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Plasmodiophora brassicae (Wor.) is an obligate plant pathogen affecting Brassicae worldwide. To date, there is very little information available on the biology and molecular basis of P. brassicae primary and secondary zoospore infections. To examine their roles, we used microscope to systematically investigate the infection differences of P. brassicae between samples inoculated separately with resting spores and secondary zoospores. The obvious development of P. brassicae asynchrony that is characterized by secondary plasmodium, resting sporangial plasmodium, and resting spores was observed at 12 days in Brassica rapa inoculated with resting spores but not when inoculated with secondary zoospores at the same time. Inoculation with resting spores resulted in much more development of zoosporangia clusters than inoculation with secondary zoospores in non-host Spinacia oleracea. The results indicated that primary zoospore infection played an important role in the subsequent development. To improve our understanding of the infection mechanisms, RNA-seq analysis was performed. Among 18 effectors identified in P. brassicae, 13 effectors were induced in B. rapa seedlings inoculated with resting spores, which suggested that the pathogen and host first contacted, and more effectors were needed. Corresponding to those in B. rapa, the expression levels of most genes involved in the calcium-mediated signaling pathway and PTI pathway were higher in plants inoculated with resting spores than in those inoculated with secondary zoospores. The ETI pathway was suppressed after inoculation with secondary zoospores. The genes induced after inoculation with resting spores were suppressed in B. rapa seedlings inoculated with secondary zoospores, which might be important to allow a fully compatible interaction and contribute to a susceptible reaction in the host at the subsequent infection stage. The primary zoospores undertook an more important interaction with plants. [ABSTRACT FROM AUTHOR]

The potential infection biology of Plasmodiophora brassicae in resistant hosts and non-hosts is still not completely understood. Clubroot resistance assay on European clubroot differentials (ECD) set revealed that ECD10 (Brassica napus) and ECD4 (Brassica rapa) show a complete resistance to the tested P. brassicae isolate in contrast to highly susceptible hosts Westar (B. napus) and ECD5 (B. rapa). Previously, we used fluorescent probe-based confocal microscopy (FCM) to refine the life cycle of P. brassicae and indicate the important time points during its infection in Arabidopsis. Here, we used FCM to systematically investigate the infection of P. brassicae in two resistant host species ECD10 and ECD4 and two non-host crops wheat and barley at each indicated time points, compared with two susceptible hosts Westar and ECD5. We found that P. brassicae can initiate the primary infection phase and produce uninucleate primary plasmodia in both resistant hosts and non-hosts just like susceptible hosts at 2 days post-inoculation (dpi). Importantly, P. brassicae can develop into zoosporangia and secondary zoospores and release the secondary zoospores from the zoosporangia in resistant hosts at 7 dpi, comparable to susceptible hosts. However, during the secondary infection phase, no secondary plasmodium was detected in the cortical cells of both resistant hosts in contrast to massive secondary plasmodia present in the cortex tissue of two susceptible hosts leading to root swelling at 15 dpi. In both non-host crops, only uninucleate primary plasmodia were observed throughout roots at 7 and 15 dpi. Quantitative PCR based on DNA revealed that the biomass of P. brassicae has no significant increase from 2 dpi in non-host plants and from 7 dpi in resistant host plants, compared to the huge biomass increase in susceptible host plants from 2 to 25 dpi. Our study reveals that the primary infection phase in the root epidermis and the secondary infection phase in the cortex tissue are, respectively, blocked in non-hosts and resistant hosts, contributing to understanding of cellular and molecular mechanisms underlying clubroot non-host and host resistance. [ABSTRACT FROM AUTHOR]