How do plants defend themselves from club rot disease?
Plasmodiophora brassicae is the causative agent of club rot disease of brassica crops such as the Canadian canola (Brassica napus). It is a soil-borne, obligate, and biotrophic pathogen that is capable of causing significant yield losses. The pathogen causes the root and stem base to swell and form characteristic clubs, which inhibit xylem and phloem roles, stunt the growth of the plant and wilting. After weeks of infection, the clubbed root, weakening the support of the plant.
P. brassicae influence glucosinolate levels in both root and aerial tissues during primary, secondary and mature gall formation stages disease development. In the opinion of Voorrips, the evidence of a correlation between IGSL content and club root susceptibility is conflicting because no relation between IGSL content and club root resistance has been found. He opined that although the auxin production from indole glucosinolates is somewhat important in clubroot development, the processes occurring during pathogenesis, the mechanisms responsible for resistance were unclear. However, Islam and Guest found that GSLs levels remain unchanged in aerial tissues but significantly increased (1.5 times) in root tissues during symptom development. The concentration difference might implicate a role for GSL in P. brassicae pathogenesis.
More so, Song reported a myrosinase mediated breakdown of GSLs to be one of the identified biological processes in resistant samples where they were up-regulated for host-defense responses. Their results implicate several phytoalexins as putatively deriving from the GSL metabolism in B. rapa roots carrying Rcr1 (the clubroot resistant gene) upon P. brassicae infection, which suggest the possibility for anti-microbial agents via the GSL-myrosinase metabolism pathway. Recently, Zhao found that in the response of A. thaliana to P. brassicae infection, the expression of GSL genes and terpenoid biosynthesis significantly increased in the metabolism pathway. Further study may be required to elucidate the resultant pathway in brassica crops.