CaPDX1, a Novel Protein, Positively Regulates Cold Stress Tolerance via Interaction with CaSnRK2.4 in Pepper (Capsicum annuum L.).

Capsicum annuum is a Solanaceae crop that is sensitive to cold, which affects its growth and development upon prolonged exposure and ultimately reduces yield. In response, a complex regulatory network of cold-responsive genes is activated. Earlier studies have shown that SnRKs play a positive role in enhancing cold tolerance in different crops, including peppers; however, the underlying molecular mechanisms and downstream targets have yet to be fully elucidated. In this study, yeast hybrid screening using CaSnRK2.4 identified a potential interacting partner CaPDX1. The interaction between CaPDX1 and CaSnRK2.4 was further confirmed through Y2H, luciferase complementation, and bimolecular fluorescence complementation assays. Subcellular localization showed that CaPDX1 and CaSnRK2.4 are localized in the nucleus as well as in the cell membrane. Silencing of CaPDX1 through VIGS showed increased susceptibility of peppers to cold stress, negatively influenced antioxidant enzymatic activities, and increased relative electrolyte leakage and malondialdehyde levels. Conversely, transient overexpression of CaPDX1 in peppers enhanced cold tolerance by reducing the accumulation of REL and MDA. Ectopic expression of CaPDX1 in Arabidopsis thaliana significantly improved its cold tolerance, accompanied by enhanced activity of antioxidant enzymes and increased chlorophyll content. In summary, these results indicate that CaPDX1 is a positive regulator of cold tolerance in pepper, and its mechanism of action involves interaction with CaSnRK2.4 and the regulation of physiological and molecular responses in pepper under cold stress.