Anti-oomycete activity and plant growth promoting properties of avocado fungal endophytes

Fungal endophytes are known as promising plant growth-promoting microorganisms. Surprisingly, despite the economic importance of avocado, the antimicrobial and plant growth-promoting properties of its fungal endophytes have seldom been investigated. Our objectives were to evaluate the anti-oomycete activity of avocado fungal endophytes and assess their potential plant growth-promoting properties, using Arabidopsis thaliana as a model plant. In total, 89 fungal isolates were obtained from the roots of avocado trees and grouped into 24 morphotypes. One isolate per morphotype was randomly selected to assess its antagonistic activity against Phytophthora cinnamomi through dual culture assays. The strongest inhibition of P. cinnamomi was induced by endophytic fungi belonging to the Fusarium, Mortierella and Penicillium genera. The six most promising fungal isolates were selected to assess their plant growth-promoting traits in co-inoculation assays with A. thaliana. All tested fungal endophytes were able to modify the plant root architecture and increase the number of lateral roots. Moreover, an accumulation of auxins was detected in the xylem and meristematic zone of plants inoculated with Mortierella sp. PC-T3-3.1 and Metapochonia sp. PC-T2-4.2, whilst auxin accumulation was restricted to the emerging lateral roots of plants inoculated with Penicillium sp. C-T0-3.1. Indole quantification showed that Penicillium sp. C-T0-3.1 produced the highest concentration of indole acetic acid (IAA) and indole butyric acid (IBA), despite the lack of auxin responsiveness in plants in the co-inoculation assays. Mortierella sp. PC-T3-3.1 and Metapochonia sp. PC-T2-4.2 produced more IAA and IBA when co-inoculated with A. thaliana than when growing alone, which suggests that sensing of plant signals could induce the production of auxin-like compounds by these two endophytes. Collectively, our findings contribute to elucidate the mechanisms underpinning the plant growth-promoting activity of avocado fungal endophytes, which will be key to harness their full metabolic potential.