Autophagy-Mediated Compartmental Cytoplasmic Deletion is Essential for Tobacco Pollen Germination and Male Fertility

In plants, macroautophagy/autophagy has mainly been associated with stress-related processes but how it impacts normal physiological and developmental processes remains largely unexplored. Pollen germination is the critical first step towards fertilization in flowering plants. It is metabolically demanding and relies on high levels of cytoplasmic reorganization activities to support a dramatic morphological transformation that underlies the development of a pollen tube as the conduit to deliver sperm for fertilization. The role of autophagy in this process remains unclear. Here we provide evidence that pollen germination is accompanied by elevated autophagic activity and successful pollen tube emergence depends on autophagy-mediated cytoplasmic deletion. Genetic and cytological experiments demonstrate that inhibition of autophagy prevents pollen germination while induces the persistence of a layer of undegraded cytoplasm at the germination aperture. Together, these results unveil a novel compartmentalized autophagy. Furthermore, high-throughput comparative lipidomic analyses show that suppressed autophagy-induced inhibition of pollen germination is accompanied by altered profiles of stored and signaling lipids. Proteomic analyses reveal that autophagy likely exert its role in pollen germination via downstream mitochondria-related pathways. These findings reveal a critical role for autophagy in initiating pollen germination and provide evidences for compartmental cytoplasmic deletion being crucial for male fertility.

在植物中，巨自噬（macroautophagy）/自噬（autophagy）的功能多与胁迫相关过程相关联，但其对植物正常生理及发育过程的影响在很大程度上仍未被系统阐明。花粉萌发是被子植物受精过程中至关重要的起始步骤，该过程代谢需求旺盛，依赖高水平的细胞质重排活动，以支持剧烈的形态转变——这一转变是花粉管形成的基础，而花粉管是输送精子完成受精的通道。目前自噬在该过程中的作用仍不明确。
本研究提供证据表明，花粉萌发伴随自噬活性升高，且花粉管的成功伸出依赖自噬介导的细胞质降解。遗传与细胞学实验证实，抑制自噬会阻碍花粉萌发，同时诱导萌发孔处残留一层未被降解的细胞质。综上，本研究揭示了一种全新的区室化自噬现象。
此外，高通量比较脂质组学分析显示，自噬抑制所导致的花粉萌发受阻，伴随储存脂质与信号脂质的谱式发生改变。蛋白质组学分析表明，自噬可能通过下游线粒体相关通路在花粉萌发过程中发挥功能。
本研究结果阐明了自噬在启动花粉萌发中的关键作用，并为区室化细胞质降解对雄性育性至关重要提供了实验依据。