Targeting the Light-Harvesting Complex I Gene Lhca4 Confers Saline-Alkali Tolerance in Rice Without Yield Penalty.

Saline-alkali stress severely limits crop yields worldwide, yet the role of photosystem I (PSI) components under such stress remains unclear. Transcriptome analysis identified Lhca4, a light-harvesting gene associated with PSI in rice, was upregulated under saline-alkali stress. Genetic analysis revealed Lhca4 functions as a negative regulator of saline-alkali tolerance. Specifically, loss of Lhca4 function enhanced stress resistance without compromising yield under non-stress conditions, while its overexpression reduced tolerance. Under stress conditions, Lhca4 knockout plants exhibited reduced ROS accumulation, elevated antioxidant activity, and maintained photosynthetic integrity, supporting its role in fine-tuning the stress response. Mechanistically, the LHCA4 protein translocates to the nucleus under these conditions, where it suppresses the expression of core retrograde signaling genes (including OsGUN4, OsHEMA1) and their downstream targets, such as photosynthesis-related genes (OsRbCS4, OsLhcb3, OsLhcb5) and ROS - scavenging genes. In the absence of Lhca4, retrograde signaling is constitutively activated, leading to the upregulation of antioxidant and metabolic genes and stabilized ROS homeostasis. Pharmacological inhibition of retrograde signaling with Norflurazon completely abolished the stress-tolerant phenotype of knockout plants. These results define a regulatory module in which Lhca4 integrates chloroplast retrograde signals with ROS metabolism to tune stress adaptation, highlighting it as a valuable target for breeding stress-resilient crops.