The study was envisaged to assess the extent of normally uncultivable endophytic bacteria in field papaya plants and in vitro established cultures adopting cultivation vs molecular analysis and microscopy. Surface-sterilized axillary shoot-buds of papaya 'Arka Surya' revealed high bacterial diversity as per 16S rRNA metagene amplicon sequencing (6 phyla, 10 classes, 21 families) with an abundance of Pseudomonas (Gammaproteobacteria), which also formed a common contaminant for in vitro cultured field explants. Molecular analysis of seedling shoot-tip-derived healthy proliferating cultures of three genotypes ('Arka Surya', 'Arka Prabhath', 'Red Lady') with regular monthly subculturing also displayed high bacterial diversity (11-16 phyla, >25 classes, >50 families, >200 genera) about 12-18 months after initial establishment. 'Arka Surya' and 'Red Lady' cultures bore predominantly Actinobacteria (75-78%) while 'Arka Prabhath' showed largely Alphaproteobacteria corroborating the slowly activated Methylobacterium sp. Bright-field direct microscopy on tissue sections and tissue homogenate and epi-fluorescence microscopy employing bacterial DNA probe SYTO-9 revealed abundant intracellular bacteria embracing the next-generation sequencing elucidated high taxonomic diversity. Phylogenetic investigation of communities by reconstruction of unobserved states- PICRUSt- functional annotation suggested significant operational roles for the bacterial-biome. Metabolism, environmental information processing, and genetic information processing constituted major Kyoto Encyclopedia of Genes and Genomes KEGG attributes. Papaya stocks occasionally displayed bacterial growth on culture medium arising from the activation of originally uncultivable organisms to cultivation. The organisms included Bacillus (35%), Methylobacterium (15%), Pseudomonas (10%) and seven other genera (40%). This study reveals a hidden world of diverse and abundant conventionally uncultivable cellular-colonizing endophytic bacteria in field shoots and micropropagating papaya stocks with high genotypic similarity and silent participation in various plant processes/pathways.
© 2018 Scandinavian Plant Physiology Society.