Black pepper (Piper nigrum L.) associated endophytic Pseudomonas putida BP25 alters root phenotype and induces defense in rice (Oryza sativa L.) against blast disease incited by Magnaporthe oryzae

Blast disease caused by Magnaporthe oryzae is an important production constraint in rice farming worldwide. Novel strategies are needed to combat blast disease as the current options like host resistance and fungicides are either unstable or not-acceptable. Black pepper associated Pseudomonas putida BP25 endogenously colonized rice-a monocot, in a density-dependent manner with altered root growth and defense activation, also showed bio-control activity against blast caused by M. oryzae. Fluorescence imaging of P. putida BP25 primed seedlings revealed extensive bacterial colonization in roots that was further confirmed in plate assay and qPCR. Although initial counts could be correlated with bacterial dose used for priming, the bacterial population stabilized 14-days post priming suggesting internal regulation of endophyte colonization. Endophytic colonization of rice triggered alteration in root morphology, possibly, due trade-off between growth and defense. P. putida BP25 significantly inhibited the mycelium of M. oryzae by both volatile and non-volatile metabolites and also seed priming protected the rice from blast. Activities of defense related peroxidase and phenols were found increased in primed plants. qPCR assay of rice defense and developmental genes indicated increased expression of Systemic Acquired Resistance (SAR) related OsPR1-1 (Pathogenesis Related protein 1-1) and down-regulation of OsPR3. In addition, rice seedlings emerged from endophyte priming showed down-regulation of stress responsive OsACO4 and OsACS6 involved in inter-nodal elongation which plays an important role in growth and development.