Moscatelli Alessandra, Idilli Aurora Irene
Dipartimento di Biologia L. Gorini, Università degli Studi di Milano, Milano, Italy.
J Integr Plant Biol. 2009 Aug;51(8):727-39. doi: 10.1111/j.1744-7909.2009.00842.x.
Although pollen tube growth is a prerequisite for higher plant fertilization and seed production, the processes leading to pollen tube emission and elongation are crucial for understanding the basic mechanisms of tip growth. It was generally accepted that pollen tube elongation occurs by accumulation and fusion of Golgi-derived secretory vesicles (SVs) in the apical region, or clear zone, where they were thought to fuse with a restricted area of the apical plasma membrane (PM), defining the apical growth domain. Fusion of SVs at the tip reverses outside cell wall material and provides new segments of PM. However, electron microscopy studies have clearly shown that the PM incorporated at the tip greatly exceeds elongation and a mechanism of PM retrieval was already postulated in the mid-nineteenth century. Recent studies on endocytosis during pollen tube growth showed that different endocytic pathways occurred in distinct zones of the tube, including the apex, and led to a new hypothesis to explain vesicle accumulation at the tip; namely, that endocytic vesicles contribute substantially to V-shaped vesicle accumulation in addition to SVs and that exocytosis does not involve the entire apical domain. New insights suggested the intriguing hypothesis that modulation between exo- and endocytosis in the apex contributes to maintain PM polarity in terms of lipid/protein composition and showed distinct degradation pathways that could have different functions in the physiology of the cell. Pollen tube growth in vivo is closely regulated by interaction with style molecules. The study of endocytosis and membrane recycling in pollen tubes opens new perspectives to studying pollen tube-style interactions in vivo.
尽管花粉管生长是高等植物受精和种子形成的前提条件，但导致花粉管发射和伸长的过程对于理解顶端生长的基本机制至关重要。人们普遍认为，花粉管伸长是通过高尔基体衍生的分泌囊泡（SVs）在顶端区域或透明区积累和融合而发生的，在该区域它们被认为与顶端质膜（PM）的一个受限区域融合，从而定义了顶端生长域。SVs在顶端的融合逆转了细胞壁物质的外部状态，并提供了新的质膜片段。然而，电子显微镜研究清楚地表明，顶端合并的质膜大大超过了伸长，并且在19世纪中叶就已经提出了质膜回收机制。最近关于花粉管生长过程中内吞作用的研究表明，不同的内吞途径发生在花粉管的不同区域，包括顶端，并导致了一个新的假说来解释顶端囊泡的积累；也就是说，除了SVs之外，内吞囊泡对V形囊泡的积累也有很大贡献，并且胞吐作用并不涉及整个顶端区域。新的见解提出了一个有趣的假说，即顶端的胞吐作用和内吞作用之间的调节有助于在脂质/蛋白质组成方面维持质膜极性，并显示出不同的降解途径，这些途径可能在细胞生理学中具有不同的功能。花粉管在体内的生长受到与花柱分子相互作用的密切调节。对花粉管内吞作用和膜循环的研究为研究体内花粉管与花柱的相互作用开辟了新的视角。
