College of Life Science, Northwest A&F University, No 3 Taicheng Road, Yangling 712100, China.
Biomass Energy Center for Arid and Semi-arid lands, Northwest A&F University, Yangling 712100, China.
J Integr Plant Biol. 2017 May;59(5):311-321. doi: 10.1111/jipb.12548.
An essential step for the distribution of carbon throughout the whole plant is the loading of sugars into the phloem in source organs. In many plants, accumulation of sugars in the sieve element-companion cell (SE-CC) complex is mediated and regulated by active processes. However, for poplar and many other tree species, a passive symplasmic mechanism of phloem loading has been proposed, characterized by symplasmic continuity along the pre-phloem pathway and the absence of active sugar accumulation in the SE-CC complex. A high overall leaf sugar concentration is thought to enable diffusion of sucrose into the phloem. In this review, we critically evaluate current evidence regarding the mechanism of passive symplasmic phloem loading, with a focus on the potential influence of active sugar transport and plasmodesmal regulation. The limited experimental data, combined with theoretical considerations, suggest that a concomitant operation of passive symplasmic and active phloem loading in the same minor vein is unlikely. However, active sugar transport could well play an important role in how passively loading plants might modulate the rate of sugar export from leaves. Insights into the operation of this mechanism has direct implications for our understanding of how these plants utilize assimilated carbon.
碳水化合物在整个植物体中分配的一个重要步骤是将糖装载到源器官的韧皮部中。在许多植物中，糖在筛分子-伴胞（SE-CC）复合体中的积累是由主动过程介导和调节的。然而，对于杨树和许多其他树种，已经提出了一种被动的韧皮部装载共质体机制，其特征是沿前韧皮部途径具有共质体连续性，并且 SE-CC 复合体中没有活性糖积累。高的总体叶片糖浓度被认为能够使蔗糖扩散到韧皮部中。在这篇综述中，我们批判性地评估了关于被动共质体韧皮部装载机制的当前证据，重点关注主动糖转运和胞间连丝调节的潜在影响。有限的实验数据，加上理论上的考虑，表明在同一小叶脉中同时进行被动共质体和主动韧皮部装载的可能性不大。然而，主动糖转运很可能在被动装载植物如何调节叶片中糖的输出速率方面发挥重要作用。对这种机制的运作的深入了解直接影响到我们对这些植物如何利用同化碳的理解。
