Defense Biopriming and Antimicrobial Activity of Endophytic Bacteria and Associated Bacillus Species Contribute to Bacterial Crown Rot Tolerance in Papaya

Bacterial crown rot (BCR) is a re-emerging disease of papaya (Carica papaya) in Southeast Asia caused by a gram-negative, facultative anaerobic bacterium, known as Erwinia mallotivora. Ultrastructural observations using SEM revealed colonization of bacterial community common in a BCR-tolerant genotype, before onset of tisue regeneration or 9regrowth9. This observation led to the hypothesis that papaya-associated bacteria might play a role in BCR tolerance. Isolation, characterization and identification using 16S rRNA sequence of bacteria associated with the tolerant 9regrowth9 genotype resulted to the discovery of two endophytic bacteria, of genera Kosakonia (isolate EBW) and Sphingomonas (isolate EBY). Papaya root-associated Bacillus isolates were also identified to belong to the B. amyloliqufaciens clade (isolate BN, BS) and B. thuringiensis clade (isolate BT). Separate inhibition assays indicated that, papaya latex and autoclaved culture metabolites (ACMs) from isolated bacteria promptly (between 18-30h) and efficiently inhibited (60-65%) E. mallotivora proliferation in vitro. Moreover, when ACMs from BN and EBW were inoculated in surface-sterilized papaya seeds, it variably retarded seed germination (20-60% reduction), depending on plant genotype, but significantly stimulated plant growth and biomass accumulation, at around two-fold increase. Interestingly, ACMs from all isolates, especially EBW, significantly reduced BCR incidence and severity in susceptible genotype at around two-fold. Finally, these results indicated that tolerance of papaya against E. mallotivora infection is probably influenced by host innate properties such as tissue regrowth and pathogen inhibition by plant latex, and acquired properties including pathogen antagonism, growth promotion and defense biopriming through metabolites from beneficial endophytic and root-assocaited bacteria.