韧皮部运输和防御作用的分子机理

韧皮部是维管系统的重要组成部分,在物质运输、信息传递、机械支持以及防御作用中发挥重要作用。近年来,在建立了一系列研究方法的基础上,对韧皮部运输和防御功能特别是碳水化合物装载和卸出机理进行了大量研究,克隆出一些特异基因,初步阐明了韧皮部装载的分子机制,取得可喜进展。本文综述了一些相关研究结果,并提出几个有待解决的问题。     

植物体内糖分子的长距离运输及其分子机制

植物器官(如叶、叶鞘、绿色的茎等)可以通过光合作用将CO₂合成为碳水化合物, 并经过长距离运输到达库组织(如新生组织、花粉、果实等)中进行贮存或利用。蔗糖是高等植物长距离运输碳水化合物的主要形式。蔗糖分子从源到库的运输包括源组织韧皮部的装载、维管束的运输和库组织韧皮部的卸载3个步骤。遗传学和分子生物学研究证明, 蔗糖转运蛋白、转化酶和单糖转运蛋白在糖分子的装载和卸载过程中发挥重要作用。该文综述了目前对光合产物运输过程及其调控分子机制的最新研究进展。     

高等植物蔗糖转运的分子调控

在高等植物中,蔗糖的合成、运输与分配是一个复杂的过程。蔗糖由源到库的运输不仅与植物的生长发育相关,还受到植物体内的激素水平以及外界环境条件变化等因素的影响。蔗糖转运蛋白介导了蔗糖在植物韧皮部的装载、运输和卸载,在某些库中的蔗糖转运和库组织分配的分子调控中起有重要的生理作用。此外,简要介绍了笔者实验室在橡胶树蔗糖转运蛋白基因研究方面的最新进展。     

水稻蔗糖转运蛋白研究进展

蔗糖转运蛋白是光合产物运输与分配调控网络中的重要节点,主要参与蔗糖从"源"到"库"的质外体运输,在蔗糖的感应、"源"器官装载、韧皮部长距离运输和"库"器官卸载中起重要作用。总结和分析了水稻蔗糖转运蛋白基因家族的组成、蛋白结构特点、表达与调控特性、生物学功能等方面的研究进展,在此基础上,提出了蔗糖转运蛋白基础理论和应用研究方面存在的不足及应予重视和加强的主要方向。     

植物体内光合同化物韧皮部装载和卸出研究进展

近年来研究表明,植物体内光合同化物的韧皮部装载和卸出均有其本途径和质外体途径,装载转运的糖类主要有：（２）棉子糖及其人类似物（以共质体方式装载）;（２）蔗糖（以质外体方式装载）。同化物的共质体卸出可通过扩散和集中作用实现,而质外体卸出则根据蔗糖在质外体是否水解而分为两种类型。卸出和装载的途径、机理因植物种类及库源关系而不同,也会受生长发育阶段及环境的变化而调整。深入研究韧皮部装载和帛出调控机制,对     

水稻蔗糖转运及其与产量形成的关系

蔗糖是植物体内主要的光合产物和运输形式,在叶片中合成并经过维管组织向库器官转运,在库组织中水解并用于合成淀粉、蛋白质和纤维素等有机物。水稻蔗糖转运对调控作物生长发育和产量形成,特别是在逆境条件下的产量稳定,都具有十分重要的作用。本文重点综述了水稻蔗糖韧皮部装载、运输和卸载机制以及关键酶的活性和基因表达调控,并讨论了其与水稻产量形成的关系。     

脱落酸在光合产物分配中的作用

作物产量最终依赖于光合产物和矿质离子在相互竞争的库器官之间的分配方式,因此,弄清光合产物分配的控制机制显然具有十分重要的意义。植物激素在光合产物分配中起着重要的作用。业已证明,生长素、赤霉素和细胞分裂素能够通过刺激生长过程或直接影响韧皮部装入和卸出或由源向库的运输而增加源和库的强度.近年来,通过对库中脱落酸浓度和生长速率之间的相关性,脱落酸对韧皮部装入和卸出以及库对糖的吸收的响影等方面的研究,证明脱落酸也参与光合产物分配的调控。本     

