Abstract
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.
Highlights
Papaya (Carica papaya) is an economically important tropical fruit, mainly cultivated for food and cosmetic products
EBY is paraphyletically related to S. endophytica YIM 65583 (Huang et al, 2012) and Sphingomonas phyllosphaerae FA2 (Rivas et al, 2004), percent identity to closest taxa is not convincingly high enough
It was observed through scanning electron microscope (SEM) that infection sites in recovering tissues from genotype 5648 were colonized by a community of bacteria at the onset of regrowth (3-4 wai)
Summary
Papaya (Carica papaya) is an economically important tropical fruit, mainly cultivated for food and cosmetic products. The industry still faces yield losses due to diseases, causing problems from plantation, transportation and processing until marketing (Ventura et al, 2004). Most importantly, emerging diseases greatly impair papaya production, and even cause up to 100% yield loss in some afflicted areas, especially in Asian tropical countries (Ploetz, 2004). A re-emerging disease of papaya, named bacterial crown rot Soft rot, canker, decline) caused by a bacterium of genus Erwinia was reported in Southeast Asia (Maktar et al, 2008). In Mindoro, Philippines, a similar soft rot disease, which has water-soaked symptoms on leaves, petioles and whorl accompanied by a foul odor was reported. The pathogen was identified under the genus Erwinia (Pordesimo and Quimio, 1965) and remained a major problem until the 1980s (Obrero, 1980)
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