerase chain reaction, also known as qRT-PCR or real-time RT-PCR) has been used in large proportions of transcriptome analyses published to date. The accuracy of the results obtained by this method strongly depends on accurate transcript normalization using stably expressed genes, known as references. Statistical algorithms have been developed recently to help validate reference genes but, surprisingly, this robust approach is under-utilized in plants. Instead, putative 'housekeeping' genes tend to be used as references without any proper validation. The concept of normalization in transcript quantification is introduced here and the factors affecting its reliability in qRT-PCR are discussed in an attempt to convince molecular biologists, and non-specialists, that systematic validation of reference genes is essential for producing accurate, reliable data in qRT-PCR analyses, and thus should be an integral component of them.

Resistances along the CO2 diffusion pathway inside leaves

Abstract: CO2 faces a series of resistances while diffusing between the substomatal cavities and the sites of carboxylation within chloroplasts. The absence of techniques to measure the resistance of individual steps makes it difficult to define their relative importance. Resistance to diffusion through intercellular airspace differs between leaves, but is usually of minor importance. Leaves with high photosynthetic capacity per unit leaf area reduce mesophyll resistance by increasing the surface area of chloroplasts exposed to intercellular airspace per unit leaf area, Sc. Cell walls impose a significant resistance. Assuming an effective porosity of the cell wall of 0.1 or 0.05, then cell walls could account for 25% or 50% of the total mesophyll resistance, respectively. Since the fraction of apoplastic water that is unbound and available for unhindered CO2 diffusion is unknown, it is possible that the effective porosity is 50% of the total resistance and a variable proportion. Most of the remaining resistance is imposed by one or more of the three membranes as mesophyll resistance can be altered by varying the expression of cooporins. The CO2 permeability of vesicles prepared from chloroplast envelopes has been reduced by RNA interference (RNAi) expression of NtAQP1, but not those prepared from the plasma membrane. Carbonic anhydrase activity also influences mesophyll resistance. Mesophyll resistance is relatively insensitive to the manipulation of any step in the pathway because it represents only part of the total and may also be countered by pleiotropic compensatory changes. The parameters in greatest need of additional measurements are Sc, mesophyll cell wall thickness, and the permeabilities of the plasma membrane and chloroplast envelope.

Overexpression of the soybean GmERF3 gene, an AP2/ERF type transcription factor for increased tolerances to salt, drought, and diseases in transgenic tobacco

Abstract: A new member of the AP2/ERF transcription factor family, GmERF3, was isolated from soybean. Sequence analysis showed that GmERF3 contained an AP2/ERF domain of 58 amino acids and two putative nuclear localization signal (NLS) domains. It belonged to a group IV protein in the ERF (ethylene response factor) subfamily as typified by a conserved N-terminal motif [MCGGAI(I/L)]. Expression of GmERF3 was induced by treatments with high salinity, drought, abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), ethylene (ET), and soybean mosaic virus (SMV), whereas there was no significant GmERF3 mRNA accumulation under cold stress treatment. GmERF3 could bind to the GCC box and DRE/CRT element, and was targeted to the nucleus when transiently expressed in onion epidermal cells. The GmERF3 protein fused to the GAL4 DNA-binding domain to activate transcription of reporter genes in yeast. Ectopic expression of the GmERF3 gene in transgenic tobacco plants induced the expression of some PR genes and enhanced resistance against infection by Ralstonia solanacearum, Alternaria alternata, and tobacco mosaic virus (TMV), and gave tolerance to high salinity and dehydration stresses. Furthermore, overexpression of GmERF3 in transgenic tobacco led to higher levels of free proline and soluble carbohydrates compared to wild-type plants under drought conditions. The overall results suggested that GmERF3 as an AP2/ERF transcription factor may play dual roles in response to biotic and abiotic stresses in plants.

