Expansins are cell-wall-loosening proteins that induce stress relaxation and extension of plant cell walls. To evaluate their hypothesized role in cell growth, we genetically manipulated expansin gene expression in Arabidopsis thaliana and assessed the consequent changes in growth and cell-wall properties. Various combinations of promoters were used to drive antisense and sense sequences of AtEXP10, which is maximally expressed in the growing leaf and at the base of the pedicel. Compared with controls, antisense lines had smaller rosettes because of shorter petioles and leaf blades and often acquired a twisted leaf morphology. Petiole cells from antisense plants were smaller than controls and their cell walls were significantly less extensible in vitro. Sense plants had slightly longer petioles, larger leaf blades, and larger cells than controls. Abscission at the base of the pedicel, where AtEXP10 is endogenously expressed, was enhanced in sense plants but reduced in antisense lines. These results support the concept that expansins function endogenously as cell-wall-loosening agents and indicate that expansins have versatile developmental roles that include control of organ size, morphology, and abscission.

/pdf/altered-expression-of-expansin-modulates-leaf-growth-and-2qe64bb7tr.pdf

Altered expression of expansin modulates leaf growth and pedicel abscission in Arabidopsis thaliana

In roots two distinct polar movements of auxin have been reported that may control different developmental and growth events. To test the hypothesis that auxin derived from the shoot and transported toward the root controls lateral root development, the two polarities of auxin transport were uncoupled in Arabidopsis. Local application of the auxin-transport inhibitor naphthylphthalamic acid (NPA) at the root-shoot junction decreased the number and density of lateral roots and reduced the free indoleacetic acid (IAA) levels in the root and [3H]IAA transport into the root. Application of NPA to the basal half of or at several positions along the root only reduced lateral root density in regions that were in contact with NPA or in regions apical to the site of application. Lateral root development was restored by application of IAA apical to NPA application. Lateral root development in Arabidopsis roots was also inhibited by excision of the shoot or dark growth and this inhibition was reversible by IAA. Together, these results are consistent with auxin transport from the shoot into the root controlling lateral root development.

Inhibition of Auxin Movement from the Shoot into the Root Inhibits Lateral Root Development in Arabidopsis

Expansive growth of plant cells is controlled principally by processes that loosen the wall and enable it to expand irreversibly. The central role of wall relaxation for cell expansion is reviewed. The most common methods for assessing the extension properties of plant cell walls ( wall extensibility') are described, categorized and assessed critically. What emerges are three fundamentally different approaches which test growing cells for their ability (a) to enlarge at different values of turgor, (b) to induce wall relaxation, and (c) to deform elastically or plastically in response to an applied tensile force. Analogous methods with isolated walls are similarly reviewed. The results of these different assays are related to the nature of plant cell growth and pertinent biophysical theory. I argue that the extensibilities' measured by these assays are fundamentally different from one another and that some are more pertinent to growth than others.

Wall extensibility: its nature, measurement and relationship to plant cell growth.

Light control of plant development is most dramatically illustrated by seedling development. Seedling development patterns under light (photomorphogenesis) are distinct from those in darkness (skotomorphogenesis or etiolation) with respect to gene expression, cellular and subcellular differentiation, and organ morphology. A complex network of molecular interactions couples the regulatory photoreceptors to developmental decisions. Rapid progress in defining the roles of individual photoreceptors and the downstream regulators mediating light control of seedling development has been achieved in recent years, predominantly because of molecular genetic studies in Arabidopsis thaliana and other species. This review summarizes those important recent advances and highlights the working models underlying the light control of cellular development. We focus mainly on seedling morphogenesis in Arabidopsis but include complementary findings from other species.

