Mechanisms and contribution of Musashi-mediated control of cell wall polysaccharide synthesis in plants – MusaWall

As the main constituent of the terrestrial plant biomass, lignocellulose represents a huge reservoir of fixed carbon and a renewable raw material that is essential for human usages. Lignocellulose biomass enclosed within secondary cell wall (SCW) is mainly composed of cellulose, hemicellulose and lignin, a complex phenolic polymer, that is deposited in specialized vascular tissues. SCW plays important roles in growth regulation, protection towards pathogens, long-distance sap transport and upright growth. Cell wall structure and composition, which is variable according to cell type, tissue and plant species, differ markedly between dicots and monocots. Even minor changes of cell wall content can trigger remarkable effects on its chemical properties and, by the way, on plant development. The synthesis and deposition of SCW polymers is controlled at the transcriptional level by a hierarchical interconnected network of transcription factors (TFs), mostly unraveled by studies in Arabidopsis. Despite a strong conservation in plant lineage, SCW regulatory network has revealed even more complex in perennials. A deeper understanding of these processes is of major interest for industry and agronomy. SCW engineering in crops or trees is a way to optimize biomass processability. Despite huge progresses, the engineering of SCW deposition has proven difficult and often impairs plant growth. While numerous works point to the role of TFs in SCW synthesis, evidence for the implication of post-transcriptional mechanisms in this process is scarce. The most convincing cases of post-transcriptional regulation of SCW synthesis involve specific microRNAs that regulate specific SCW biosynthesis genes. In this context, the coordinator/P1 and partners (P2/P3) teams have recently identified an Arabidopsis RNA-binding protein family, sharing functional similarities with the animal translation regulator Musashi/MSI, named as Musashi-like/MSIL. Results from the consortium indicate that MSILs are implicated in the cell-specific control of SCW synthesis in Arabidopsis, revealing a specific role for MSIL in the translational control of glucuronoxylan methylation and lignin deposition in the interfascicular fiber cells of the inflorescence stem. Such a translational regulation represents an additional layer of control of cell wall polymers synthesis, that opens new directions of research and alternatives for SCW engineering. MusaWall is a pioneer project that aims to study the mechanisms by which MSIL regulate SCW synthesis in Arabidopsis thaliana (P1), but also in two plant models that present SCWs with different chemical structures, Brachypodium distachyon (a grass model, P2) and Eucalyptus grandis (a model of hardwood tree, P3). In particular, 1) by using state-of-the-art genetic approaches, we will generate msil-loss-of-function lines in the grass model Brachypodium distachyon and the model of hardwood tree Eucalyptus grandis; 2) we wish to analyze the impact of MSIL in cell wall polymers synthesis in the grass and tree models and compare with Arabidopsis data; 3) in Arabidopsis more particularly, we wish to find MSIL targets that are relevant to SCW polymers synthesis and also to assess the connection existing between the MSILs and the m6A mRNA modification pathway in the SCW synthesis; 4) at last, we will analyze the MSIL-dependent regulatory network involved in SCW synthesis in interfascicular fiber cells, through the identification of actors thanks to a genetic screen in Arabidopsis . By applying a unique combination of molecular, biochemical, genetic and chemotyping approaches, MusaWall will shed light on the functional conservation of MSIL function in plants and will contribute to the identification of novel regulators with high biotechnology interest for the improvement of bioproducts conversion.