Sieve element unloading: cellular pathway, mechanism and control
Sieve element unloading: cellular pathway, mechanism and control
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筛元卸载：细胞途径、机制和控制
DOI：
10.1111/j.1399-3054.1990.tb02095.x
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发表时间：
1990
影响因子：
6.4
通讯作者：
J. Patrick
中科院分区：
文献类型：
--
作者：
J. Patrick
韧皮部 - 移动溶质的运输和分布主要由位于源 - 运输 - 汇系统汇端的运输过程决定。穿过筛元边界的运输，即筛元卸载，是一系列水槽运输过程中的第一个过程。从筛元件卸载溶质可以遵循脱辅基或共质途径。据推测，卸载途径与汇功能相结合，并且质外体卸载仅限于通过共质体的运动与汇功能不兼容的情况。这些情况包括渗透活性溶质相对于筛元件和汇池之间的浓度和膨胀梯度的轴向运输和存储。与库功能的改变相结合，细胞的卸载途径可以在茎和可能的其他库中切换。回顾了用于阐明筛元件卸载机制的实验系统和方法。向质外体卸载的通量很大程度上可以通过轴向汇中的膜扩散来解释。然而，存储汇中较高的通量表明对某种形式的便利运输的依赖。质子蔗糖同向转运被认为是促进溶质穿过筛元件质膜到达库质外体的可能机制。通过共质体的卸载可以通过扩散或质量流发生。后一种机制用于消散韧皮部水，从而防止筛元件膨胀的潜在升高，否则会减慢韧皮部进入水槽的速度。增加了通电胞间连丝运输的可能性。筛元件卸载必须与卸载溶质的后续分隔和代谢相结合。溶质水平是控制筛元件卸载的明显基础，但被发现为质量作用机制提供了有限的范围。质外体、细胞途径、筛元件、溶质运输、共质体。转化为膨压信号后，溶质水平可以调节卸载速率、代谢和形成膨压稳态部分的区隔，而与卸载途径无关。
The transport and distribution of phloem – mobile solutes is predominantly determined by transport processes located at the sink end of the source – transport – sink system. Transport across the sieve element boundary, sieve element unloading, is the first of a series of sink transport processes. Unloading of solutes from the sieve elements may follow an apo- or symplastic route. It is speculated that the unloading pathway is integrated with sink function and that apoplastic unloading is restricted to situations in which movement through the symplast is not compatible with sink function. These situations include axial transport and storage of osmotically active solutes against concentration and turgor gradients between the sieve elements and sink cells. Coupled with alteration in sink function, the cellular pathway of unloading can switch in stems and possibly other sinks. Experimental systems and approaches used to elucidate the mechanism of sieve element unloading are reviewed. Unloading fluxes to the apoplast can largely be accounted for by membrane diffusion in axial sinks. However, the higher fluxes in storage sinks suggests dependence on some form of facilitated transport. Proton sucrose symport is assessed to be a possible mechanism for facilitated efflux of solutes across the sieve element plasma membrane to the sink apoplast. Unloading through the symplast may occur by diffusion or mass flow. The latter mechanism serves to dissipate phloem water and hence prevent the potential elevation of sieve element turgor that would otherwise slow phloem import into the sink. The possibility of energised plasmodesmatal transport is raised. Sieve element unloading must be integrated with subsequent compartmentation and metabolism of the unloaded solute. Solute levels are an obvious basis for control of sieve element unloading, but are found to offer limited scope for a mass action mechanism. Apoplastic, cellular pathway, sieve element, solute transport, symplastic. Translated into a turgor signal, solute levels could regulate the rate of unloading, metabolism and compartmentation forming part of a turgor homeostat irrespective of the pathway of unloading.