Genetic and Molecular Characterization of Fusarium Wilt and Cotton Interactions

Cotton (Gossypium spp.) is an economically important crop that provides much of the world’s supply of fiber, feed, foodstuff, oil, and biofuel products. However, several fungal diseases represent expanding threats to cotton production. Fusarium wilt of cotton, caused by Fusarium oxysporum f. sp. vasinfectum (Fov), is a major destructive disease of cotton. The conventional biological and chemical controls are not efficient against newly emerging Fov isolates. Thus, understanding the genetic basis of Fusarium wilt and cotton interactions could provide insights to control the disease. To respond to pathogen attacks, plants have evolved a sophisticated immune system. To date, a large number of plant immune receptors conferring resistance to fungal pathogens belong to the plasma membrane-localized receptor-like proteins (RLPs) with an extracellular domain that recognizes specific pathogen signatures. Upon perception of pathogen signatures, RLPs induce a series of defense responses, including massive transcriptional re-programming, in fending off pathogen attacks. In this study, I used an integrative genomic and molecular approach to identify and understand the functions of cotton RLPs in response to Fov infections at the whole genome level. Based on the in-silico prediction of RLP candidates in different cotton genomes, I systemically silenced 39 RLPs in upland cotton (G. hirsutum) with virus-induced gene silencing (VIGS). Seven of these showed reduced resistance to Fov infections, whereas silencing of one showed reduced susceptibility to Fov infections, suggesting that RLPs could play both positive and negative roles in cotton response to Fov. Silencing of two RLPs displayed reduced immune responses, including mitogen-activated protein kinase (MAPK) phosphorylation, reactive oxygen species (ROS) burst and ethylene biosynthesis, to the conserved pathogen-associated molecular patterns (PAMPs), supporting a molecular mechanism of these RLPs in cotton resistance to Fov. Plant immunity is triggered by recognition of pathogen signatures or PAMPs. I isolated the cell wall components of different Fusarium oxysporum isolates (FoCWEs) and tested their ability to trigger immune responses in both cotton and the model plant Arabidopsis. Significantly, FoCWE triggers MAPK activation, ROS burst, ethylene biosynthesis, growth inhibition, and stomatal closure in cotton and Arabidopsis. In addition, FoCWE protects cotton seeds against infections by virulent isolates of Fov, and Arabidopsis plants against Pseudomonas syringae pv tomato DC3000. The data indicate that FoCWE is a classical PAMP that is potentially recognized by a conserved pattern recognition receptor (PRR) in different plant species. Analysis of available PRR mutants in Arabidopsis indicates that FoCWE is perceived by a potential novel PRR. Further characterization of the eliciting component within FoCWE demonstrated that it is a heatstable and water-soluble PAMP. In summary, these findings shed light on the molecular basis of cotton-Fov interactions, and provide evidence that RLPs are key genes that can be used in breeding and genetic engineering of cotton resistance to fungal wilt pathogens. In addition, my data indicate that FoCWE can be used as a biocontrol agent to protect cotton against Fov infections. To conclude, my work provides new tools to develop different strategies to control Fusarium wilt of cotton