Induction of auxin biosynthetic enzymes by jasmonic acid and in clubroot diseased Chinese cabbage plants

Nitrilase (NIT) and myrosinase are important enzymes for auxin biosynthesis in Brassicaceae, which is increased during clubroot disease. Therefore, NIT and myrosinase levels during club development and possible regulation mechanisms were investigated. In addition, the occurrence of different nitrilase isoforms in Chinese cabbage has been shown. Nitrilase activity was enhanced in infected roots during later stages of club development (35–42 days after inoculation). However, no differences in nitrilase mRNA levels between infected and healthy roots were found during symptom development. Myrosinase expression was increased in clubbed roots at slightly earlier time points (28 days after inoculation) and also at later time points during infection. The activities of tryptophan oxidizing enzyme (TrpOxE), which catalyzes the first step in tryptophan-dependent auxin biosynthesis in Brassicaceae, and nitrilase were enhanced after treatment with jasmonic acid (JA) and methyl jasmonate. Similarly, the amount of myrosinase mRNA was increased by JA. During clubroot disease the endogenous concentration of JA increased in infected roots 3–5 weeks after inoculation. From our results it can be concluded that: (1) de novo indole-3-acetic acid (IAA) biosynthesis plays a role for symptom development of clubroot disease in Brassicaceae during later developmental stages; and (2) JA which increased during club development, may be involved in the up-regulation of three enzymes important for IAA synthesis.