RNA-Sequencing, Physiological and RNAi Analyses Provide Insights into the Response Mechanism of the ABC-Mediated Resistance to Verticillium dahliae Infection in Cotton.

Verticillium wilt that is caused by Verticillium dahliae, does result in massive annual yield losses and fiber quality decline in cotton. Control by conventional mechanisms is not possible due to a wide host range and the longevity of dormant fungi in the soil in the case of absence of a suitable host. Plants have developed various mechanisms to boost their immunity against various diseases, and one is through the induction of various genes. In this research, we carried out RNA sequencing and then identified the members of the adenosine triphosphate (ATP)-binding cassette (ABC) proteins to be critical in enhancing resistance to V. dahliae infection. A total of 166 proteins that are encoded by the ABC genes were identified in Gossypium raimondii with varying physiochemical properties. A novel ABC gene, Gorai.007G244600 (ABCF5), was found to be highly upregulated, and its homolog in the tetraploid cotton Gh_D11G3432 (ABCF5), was then silenced through virus induced gene silencing (VIGS) in G. hirsutum, tetraploid upland cotton. The mutant cotton seedlings ability to tolerate V. dahliae infection was significantly reduced. Based on the evaluation of oxidant enzymes, hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) showed significantly increased levels in the leaves of the mutant compared to the wild type. In addition, antioxidant enzymes, peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) concentrations were reduced in the mutant cotton leaves after treatment with V. dahliae fungi as compared to the wild type. Moreover, expression levels of the biotic stress genes, cotton polyamine oxidase (GhPAO), cotton ribosomal protein L18 (GhRPL18), and cotton polygalacturonase-inhibiting protein-1 (GhPGIP1), were all downregulated in the mutant but they were highly upregulated in the various tissues of the wild cotton seedlings. This research has shown that ABC genes could play an important role in enhancing the immunity of cotton to V. dahliae infection, and thus can be explored in developing more resilient cotton genotypes with improved resistance to V. dahliae infection in cotton.