The correlation between proline/P5C cycle and the response to avirulent pathogen infection in Arabidopsis

Proline/P5C cycle between mitochondria and cytosol, play important roles in energy supply and ROS (Reactive oxygen species) generation in mammalian cell. Recently, in plant, proline-dependent ROS via proline/P5C cycle was proposed to be involved in hypersensitive reaction during plant response to avirulent pathogens. However, much remains to be elucidated about the regulation of proline/P5C cycle upon pathogen infection. Here, we reported the isolation and characterization of an Arabidopsis mutant proline resistance 2 (pre2), with a single dominant mutation in a single gene. Our results showed that the proline resistance phenotype of pre2 is not due to decreased intracellular proline content, when treated with exogenous proline. Upon proline treatment, pre2 showed reduced induction of PDH transcript level and enhanced induction of that of P5CDH, accompanied by lower level of mitochondrial ROS, suggesting an attenuated proline/P5C cycle activity. Proline-induced SA (Salicylic acid) signaling was also less activated in pre2, as evidenced by reduced free SA content and PR1 transcript level, compared to the WT. On the other hand, SA activation on the proline/P5C activity is to a lesser extent in pre2 than in WT. Significantly, pre2 demonstrated increased susceptibility to infection by avirulent pathogen Pst. DC3000 (avrRps4), accompanied also by lesser induced proline/P5C cycle activity by the pathogen. Our results indicated that there is a correlation between proline/P5C cycle and plant response to avirulent pathogen.