Light Quality-Mediated Petiole Elongation in Arabidopsis during Shade Avoidance Involves Cell Wall Modification by Xyloglucan Endotransglucosylase/Hydrolases      

Abstract

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.

Dynamic Antagonism between Phytochromes and PIF Family Basic Helix-Loop-Helix Factors Induces Selective Reciprocal Responses to Light and Shade in a Rapidly Responsive Transcriptional Network in Arabidopsis

“…Direct PIF regulation of cell wall metabolism, possibly related to the rapid growth responses to shade, is also suggested by the presence of XTR7/XTH15 (a xyloglucan transglycosylaseencoding gene; Rose et al, 2002) in this group. This is consistent with the observation that xth15 knockout mutants show reduced shade-induced growth responses (Sasidharan et al, 2010). Collectively, the data focus attention on a subset of genes that provide potential targets for future investigations aimed at securing more detailed insight into central cellular processes involved in the initiation of the shade avoidance response.…”

Plant neighbor detection through touching leaf tips precedes phytochrome signals

“…Interestingly, microarray analysis on petioles from canopy plants at LAI 1.0 showed no induced expression of genes known to be involved in shade avoidance (Dataset 1 and Table S1). Accordingly, a quantitative RT-PCR time course throughout canopy development indicated that expression of the low R:FR-inducible marker genes ATHB2 and XTH15 (15,16) was not induced at the onset of hyponasty (LAI 0.65). These genes were only up-regulated after petiole elongation occurred in canopy plants at an LAI >1.27, which corresponded with a R:FR <1.3 ( Fig.…”

Neighbor Detection Induces Organ-Specific Transcriptomes, Revealing Patterns Underlying Hypocotyl-Specific Growth

“…Notably, the expression of XTH (XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE) genes regulated by PIFs and/or IAA and/or BL has been linked to growth promotion (Hornitschek et al, 2009;Sasidharan et al, 2010;Keuskamp et al, 2011;de Wit et al, 2015;Pedmale et al, 2016). Our organ-specific analysis allowed us to test whether shade-induced expression of XTH genes selectively occurred in the fast growing hypocotyl.…”