Abstract:

Plasmodesmata (PDs) are cytoplasmic structures that link adjacent cells to form the symplast of a plant. PDs are involved extensively in a plant's life by mediating symplastic transport of a wide range of ions and molecules. Major components of a plasmodesma (PD) include a plasma membrane, a desmotubule, and a cytoplasmic annulus, all of which are readily detectable by electron microscopy. Both the plasma membrane and the desmotubule contain proteinaceous particles, thought to be involved in altering the size of the cytoplasmic annulus. Cytoskeleton elements (actin and myosin) are essential for maintaining the integrity of PDs. Together with these elements, calcium-binding proteins probably play a significant role in regulating PD function. Symplastic transport occurs through the cytoplasmic annulus for the great majority of solutes, while other substances may traverse through the desmotubule internal compartment, the desmotubule shell, or the plasma membrane. The symplast is subdivided into several domains with varying molecular size exclusion limits (ranging from <1 kD to >10 kD). Plasmodesmata can be either primary or secondary; the former are developed during new wall formation and the latter are made in existing walls. The dynamic nature of plasmodesmata is also reflected by their changing frequencies, which, in turn, depend on the developmental and physiological status of the tissue or the entire plant. While diffusion is the major mechanism of symplastic transport, plasmodesmata are selective for certain ions and molecules. Upon viral infection, viral movement proteins interact with PD receptor proteins and, as a result of yet unknown mechanisms, the plasmodesmata are remarkably dilated to allow viral movement proteins and the bound viral genome to enter healthy cells. Some proteins of plant origin are also able to traverse plasmodesmata, presumably in ways similar to viral movement proteins. Some of these plant proteins are probably signal molecules contributing to cell differentiation and other activities. Other proteins move cell-to-cell in a non-specific manner.

胞间连丝：共质体运输的动态通道
马丰山1^，2* Carol A.PETERSON ¹

^{(1. Department of Biology, Unixersity of Waterloo, Waterloo,Ontario N2L 3G1, Canada;2.Department of Biology, Unixersity of  Guelph,Guelph,Ontario N1G2W1,Ganada)}

摘要：胞间连丝作为一种细胞质结构将相邻的细胞连系起来而形成植物的共质体。胞间连丝通过调控许多离子和分子的共质体运输而广泛地参与植物的生命活动。胞间连丝的主要构成部分是细胞质膜、连丝小管、以及位于二者之间的环层细胞质。这三者都很容易在电子显微镜下观察到。细胞骨架的成分(肌动蛋白和肌球蛋白)起到稳定胞间连丝的作用。同时，钙结合蛋白可能具有调节胞间连丝功能的作用。在胞间连丝里，环层细胞质为大多数溶质提供共质体运输的通道，而有些物质的共质体运输则可能是通过连丝小管的内腔、连丝小管的壳层、甚或是细胞质膜来实现的。共质体可以细分为数个区块，它们各自允许不同大小的分子(从低于1000到高于10000道尔顿)通过。从发生上看，胞间连丝可以是初生的，也可以是次生的。前者是伴随着新细胞壁的形成而产生的，而后者则是在已有的细胞壁上产生的。胞间连丝的动态性质还表现在它们的频率是处于变化之中的，这是由组织或植物整体的发育和生理状态决定的。虽然共质体运输的基本形式是扩散，但胞间连丝对于某些离子和分子却是有选择性的。在病毒感染细胞时，病毒的移动蛋白作用于胞间连丝的受体蛋白，结果，胞间连丝被显著地扩张(其机理尚不清楚)。于是，病毒的移动蛋白连同与之结合在一起的病毒基因组进入毗邻的健康细胞。一些植物源性的蛋白质也能够通过胞间连丝来运输；推测其方式类似于病毒的移动蛋白。有些植物蛋白质本身就是信号分子，它们调节分化和其他活动。与此相反，还有一些植物蛋白质的共质体运输却并不是通过特异的方式来实现的。

关键词： 细胞骨架；移动蛋白；韧皮部；胞间连丝；蛋白质；可过分子限度；共质体运输；病毒

Key words: cytoskeleton, movement protein, phloem, plasmodesmata, protein, size exclusion limit, symplastic transport, virus