Influence of metal stress on production of volatile inhibitory compounds by endophytes against Ganoderma boninense

Five endophytic fungal isolates (Bipolaris sp. LF7, Diaporthe miriciae LF9, Trichoderma asperellum LF11, Phomopsis asparagi LF15, Saccharicola bicolour LF22) demonstrated inhibitory effects towards the pathogen Ganoderma boninense (GB) in double-plate assay. Among these, isolate T. asperellum LF11 has the strongest inhibitory effect (50.68%), followed by D. miriciae LF9 (32.19%), P. asparagi LF15 (19.86%) and Bipolaris sp. LF7 (19.86%), and isolate S. bicolour LF22 (17.81%). The strength of inhibition varied under the influence of metal types (Al3+, Cu2+, Pb2+, Zn2+, Cd2+) and concentrations (0, 10 and 25 mg L-1 for all metals; additional 50 mg L-1 for Al3+ and Cu2+) with the strongest inhibition by D. miriciae LF9 and T. asperellum LF11 in 10 mg L-1 Al3+ (47.89%) and Pb2+ (48.15%), respectively. Volatile compounds by these two isolates were further characterised via the SPME-GCMS approach, revealing monoterpenes (gamma-terpinene, alpha-thujene, carene, beta-phellandrene) and sesquiterpene (bergamotene) as primary volatile compounds. The number and type of volatile organic compounds (VOCs) produced by endophytes differed under metal stress. Among the many VOCs produced by the endophytes, gamma-terpinene and terpinen-4-ol by D. miriciae LF9 as well as 6-PAP by T. asperellum LF11 were strongly associated with the antifungal activities against GB. To summarise, metal stress was discovered to increase the number of volatiles and their concentrations produced by the endophytes; and that the efficiencies of D. miriciae LF9 and T. asperellum LF11 in inhibiting GB were not impaired under metal stress.