Plant Growth Promoting Rhizobacteria (PGPRs) Alter Plant Host Somatic Mutation Frequencies

Plant growth promoting rhizobacteria (PGPRs) are a group of soil bacteria which can induce positive growth in plants by different mechanisms. This work intends to find the effect of PGPRs on two classes of somatic mutations in the host, frame shift mutation (FSM) and somatic homologous recombination (SHR) and, compare the same with that of a pathogen. Somatic mutations in plants are important as they are an adaptation strategy to overcome stressful conditions and also get passed on to the next generations. The mutation detector Arabidopsis thaliana lines carrying a non-functional β-glucuronidase gene (GUS) were used to score the mutation events. One day-old mutant seedlings were co-cultivated with the PGPRs (Rhizobium leguminosarum and Pseudomonas fluorescens) and the pathogenic strain (P. syringae) for two different post-infection durations (4 h and 48 h). A reversion of the mutated GUS to its functional form resulted in blue spots in the host plant. Based on the number of blue spots seen, the mutation frequencies were estimated. An increase in FSM was observed in plants co-cultivated with R. leguminosarum for 4 h as well as 48 h. R. leguminosarum suppressed SHR frequency 4 h-post infection, which significantly increased at 48 h. In contrast, P. fluorescens infection lead to a temporal suppression of FSM and induction of SHR at 4 h. Subsequently, the SHR rates reduced significantly, i.e. lower than the uninfected controls at 48 h. The pathogenic strain P. syringae temporally increased FSM in plants and also enhanced SHR rates in plants 4 h post-infection, which also subsequently reduced 48 h post infection. To the best of our knowledge, there are no other reports comparing the effect of PGPRs on host somatic mutation rates.