灵武长枣果实同化物韧皮部卸载和运输途径研究

韧皮部卸载和韧皮部后运输在调节同化物在果实中的分配和积累方面起着至关重要的作用,而且很大程度上决定着果实的产量和质量。为探讨灵武长枣果实同化物韧皮部卸载和运输途径,以4个时期灵武长枣果实为实验材料,对各个发育时期果实维管束的显微结构进行观察,并综合运用荧光染料活细胞示踪与激光共聚焦扫描显微镜技术实时观察果实内韧皮部同化物卸载路径的变化,为灵武长枣果实同化物积累和品质调控奠定基础。结果显示：(1)膨大前期不仅果实的韧皮部中具有明显的CF绿色荧光,同时在周围薄壁细胞中也分布着CF绿色荧光,筛管伴胞复合体和周围薄壁细胞之间存在着共质体联系。(2)快速膨大期,CF绿色荧光主要局限于果实的韧皮部中,在韧皮部周围薄壁细胞中分布较少,筛管伴胞复合体与周围薄壁细胞之间主要以共质体隔离为主,但也存在着一定的共质体联系。(3)着色期和完熟期,CF绿色荧光局限于果实的韧皮部中,在韧皮部周围薄壁细胞中基本没有CF绿色荧光,果实筛管伴胞复合体与周围薄壁细胞之间是共质体隔离状态,但引入CFDA的同时引入具有质膜通透作用的洋地黄皂苷时,周围薄壁细胞中CF绿色荧光分布明显增加。研究认为,灵武长枣在膨大前期果实韧皮部同化物为共质体卸载途径,快速膨大期果实主要以质外体途径运输同化物,但也通过共质体卸出同化物,着色期和完熟期果实通过质外体途径运输同化物。     

水稻蔗糖转运载体蛋白(OsSUT2)非跨膜区域特征性序列的融合表达及其抗体制备

植物光合作用产生的蔗糖是植物生长发育的主要碳源物质,还是诱导植物生长发育过程中诸多相关基因表达的特异信号分子[1].蔗糖分子在植物器官及组织间的生理分配维持着整个植物体的正常生长发育[2].植物蔗糖转运载体(sucrosetransporter,SUT)是一类担负着蔗糖分子在细胞间的转运及信号转导的功能性蛋白家族,它在蔗糖的韧皮部装载、沿韧皮部的再吸收、韧皮部卸载和向库器官的转运等跨膜运输以及蔗糖特异信号感应过程中发挥着重要的生理功能[3～5].植物蔗糖转运载体蛋白分布于植物细胞质膜上,该转运载体蛋白含有12个疏水性跨膜结构域,在其氨…     

多年生植物韧皮部发育的季节变化和筛分子寿命

近几十年来,韧皮部的解剖学和细胞学研究有了很大的进展。在韧皮部的发育、季节变化和筛分子寿命等方面也积累了不少资料。研究韧皮部发育的季节变化可为了解植物生长发育规律提供资料,同时也是研究外界因素对植物生长发育的影响所必需的。筛分子寿命更是研究运输生理的学者所特别关注的问题。研究韧皮部发育的季节变化和确定筛分子寿命通常运用周年采样、常规解剖学制片的方     

Ecophysiology of phloem loading in source leaves

The nature of phloem loading of photosynthesis products – either symplastic or apoplastic – has been a matter of debate over the last two decades. This controversy was reconciled by proposing a multiprogrammed loading mechanism. Different modes of phloem loading were distinguished on the basis of the variety of plasmodesmatal connectivity between the minor vein elements. Physiological evidence for at least two phloem loading mechanisms as well as recent support for coincidence between plasmodesmatal connectivity and the loading mechanism is shortly reviewed. The present paper attempts to correlate the plasmodesmatal connectivity between sieve element/companion cell complex and the adjacent cells (the minor vein configuration) – and implicitly the associate phloem loading mechanisms – with different types of climate. The minor vein configuration is a family characteristic. This enables one to relate vein configuration with ecosystem using the family distribution over the globe. The uneven distribution of vein types between terrestrial ecosystems indicates that apoplastic phloem loading predominates in cold and dry climate zones. Projection of the minor vein configuration on the Takhtajan system of flowering plants suggests evolution from apoplastic to symplastic phloem loading. Accordingly, the distribution of minor vein configurations suggests that drought and temperature stress have led to the transformation of the ancient symplastic mode into the more advanced apoplastic mode of loading.     

