Black pepper rhizomicrobiome: Spectrum of plant health indicators, critical environmental factors and community compartmentation in Vietnam

The biodiversity of the black pepper rhizosphere is a source of economically relevant microbial biocontrol agents that, if properly understood, can be used to improve black pepper pathogen suppression. However, the char-acterization of this system is poor in Vietnam, where most black pepper is grown. Here, we evaluated black pepper rhizosphere bacterial communities, from healthy and unhealthy orchards in two Vietnamese provinces for the detection of biomarkers and keystone species indicative of plant health status. Rhizosphere samples were collected for 16S rRNA metabarcoding and physicochemical analyses. Results revealed that unhealthy orchards had significantly higher electrical conductivity (EC), clay and available nitrogen. Richness and alpha diversity indices were significantly higher in unhealthy orchards in both provinces. Health state of orchards was the best individual predictor of alpha diversity compared to geographic and physicochemical variables. The composi-tional structure of the healthy orchards was also significantly different from the unhealthy orchards, though the interaction between orchard state and physicochemical variables explained a far higher proportion of the bac-terial community dissimilarities. Actinobacteriota, Proteobacteria and Chloroflexi were the dominant phyla across orchards. Biomarkers of unhealthy orchards included Gemmatimonas, Conexibacter and Byrobacter, among others; biomarkers of healthy orchards included Solirubrobacter, Altererythrobacter and Crossiella. Bacterial co-occurrence networks were more complex in the healthy than unhealthy orchards, and available nutrients and EC notably influenced the assembly of subcommunities in the unhealthy orchards. Gemmatimonas, a potential plant growth antagonist, was detected as a keystone species in unhealthy orchards of both provinces. Overall, this study presents biomarkers and keystone species that may contribute to black pepper rhizosphere pathogen management and suggests that appropriate control of soil nutrients may mitigate the establishment of pathogenic bacterial communities.