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Abstract

In all organisms, tailoring development to the environment relies on the proper integration of intracellular and extracellular cues. The cell wall, the extracellular matrix of plants, directly influences the growth and shape of the cells. As a result, the biophysical properties of the cell wall are permanently controlled and the resultant information is transduced to the cell interior to accommodate the properties of the cell wall to growth. However, little is known about the cell wall signaling mechanisms or the molecular components that are part of them.

Recently, it was demonstrated that cell wall modifications could modulate the activity of the brassinosteroid (BR) signaling pathway, a feedback mechanism presumably mediating cell wall homeostasis. The integration of cell wall and BR signaling depends on the LRR receptor-like protein RLP44, which is able to interact with the BR receptor BRI1 and its co-receptor BAK1. In addition, RLP44 is important for the maintenance of procambial cell identity through PSK signaling by interacting with the PSK receptor, PSKR1. Therefore, RLP44 balances BR and PSK signaling to control vascular cell fate.

We could show that the RLP44 cytoplasmic domain is highly conserved and contains four amino acids predicted to be phosphorylated. Among those residues, Ser-268 and Ser-270 play an important role in shifting the localization of RLP44 and therefore control its function. Mechanistically, phosphorylation affects subcellular localization of the protein by exerting a negative effect on its clathrin-mediated endocytosis. Besides, RLP44 phosphorylation may alter its interaction with BR receptors, affecting signaling integration. Moreover, we demonstrated that RLP44 phosphorylation occurs in a BL-dependent manner and the phosphorylation of cytoplasmic RLP44 residues is required for activation of BR signaling. However, the kinases involved in RLP44 phosphorylation are not defined yet, although BRI1 and BAK1, but not PSKR1 might play a critical role. Contrary to the situation with BRI1, RLP44 phosphorylation is PSKR1-independent and modification of its cytoplasmic domain neither influences either the interaction with PSKR1 nor the RLP44 responsiveness to PSK. Taken together, we hypothesize that RLP44 phosphorylation might play a crucial role in regulating the integration of PSK and BL signaling in a BRI1- and Cell Wall-dependent manner. In addition, we identified in a suppressor screen of RLP44 overexpressing plants (RLP44ox), four mutants (RRE 9.2, RRE 11.1, RRE 24.1 and RRE 38.6) involved in the integration of CW changes into the BRI1- and/or PSKR1-dependent signaling. Those mutants possess an altered cell wall when compared to RLP44ox or wild-type, which might reflect an unbalanced cell wall signaling. In addition, RRE 24.1 and RRE 38.6 are PSK-insensitive and show an altered RLP44ox xylem phenotype, characterized by an increased number of xylem cell. Thus, RRE 24.1 and RRE 38.6 are promising candidates to study the integration of CW and PSK signaling and shed light on the biochemical mechanism that governs RLP44 function.