Cell Wall Signalling in Arabidopsis thaliana

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How the development of complex multicellular organisms is accomplished through coordinated cell proliferation, differentiation, and growth is a fundamental biological question. In animals, stem cell maintenance and cell identity specification are in part controlled by the extracellular matrix (ECM). The rigid, yet dynamic ECM of plants, the cell wall, is known to guide morphogenesis through selective restriction of cell expansion. To control growth, but also to respond to extrinsic perturbations, cell wall state is believed to be under constant surveillance by cell surface receptors connected to intracellular signalling. However, very little is known about how cell wall properties are sensed, how signals are transduced, and how cell wall-mediated feedback signalling intersects with the regulatory mechanisms of plant development. Based on our recent discoveries, we hypothesize that cell wall signalling pathways play major roles in development by controlling the maintenance of cell fate, reminiscent of the regulatory role of the ECM in animals. We discovered a novel cell wall signalling pathways that depends on the plasma membrane localized receptor-like protein RLP44. RLP44 interacts with the pectate component of the cell wall and connects two well-known signalling pathways, brassinosteroid (BR) and phytosulfokine (PSK) signalling, by interacting with the respective receptor complexes and promoting their activity. Interestingly, RLP44-promoted PSK signalling is required for the maintenance of procambial cell identity in the root, a novel function for this pathway. Mimicking the loss of RLP44, PSK-related mutants show ectopic xylem in the position of procambium, whereas RLP44 mutants can be rescued by exogenous PSK. Here, we want to address mainly two questions. First, how does the BRI1-RLP44-PSK signalling module promote procambial identity? Building on extensive preliminary results, we want to test the hypothesis that PSK signalling acts in part through the control of protein translation. In addition, we want to dissect how PSK signalling intersects with the known regulators of procambial identity, namely the mutually inhibitory auxin and cytokinin morphogenetic fields. Second, does RLP44-mediated cell wall sensing have a direct effect on intracellular networks regulating growth and development? To this end, we will leverage our finding that RLP44 directly interacts with pectate and assess the influence of cell wall binding on signalling readouts as well as on protein-protein interactions in the receptor complexes.