Integrating pests and pathogens into the climate change/food security debate

Abstract: While many studies have demonstrated the sensitivities of plants and of crop yield to a changing climate, a major challenge for the agricultural research community is to relate these findings to the broader societal concern with food security. This paper reviews the direct effects of climate on both crop growth and yield and on plant pests and pathogens and the interactions that may occur between crops, pests, and pathogens under changed climate. Finally, we consider the contribution that better understanding of the roles of pests and pathogens in crop production systems might make to enhanced food security. Evidence for the measured climate change on crops and their associated pests and pathogens is starting to be documented. Globally atmospheric [CO(2)] has increased, and in northern latitudes mean temperature at many locations has increased by about 1.0-1.4 degrees C with accompanying changes in pest and pathogen incidence and to farming practices. Many pests and pathogens exhibit considerable capacity for generating, recombining, and selecting fit combinations of variants in key pathogenicity, fitness, and aggressiveness traits that there is little doubt that any new opportunities resulting from climate change will be exploited by them. However, the interactions between crops and pests and pathogens are complex and poorly understood in the context of climate change. More mechanistic inclusion of pests and pathogen effects in crop models would lead to more realistic predictions of crop production on a regional scale and thereby assist in the development of more robust regional food security policies.

Climate change and the flowering time of annual crops

Abstract: Crop production is inherently sensitive to variability in climate. Temperature is a major determinant of the rate of plant development and, under climate change, warmer temperatures that shorten development stages of determinate crops will most probably reduce the yield of a given variety. Earlier crop flowering and maturity have been observed and documented in recent decades, and these are often associated with warmer (spring) temperatures. However, farm management practices have also changed and the attribution of observed changes in phenology to climate change per se is difficult. Increases in atmospheric [CO(2)] often advance the time of flowering by a few days, but measurements in FACE (free air CO(2) enrichment) field-based experiments suggest that elevated [CO(2)] has little or no effect on the rate of development other than small advances in development associated with a warmer canopy temperature. The rate of development (inverse of the duration from sowing to flowering) is largely determined by responses to temperature and photoperiod, and the effects of temperature and of photoperiod at optimum and suboptimum temperatures can be quantified and predicted. However, responses to temperature, and more particularly photoperiod, at supraoptimal temperature are not well understood. Analysis of a comprehensive data set of time to tassel initiation in maize (Zea mays) with a wide range of photoperiods above and below the optimum suggests that photoperiod modulates the negative effects of temperature above the optimum. A simulation analysis of the effects of prescribed increases in temperature (0-6 degrees C in +1 degree C steps) and temperature variability (0% and +50%) on days to tassel initiation showed that tassel initiation occurs later, and variability was increased, as the temperature exceeds the optimum in models both with and without photoperiod sensitivity. However, the inclusion of photoperiod sensitivity above the optimum temperature resulted in a higher apparent optimum temperature and less variability in the time of tassel initiation. Given the importance of changes in plant development for crop yield under climate change, the effects of photoperiod and temperature on development rates above the optimum temperature clearly merit further research, and some of the knowledge gaps are identified herein.

The role of ABA in triggering ethylene biosynthesis and ripening of tomato fruit.

Abstract: In order to understand more details about the role of abscisic acid (ABA) in fruit ripening and senescence of tomato, two cDNAs (LeNCED1 and LeNCED2) which encode 9-cis-epoxycarotenoid dioxygenase (NCED) as a key enzyme in ABA biosynthesis, two cDNAs (LeACS2 and LeACS4) which encode 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, and one cDNA (LeACO1) which encodes ACC oxidase involved in ethylene biosynthesis were cloned from tomato fruit using a reverse transcription-PCR (RT-PCR) approach. The relationship between ABA and ethylene during ripening was also investigated. Among six sampling times in tomato fruits, the LeNCED1 gene was highly expressed only at the breaker stage when the ABA content becomes high. After this, the LeACS2, LeACS4, and LeACO1 genes were expressed with some delay. The change in pattern of ACO activity was in accordance with ethylene production reaching its peak at the pink stage. The maximum ABA content preceded ethylene production in both the seeds and the flesh. The peak value of ABA, ACC, and ACC oxidase activity, and ethylene production all started to increase earlier in seeds than in flesh tissues, although they occurred at different ripening stages. Exogenous ABA treatment increased the ABA content in both flesh and seed, inducing the expression of both ACS and ACO genes, and promoting ethylene synthesis and fruit ripening, while treatment with fluridone or nordihydroguaiaretic acid (NDGA) inhibited them, delaying fruit ripening and softening. Based on the results obtained in this study, it was concluded that LeNCED1 initiates ABA biosynthesis at the onset of fruit ripening, and might act as an original inducer, and ABA accumulation might play a key role in the regulation of ripeness and senescence of tomato fruit.

