The microbiota of avocado floral nectar inhibits pathogens and improves plant fitness

Floral nectar-living microbes contribute to flower protection and pollinator health and are primarily determined by nectar chemical composition. Microbial communities in non-hexose-rich nectars and their ability to inhibit pathogens and modulate plant development have been poorly explored. We used metabarcoding to examine the richness and relative abundance of bacteria and fungi from avocado, a globally important crop with a unique nectar chemical composition, whose production is severely affected by diseases and low pollination rates. We also explored the antagonistic activity of the nectar microbial culturable fraction and its volatile organic compounds (VOCs) against avocado pathogens Phytophthora cinnamomi and Colletotrichum gloeosporioides, and against the most devastating honeybee pathogens Ascosphaera apis and Paenibacillus larvae. Furthermore, we experimentally analyzed the effects of microbial isolates and their VOCs on plant growth and the activation of jasmonic acid (JA) defense responses in Arabidopsis thaliana. Pseudomonas, Acinetobacter, Protomyces, and Vishniacozyma were the dominant microbial genera inhabiting avocado nectar. From 43 evaluated isolates, 17 bacteria and three yeasts inhibited the plant and honeybee pathogens, promoted the growth of A. thaliana seedlings, and induced JA signaling. Microbial VOCs emitted by all tested isolates promoted lateral root formation and increased plant biomass. Collectively, our findings highlight the selectivity of avocado nectar over its microbiota, which could directly impact plant fitness and contribute to the health of its pollinators.