Changes in gene expression of putative isopentenyltransferase during clubroot development in Chinese cabbage (Brassica rapa L.)

The obligate biotroph Plasmodiophora brassicae induces club formation on roots of various species of the Brassicaceae family. This hypertrophy is dependent on increased amount of auxins and cytokinins. To investigate the involvement of cytokinin biosynthesis by the plant in clubroot development, we isolated cDNA fragments of five putative cytokinin synthase genes from Brassica rapa (BrIPT1, 3, 5, 7, and 8) and examined their expression using Northern blot analysis. Observations at 5-day intervals revealed the first visible club formation (only near the base of the root) at 20 days post-inoculation (dpi). Compared with control roots, the expression of BrIPT1, 3, 5, and 7 was increased at 20 dpi in parts of the inoculated root that lacked visible symptoms (>2 cm below the base of the root) but at which large clubs formed later in the progress of the disease. However, these genes were less expressed in clubs that formed 0-2 cm below the base of the root than in the control at 20 dpi. These results suggest that P. brassicae infection transiently stimulates the transcription of BrIPT1, 3, 5, and 7 before club formation, after which the expression is repressed in clubs. No signals corresponding to BrIPT8 were detected in either inoculated or uninoculated roots. These results suggest that cytokinin biosynthesis by the plant is related to the primary development of clubroot disease, although it is unlikely to contribute to club enlargement.