Interactions between Receptor Kinases and PXY SUMOylation define Radial Pattern in Arabidopsis

Abstract

In plant development, a receptor kinase may be active in disparate cell types, with each requiring 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 cambium. Such diverse expression and functionality raise the question of how ER signalling can specify such distinct cell behaviours. One explanation is that cell-type specific interactions with co-receptors, ligands, or other proteins modulate signalling. However, little is known about ER interactors in the vascular cambium, a bifacial stem cell niche that generates phloem and xylem. Combinatorial mutations between ER, ERL1 and ERL2 and receptor kinases of a second family, PXY, PXL1, and PXL2, show severe cambial defects. Here I discovered that PXY and PXL proteins form heterodimers 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 phenotypes that required the presence of ER or ERL2. These 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 heterodimers at the centre of the accumulation of this versatile biomaterial and carbon sink.

The cambium and procambium are responsible for producing the majority of biomass in vascular plants. These meristems form a bifacial stem cell population, from which xylem and phloem are specified on opposing sides through positional signals. The PHLOEM INTERCALATED WITH XYLEM (PXY) receptor kinase plays a key role in promoting vascular cell division and organization. SUMO is a common post-translational protein decoration that affects many different biological processes such as plant immunity, and resistance to biotic and abiotic stresses. Here, PXY was found to be SUMOylated and SUMOylation sites were predicted. Mutation of PXY SUMO sites resulted in vascular defects including disorganized tissues, increased cell division and potential xylem differentiation defects. These data collectively indicate PXY SUMO is likely to be a negative regulator of PXY signaling and play crucial roles in vascular development.

Gaining a comprehensive understanding of how PXY works at the plasma membrane may guide us in manipulating the system to increase or decrease signal transduction. A structural understanding of how PXY functions is therefore essential. The structure of the extracellular domain of PXY has been solved, but to fully understand its function, the structure of the cytoplasmic kinase domain would likely be informative. This could assist in understanding aspects of protein turnover and interactions with co-receptors.