Light control of seedling development

SUMMARY Expansive growth of plant cells is controlled principally by processes that loosen the wall and enable it to expand irreversibly. The central role of wall relaxation for cell expansion is reviewed. The most common methods for assessing the extension properties of plant cell walls ('wall extensibility') are described, categorized and assessed critically. What emerges are three fundamentally different approaches which test growing cells for their ability (a) to enlarge at different values of turgor, (b) to induce wall relaxation, and (c) to deform elastically or plastically in response to an applied tensile force. Analogous methods with isolated walls are similarly reviewed. The results of these different assays are related to the nature of plant cell growth and pertinent biophysical theory. I argue that the 'extensibilities' measured by these assays are fundamentally different from one another and that some are more pertinent to growth than others.

/pdf/tansley-review-no-46-wall-extensibility-its-nature-btwwk47rfz.pdf

Tansley Review No. 46 Wall extensibility: its nature, measurement and relationship to plant cell growth

The effect of red (R) and far-red (FR) light on stem elongation and indole-3-acetic acid (IAA) levels was examined in dwarf and tall Pisum sativum L. seedlings. Red light reduced the extension-growth rate of etiolated seedlings by 70–90% after 3 h, and this inhibition was reversible by FR. Inhibition occurred throughout the growing zone. After 3 h of R, the level of extractable IAA in whole stem sections from the growing zone of etiolated plants either increased or showed no change. By contrast, extractable IAA from epidermal peels consistently decreased 3 h after R treatments. Decreases of 40% were observed for epidermal peels from the top 1 cm of tall plants receiving 3 h R. Brief R treatments resulted in smaller decreases in epidermal IAA levels and these decreases were not as great when FR followed R. In lightgrown plants, end-of-day FR stimulated growth during the following dark period in a photoreversible manner. The uppermost 1 cm of expanding third internodes was most responsive to the FR. Extractable IAA from epidermal peels from the upper 1 cm of third internodes increased by 30% or more 5 h after FR. When R followed the FR the increases were smaller. Levels of IAA in whole stem sections did not change and were twofold greater than in dark-grown plants. In both dark- and light-grown tall plants, IAA levels were lower in epidermal peels than in whole stem segments. These results provide evidence that IAA is compartmentalized at the tissue level within the growing stem and that phytochrome regulation of stem elongation rates may be partly based on modulating the level of IAA within the epidermis.

Indole-3-acetic acid levels after phytochrome-mediated changes in the stem elongation rate of dark- and light-grown Pisum seedlings.

Biophysical parameters related to gibberellin (GA)-dependent stem elongation were examined in dark-grown stem-length genotypes of Pisum sativum L. The rate of internode expansion in these genotypes is altered due to recessive mutations which affect either the endogenous levels of, or response to, GA. The GA deficient dwarf L181 (ls), two GA insensitive semierectoides dwarfs NGB5865 and NGB5862 (lka and lkb, respectively) and the slender' line L197 (la crys), which is tall regardless of GA content, were compared to the wild-type tall cultivar, Torsdag. Osmotic pressure, estimated by vapor pressure osmometry, and turgor pressure, measured directly with a pressure probe, did not correlate with the differences in growth rate among the genotypes. Mechanical wall properties of frozen-thawed tissue were measured using a constant force assay. GA deficiency resulted in increased wall stiffness judged both on the basis of plastic compliance and plastic extensibility normalized for equal stem circumference. Plastic compliance was not reduced in the GA insensitive dwarfs, though lka reduced circumference-normalized plasticity. In contrast, in vivo wall relaxation, determined by the pressure-block technique, differed among genotypes in a manner which did correlate with extension rates. The wall yield threshold was 1 bar or less in the tall lines, but ranged from 3 to 6 bars in the dwarf genotypes. The results with the ls mutant indicate that GA enhances stem elongation by both decreasing the wall yield threshold and increasing the wall yield coefficient. In the GA-insensitive mutants, lka and lkb, the wall yield threshold is substantially elevated. Plants possessing lka may also possess a reduced wall yield coefficient.

Physical basis for altered stem elongation rates in internode length mutants of Pisum.

