A Phytochrome B-PIF4-MYC2 Module Tunes Secondary Cell Wall Thickening in Response to Shaded Lighting

Secondary cell walls (SCW) in stem xylem cells provide mechanical strength and structural support for growth. SCW thickening is light- regulated and varies under different light growth conditions. Our previous study revealed that blue light enhances SCW thickening through the activity of MYC2 directed by CRYPTOCHROME1 (CRY1) signaling in stem xylary fiber cells. In this study, we demonstrate that the low ratio of red: far-red light (R:FR) of the shaded light condition inhibits SCW thickening in the inflorescence stem of Arabidopsis. Phytochrome B (PHYB) plays a dominant role in perceiving the R:FR balance. Under white and red-light conditions, phyB mutants display thinner SCWs in xylary fibers, but thicker SCWs are deposited in the PHYTOCHROME INTERACTING FACTORS (PIFs) quadruple mutant pif1pif3pif4pif5 (pifq), suggesting involvement of the PHYB-PIFs signaling module in regulating SCW thickening. Interaction of PIF4 with MYC2 affects MYC2 localization in nuclei and inhibits its transactivation activity on the NST1 promoter. Shade conditions mediate the PIF4 interaction with MYC2 to regulate SCW thickening. Genetic analysis confirms that the regulation of SCW thickening by PIFs is dependent on MYC2 function. Together, these data reveal a molecular mechanism for the effect of shaded light inhibition on SCW thickening in stems of Arabidopsis.

A Transcriptomic Analysis of Xylan Mutants Does Not Support the Existence of A Secondary Cell Wall Integrity System in Arabidopsis

Yeast have long been known to possess a cell wall integrity (CWI) system, and recently an analogous system has been described for the primary walls of plants (PCWI) that leads to changes in plant growth and cell wall composition. A similar system has been proposed to exist for secondary cell walls (SCWI). However, there is little data to support this. Here, we analysed the stem transcriptome of a set of cell wall biosynthetic mutants in order to investigate whether cell wall damage, in this case caused by aberrant xylan synthesis, activates a signalling cascade or changes in cell wall synthesis gene expression. Our data revealed remarkably few changes to the transcriptome. We hypothesise that this is because cells undergoing secondary cell wall thickening have entered a committed programme leading to cell death, and therefore a SCWI system would have limited impact. The absence of transcriptomic responses to secondary cell wall alterations may facilitate engineering of the secondary cell wall of plants.