Heterotrophic Nutrition Strategies of the Parasitic Plant Cuscuta

葫芦属(Cuscuta Spp)是全寄生植物，没有根和叶，几乎侵染所有双子叶植物。葫芦属(Cuscuta Spp)将寄主植物的枝条缠绕起来，形成专门的穿透器官吸器，以获得水分、营养物质和同化物。这些多细胞器官在7至9天内侵入宿主组织并附着在宿主的血管系统上。当连接完成时，葫芦丝细胞直接连接到木质部的管状细胞以及寄主的韧皮部细胞。特别是，寄主和寄生细胞之间的韧皮部-韧皮部连接是由种间胞间连丝(PDs)建立的。由于这一过程，包括PD形成和韧皮部连接，至少需要7天，因此还不清楚是否需要其他例如质外体取食策略。此外，还不清楚是否为了尽可能有效地吸收养分，通常会发生额外的质外体营养。在我们自己的前期工作中，我们用反枝子侵染拟南芥，观察到双向蔗糖转运蛋白AtSWEET10(蔗糖最终将被输出转运蛋白10)在拟南芥中的转录在感染早期增加。此外，在转基因拟南芥植株中的报告基因分析表明，AtSWEET10启动子是由反枝莲吸器穿透诱导的，并且另一种蔗糖转运蛋白AtSUC2的基因表达也上调。在反枝藻中，我们鉴定了CrSUT1(蔗糖转运蛋白1)是反枝藻的第一个蔗糖转运蛋白，并通过原位杂交证实了基因在吸器前细胞中的表达。在感染的早期阶段，CrSUT1似乎负责将质外体蔗糖吸收到吸器前。综上所述，这些初步数据表明葫芦科植物Cuscuta spp.在通过PDs建立共体连接之前，在感染的早期阶段使用策略来摄取质外体糖。韧皮部细胞之间随后的连接过程包括种间PD的形成也尚不清楚，因此我们的目标是利用葫芦科-寄主植物模型来研究种间PD形成的详细分子机制以及葫芦科的寄生和异养取食策略。因此，在拟议的项目中，我们追求两个主要目标：A)揭示Cuscuta spp.的早期取食策略，该策略独立于PD介导的细胞-细胞接触发生，并且在宿主渗透的最初几天特别重要；B)跨PD的营养通量的功能研究以及宿主和寄生细胞之间PD形成的相关发育步骤。

Cuscuta spp. are holoparasitic plants without roots and leaves that infect almost all dicotyledonous plants. Cuscuta spp. enwind the shoots of their host plants and develop specialized penetrating organs, the haustoria, to gain access to water, nutrients and assimilates. These multicellular organs invade the host's tissues and attach to the host's vasculature within seven to nine days. When the connection is complete, Cuscuta cells are directly connected to the tracheae of the xylem as well as to the phloem cells of the hosts. In particular, the phloem-phloem junction between host and parasite cells is established by interspecific plasmodesmata (PDs). Since this process, including PD formation and phloem connection, takes at least seven days, it is unclear whether other e.g. apoplastic feeding strategies are required. Furthermore, it is unclear whether an additional apoplastic nutrition generally takes place in order to enable nutrient absorption as efficiently as possible. In our own preliminary work, we infected Arabidopsis thaliana with Cuscuta reflexa and observed that the transcript of the bidirectional sucrose transporter AtSWEET10 (SUCROSE WILL EVENTUALLY BE EXPORTED TRANSPORTER 10) in A. thaliana is increased in early infection stages. Furthermore, reporter gene assays in transgenic A. thaliana plants revealed that the AtSWEET10 promoter is induced by penetration of the C. reflexa haustorium and gene expression for another sucrose transporter, AtSUC2, is also upregulated. In C. reflexa we identified CrSUT1 (SUCROSE TRANSPORTER 1) as the first sucrose transporter of C. reflexa and demonstrated gene expression in prehaustorial cells by in situ hybridization. CrSUT1 appears to be responsible for uptake of apoplastic sucrose into the prehaustorium during the early stages of infection. Taken together, these preliminary data indicate that Cuscuta spp. use strategies to feed on apoplastic sugars at the early infection stages before the symplastic connections via PDs are established. The subsequent joining process including interspecific PD formation between phloem cells is also not yet understood, and we therefore aim to use the Cuscuta – host plant model to study the detailed molecular mechanisms of interspecific PD formation as well as the parasitic and heterotrophic feeding strategies of Cuscuta. In the proposed project we therefore pursue two main goals: A) to unravel the early feeding strategies of Cuscuta spp., which occur independently of PD-mediated cell-cell contact and are especially important during the first days of host penetration; and B) functional studies of nutrient flux across PDs and the relevant developmental steps of PD formation between host and parasite cells.