The beneficial endophyte Trichoderma hamatum isolate DIS 219b promotes growth and delays the onset of the drought response in Theobroma cacao

Abstract: Theobroma cacao (cacao) is cultivated in tropical climates and is exposed to drought stress. The impact of the endophytic fungus Trichoderma hamatum isolate DIS 219b on cacao’s response to drought was studied. Colonization by DIS 219b delayed drought-induced changes in stomatal conductance, net photosynthesis, and green fluorescence emissions. The altered expression of 19 expressed sequence tags (ESTs) (seven in leaves and 17 in roots with some overlap) by drought was detected using quantitative real-time reverse transcription PCR. Roots tended to respond earlier to drought than leaves, with the drought-induced changes in expression of seven ESTs being observed after 7 d of withholding water. Changes in gene expression in leaves were not observed until after 10 d of withholding water. DIS 219b colonization delayed the drought-altered expression of all seven ESTs responsive to drought in leaves by >3 d, but had less influence on the expression pattern of the drought-responsive ESTs in roots. DIS 219b colonization had minimal direct influence on the expression of drought-responsive ESTs in 32-d-old seedlings. By contrast, DIS 219b colonization of 9-d-old seedlings altered expression of drought-responsive ESTs, sometimes in patterns opposite of that observed in response to drought. Drought induced an increase in the concentration of many amino acids in cacao leaves, while DIS 219b colonization caused a decrease in aspartic acid and glutamic acid concentrations and an increase in alanine and g-aminobutyric acid concentrations. With or without exposure to drought conditions, colonization by DIS 219b promoted seedling growth, the most consistent effects being an increase in root fresh weight, root dry weight, and root water content. Colonized seedlings were slower to wilt in response to drought as measured by a decrease in the leaf angle drop. The primary direct effect of DIS 219b colonization was promotion of root growth, regardless of water status, and an increase in water content which it is proposed caused a delay in many aspects of the drought response of cacao.

Metabolic responses to salt stress of barley (Hordeum vulgare L.) cultivars, Sahara and Clipper, which differ in salinity tolerance

Abstract: Plants show varied cellular responses to salinity that are partly associated with maintaining low cytosolic Na+ levels and a high K+/Na+ ratio. Plant metabolites change with elevated Na+, some changes are likely to help restore osmotic balance while others protect Na+-sensitive proteins. Metabolic responses to salt stress are described for two barley (Hordeum vulgare L.) cultivars, Sahara and Clipper, which differed in salinity tolerance under the experimental conditions used. After 3 weeks of salt treatment, Clipper ceased growing whereas Sahara resumed growth similar to the control plants. Compared with Clipper, Sahara had significantly higher leaf Na+ levels and less leaf necrosis, suggesting they are more tolerant to accumulated Na+. Metabolite changes in response to the salt treatment also differed between the two cultivars. Clipper plants had elevated levels of amino acids, including proline and GABA, and the polyamine putrescine, consistent with earlier suggestions that such accumulation may be correlated with slower growth and/or leaf necrosis rather than being an adaptive response to salinity. It is suggested that these metabolites may be an indicator of general cellular damage in plants. By contrast, in the more tolerant Sahara plants, the levels of the hexose phosphates, TCA cycle intermediates, and metabolites involved in cellular protection increased in response to salt. These solutes remain unchanged in the more sensitive Clipper plants. It is proposed that these responses in the more tolerant Sahara are involved in cellular protection in the leaves and are involved in the tolerance of Sahara leaves to high Na+.

