Influence of native endophytic bacteria on the growth and bacterial crown rot tolerance of papaya (Carica papaya)

The native plant microbiome is composed of diverse microbial communities that influence overall plant health, with some species known to promote plant growth and pathogen resistance. Here, we show the antibacterial and growth promoting activities of autoclaved culture metabolites (ACM) from native endophytic bacteria (NEB). These NEB were isolated from a papaya cultivar (var. Cariflora) that is tolerant to bacterial crown rot (BCR) caused by Erwinia mallotivora. In this cultivar, bacterial colonization in tissues recovering from the disease was observed before onset of tissue regeneration or 'regrowth'. We further isolated and characterized these bacteria and were able to identify two culturable stem NEB related to plant endophytic genera Kosakonia sp. (ex. Enterobacter sp., isolate EBW), and to Sphingomonas sp. (isolate EBY). We also identified root NEB under genus Bacillus (isolates BN, BS, and BT). Inhibition assays indicated that ACM from these NEB promptly (within 18-30 h) and efficiently inhibited (60-65% reduction) E. mallotivora proliferation in vitro. When surface-sterilized papaya seeds were soaked in ACM from isolates EBY and EBW, germination was variably retarded (20-60% reduction) depending on plant genotype, but plant biomass accumulation was significantly stimulated, at around two-fold increase. Moreover, greenhouse experiments show that ACM from all isolates, especially isolate EBW, significantly reduced BCR incidence and severity in a susceptible genotype (var. Solo), at around two-fold. In general, our observations of pathogen antagonism and plant growth promotion leading to disease reduction, suggested the influence of native endophytic bacteria to increased fitness in plants, and tolerance against the re-emerging crown rot disease of papaya.