Phosphorylation of RLP44: Shifting between subcellular localization and receptor complexes

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

A plant virus causes symptoms through the deployment of a host-mimicking protein domain to attract the insect vector.

During compatible plant-virus interactions, viruses can interfere with the normal developmental program of their hosts, leading to the appearance of phenotypes that we usually identify as ‘’symptoms of infection’’ (leaf curling and yellowing, stunting, dwarfism, necrosis). Despite their relevance, the molecular mechanisms underlying symptom induction and their biological meaning, if any, remain poorly understood. By using tomato yellow leaf curl virus (TYLCV, Geminivirus) as model, we have isolated C4 as the main protein responsible for the induction of TYLCV-associated symptoms in tomato. C4, by mimicking a host protein domain, the Conserved C-termini in LAZY1 protein family (CCL) domain, physically interacts with the RCC1-like domain-containing plant proteins (RLDs). By interacting with the RLDs through the CCL-like domain, C4 displaces one endogenous interactor, LAZY (LZY), interfering with RLD functions in processes such as auxin signaling and endomembrane trafficking, which correlates with the manifestation of symptoms. Surprisingly, we observed that appearance of C4-mediated symptoms in tomato plants plays no major role in viral replication nor movement, but they serve as attractants for the insect vector, the whitefly Bemisia tabaci, which preferentially feeds on tomato plants exhibiting strong symptoms of viral infection. These results suggest that, during plant-virus co-evolution, symptoms may have appeared as a strategy to promote viral transmission by the insect vector, at least in some specific plant-virus-vector pathosystems.Work in RLD’s lab is partially funded by the Excellence Strategy of the German Federal and State Governments, the ERC-COG GemOmics (101044142), the DeutscheForschungsgemeinschaft (DFG, German Research foundation) (project numbers LO 2314/1-1 and SBF 1101/3, C08), and a Royal Society Newton Advance grant (NA140481 – NAF\R2\180857). EA is the recipient of a Marie Skłodowska-Curie Grant from the European Union’s Horizon 2020 Research and Innovation Program (Grant 896910-GeminiDECODER). Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech