Interplay of MLO and exocyst complex proteins in localized secretion in plant cells

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Sessile multicellular life and development of plants relies among other factors on the ability to establish, sustain and modify cellular polarity during cell division, elongation, differentiation, reproduction and pathogen defence. Despite considerable progress in uncovering fundamental molecular processes involved in the regulation of eukaryotic cell polarity, the understanding of this phenomenon in plants is still fragmentary at best. Our two labs converged from different starting points on the possibility of an unexpected co-ordinated involvement of exocyst and MLO proteins in the regulation of targeted exocytosis (secretion) to specific plant cell cortical domains. Exocyst is an evolutionarily conserved cytoplasmic secretory vesicle-tethering complex implicated in the targeting of secretory pathway components to specific plasma membrane (PM) domains, which is also present and active in plants. On the contrary, the basic molecular function of plant integral membrane MLO proteins, identified first as responsible for the regulation of antifungal defence in angiosperms, remains unknown, despite many efforts to characterize their mode of action. However, recently obtained circumstantial evidence points to the possibility that they are also involved in the regulation of targeted secretion processes in plants. Unpublished observations in our two labs revealed that Arabidopsis loss-of-function mutants of MLOs and the exocyst subunit EXO70H4 have almost identical phenotypic defects in the biogenesis of secondary cell walls of leaf hairs (trichomes), in particular absence of directed deposition of the carbohydrate polymer callose. Moreover, preliminary data indicate a possible reciprocal inter-dependence of MLOs and exocyst proteins regarding their respective subcellular localization. In our joint project, we aim to elucidate the molecular mechanism(s) of MLO protein activity in the context of exocyst function in polarized exocytosis. To this end, we will use trichomes as a model system for secondary cell wall biogenesis (localized callose deposition), and we will deploy a combination of genetic (mutants; transgenes), biochemical (proteomic) and cell biological approaches, all of which are well established in both our labs. Since MLO proteins as well as the exocyst complex modulate also polarized cell wall modification in response to pathogen attack, we will use the combined expertise of our labs to address the presumed interplay of these proteins also in this biological context. Our joint efforts should substantially contribute to the understanding of the enigmatic molecular mechanism of MLO proteins and the exocyst complex, both in the course of normal plant development and in pathogenic biotic interactions.