To the core of the cell wall integrity pathways in land plants

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In plants, cell growth is a complex process that requires the turgor-derived stretching of the pre-existing cell wall to be compensated by the secretion of new cell wall material. Higher plants have developed cell wall integrity mechanisms that monitor and coordinate the performance of their extracellular cell wall with their intracellular activities to avoid growth accidents. These mechanisms are orchestrated by a complex and robust interplay between the transmembrane malectin-like receptors (MLRs), their secreted peptide ligands of the Rapid Alkalinisation Factor (RALF) family and downstream effectors such as the receptor-like cytoplasmic kinase MARIS (MRI) of the PTI1-like family. In the flowering plant model Arabidopsis thaliana, the MLRs AtFERONIA (AtFER) and its closest homologues AtANXUR1/2 (AtANX1/2) function upstream of AtMRI to govern the cell wall integrity pathways of the tip-growing root hairs and pollen tubes, respectively. However, these pathways remain particularly hard to decipher due to complex, sometimes partial, functional redundancy between the multigene family members of regulators.Recently, we demonstrated that cell wall integrity of the tip-growing rhizoids in the early diverging land plant Marchantia polymorpha, also required the receptor-like kinases MpFER and MpMRI, both unique representatives of the MLR and PTI1-like families in Marchantia. Thus, a genetic program maintaining rhizoid cell wall integrity was established early during land plant evolution and then adopted by vascular plants for root hair growth control and much later by pollen tubes, the male gametophyte of flowering plants.Here, we propose to exploit the morphological and genetic simplicity of Marchantia and the conservation of the MLR/PTI1-like genetic pathway by using MpFER and MpMRI as anchor points for protein-protein interactions and genetic screens combined with Next Generation Sequencing strategies. These screens will provide us with new interactors and downstream components of the MLR/PTI1-like module that governs cell wall integrity mechanisms. Comparatively studying the roles of these new regulators (and their homologues) in both Marchantia and Arabidopsis in the context of tip-growth and other developmental/environmental responses will be fundamental for apprehending cell growth control in plants.