Cotton Major Latex Protein 28 Functions as a Positive Regulator of the Ethylene Responsive Factor 6 in Defense against Verticillium dahliae

In this study, we identified a defense-related major latex protein (MLP) from upland cotton (designated GhMLP28) and investigated its functional mechanism. GhMLP28 transcripts were ubiquitously present in cotton plants, with higher accumulation in the root. Expression of the GhMLP28 gene was induced by Verticillium dahliae inoculation and was responsive to defense signaling molecules, including ethylene, jasmonic acid, and salicylic acid. Knockdown of GhMLP28 expression by virus-induced gene silencing resulted in increased susceptibility of cotton plants to V. dahliae infection, while ectopic overexpression of GhMLP28 in tobacco improved the disease tolerance of the transgenic plants. Further analysis revealed that GhMLP28 interacted with cotton ethylene response factor 6 (GhERF6) and facilitated the binding of GhERF6 to GCC-box element. Transient expression assay demonstrated that GhMLP28 enhanced the transcription factor activity of GhERF6, which led to the augmented expression of some GCC-box genes. GhMLP28 proteins were located in both the nucleus and cytoplasm and their nuclear distribution was dependent on the presence of GhERF6. Collectively, these results demonstrate that GhMLP28 acts as a positive regulator of GhERF6, and synergetic actions of the two proteins may contribute substantially to protection against V. dahliae infection in cotton plants.