Light quality-mediated petiole elongation in Arabidopsis during shade avoidance involves cell wall modification by Xyloglucan Endotransglucosylase/Hydrolases.
Sasidharan R, Chinnappa CC, Staal M, Elzenga JTM, Nishitani K, Yokoyama R, Voesenek L, Pierik R
Plant Physiol. 154: 978-990
Some plants can avoid shaded conditions via rapid shoot elongation, thus growing into better lit areas in a canopy. Cell wall modifying mechanisms promoting this elongation response are therefore important regulatory points during shade voidance. 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 thaliana. In response to two shade cues, low R/FR (implying neighbour 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 wall modifying proteins like expansins and XTHs. Acid-induced extension, expansin susceptibility and extractable expansin activity was similar in petioles from white light and shade treated plants. XTH activity, however, was high in petioles exposed to shade treatments. Five XTH (9, 15, 16, 17 and 19) genes were positively regulated by low red to far-red (R/FR) conditions while the latter four and XTH 22 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.