Putative role of IAA during the early response of Brassica napus L. to Plasmodiophora brassicae

This study examined the role of indole-3-acetic acid (IAA) during the early stage of infection of rapeseed (Brassica napus L.) by the obligate biotrophic protist Plasmodiophora brassicae Woron., which causes clubroot disease. Treatment of infected B. napus seedlings with exogenous IAA promoted the development of clubroot disease, including an increase in the number and size of galls, whereas treatment with N-1-naphthylphthalamic acid (NPA) attenuated the disease and altered the location and size of the root nodules, resulting in a reduced disease index. An increased level of free IAA was detected in the roots at 3 and 7 days after inoculation (DAI) compared with the control, suggesting the activation of IAA signaling in the early stage of P. brassicae infection. Moreover, IAA treatment caused no significant change in the germination rate of the resting spores but resulted in enhanced root hair infection. RT-PCR and quantitative real-time PCR (qPCR) were used to analyze the expression of IAA homeostasis-related genes (3, 7, and 10 DAI) to identify gene expression patterns in the roots and leaves of B. napus after inoculation. Five IAA biosynthesis-related genes examined in this study were induced in the roots after inoculation with P. brassicae but not in the leaves. The rapid induction of BnAAO4 expression at 3 DAI may be responsible for the overproduction of IAA during the early stages of infection. This study suggests that IAA acts as a signaling molecule that putatively stimulates root hair infection in the early response of B. napus to P. brassicae infection.