The root endophytic fungus Serendipita indica improves resistance of Banana to Fusarium oxysporum f. sp. cubense tropical race 4

Serendipita indica is a root-colonizing basidiomycete that has received considerable attention in recent decades on account of its wide host range and the diverse benefits it imparts to host plants, including banana (Musa spp.). Banana is globally threatened by the fungal pathogen Fusarium oxysporum f. sp. cubense (Foc). In this study, to examine whether S. indica can improve the Foc resistance of banana, S. indica colonized (S+) and non-colonized (CK) ‘Tianbaojiao’ banana plants were inoculated with Foc tropical race 4 (TR4). Chlorophyll fluorescence parameters in leaves and antioxidant enzyme activities in roots of S+, CK, FocTR4-inoculated S+ plants (SF), and FocTR4-inoculated CK plants (F+) were determined and symptom appearance in corms of SF and F+ plants was observed. An analysis of antagonistic activity between S. indica and FocTR4 was performed and results showed that no significant differences in chlorophyll fluorescence parameters were displayed between CK and S+. No significant differences in Y(II) (effective quantum yield of photochemical energy in PSII) and Y(NPQ) (quantum yield of regulated energy dissipation in PSII) were exhibited among the four groups. However, the Fv/fm (ratio of variable to maximal fluorescence) values were significantly decreased (p < 0.01) in F+ and SF, and the Fv/fm value of SF was significantly improved than that of F+ (p < 0.05). The Y(NO) (quantum yield of non-regulated energy dissipation in PSII) was remarkable improved (p < 0.01) in F+ but not in SF. Although infection symptoms in SF were significantly alleviated compared with those of F+, no obvious antagonistic effect was observed between S. indica and FocTR4, indicating that the enhancement of FocTR4 resistance in banana was not directly induced by S. indica. Colonization by S. indica increased superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) activities in S+. After FocTR4 inoculation, SOD activity was significantly compromised in F+ but not in SF; POD activity was drastically increased in both F+ and SF, and that in SF was significantly higher than in F+; CAT activity was significantly improved in S+, but no obvious change was observed in F+ and SF when compared with CK. Moreover, APX and glutathione reductase (GR) activities were enhanced dramatically in SF but not in F+ compared with CK. Results obtained in this study indicated that the beneficial effect of S. indica colonization on FocTR4 resistance of banana might be achieved, at least partly, through regulation of antioxidant enzyme activities.