NpWRKY38 Negatively Regulates Drought Tolerance by Impairing ABA/SA Signaling as Well as Photosynthesis in Narenga porphyrocoma
Narenga porphyrocoma, a wild relative of sugarcane with high drought tolerance, is a valuable germplasm resource for mining drought-tolerance genes. Weighted gene co-expression network analysis (WGCNA) identified NpWRKY38, a WRKY transcription factor that was significantly down-regulated under drought stress, a response that was further recapitulated by exogenous abscisic acid (ABA) treatment and consistent with the presence of ABA-responsive elements in its promoter. NpWRKY38 overexpression in rice resulted in a significant increase in endogenous ABA content, yet led to insufficient stomatal closure, an attenuated response to exogenous ABA, and a significant reduction in drought tolerance. Transcriptomic analysis revealed that stress-related signaling pathways (e.g., MAPK signaling and plant hormone signal transduction) were activated in the OE lines, whereas photosynthesis, carbon fixation, and chlorophyll biosynthesis pathways were globally suppressed. Specifically, the ABA biosynthetic gene NCED was up-regulated, whereas the positive signaling regulator ABI5 was down-regulated and the negative regulator PP2C was up-regulated, thereby impairing ABA signal transduction. In addition, genes involved in salicylic acid (SA) biosynthesis and signaling (PAL, C4H, NPR4, WRKY45) were broadly down-regulated concurrently with reduced antioxidant enzyme activities. Furthermore, NpWRKY38 overexpression exacerbated drought-induced chloroplast damage, as evidenced by a greater reduction in SPAD values and severe chloroplast ultrastructural abnormalities, including swelling, disorganized thylakoid lamellae, and disrupted grana. These three pathways, namely impaired ABA/SA signaling, suppressed photosynthesis, and chloroplast damage, collectively exacerbated the drought-sensitive phenotype of the OE lines. This study demonstrates that NpWRKY38 acts as a multi-pathway negative regulatory mechanism by interfering with ABA signal transduction, suppressing the SA pathway, down-regulating photosynthetic genes, and compromising chloroplast integrity, providing a potential target for drought tolerance breeding in sugarcane.