Increase in Indole 3-acetic Acid (IAA) level and Nitrilase Activity in Turnips Induced by Plasmodiophora brassicae Infection

Indole acetic acid (IAA) level and nitrilase activity were measured in infected and healthy turnips, Brassica campestris L., subsp. Rapa, over a time course to confirm increases in IAA levels upon infection by Plasmodiophora brassicae and to investigate underlying mechanisms. Healthy and infected seedlings were assayed from 20 to 90 days after sowing. IAA levels in both roots fluctuated similarly over days 20-35. By day 45, IAA in infected roots increased to five-fold over healthy roots then decreased to the level of healthy roots by day 90. Nitrilase activity was negligible on days 20 and 25 but increased thereafter in infected and healthy roots. However, activity in healthy roots decreased substantially by day 40 while infected roots showed a continued increase to day 45 then decreased to a low level. These findings suggest that IAA concentration increases after P. brassicae infection, possibly due to IAA synthesis via pathways involving nitrilase.