Crops and climate change: progress, trends, and challenges in simulating impacts and informing adaptation

Abstract: Assessments of the relationships between crop productivity and climate change rely upon a combination of modelling and measurement. As part of this review, this relationship is discussed in the context of crop and climate simulation. Methods for linking these two types of models are reviewed, with a primary focus on large-area crop modelling techniques. Recent progress in simulating the impacts of climate change on crops is presented, and the application of these methods to the exploration of adaptation options is discussed. Specific advances include ensemble simulations and improved understanding of biophysical processes. Finally, the challenges associated with impacts and adaptation research are discussed. It is argued that the generation of knowledge for policy and adaptation should be based not only on syntheses of published studies, but also on a more synergistic and holistic research framework that includes: (i) reliable quantification of uncertainty; (ii) techniques for combining diverse modelling approaches and observations that focus on fundamental processes; and (iii) judicious choice and calibration of models, including simulation at appropriate levels of complexity that accounts for the principal drivers of crop productivity, which may well include both biophysical and socio-economic factors. It is argued that such a framework will lead to reliable methods for linking simulation to real-world adaptation options, thus making practical use of the huge global effort to understand and predict climate change.

Influence of Arabidopsis thaliana accessions on rhizobacterial communities and natural variation in root exudates

Abstract: Plant species is considered to be one of the most important factors in shaping rhizobacterial communities, but specific plant-microbe interactions in the rhizosphere are still not fully understood. Arabidopsis thaliana, for which a large number of naturally occurring ecotype accessions exist, lacks mycorrhizal associations and is hence an ideal model for rhizobacterial studies. Eight Arabidopsis accessions were found to exert a marked selective influence on bacteria associated with their roots, as determined by terminal-restriction fragment length polymorphism (T-RFLP) and ribosomal intergenic spacer analysis (RISA). Community differences in species composition and relative abundance were both significant (P <0.001). The eight distinct and reproducible accession-dependent community profiles also differed from control bulk soil. Root exudates of these variants were analysed by high performance liquid chromatography (HPLC) to try to establish whether the unique rhizobacterial assemblages among accessions could be attributed to plant-regulated chemical changes in the rhizosphere. Natural variation in root exudation patterns was clearly exhibited, suggesting that differences in exudation patterns among accessions could be influencing bacterial assemblages. Other factors such as root system architecture are also probably involved. Finally, to investigate the Arabidopsis rhizosphere further, the phylogenetic diversity of rhizobacteria from accession Cvi-0 is described.

Gibberellin as a factor in floral regulatory networks

Abstract: Gibberellins (GAs) function not only to promote the growth of plant organs, but also to induce phase transitions during development. Their involvement in flower initiation in long-day (LD) and biennial plants is well established and there is growing insight into the mechanisms by which floral induction is achieved. The extent to which GAs mediate the photoperiodic stimulus to flowering in LD plants is, with a few exceptions, less clear. Despite evidence for photoperiod-enhanced GA biosynthesis in leaves of many LD plants, through up-regulation of GA 20-oxidase gene expression, a function for GAs as transmitted signals from leaves to apices in response to LD has been demonstrated only in Lolium species. In Arabidopsis thaliana, as one of four quantitative floral pathways, GA signalling has a relatively minor influence on flowering time in LD, while in SD, in the absence of the photoperiod flowering pathway, the GA pathway assumes a major role and becomes obligatory. Gibberellins promote flowering in Arabidopsis through the activation of genes encoding the floral integrators SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), LEAFY (LFY), and FLOWERING LOCUS T (FT) in the inflorescence and floral meristems, and in leaves, respectively. Although GA signalling is not required for floral organ specification, it is essential for the normal growth and development of these organs. The sites of GA production and action within flowers, and the signalling pathways involved are beginning to be revealed.

Specific functions of individual class III peroxidase genes

Abstract: In higher plants, class III peroxidases exist as large multigene families (e.g. 73 genes in Arabidopsis thaliana). The diversity of processes catalysed by peroxidases as well as the large number of their genes suggests the possibility of a functional specialization of each isoform. In addition, the fact that peroxidase promoter sequences are very divergent and that protein sequences contain both highly conserved domains and variable regions supports this hypothesis. However, two difficulties are associated with the study of the function of specific peroxidase genes: (i) the modification of the expression of a single peroxidase gene often results in no visible mutant phenotype, because it is compensated by redundant genes; and (ii) peroxidases show low substrate specificity in vitro resulting in an unreliable indication of peroxidase specific activity unless complementary data are available. The generalization of molecular biology approaches such as whole transcriptome analysis and recombinant DNA combined with biochemical approaches provide unprecedented tools for overcoming these difficulties. This review highlights progress made with these new techniques for identifying the specific function of individual class III peroxidase genes taking as an example the model plant A. thaliana, as well as discussing some other plants.

