Light regulates xylem cell differentiation via PIF in Arabidopsis

Water transport is required for photosynthesis; thus plant vascular development must proceed such that water demands can be met during formation of new organs. Precise regulation of the balance between cell proliferation and differentiation in the cambial meristem defines formation of xylem and phloem, which constitute the vascular tissues. The TDIF-PXY/TDR signalling pathway is central to this process1. Peptide ligand, TDIF, encoded by CLE41 and CLE44 2, activates PXY/TDR receptors to maintain proliferative cambium. Cambium cells differentiate to xylem or phloem in the absence of active TDIF-PXY/TDR complexes. Vascular development is stimulated by light. After gemination, light induces the establishment of photoautotrophic growth via photosynthesis. Consequently, the differentiation of vascular cells must occur to favour the transport of water and minerals from the soil to the green tissues and newly developing organs. Plants perceive light through the action of photoreceptors, which modulate the activity of the transcription factors that orchestrate development. Despite the association between light signalling and vascular development, molecular mechanisms linking the two are unknown. Our work shows that vascular differentiation is inhibited in the dark by a mechanism that depends on PIF transcription factors. The dark accumulation of PIFs is necessary for CLE44 induction and thus maintenance of undifferentiated vasculature. In illuminated environments, PIF inactivation by photoreceptors causes a decrease in CLE44 expression. CLE44 reduction, in turn, leads to reduced PXY/TDR signalling which induces the xylem differentiation required to fulfil the water demands associated with photoautotrophic development. Arabidopsis seedlings were grown in the dark for 5 days, then exposed to white light for varying lengths of time (15, 30, 60, 150, and 360 min) before tissue collection. Three biological replicates were taken for each light condition.