Red light causes a reduction in the extension growth of dark-grown seedlings The involvement of gibberellin in this process was tested by screening a number of gibberellin synthesis and gibberellin response mutants of Pisum sativum L for the kinetic response of stem growth inhibition by red light Gibberellin deficient dwarfs, produced by mutant alleles at the Le, Na, and Ls loci, and gibberellin response mutants produced by mutant alleles at the La and Cry2, Lka, and Lkb loci were tested Extension growth of expanding third internodes of dark-grown seedlings was recorded with high resolution using angular position transducers Seedlings were treated with red light at a fluence rate of 4 micromoles per square meter per second either continuously or for 75 seconds, and the response was measured over 9 hours With certain small exceptions, the response to the red light treatments was similar in all the mutants and wild types examined The lag time for the response was approximately 1 hour and a minimum in growth rate was reached by 3 to 4 hours after the onset of the light treatment Growth rate depression at this point was about 80% Seedlings treated with 75 seconds red light recovered growth to a certain extent Red/far-red treatments indicated that the response was mediated largely by phytochrome The similar responses to red light among these wild-type and mutant genotypes suggest that the short-term (ie 9 hour) response to red light is not mediated by either a reduction in the level of gibberellin or a reduction in the level or affinity of a gibberellin receptor

/pdf/genetic-analysis-of-the-role-of-gibberellin-in-the-red-light-2346c2iha5.pdf

Genetic Analysis of the Role of Gibberellin in the Red Light Inhibition of Stem Elongation in Etiolated Seedlings

Phytochrome influences stem elongation and the mechanism for this is not understood. The levels of indole‐3‐acetic acid (IAA) were analyzed in an leLv genotype of Pisum sativum L. which responded to end‐of‐day far‐red light by doubling growth rate. The IAA levels in epidermal peels increased 40% after far‐red light whereas IAA levels of the entire stem tissue changed insignificantly. This increase was reversible by red light. Under light‐grown conditions, the lv mutation increases stem elongation rates by 2–3‐fold and is thought to block the transduction of a phytochrome signal. Analysis of the short‐term stem elongation kinetics of dark‐ and light‐grown Lv and lv seedlings suggests that lv blocks the action of the light‐stable form of phytochrome. The higher growth rate of lv plants was found to be associated with abnormally high epidermal IAA levels typical of far‐red treated Lv plants. End‐of‐day far‐red treatments did not substantially increase epidermal IAA levels in lv plants. These observations support the view that phytochrome regulation of stem elongation may occur in part through modulation of epidermal IAA levels. The lv mutation may result in increased internode growth in part by blocking the ability of phytochrome to decrease epidermal IAA levels.

PHYTOCHROME REGULATION OF STEM GROWTH AND INDOLE‐3‐ACETIC ACID LEVELS IN THE lv AND Lv GENOTYPES OF Pisum

The exposure of dark-grown seedlings to light results in a dramatic decrease in the rate of stem elongation, which is one of the many facets of the phenomenon of de-etiolation. While the spectral characteristics of the response are now relatively well known the transduction chain between the reception of the light and the ultimate growth response is not well understood. Light could have a direct effect on the cell membrane, leading to changes in the cell wall which affect growth, or light could act by altering the level of a plant hormone and the changed hormonal level would then directly or indirectly bring about changes in the cell wall. The objective of this research was to determine whether the light-induced decrease in stem elongation rate could, at least to some extent, be hormonally mediated.

Friedrich J. Behringer

Papers

Indole-3-acetic acid levels after phytochrome-mediated changes in the stem elongation rate of dark- and light-grown Pisum seedlings.

Physical basis for altered stem elongation rates in internode length mutants of Pisum.

Genetic Analysis of the Role of Gibberellin in the Red Light Inhibition of Stem Elongation in Etiolated Seedlings

PHYTOCHROME REGULATION OF STEM GROWTH AND INDOLE‐3‐ACETIC ACID LEVELS IN THE lv AND Lv GENOTYPES OF Pisum

The role of indole-3-acetic acid in mediating changes in stem elongation of etiolated pisum seedlings following exposure to light