Identification of Pepper AAP Gene Family and Functional Characterization of CaAAP6 in Drought Stress

Amino acid permeases (AAPs) facilitate the uptake and transmembrane transport of amino acids and play critical roles in regulating plant growth, development, and stress responses. However, the characteristics and functions of the AAP gene in pepper remain unclear. In this study, nine CaAAP genes were identified within the pepper (Capsicum annuum) genome and designated as CaAAP1-CaAAP9. Phylogenetic analysis revealed that CaAAP6, CaAAP7, and CaAAP9 belong to subfamily I, CaAAP8 is the sole member of subfamily III-E, and the remaining genes are classified within subfamily II. Collinearity analysis indicated the presence of both tandem and segmental duplication events among the nine CaAAP genes in pepper, as well as four and seven duplicated gene pairs between Arabidopsis and tomato, respectively. Expression profiling demonstrated that CaAAP genes exhibit pronounced tissue-specific expression patterns and differential expression in response to exogenous plant hormones and various abiotic stress treatments. Notably, drought stress elicited a significant up-regulation of CaAAP6 expression. Functional characterization confirmed that CaAAP6 localizes to the cell membrane. Silencing of CaAAP6 resulted in a significant reduction in plant drought tolerance, accompanied by disrupted tissue architecture, and stomatal closure. Furthermore, the concentrations of H₂O₂, O₂^•-, and MDA exhibited increases of 76.19%, 108.52%, and 20.23%, respectively. Concurrently, the activities of SOD, POD, and CAT decreased by 42.32%, 64.09%, and 109.64%, respectively. Additionally, the transcriptional levels of the CaRD22, CaRD29B, and CaDREB2A genes were reduced by 135.56%, 67.19%, and 37.91%, respectively, compared to the control plants. Collectively, these findings provide initial functional evidence that CaAAP6 contributes to drought tolerance in pepper.