Petiole hyponasty: an ethylene-driven, adaptive response to changes in the environment

Hyponastic (upwardly bending) growth by leaves is a response of numerous plant species to adverse environmental conditions. This review summarises current knowledge on hyponasty with a particular focus on the role of ethylene in regulating this phenomenon and its possible adaptive significance

Ethylene-mediated regulation of A2-type CYCLINs modulates hyponastic growth in arabidopsis

Upward leaf movement (hyponastic growth) is frequently observed in response to changing environmental conditions and can be induced by the phytohormone ethylene. Hyponasty results from differential growth (i.e. enhanced cell elongation at the proximal abaxial side of the petiole relative to the adaxial side). Here, we characterize Enhanced Hyponasty-D, an activation-tagged Arabidopsis (Arabidopsis thaliana) line with exaggerated hyponasty. This phenotype is associated with overexpression of the mitotic cyclin CYCLINA2;1 (CYCA2;1), which hints at a role for cell divisions in regulating hyponasty. Indeed, mathematical analysis suggested that the observed changes in abaxial cell elongation rates during ethylene treatment should result in a larger hyponastic amplitude than observed, unless a decrease in cell proliferation rate at the proximal abaxial side of the petiole relative to the adaxial side was implemented. Our model predicts that when this differential proliferation mechanism is disrupted by either ectopic overexpression or mutation of CYCA2;1, the hyponastic growth response becomes exaggerated. This is in accordance with experimental observations on CYCA2;1 overexpression lines and cyca2;1 knockouts. We therefore propose a bipartite mechanism controlling leaf movement: ethylene induces longitudinal cell expansion in the abaxial petiole epidermis to induce hyponasty and simultaneously affects its amplitude by controlling cell proliferation through CYCA2;1. Further corroborating the model, we found that ethylene treatment results in transcriptional down-regulation of A2-type CYCLINs and propose that this, and possibly other regulatory mechanisms affecting CYCA2;1, may contribute to this attenuation of hyponastic growth

Alterations in auxin homeostasis suppress defects in cell wall function.

The plant cell wall is a highly dynamic structure that changes in response to both environmental and developmental cues. It plays important roles throughout plant growth and development in determining the orientation and extent of cell expansion, providing structural support and acting as a barrier to pathogens. Despite the importance of the cell wall, the signaling pathways regulating its function are not well understood. Two partially redundant leucine-rich-repeat receptor-like kinases (LRR-RLKs), FEI1 and FEI2, regulate cell wall function in Arabidopsis thaliana roots; disruption of the FEIs results in short, swollen roots as a result of decreased cellulose synthesis. We screened for suppressors of this swollen root phenotype and identified two mutations in the putative mitochondrial pyruvate dehydrogenase E1α homolog, IAA-Alanine Resistant 4 (IAR4). Mutations in IAR4 were shown previously to disrupt auxin homeostasis and lead to reduced auxin function. We show that mutations in IAR4 suppress a subset of the fei1 fei2 phenotypes. Consistent with the hypothesis that the suppression of fei1 fei2 by iar4 is the result of reduced auxin function, disruption of the WEI8 and TAR2 genes, which decreases auxin biosynthesis, also suppresses fei1 fei2. In addition, iar4 suppresses the root swelling and accumulation of ectopic lignin phenotypes of other cell wall mutants, including procuste and cobra. Further, iar4 mutants display decreased sensitivity to the cellulose biosynthesis inhibitor isoxaben. These results establish a role for IAR4 in the regulation of cell wall function and provide evidence of crosstalk between the cell wall and auxin during cell expansion in the root

Mutations in the auxin biosynthetic genes <i>WEI8</i> and <i>TAR2</i> suppress <i>fei1 fei2</i>.

<p>Phenotypes of the root tips of the indicated seedlings four days after transfer from MS medium containing 0% sucrose to MS medium containing 4.5% sucrose. Scale bar = 1 mm.</p

Effect of <i>iar4-5</i> on floral phenotypes of <i>fei1 fei2</i>.

<p>(<b>A</b>) Silique length from indicated genotypes. Plants were grown on soil under 16h light regime for four weeks at 22°C. The first five fully developed siliques from the primary inflorescences were used for the analysis. Values are the mean ± SE (n = 25). (<b>B</b>) Stamen length and (<b>C</b>) Carpel:stamen ratio of indicated genotypes. Stamens and carpels were removed from individual flowers and measured. Values are the mean ± SE (n>11). Different letters indicate significant differences between groups. Data were analyzed with one-way ANOVA and Tukey's post-hoc comparisons; <i>P</i><0.05.</p

Positional cloning of <i>SHOU2</i>.

<p>(<b>A</b>) <i>shou2</i> was mapped to a region on chromosome 1 between markers F3I6.D and F3I6.F as described in the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0098193#s4" target="_blank">methods</a>. The name of each DNA marker is shown above and the number of recombinants is indicated below the line. Open reading frames located between markers F3I6.D and F3I6.F are shown below BAC F316. (<b>B</b>) Structure of <i>SHOU2</i> gene. Boxes represent exons and lines represent the introns. The positions and changes of the three <i>shou</i> alleles are indicated. The triangle indicates the position of the DNA insertion in <i>shou2-3</i>. The corresponding allele numbers for the <i>iar4</i> designations are shown in parentheses.</p

Mutations in <i>IAR4</i> confer resistance to isoxaben.

<p>(A) Root phenotypes of indicated seedlings germinated and grown for five days on MS medium with 0% sucrose and then transferred for 48 hours to MS medium with 0% sucrose containing either 0 nM (DMSO), 1 nM or 2 nM isoxaben. Representative root tips were imaged using dark field microscopy. Scale bar = 0.5 mm. (B) Quantification of the root tip swelling from (A). The width of the roots was measured at the level of the youngest root hair using ImageJ software <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0098193#pone.0098193-Abramoff1" target="_blank">[48]</a>. Values are the means (n>8) ± SE. Data were analyzed by unpaired Student's t-test. Asterisks indicate significant differences relative to the wild type; <i>P</i><0.05.</p

Model of the FEI pathway.

<p>Hypothetical model depicting the role of both auxin and the FEI pathway in regulating cell expansion. See text for additional details.</p

Isolation of the <i>shou2</i> suppressor.

<p>(<b>A</b>) Top: Phenotypes of indicated seedlings grown on MS medium containing 4.5% sucrose for three weeks. The bottom panels show a close-up of the root tips. Scale bar = 1 mm. (<b>B</b>) Quantification of total root length from (A). Values are the means (n = 150) ± SE. Different letters indicate significant differences between groups. Data were analyzed with one-way ANOVA and Tukey's post-hoc comparisons; <i>P</i><0.05. (<b>C</b>) Transverse sections through the root elongation zone of wild type, <i>fei1 fei2, fei1 fei2 shou2-1</i>, and <i>fei1 fei2 shou2-2</i>. Scale bar = 50 µm.</p

<i>iar4</i> suppresses lignin accumulation in cell wall mutants.

<p>Phloroglucinol stain for lignin (red) accumulation in root tips of seedlings of the indicated genotype grown on 4.5% sucrose for 2 weeks. Scale bar = 1 mm.</p