From Arabidopsis to rice: pathways in pollen development

Abstract: The control of male fertility is of vital importance for crop breeding, hybrid generation, and the control of pollen release. Recent development in the analysis of Arabidopsis male sterile mutants has meant that there is a greater understanding of the gene regulatory networks controlling maternal development of the anther and the resultant sporophytes. With the advent of the genome sequence and tools to allow the analysis of gene function, this knowledge base is now extending into the monocot crop rice. This has shown high levels of similarity between the networks of pollen development in Arabidopsis and rice, which will serve as valuable tools to understand and manipulate this developmental pathway further in plants.

Plant cell walls throughout evolution: towards a molecular understanding of their design principles

Abstract: Throughout their life, plants typically remain in one location utilizing sunlight for the synthesis of carbohydrates, which serve as their sole source of energy as well as building blocks of a protective extracellular matrix, called the cell wall. During the course of evolution, plants have repeatedly adapted to their respective niche, which is reflected in the changes of their body plan and the specific design of cell walls. Cell walls not only changed throughout evolution but also are constantly remodelled and reconstructed during the development of an individual plant, and in response to environmental stress or pathogen attacks. Carbohydrate-rich cell walls display complex designs, which together with the presence of phenolic polymers constitutes a barrier for microbes, fungi, and animals. Throughout evolution microbes have co-evolved strategies for efficient breakdown of cell walls. Our current understanding of cell walls and their evolutionary changes are limited as our knowledge is mainly derived from biochemical and genetic studies, complemented by a few targeted yet very informative imaging studies. Comprehensive plant cell wall models will aid in the re-design of plant cell walls for the purpose of commercially viable lignocellulosic biofuel production as well as for the timber, textile, and paper industries. Such knowledge will also be of great interest in the context of agriculture and to plant biologists in general. It is expected that detailed plant cell wall models will require integrated correlative multimodal, multiscale imaging and modelling approaches, which are currently underway.

Differentially expressed genes between drought-tolerant and drought-sensitive barley genotypes in response to drought stress during the reproductive stage

Abstract: Drought tolerance is a key trait for increasing and stabilizing barley productivity in dry areas worldwide. Identification of the genes responsible for drought tolerance in barley (Hordeum vulgare L.) will facilitate understanding of the molecular mechanisms of drought tolerance, and also facilitate the genetic improvement of barley through marker-assisted selection or gene transformation. To monitor the changes in gene expression at the transcriptional level in barley leaves during the reproductive stage under drought conditions, the 22K Affymetrix Barley 1 microarray was used to screen two drought-tolerant barley genotypes, Martin and Hordeum spontaneum 41-1 (HS41-1), and one drought-sensitive genotype Moroc9-75. Seventeen genes were expressed exclusively in the two drought-tolerant genotypes under drought stress, and their encoded proteins may play significant roles in enhancing drought tolerance through controlling stomatal closure via carbon metabolism (NADP malic enzyme, NADP-ME, and pyruvate dehydrogenase, PDH), synthesizing the osmoprotectant glycine-betaine (C-4 sterol methyl oxidase, CSMO), generating protectants against reactive-oxygenspecies scavenging (aldehyde dehydrogenase,ALDH, ascorbate-dependent oxidoreductase, ADOR), and stabilizing membranes and proteins (heat-shock protein 17.8, HSP17.8, and dehydrin 3, DHN3). Moreover, 17 genes were abundantly expressed in Martin and HS41-1 compared with Moroc9-75 under both drought and control conditions. These genes were possibly constitutively expressed in drought-tolerant genotypes. Among them, seven known annotated genes might enhance drought tolerance through signalling [such as calcium-dependent protein kinase (CDPK) and membrane steroid binding protein (MSBP)], anti-senescence (G2 pea dark accumulated protein, GDA2), and detoxification (glutathione S-transferase, GST) pathways. In addition, 18 genes, including those encoding D l -pyrroline-5carboxylate synthetase (P5CS), protein phosphatase 2C-like protein (PP2C), and several chaperones, were differentially expressed in all genotypes under drought; thus they were more likely to be general drought-responsive genes in barley. These results could provide new insights into further understanding of drought-tolerance mechanisms in barley.

