Sink plasmodesmata as gateways for phloem unloading.: Myosin VIII and calreticulin as molecular determinants of sink strength?
Sink plasmodesmata as gateways for phloem unloading.: Myosin VIII and calreticulin as molecular determinants of sink strength?
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DOI：
10.1104/pp.126.1.39
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发表时间：
2001-05-01
期刊：
影响因子：
7.4
通讯作者：
Volkmann, D
中科院分区：
文献类型：
--
作者：
Baluska, F;Cvrcková, F;Volkmann, D
韧皮部介导的光同化物运动是植物中最关键的过程之一。光合作用活跃的叶片（源）产生过量的光同化物，这些光同化物通过韧皮部的筛子元素出口到光合作用不活跃的组织（汇）。例如，生长中的根尖是异养汇器官，依赖于地上源器官持续提供光同化物。因此，根尖是研究韧皮部光同化物质卸载机制和汇强度的理想对象。汇强度最简单的定义考虑了异养器官进口、加工和储存光同化物的竞争能力（Herbers和Sonnenwald， 1998）。不幸的是，决定汇强度的因素和分子仍然存在争议。尽管如此，人们普遍认为，将高等植物的大多数细胞连接成一个共质连续体的胞间连丝，在很大程度上有助于韧皮部向汇组织卸载。这一点已在根尖和其他几种碳库组织中得到证实，如马铃薯（Solanum tuberosum）块茎和农杆菌肿瘤（Fisher和Oparka, 1996； Pradel等人，1999；Oparka和Santa Cruz， 2000）。胞间连丝也参与叶片的这种输出（Stitt, 1996），强调了它们在源-汇相互作用中的首要重要性。玉米（Zea mays）和拟南芥（Arabidopsis）的根尖是研究胞间连丝在韧皮部光同化物质向汇组织卸载中的作用的理想模型对象。尽管人们接受根尖的韧皮部卸载的同质途径，但计算出的间连丝的数量不足以支持快速生长的根尖的广泛碳需求（Bret-Harte和Silk， 1994）。因此，这些作者得出结论，要么是胞间连丝的通透性受到积极调节，要么是存在其他转运机制将其转运到顶端分生组织。支持第一种可能性的一种可能的解释来自于最近对胞间连丝的研究，这些研究表明这些复杂的细胞壁“通道”是可进入的（van Bel and Kesteren, 1999; Jackson, 2000; Zambryski and Crawford, 2000）。
Phloem-mediated movement of photoassimilates is one of the most critical processes in plants. Photosynthetically active leaves (source) produce an excess of photoassimilates that are exported, via sieve elements of the phloem, into photosynthetically inactive tissues (sink). For instance, growing root apices are heterotrophic sink organs that are dependent on the continuous supply of photoassimilates from the above-ground source organs. Thus, root apices represent an ideal object to study mechanisms of the phloem unloading of photoassimilates and of the sink strength. The simplest definition of sink strength considers the competitive ability of heterotrophic organs to import, process, and store photoassimilates (Herbers and Sonnenwald, 1998). It is unfortunate that both factors and molecules that determine the sink strength remain controversial. Nevertheless, general agreement exists that plasmodesmata, which interconnect most cells of higher plants into a symplasmic continuum, substantially contribute to phloem unloading into sink tissues. This has been shown for root apices and for several other sink tissues such as potato (Solanum tuberosum) tubers and Agrobacterium tumefaciens sp. tumors (Fisher and Oparka, 1996; Pradel et al., 1999; Oparka and Santa Cruz, 2000). Plasmodesmata also participate in Suc export from leaves (Stitt, 1996), highlighting their prime importance for source-sink interactions. Maize (Zea mays) and Arabidopsis root apices serve as excellent model objects to study the role of plasmodesmata in unloading of photoassimilates from phloem elements into sink tissues. Although a symplasmic pathway for phloem unloading is accepted for root apices, the number of plasmodesmata that have been calculated to be present is not sufficient to support the extensive carbon demand of rapidly growing root apices (Bret-Harte and Silk, 1994). Thus, these authors concluded that either plasmodesmata permeability is actively regulated or that alternative transport mechanisms exist for Suc transport to the apical meristem. A possible explanation for this paradox, in favor of the first possibility, comes from the recent studies on plasmodesmata that reveal that these complex cell wall “tunnels” are gateable (van Bel and Kesteren, 1999; Jackson, 2000; Zambryski and Crawford, 2000).