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

植物固着的多细胞生命和发育除其他因素外，还取决于在细胞分裂、伸长、分化、繁殖和病原防御过程中建立、维持和修改细胞极性的能力。尽管在揭示参与真核细胞极性调控的基本分子过程方面取得了相当大的进展，但对植物中这一现象的理解充其量仍是零碎的。我们的两个实验室从不同的起点一致认为，胞囊和MLO蛋白可能意外地协同参与调控特定植物细胞皮质区的靶向胞吐(分泌)。外囊是一种进化上保守的细胞质分泌囊泡-系留复合体，参与将分泌途径成分定位于特定的质膜(PM)结构域，该结构域也存在于植物中并具有活性。相反，植物完整膜MLO蛋白的基本分子功能仍然未知，尽管人们对其作用方式进行了许多研究，但该蛋白最先被鉴定为负责调节被子植物的抗真菌防御。然而，最近获得的间接证据表明，它们也可能参与植物靶向分泌过程的调节。我们两个实验室未发表的观察结果显示，拟南芥MLOS功能缺失突变体和胞外胞外亚单位EXO70H4在叶毛(毛状体)次生细胞壁的生物发生中具有几乎相同的表型缺陷，特别是缺乏碳水化合物聚合物胼胝质的定向沉积。此外，初步数据表明，MLO和胞外蛋白在各自的亚细胞定位上可能相互依赖。在我们的联合项目中，我们的目标是在极化胞吐中胞囊功能的背景下阐明MLO蛋白活性的分子机制(S)。为此，我们将使用毛状体作为次生细胞壁生物发生(局部老茧沉积)的模型系统，并将采用遗传(突变体；转基因)、生化(蛋白质组)和细胞生物学方法的组合，所有这些方法都在我们两个实验室建立得很好。由于MLO蛋白质以及胞外囊复合体也调节极化的细胞壁修饰以响应病原体的攻击，我们将利用我们实验室的联合专业知识来解决这些蛋白质在这一生物学背景下可能的相互作用。我们的共同努力将大大有助于理解MLO蛋白和外囊复合体的神秘分子机制，无论是在植物的正常发育过程中，还是在病原生物相互作用中。

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 Unpublished observations in our two labs revealed that Arabidopsis loss-of-function mutants of MLOs and the exocyst subunit EXO70H4 have almost identical phenotypic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.