Wheat (Triticum aestivum) NAM proteins regulate the translocation of iron, zinc, and nitrogen compounds from vegetative tissues to grain

Abstract: The NAM-B1 gene is a NAC transcription factor that affects grain nutrient concentrations in wheat (Triticum aestivum). An RNAi line with reduced expression of NAM genes has lower grain protein, iron (Fe), and zinc (Zn) concentrations. To determine whether decreased remobilization, lower plant uptake, or decreased partitioning to grain are responsible for this phenotype, mineral dynamics were quantified in wheat tissues throughout grain development. Control and RNAi wheat were grown in potting mix and hydroponics. Mineral (Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn) and nitrogen (N) contents of organs were determined at regular intervals to quantify the net remobilization from vegetative tissues and the accumulation of nutrients in grain. Total nutrient accumulation was similar between lines, but grain Fe, Zn, and N were at lower concentrations in the NAM knockdown line. In potting mix, net remobilization of N, Fe, and Zn from vegetative tissues was impaired in the RNAi line. In hydroponics with ample nutrients, net remobilization was not observed, but grain Fe and Zn contents and concentrations remained lower in the RNAi line. When Fe or Zn was withheld post-anthesis, both lines demonstrated remobilization. These results suggest that a major effect of the NAM genes is an increased efflux of nutrients from the vegetative tissues and a higher partitioning of nutrients to grain.

Releasing the brakes of plant growth: how GAs shutdown DELLA proteins

Abstract: Bioactive gibberellins (GAs) are tetracyclic diterpenoid plant hormones that promote important processes of plant growth and development, such as seed germination, growth through elongation, and floral transition. Thus, mutant plants that are affected in GA biosynthesis or signalling exhibit altered seed germination and, at the adult stage, are dwarf and dark green and also show delayed flowering. The components of the GA metabolism and signalling pathways are reviewed here and recent findings regarding the regulation and possible mode of action of DELLA proteins are discussed.

Photosynthesis limitations during water stress acclimation and recovery in the drought-adapted Vitis hybrid Richter-110 (V. berlandieri×V. rupestris)

Abstract: The hybrid Richter-110 (Vitis berlandieri3Vitis rupestris) has the reputation of being a genotype strongly adapted to drought. A study was performed with plants of R-110 subjected to sustained water-withholding to induce acclimation to two different levels of water stress, followed by rewatering to induce recovery. The goal was to analyse how photosynthesis is regulated during acclimation to water stress and recovery. In particular, the regulation of stomatal conductance (gs), mesophyll conductance to CO2 (gm), leaf photochemistry (chlorophyll fluorescence and thermoluminescence), and biochemistry (Vc,max) were assessed. During water stress, gs declined to 0.1 and less than 0.05 mol CO2 m 22 s 21 in moderately and severely water-stressed plants, respectively, and was kept quite constant during an acclimation period of 1-week. Leaf photochemistry proved to be very resistant to the applied water-stress conditions. By contrast, gm and Vc,max were affected by water stress, but they were not kept constant during the acclimation period. gm was initially unaffected by water stress, and Vc,max even increased above control values. However, after several days of acclimation to water stress, both parameters declined below (gm )o r at (Vc,max) control values. For the latter two parameters there seemed to be an interaction between water stress and cumulative irradiance, since both recovered to control values after several cloudy days despite water stress. A photosynthesis limitation analysis revealed that diffusional limitations and not biochemical limitations accounted for the observed decline in photosynthesis during water stress and slow recovery after rewatering, both in moderately and severely stressed plants. However, the relative contribution of stomatal (SL) and mesophyll conductance (MCL) limitations changes during acclimation to water stress, from predominant SL early during water stress to similar SL and MCL after acclimation. Finally, photosynthesis recovery after rewatering was mostly limited by SL, since stomatal closure recovered much more slowly than gm.

