A receptor kinase complex refines cambium activity in Arabidopsis

In plant development, key receptor kinases are often active in disparate cell types, with each requiring vastly different signalling outputs. The ERECTA (ER) receptor kinase and its homologs ERL1 and ERL2 exemplify this pleiotropy. In Arabidopsis, they influence stomatal patterning, shoot meristem function, ovule morphogenesis, xylem fiber differentiation, and cell division in the vascular cambium1-6. Such diverse expression and functionality raises the question of how ER signalling can specify such distinct cell behaviours. One mechanism is via cell-type specific interactions with co-receptors, ligands, or other proteins that modulate signalling. However, little is known about ER interactors in the vascular cambium, a bifacial stem cell niche that generates phloem and xylem (Figure 1A). Combinatorial mutations between ER, ERL1 and ERL2 and receptor kinases of a second family, PXY, PXL1, and PXL2, show severe cambial defects5,7, but the mechanism underpinning these phenotypes is not known. Here we discovered that PXY proteins form protein complexes with ER and ERL2. PXY signalling can be manipulated by altering levels of its cognate ligand, TDIF. In genetic analysis, plant lines in which TDIF levels were altered had dramatic phenotypic changes that required the presence of ER or ERL2. Our results demonstrate that PXY signalling mediated cambium regulation depends on ER signalling and explains ER function in the cambium. Because the cambium produces xylem, which constitutes the wood in vascular plants, our findings position PXY-ER complexes at the centre of the accumulation of this versatile biomaterial and essential carbon sink. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=168 SRC="FIGDIR/small/594489v1_fig1.gif" ALT="Figure 1"> View larger version (79K): org.highwire.dtl.DTLVardef@289bb6org.highwire.dtl.DTLVardef@6d0fdorg.highwire.dtl.DTLVardef@1829836org.highwire.dtl.DTLVardef@d70b3b_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFigure 1.C_FLOATNO PXY forms heterodimers with ER and ERL2 (A) Transverse sections of a wild type hypocotyl showing the region of secondary growth. Black rectangle marks the area of cambium shown in the diagrammatic representation on the lower left. (B-D) Localisation of PXY-CFP (B), ER-YFP (C) and overlap (D) in Nicotiana epidermal cells. (E) FRET signal. (F) Graph showing FRET efficiencies of PXY-CFP and ER/ERL1/ERL2-YFP pairs. (G-H) Coimmunoprecipitation of PXY-FLAG with ER-HA or ERL2-HA and vice versa in Arabidopsis cells. Scales (A) are 50 M. C_FIG