Comparing the infection biology of Plasmodiophora brassicae primary and secondary zoospores in host and non-host

Plasmodiophora brassicae (Wor.) is an obligate plant pathogen affecting Brassicae worldwide. The cycle of P.brassicae consists of a primary phase by primary zoospores infection in root hairs and followed a secondary phase by secondary zoospores infection in the root cortex. To date, there is very little information available on the biology and molecular basis of primary and secondary zoospores infection. To examine the role, we investigated the infection from the samples of inoculation with resting spores (source of primary zoospores) and secondary 18 zoospores separately. Our study reconfirm that the primary infection phase in the root hairs and epidermis can complete in non-hosts, releasing secondary zoospores can further infect host and non-host. This particular mode of infection made it was not suitable for controlling clubroot by intercrop. However, reinfection of secondary zoospores in the root hairs and epidermis of nonhost were obviously limited compared with that of host. The secondary zoospores populations through repetitive cycles in the root hairs and epidermis likely influence the subsequent cortical infection. The development asynchrony of P.brassicae characterized by elevating proportions of multinucleate secondary plasmodum, resting sporangial plasmodium and resting spore were more obvious by inoculating primary zoospores than secondary zoospores in host. We first found resting spores could form at 12dpi. Otherwises, zoosporangia group by inoculating primary zoospores were observed in nonhost. The results 30 indicated that the initiation of host response to P.brassicae mainly occurred by primary zoospores infection. In view of effector was key to understand how the pathogen manipulates the host, we identified the secreted proteins by bioinformatics approach. Here, we first found that effectors were induced by primary zoospores infection, but no effectors were only induced by secondary zoospores during early infection phase. The results further proved that the interaction of P.brassicae and plants mainly occurred during primary infection phase, which was beneficial to inhibit host defense and establish subsequent infection. Our results may lead to better understanding of the molecular mechanism underlying the life cycle of P. brassicae and the interaciton of P.brassicae and host.