PR protein accumulation in seminal roots of barley and wheat in response to fungal infection -: the importance of cortex senescence

The accumulation of PR proteins in the roots of barley and wheat was investigated in response to root infection with the necrotrophic pathogen Bipolaris sorokiniana. Analyses of protein extracts using both biochemical methods and immunohistological localization on tissue sections were performed. Several PR proteins, particularly PR-I and PR-5, accumulated in response to infection. The pattern was similar to that observed in earlier reports after infection with, for example, Blumeria graminis and Drechslera teres. Leaf tissue from root-inoculated plants were also analysed and it was found that isoforms of PR-3 and PR-5 proteins were differently expressed in leaves and roots, indicating tissue-specific expression. Tissue differences were observed within the roots as PR-3 proteins was constitutively expressed in the root cortex but not in the stele. PR protein accumulation in the leaves after root inoculation indicate systemic induction. Evidence was also found for systemic induction within the roots as PR proteins accumulated in root tissues that were distant from the infection point. Infection of roots close to the root tip caused a stronger accumulation of PR proteins compared with infection of older segments of seminal roots. This was particularly evident in wheat where almost no effect of inoculation of older root segments could be detected. The patterns of PR protein accumulation in seminal roots are discussed in relation to root cortical cell death, a phenomenon that has earlier been found to occur at a progressively increasing rate from the root tip and upwards.