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, as well as their ability to inhibit pathogens and modulate plant development, have been poorly explored. Here, 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 the incidence of diseases and by low pollination rates. We also explored the antagonistic activity of the nectar microbial culturable fraction and their 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 emitted VOCs on plant growth and the activation of the jasmonic acid (JA) defense-responses in the model plant 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. The effect of microbial VOCs in promoting plant growth was noteworthy as 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 the plant fitness and contribute to its pollinators´ health.