Phytomelatonin: a review

Abstract: Melatonin (N-acetyl-5-methoxytryptamine) has been detected in a number of plant species. Indeed, there exists evidence that this classically-considered animal indole is actually both synthesized in and taken up by plants. Among the actions that melatonin may carry out in plant tissues, its role as an antioxidant or growth promoter is most strongly supported by the experimental evidence. Other suggested functional implications include the co-ordination of photoperiodic responses and regulation of plant reproductive physiology, defence of plant cells against apoptosis induced by harsh environmental conditions, its participation as a free radical scavenging agent and/or up-regulator of certain protective enzymes in the senescent process. This review presents a detailed summary of the investigations that have been performed to date in the plant melatonin (phytomelatonin) field. The purpose of this summary is to bring the reader up to date on what is known about melatonin in plants and to encourage plant scientists to investigate this novel research topic; this would certainly assist in solving the numerous questions that still remain regarding the role of melatonin in plants.

Male gametophyte development: a molecular perspective

Abstract: Pollen grains represent the highly reduced haploid male gametophyte generation in flowering plants, consisting of just two or three cells when released from the anthers. Their role is to deliver twin sperm cells to the embryo sac to undergo fusion with the egg and central cell. This double fertilization event along with the functional specialization of the male gametophyte, are considered to be key innovations in the evolutionary success of flowering plants. This review encompasses important recent advances in our understanding of the molecular mechanisms controlling male gametophyte development. A brief overview of pollen development is presented, followed by a discussion of genome-wide transcriptomic studies of haploid gene expression. The progress achieved through genetic analysis of landmark events of male gametogenesis is discussed, with a focus on sperm cell production, and an emerging model of the regulatory network governing male germline development is presented. The review concludes with a perspective of the impact these data will have on future research strategies to further develop our understanding of the gametophytic control of pollen development.

Post-veraison sunlight exposure induces MYB-mediated transcriptional regulation of anthocyanin and flavonol synthesis in berry skins of Vitis vinifera

Abstract: Anthocyanins, flavan-3-ols, and flavonols are the three major classes of flavonoid compounds found in grape berry tissues. Several viticultural practices increase flavonoid content in the fruit, but the underlying genetic mechanisms responsible for these changes have not been completely deciphered. The impact of post-veraison sunlight exposure on anthocyanin and flavonol accumulation in grape berry skin and its relation to the expression of different transcriptional regulators known to be involved in flavonoid synthesis was studied. Treatments consisting of removing or moving aside the basal leaves which shade berry clusters were applied. Shading did not affect sugar accumulation or gene expression of HEXOSE TRANSPORTER 1, although in the leaf removal treatment, these events were retarded during the first weeks of ripening. Flavonols were the most drastically reduced flavonoids following shading and leaf removal treatments, related to the reduced expression of FLAVONOL SYNTHASE 4 and its putative transcriptional regulator MYB12. Anthocyanin accumulation and the expression of CHS2, LDOX, OMT, UFGT, MYBA1, and MYB5a genes were also affected. Other regulatory genes were less affected or not affected at all by these treatments. Non-transcriptional control mechanisms for flavonoid synthesis are also suggested, especially during the initial stages of ripening. Although berries from the leaf removal treatment received more light than shaded fruits, malvidin-3-glucoside and total flavonol content was reduced compared with the treatment without leaf removal. This work reveals that flavonol-related gene expression responds rapidly to field changes in light levels, as shown by the treatment in which shaded fruits were exposed to light in the late stages of ripening. Taken together, this study establishes MYB-specific responsiveness for the effect of sun exposure and sugar transport on flavonoid synthesis.

Analysing nitrogen responses of cereals to prioritize routes to the improvement of nitrogen use efficiency

Abstract: The efficient use of fertilizer nitrogen (N) is crucial to sustainable human nutrition. All crops receive significant amounts of additional N in temperate environments, through fixation or fertilizer use. This paper reviews progress towards the efficient use of fertilizer N by winter wheat (Triticum aesitivum L.) and spring barley (Hordeum vulgare L.) in the UK, acknowledging that on-farm this is governed by economics. Recent multi-site N response experiments on old and modern varieties show that yield improvements since the 1980s have been accompanied by increases in economic optimum N amounts for wheat but not for spring barley. On-farm N use efficiency (NUE) has increased for barley because increased yields with optimum N were associated with compensatory decreases in grain N concentration, whereas on-farm NUE has not increased for wheat because grain N concentration has not changed and improvements in N capture were insufficient to make up for the increased yield. Genetic effects on NUE are shown to differ markedly depending on whether they are determined at a single N rate, as in variety trials, or with optimum N amounts. It is suggested that, in order to elicit faster improvement in NUE on farms, breeding and variety testing should be conducted at some sites with more than one level of applied N, and that grain N%, N harvest index, and perhaps canopy N ratio (kg N ha -1 green area) should be measured more widely. It is also suggested that, instead of using empirical functions, N responses might be analysed more effectively using functions based on explanations of yield determination for which the parameters have some physiological meaning.

