Orchid Root Endophytes: Isolation, Identification, and Growth-Promoting Traits

Orchid root-associated fungal endophytes (ORAFEs) represent an important component of fungal biodiversity with potential applications in orchid conservation and sustainable production. ORAFEs were isolated from the roots of four orchid species in Veracruz, Mexico: Gongora galeata, Maxillaria densa, Maxillaria tenuifolia, and Maxillaria variabilis to investigate their diversity and plant-growth-promoting (PGP) properties. Taxonomic identification relied on morphological and molecular phylogenetic analyses using ITS, TEF1-α, β-tubulin (TUB2), and actin (ACT). Fifty-six isolates were grouped into 20 morphotypes representing 15 genera, including Aspergillus, Biscogniauxia, Chaetomium, Colletotrichum, Thelonectria, Diaporthe, Fusarium, Geotrichum, Hypoxylon, Lasiodiplodia, Mucor, Periconia, Pestalotiopsis, Trichoderma, Xylaria, and two isolates that may represent undescribed taxa requiring further polyphasic taxonomic investigation. Functional experiments indicated a wide range of PGP activities: ten strains solubilized phosphate, eight produced siderophores, twelve synthesized indole-3-acetic acid, fourteen produced pectinases, six fungi exhibited antibacterial activity against Bacillus subtilis, and five inhibited Staphylococcus aureus. Furthermore, Trichoderma atroviride TR1-T2 and Xylaria multiplex XY1-D42 showed antagonistic activity against Phytopythium vexans, characterized by a contact-dependent displacement effect. Five strains were chosen for further research as orchid bioinoculants based on their PGP and antagonistic traits: Chaetomium cochliodes CH1-V10, Mucor pseudolusitanicus MU2-T6, Thelonectria veuillotiana TH1-T3, Trichoderma atroviride TR1-T2, and Xylaria multiplex XY1-D42. These endophytes have the potential to improve in vitro propagation and facilitate ex vitro acclimatization of orchids, highlighting their importance in sustainable orchid production.