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

植物的无柄多细胞生命和发育依赖于在细胞分裂、伸长、分化、繁殖和病原体防御期间建立、维持和改变细胞极性的能力等因素。尽管在揭示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然是零碎的。我们的两个实验室从不同的出发点会聚在一起，研究外囊蛋白和MLO蛋白在特定植物细胞皮质结构域的靶向胞吐（分泌）调节中意外协调参与的可能性。外囊是一种进化上保守的细胞质分泌囊泡-栓系复合物，涉及将分泌途径组分靶向特定质膜（PM）结构域，其也存在于植物中并具有活性。相反，植物膜MLO蛋白的基本分子功能，首先确定为负责被子植物中的抗真菌防御的调节，仍然未知，尽管许多努力来表征其作用模式。然而，最近获得的间接证据表明，它们也可能参与调节植物中的靶向分泌过程。在我们的两个实验室中未发表的观察结果显示，拟南芥MLO和外囊亚基EXO 70 H4的功能丧失突变体在叶毛（毛状体）的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和外囊蛋白关于其各自的亚细胞定位可能相互依赖。在我们的联合项目中，我们的目标是阐明MLO蛋白活性的分子机制（S）的背景下，胞吐功能极化胞吐。为此，我们将使用毛状体作为次生细胞壁生物发生（局部胼胝质沉积）的模型系统，并且我们将部署遗传（突变体;转基因），生物化学（蛋白质组学）和细胞生物学方法的组合，所有这些方法都在我们的实验室中得到了很好的建立。由于MLO蛋白以及外囊复合物也调节响应病原体攻击的极化细胞壁修饰，我们将利用我们实验室的综合专业知识来解决这些蛋白质在这种生物背景下的假定相互作用。我们的共同努力应大大有助于理解的神秘分子机制的MLO蛋白和外囊复合物，无论是在正常的植物发育过程中，在致病性生物相互作用。

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 Unpublished observations in our two labs revealed that Arabidopsis loss-of-function mutants of MLOs and the exocyst subunit EXO70H4 have almost identical phenotypic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