Estimating mesophyll conductance to CO2: methodology, potential errors, and recommendations

Abstract: The three most commonly used methods for estimating mesophyll conductance (gm) are described. They are based on gas exchange measurements either (i) by themselves; (ii) in combination with chlorophyll fluorescence quenching analysis; or (iii) in combination with discrimination against 13 CO2. To obtain reliable estimates of gm, the highest possible accuracy of gas exchange is required, particularly when using small leaf chambers. While there may be problems in achieving a high accuracy with leaf chambers that clamp onto a leaf with gaskets, guidelines are provided for making necessary corrections that increase reliability. All methods also rely on models for the calculation of gm and are sensitive to variation in the values of the model parameters. The sensitivity to these factors and to measurement error is analysed and ways to obtain the most reliable gm values are discussed. Small leaf areas can best be measured using one of the fluorescence methods. When larger leaf areas can be measured in larger chambers, the online isotopic methods are preferred. Using the large CO2 draw-down provided by big chambers, and the isotopic method, is particularly important when measuring leaves with high gm that have a small difference in [CO2] between the substomatal cavity and the site of carboxylation in the chloroplast (Ci2Cc gradient). However, equipment for the fluorescence methods is more easily accessible. Carbon isotope discrimination can also be measured in recently synthesized carbohydrates, which has its advantages under field conditions when large number of samples must be processed. The curve-fitting method that uses gas exchange measurements only is not preferred and should only be used when no alternative is available. Since all methods have their weaknesses, the use of two methods for the estimation of gm, which are as independent as possible, is recommended.

The role of auxin and gibberellin in tomato fruit set

Abstract: The initiation of tomato fruit growth, fruit set, is very sensitive to environmental conditions. Therefore, an understanding of the mechanisms that regulate this process can facilitate the production of this agriculturally valuable fruit crop. Over the years, it has been well established that tomato fruit set depends on successful pollination and fertilization, which trigger the fruit developmental programme through the activation of the auxin and gibberellin signalling pathways. However, the exact role of each of these two hormones is still poorly understood, probably because only few of the signalling components involved have been identified so far. Recent research on fruit set induced by hormone applications has led to new insights into hormone biosynthesis and signalling. The aim of this review is to consolidate the current knowledge on the role of auxin and gibberellin in tomato fruit set.

Exploiting the potential of plants with crassulacean acid metabolism for bioenergy production on marginal lands

Abstract: Crassulacean acid metabolism (CAM) is a photosynthetic adaptation that facilitates the uptake of CO(2) at night and thereby optimizes the water-use efficiency of carbon assimilation in plants growing in arid habitats. A number of CAM species have been exploited agronomically in marginal habitats, displaying annual above-ground productivities comparable with those of the most water-use efficient C(3) or C(4) crops but with only 20% of the water required for cultivation. Such attributes highlight the potential of CAM plants for carbon sequestration and as feed stocks for bioenergy production on marginal and degraded lands. This review highlights the metabolic and morphological features of CAM that contribute towards high biomass production in water-limited environments. The temporal separation of carboxylation processes that underpins CAM provides flexibility for modulating carbon gain over the day and night, and poses fundamental questions in terms of circadian control of metabolism, growth, and productivity. The advantages conferred by a high water-storage capacitance, which translate into an ability to buffer fluctuations in environmental water availability, must be traded against diffusive (stomatal plus internal) constraints imposed by succulent CAM tissues on CO(2) supply to the cellular sites of carbon assimilation. The practicalities for maximizing CAM biomass and carbon sequestration need to be informed by underlying molecular, physiological, and ecological processes. Recent progress in developing genetic models for CAM are outlined and discussed in light of the need to achieve a systems-level understanding that spans the molecular controls over the pathway through to the agronomic performance of CAM and provision of ecosystem services on marginal lands.