The mechanism of phloem loading in rice (Oryza sativa)

Carbohydrates, mainly sucrose, that are synthesized in source organs are transported to sink organs to support growth and development. Phloem loading of sucrose is a crucial step that drives long-distance transport by elevating hydrostatic pressure in the phloem. Three phloem loading strategies have been identified, two active mechanisms, apoplastic loading via sucrose transporters and symplastic polymer trapping, and one passive mechanism. The first two active loading mechanisms require metabolic energy, carbohydrate is loaded into the phloem against a concentration gradient. The passive process, diffusion, involves equilibration of sucrose and other metabolites between cells through plasmodesmata. Many higher plant species including Arabidopsis utilize the active loading mechanisms to increase carbohydrate in the phloem to higher concentrations than that in mesophyll cells. In contrast, recent data revealed that a large number of plants, especially woody species, load sucrose passively by maintaining a high concentration in mesophyll cells. However, it still remains to be determined how the worldwide important cereal crop, rice, loads sucrose into the phloem in source organs. Based on the literature and our results, we propose a potential strategy of phloem loading in rice. Elucidation of the phloem loading mechanism should improve our understanding of rice development and facilitate its manipulation towards the increase of crop productivity.     

Transport of photoassimilates in plants under unfavourable environmental conditions

Transport of photoassimilates linking functionally plant, as a whole system, is discussed as a target for different environmental stresses. Anatomical, physiological and biochemical aspects of phloem transport, phloem loading and unloading are taken into consideration. In the light of modern theoríes of assimilate transport some historical hypotheses are also shown, due to their input into the progress of transport science. The role of phloem unloading in plant acclimation to environment stress is not clear, however changes in source/sink ratio was often observed as the effect of stress. The blockage of sieve tubes found as the result of given stress may be of secondary importance. On the other hand, phloem loading process seems to be an important target for different environmental stresses.     

The phloem pathway: New issues and old debates

The phloem is a central actor in plant development and nutrition, providing nutrients and energy to sink organs and integrating interorgan communication. A comprehensive picture of the molecules trafficking in phloem sap is being made available, with recent surveys of proteins, RNAs, sugars, and other metabolites, some of which are potentially acting as signals. In this review, we focus on recent breakthroughs on phloem transport and signalling. A case study was phloem loading of sucrose, acting both as a nutrient and as a signal, whose activity was shown to be tightly regulated. Recent advances also described actors of macromolecular trafficking in sieve elements, including chaperones and RNA binding proteins, involved potentially in the formation of ribonucleoprotein complexes. Likewise, long distance signalling appeared to integrate electrical potential waves, calcium bursts and potentially the generation of reactive oxygen species. The ubiquitin–proteasome system was also proposed to be on action in sieve elements for signalling and protein turnover. Surprisingly, several basic processes of phloem physiology are still under debate. Hence, the absence in phloem sap of reducing sugar species, such as hexoses, was recently challenged with observations based on an analysis of the sap from Ranunculaceae and Papaveraceae. The possibility that protein synthesis might occur in sieve elements was again questioned with the identification of components of the translational machinery in Pumpkin phloem sap. Altogether, these new findings strengthen the idea that phloem is playing a central role in interorgan nutrient exchanges and communication and demonstrate that the ways by which this is achieved can obey various patterns among species.     

