Heterotrophic Nutrition Strategies of the Parasitic Plant Cuscuta

点击翻译按钮获取中文摘要

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.