Plant Cell Wall Proteomics as a Strategy to Reveal Candidate Proteins Involved in Extracellular Lipid Metabolism.

Plant cell walls are composite structures surrounding cells and involved in both mechanical support and perception of their environment. They are mainly composed of polysaccharides (90-95% of their mass) and proteins (5-10%). The cell wall proteins (CWPs) contribute to the arrangements and modifications of polymer networks and to cell-to-cell communication. The structure and composition of cell walls are not uniform in the whole plants, but rather specialized in different cell types to fulfil different functions. As examples, two kinds of cells are covered with extracellular structures composed of lipids: epidermal cells of aerial organs synthesize a cuticle on their outside surface whereas endodermal root cells form a suberin surrounding strip. In both cases, these particular hydrophobic layers contribute to the protection of the cells against attacks by pathogens or abiotic stresses and regulate physiological processes. If the intracellular biosynthesis of the molecules forming these layers starts to be welldescribed, the mechanisms of their assembly beyond the plasma membrane remain largely unknown. In this review, this issue is addressed on the basis on cell wall proteomics data which has allowed the identification of many CWPs possibly related to lipid metabolism during the last years, in particular in Arabidopsis thaliana and tomato. These data are combined with transcriptomics and genetics studies. The main known roles of extracellular proteins related to lipid metabolism are discussed.

The Arabidopsis Lipid Transfer Protein 2 (AtLTP2) Is Involved in Cuticle-Cell Wall Interface Integrity and in Etiolated Hypocotyl Permeability.

Plant non-specific lipid transfer proteins (nsLTPs) belong to a complex multigenic family implicated in diverse physiological processes. However, their function and mode of action remain unclear probably because of functional redundancy. Among the different roles proposed for nsLTPs, it has long been suggested that they could transport cuticular precursor across the cell wall during the formation of the cuticle, which constitutes the first physical barrier for plant interactions with their aerial environment. Here, we took advantage of the Arabidopsis thaliana etiolated hypocotyl model in which AtLTP2 was previously identified as the unique and abundant nsLTP member in the cell wall proteome, to investigate its function. AtLTP2 expression was restricted to epidermal cells of aerial organs, in agreement with the place of cuticle deposition. Furthermore, transient AtLTP2-TagRFP over-expression in Nicotiana benthamiana leaf epidermal cells resulted in its localization to the cell wall, as expected, but surprisingly also to the plastids, indicating an original dual trafficking for a nsLTP. Remarkably, in etiolated hypocotyls, the atltp2-1 mutant displayed modifications in cuticle permeability together with a disorganized ultra-structure at the cuticle-cell wall interface completely recovered in complemented lines, whereas only slight differences in cuticular composition were observed. Thus, AtLTP2 may not play the historical purported nsLTP shuttling role across the cell wall, but we rather hypothesize that AtLTP2 could play a major structural role by maintaining the integrity of the adhesion between the mainly hydrophobic cuticle and the hydrophilic underlying cell wall. Altogether, these results gave new insights into nsLTP functions.