Phloem loading and unloading of sugars and amino acids

In terrestrial higher plants, phloem transport delivers most nutrients required for growth and storage processes. Some 90% of plant biomass, transported as sugars and amino nitrogen (N) compounds in a bulk flow of solution, is propelled though the phloem by osmotically generated hydrostatic pressure differences between source (net nutrient export) and sink (net nutrient import) ends of phloem paths. Source loading and sink unloading of sugars, amino N compounds and potassium largely account for phloem sap osmotic concentrations and hence pressure differences. A symplasmic component is characteristic of most loading and unloading pathways which, in some circumstances, may be interrupted by an apoplasmic step. Raffinose series sugars appear to be loaded symplasmically. However, sucrose, and probably certain amino acids, are loaded into minor veins from source leaf apoplasms by proton symporters localized to plasma membranes of their sieve element/companion cell (se/cc) complexes. Sucrose transporters, with complementary kinetic properties, are conceived to function as membrane transporter complexes that respond to alterations in source/sink balance. In contrast, symplasmic unloading is common for many sink types. Intervention of an apoplasmic step, distal from importing phloem, is reserved for special situations. Effluxers that release sucrose and amino acids to the surrounding apoplasm in phloem loading and unloading are yet to be cloned. The physiological behaviour of effluxers is consistent with facilitated membrane transport that can be energy coupled. Roles of sucrose and amino acid transporters in phloem unloading remain to be discovered along with mechanisms regulating symplasmic transport. The latter is hypothesized to exert significant control over phloem unloading and, in some circumstances, phloem loading.     

Sucrose transporters and plasmodesmal regulation in passive phloem loading

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.     

灵武长枣果实维管束韧皮部及周围细胞的超微结构特征

为了探讨灵武长枣果实光合同化物韧皮部卸载和运输的途径,该研究采用透射电镜技术,对不同发育时期灵武长枣果实维管束韧皮部及其周围薄壁细胞的超微结构特征进行了分析。结果表明:筛管/伴胞复合体及其周围韧皮薄壁细胞间在果实膨大前期富含胞间连丝,而韧皮薄壁细胞与周围库细胞以及相邻库细胞间几乎不存在胞间连丝,形成共质体隔离;筛管/伴胞复合体及其与周围薄壁细胞间在果实快速膨大期也存在胞间连丝,但与果实膨大前期相比明显减少;果实着色期,SE/CC复合体及其与周围薄壁细胞间胞间连丝较少,并且出现阻塞现象;果实完熟期,筛管和伴胞之间几乎没有胞间连丝,有的筛管之间有少量胞间连丝,但却出现了阻塞现象,果肉库薄壁细胞与韧皮薄壁细胞间因胞间连丝阻塞现象而形成共质体隔离。综上结果认为,在果实发育的膨大前期阶段,光合同化物以共质体途径经筛分子卸出,卸出后可能以质外体途径进入液泡贮藏与利用;果实快速膨大期,光合同化物的卸出与运输采用共质体和质外体共存的途径;果实着色期和完熟期,光合同化物从筛分子卸出到贮藏薄壁细胞的运输均以质外体途径为主。     

Phloem loading. A reevaluation of the relationship between plasmodesmatal frequencies and loading strategies

The incidence of plasmodesmata in the minor vein phloem of leaves varies widely between species. On this basis, two pathways of phloem loading have been proposed: symplastic where frequencies are high, and apoplastic where they are low. However, putative symplastic-loading species fall into at least two categories. In one, the plants translocate raffinose-family oligosaccharides (RFOs). In the other, the primary sugar in the phloem sap is sucrose (Suc). While a thermodynamically feasible mechanism of symplastic loading has been postulated for species that transport RFOs, no such mechanism is known for Suc transporters. We used p-chloromercuribenzenesulfonic acid inhibition of apoplastic loading to distinguish between the two pathways in three species that have abundant minor vein plasmodesmata and are therefore putative symplastic loaders. Clethra barbinervis and Liquidambar styraciflua transport Suc, while Catalpa speciosa transports RFOs. The results indicate that, contrary to the hypothesis that all species with abundant minor vein plasmodesmata load symplastically, C. barbinervis and L. styraciflua load from the apoplast. C. speciosa, being an RFO transporter, loads from the symplast, as expected. Data from these three species, and from the literature, also indicate that plants with abundant plasmodesmata in the minor vein phloem have abundant plasmodesmata between mesophyll cells. Thus, plasmodesmatal frequencies in the minor veins may be a reflection of overall frequencies in the lamina and may have limited relevance to phloem loading. We suggest that symplastic loading is restricted to plants that translocate oligosaccharides larger than Suc, such as RFOs, and that other plants, no matter how many plasmodesmata they have in the minor vein phloem, load via the apoplast.