Induced systemic resistance using non-pathogenic microorganisms for the control of black sigatoka in banana

Abstract

Black sigatoka is a devastating disease of banana caused by Mycosphaerella fijiensis. Existing cultural and chemical strategies to control the disease have not been successful. Pseudomonas fluorescence, a plant growth promoting rhizobacteria, and an endophyte, Fusarium oxysporum (V5W2), were assessed for efficacy in controlling Black sigatoka. Two month-old disease-free banana plantlets of cvs. Mpologoma (AAAEA) (susceptible to black sigatoka), Nfuuka (AAA-EA) (moderately tolerant) and Kayinja (ABB) (tolerant) were treated with spore suspensions of 1.5´105 and 1.5´106 spores/ml of P. fluorescens and F. Oxysporum, V5W2, and tested vs. an untreated control. The plants were then challenge-inoculated with a M. fijiensis suspension of 5.7×105 mycelial fragments/ml. Black sigatoka disease was assessed every five days until 60 days after infection (DAI). Disease was assessed visually as percent infected leaf area. Results showed significant reductions of disease symptoms in P. fluorescens and F. oxysporum-treated plants compared to the control that was only inoculated with M. fijiensis. For P. fluorescens, results on gene expression showed that a higher expression of the Non-expressor of Pathogenesis Related gene 1B (NPR1B) was exhibited at 9 DAI than at 17 and 23 DAI. Jasmonate/Ethylene responsive genes Hevein-like protein (Hel) and Allene Oxide Synthase (AOS) were expressed at 23 DAI, with a higher expression in leaves treated with both P.fluorescens and M.fijiensis. For F. oxysporum (V5W2), NPR1B was expressed at all time points, i.e., 0, 9, 17 and 23 DAI. Chitinase (PR3) was also expressed on all tested time points and showed an increasing trend in expression from 9 to 23 DAI. Expression of NPR1B in plants pretreated with P. fluorescens or F. oxysporum (V5W2) then challenged with M. fijiensis, as well as the up-regulation of the above-mentioned defense genes downstream of NPR1B indicate that P. fluorescens and F. oxysporum can induce resistance against Black sigatoka in banana. Key words: AAA-EA and ABB banana genomes, Chitinase (PR3), Hevein-like protein (Hel) and Allene Oxide Synthase (AOS), Non-expressor of Pathogenesis Related gene 1B (NPR1B)