Some plants can avoid shaded conditions via rapid shoot elongation, thus growing into better lit areas in a canopy. Cell wallmodifying mechanisms promoting this elongation response, therefore, are important regulatory points during shade avoidance. Two major cell wall-modifying protein families are expansins and xyloglucan endotransglucosylase/hydrolases (XTHs). The role of these proteins during shade avoidance was studied in Arabidopsis (Arabidopsis thaliana). In response to two shade cues, low red to far-red light (implying neighbor proximity) and green shade (mimicking dense canopy conditions), Arabidopsis showed classic shade avoidance features: petiole elongation and leaf hyponasty. Measurement of the apoplastic proton flux in green shade-treated petioles revealed a rapid efflux of protons into the apoplast within minutes, unlike white light controls. This apoplastic acidification probably provides the acidic pH required for the optimal activity of cell wallmodifying proteins like expansins and XTHs. Acid-induced extension, expansin susceptibility, and extractable expansin activity were similar in petioles from white light-and shade-treated plants. XTH activity, however, was high in petioles exposed to shade treatments. Five XTH genes (XTH9, were positively regulated by low red to far-red light conditions, while the latter four and XTH22 showed a significant up-regulation also in response to green shade. Consistently, knockout mutants for two of these XTH genes also had reduced or absent shade avoidance responses to these light signals. These results point toward the cell wall as a vital regulatory point during shade avoidance.

Shade avoidance in plants involves rapid shoot elongation to grow toward the light. Cell wall-modifying mechanisms are vital regulatory points for control of these elongation responses. Two protein families involved in cell wall modification are expansins and xyloglucan endotransglucosylase/hydrolases. We used an alpine and a prairie ecotype of Stellaria longipes differing in their response to shade to study the regulation of cell wall extensibility in response to low red to far-red ratio (R/FR), an early neighbor detection signal, and dense canopy shade (green shade: low R/FR, blue, and total light intensity). Alpine plants were nonresponsive to low R/FR, while prairie plants elongated rapidly. These responses reflect adaptation to the dense vegetation of the prairie habitat, unlike the alpine plants, which almost never encounter shade. Under green shade, both ecotypes rapidly elongate, showing that alpine plants can react only to a deep shade treatment. Xyloglucan endotransglucosylase/hydrolase activity was strongly regulated by green shade and low blue light conditions but not by low R/FR. Expansin activity, expressed as acid-induced extension, correlated with growth responses to all light changes. Expansin genes cloned from the internodes of the two ecotypes showed differential regulation in response to the light manipulations. This regulation was ecotype and light signal specific and correlated with the growth responses. Our results imply that elongation responses to shade require the regulation of cell wall extensibility via the control of expansin gene expression. Ecotypic differences demonstrate how responses to environmental stimuli are differently regulated to survive a particular habitat.

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y-Aminobutyric acid (GABA), a nonprotein amino acid, is often accumulated in plants following environmental stimuli that can also cause ethylene production. We have investigated the relationship between GABA and ethylene production in excised sunflower (Helianfhus annuus 1.) tissues. Exogenous GABA causes up to a 14-fold increase in the ethylene production rate after about 1 2 h. Cotyledons fed with [14C]GABA did not release substantiaf amounts of radioactive ethylene despite its chemical similarity to 1-aminocyclopropane-1-carboxylic acid (ACC), indicating that GABA is not likely to be an alternative precursor for ethylene.GABA causes increases in ACC synthase mRNA accumulation, ACC levels, ACC oxidase mRNA levels, and in vitro ACC oxidase activity. In the presence of aminoethoxyvinylglycine or a-aminoisobutyric acid, GABA did not stimulate ethylene production. We therefore conclude that GABA stimulates ethylene biosynthesis mainly by promoting ACC synthase transcript abundance. Possible roles of GABA as a signal transducer are suggested.

Using two ecotypes of Stellaria longipes an alpine form with low plasticity and a prairie form with high plasticity, we investigated whether ethylene was involved in the response to wind stress and might be important in controlling plasticity of stem elongation. Stem growth inhibition was positively correlated with concentration of ethephon application and elevation in ambient ethylene in alpine ecotypes, whereas stem growth in prairie plants was stimulated by low ethephon concentrations. When treated with high AVG, the effects were reversed: alpine plant growth was promoted and prairie plant growth was inhibited. Prairie plants exhibited a daily rhythm in ethylene evolution which increased and peaked at 1500 h, and which was absent in alpine plants. Ethylene evolution did not change significantly during the first 2 weeks of growth in alpine plants, whereas ethylene in prairie plants increased significantly during periods of rapid stem elongation. Wind treatment inhibited growth in both ecotypes, but only alpine plants showed a recovery of growth to control levels when wind stressed plants were pretreated with STS. In addition, only alpine plants showed an increase in ethylene evolution in response to wind simulation, whereas prairie plant ethylene evolution did not deviate from rhythms observed in unstressed plants. We concluded that ethylene dwarfs stems in alpine S. longipes in response to wind stress. However, low levels of ethylene may stimulate growth in prairie ecotypes and act independently of wind stress intensity. The contrasting ability to synthesize and respond to ethylene can account for part of the difference in plasticity documented between the two ecotypes.

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showed a lower level of methylation, and were taller than the Changes in cytosine methylation are known to occur in rehigh R/FR light grown counterparts. In addition, prairie sponse to various environmental stimuli, therefore, we looked at methylation changes in relation to stem elongation. More ecotype plants demonstrated lower day 4 methylation levels, specifically, we investigated the response of genomic cytosine as well as longer day 7 stem lengths, than the alpine ecotype methylation to irradiance-mediated plasticity of stem elonga-plants within the same R/FR light treatments. To investigate if the degree of methylation was a crucial factor in controlling tion in two ecotypes of Stellaria longipes. Ramets of S. the stem elongation response, ramets of both alpine and longipes were grown under high and low ratios of red/far-red prairie plants were grown in MS media supplemented with light (F/FR; 3.7 and 0.7, respectively). Stem elongation and methylated cytosine content were measured over a period of 7 5-azacytidine (5-AzaC), and grown for 14 days under a R/FR ratio of 3.7 and two different PAR values. 5-AzaC treatments days. Ramets of S. longipes demonstrated the highest level of demethylation after 4 days of long-day warm (LDW) treat-demonstrated that the prairie ecotype plants required greater doses of 5-AzaC, and thus lower methylation levels, than the ment, which coincides with the first day of rapid stem elongation initiation. The extent of demethylation associated with alpine ecotype plants in order to promote maximal stem elongation. These observations suggest that DNA demethyla-day 4 depended upon the relative ratio of R/FR light. In tion is involved in the shade-avoidance response. particular, those plants treated with low R/FR light ratios demethylating agent. In suspension cultures of tobacco, 0.