Intracellular Bacteria in Plants: Elucidation of Abundant and Diverse Cytoplasmic Bacteria in Healthy Plant Cells Using In Vitro Cell and Callus Cultures

Abstract

This study was initiated to assess whether the supposedly axenic plant cell cultures harbored any cultivation-recalcitrant endophytic bacteria (CREB). Adopting live-cell imaging with bright-field, fluorescent and confocal microscopy and bacterial 16S-rRNA gene taxonomic profiling, we report the cytoplasmic association of abundant and diverse CREBs in long-term actively maintained callus and cell suspension cultures of different plant species. Preliminary bright-field live-cell imaging on grape cell cultures showed abundant intracellular motile micro-particles resembling bacteria, which proved uncultivable on enriched media. Bacterial probing employing DNA stains, transmission electron microscopy, and Eubacterial FISH indicated abundant and diverse cytoplasmic bacteria. Observations on long-term maintained/freshly established callus stocks of different plant species—grapevine, barley, tobacco, Arabidopsis, and medicinal species—indicated intracellular bacteria as a common phenomenon apparently originating from field shoot tissues.Cultivation-independent 16S rRNA gene V3/V3–V4 amplicon profiling on 40-year-old grape cell/callus tissues revealed a high bacterial diversity (>250 genera), predominantly Proteobacteria, succeeded by Firmicutes, Actinobacteria, Bacteriodetes, Planctomycetes, and 20 other phyla, including several candidate phyla. PICRUSt analysis revealed diverse functional roles for the bacterial microbiome, majorly metabolic pathways. Thus, we unearth the widespread association of cultivation-recalcitrant intracellular bacteria “Cytobacts” inhabiting healthy plant cells, sharing a dynamic mutualistic association with cell hosts.