CaARP1/CaSGT1 Module Regulates Vegetative Growth and Defense Response of Pepper Plants against Phytophthora capsici

Pepper (<i>Capsicum annuum</i> L.) suffers severe quality and yield loss from oomycete diseases caused by <i>Phytophthora capsici</i>. CaSGT1 was previously determined to positively regulate the immune response of pepper plants against <i>P. capsici</i>, but by which mechanism remains elusive. In the present study, the potential interacting proteins of CaSGT1 were isolated from pepper using a yeast two-hybrid system, among which CaARP1 was determined to interact with CaSGT1 via bimolecular fluorescence complementation (BiFC) and microscale thermophoresis (MST) assays. CaARP1 belongs to the auxin-repressed protein family, which is well-known to function in modulating plant growth. The transcriptional and protein levels of CaARP1 were both significantly induced by infection with <i>P. capsici</i>. Silencing of <i>CaARP1</i> promotes the vegetative growth of pepper plants and attenuates its disease resistance to <i>P. capsici</i>, as well as compromising the hypersensitive response-like cell death in pepper leaves induced by PcINF1, a well-characterized typical PAMP from <i>P. capsici</i>. Chitin-induced transient expression of <i>CaARP1</i> in pepper leaves enhanced its disease resistance to <i>P. capsici</i>, which is amplified by <i>CaSGT1</i> co-expression as a positive regulator. Taken together, our result revealed that CaARP1 plays a dual role in the pepper, negatively regulating the vegetative growth and positively regulating plant immunity against <i>P. capsici</i> in a manner associated with CaSGT1.