Fungal endophytes alleviate drought-induced oxidative stress in mandarin (Citrus reticulata L.): Toward regulating the ascorbate-glutathione cycle

Fungal endophytes grow symbiotically inside a diverse array of plants, and improve viability of the hosts in various mutualistic manners. We investigated the effects of inoculating mandarin endophytic fungi, introduced formerly by our group, i.e. Penicillium citrinum, Aurobassium pullunts and Dothideomycetes sp., individually and in combination with each other on the reactive oxygen species (ROS) scavenging and antioxidant functions in plants under drought stress. To do so, mandarin (Citrus reticulata) scions grafted on Bakraii [Citrus reticulata x Citrus limetta] (Mandarin/Bakraii) were subjected to four irrigations interval including 2, 4, 6 and 8 days, in present/absent of the fungal endophytes. The results indicated that drought stress significantly reduced growth, chlorophyll, carotenoid content and chlorophyll fluorescence (Fv/Fm) of the plants lacking endophytes. Combinatory applications with three fungal endophytes (P. citrinum + A. pullunts + Dothideomycetes sp.) significantly improved the above-mentioned parameters under drought stress. H2O2, O-2(-) and lipid peroxidation level were significantly reduced in the plants inoculated with fungal endophytes. Drought stress significantly increased activities of ascorbate peroxidase (APX), superoxide dismutase (SOD) and glutathione reductase (GR) and levels of ascorbate (ASA) and glutathione (GSH) and decreased activities of catalase (CAT), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR). Fungal endophytes inoculated drought stressed seedlings enhanced the above-mentioned indicators as compared to the drought-stressed plants without fungal endophytes, as well as in the ratios of reduced ascorbate/dehydroascorbic acid (AsA/DHA) and reduced glutathione/oxidized glutathione (GSH/GSSG). Overall, fungal endophyte inoculation improved drought tolerance and reduced the accumulation of ROS by increasing their scavenging via improving the redox state of ascorbate and glutathione and promotion of antioxidant enzymes activity.