Analyse der Verticillium longisporum induzierten Seneszenz und Transdifferenzierung in Arabidopsis thaliana

Abstract

Verticillium longisporum (Vl43) is a soil born fungus and its main host is rape (Brassica napus). The main disease symptoms are early senescence, stunting, yellow veins and curled leaves. Vl43 infects the plant via the roots and proliferates systemically through the vascular system. In this work, an in vitro-assay has been established successfully, which allowed a cell-biological characterization of Verticillium proliferation in the Arabidopsis root system and provides the bases for mutant screens. Analyses of infected roots showed that V. longisporum enters the vascular system at zones without a fully established endodermis like the root tip area. The endodermis forms a barrier and, therefore, prevents fungal colonization of the vascular system in the differentiation zone of the root. Cell-biological analysis of Vl43 infected Arabidopsis leaves revealed pathogen induced transdifferentiation of bundle sheath cells to de novo formed xylem.The focus of this work were analyses of induced- early senescence and characterization of transdifferentiation of bundle sheath cells to de novo xylem. The results indicate that V. longisporum actively induces plant senescence during the necrotrophic phase of infection. A strong reduction of cytokinin concentrations at early time points of infection may be a reason for early senescence of infected plants. The observed modification of cytokinin homeostasis is possibly caused by higher expression levels of cytokinin oxidases. However, the underlying mechanism or regulation of this process is still unknown. Stabilization of cytokinin homeostasis both by pharmacological treatments and by a genetic approach resulted in a reduced senescence and, therefore, in an inhibition of fungal proliferation. The analyses show for the first time pathogen induced transdifferentiation in an infected host plant. Transdifferentiation of bundle sheath cells correlates with the yellow vein symptom and may be a part of the stunting phenotype. The genetic mechanism of transdifferentiation is based on the same processes as natural xylem development, which is regulated by vascular related VASCULAR NAC DOMAIN transcription factors (VNDs). Analyses to study the biological role of pathogen induced transdifferentiation revealed that Verticillium does not colonize the newly formed xylem. Therefore, it is not likely that transdifferentiation is induced to support fungal colonization of host plants. De novo xylem may play a role in water supply. However, suppression of the transdifferentiation by repression of VND7 function did not result in wilting of Arabidopsis.Interestingly, suppression of transdifferentiation resulted in a slightly different pattern of symptom development. After infection the transgenic plants with suppressed transdifferentiation showed a physiological partitioning of the rosette, which was not seen in infected wild type plants. The source area showed senescence, while the sink area was still green and relatively healthy. Our hypothesis is that transdifferentiation is a process to disturb the reimport of nutrients into the plant sink area. The fungus may use these nutrients during its necrotrophic phase. A C14-glucose transport experiment provided first evidence on altered source-sink ratios in infected plants.