SlSTOP1-regulated SlHAK5 expression confers Al tolerance in tomato by facilitating citrate secretion from roots

Abstract SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1) is a core transcription factor that regulates the expression of aluminum (Al)-resistance genes to manage Al toxicity in plants. However, the genome-wide roles of SlSTOP1 in the Al stress response of tomato (Solanum lycopersicum) remain largely unknown. Here, we report that SlSTOP1 is crucial for Al tolerance in tomato, as loss-of-function mutants of SlSTOP1 displayed hypersensitivity to Al stress. Aluminum stress had no effect on SlSTOP1 mRNA expression, but promoted accumulation of SlSTOP1 protein in the nucleus. Through integrated DNA affinity purification sequencing and RNA sequencing analysis, we identified 39 SlSTOP1-targeted Al-responsive genes, some of which are homologous to known Al resistance genes in other plant species, suggesting that these SlSTOP1-targeted genes play essential roles in Al resistance in tomato. Furthermore, using peak enrichment analysis of SlSTOP1-targeted sequences, we identified a cis-acting element bound by SlSTOP1 and validated this finding via dual-luciferase reporter and electrophoretic mobility shift assay (EMSA). Additionally, we demonstrated SlHAK5 is one of direct targets of SlSTOP1 and functionally characterized it in terms of Al stress tolerance. Compared with wild-type plants, Slhak5 mutants developed by CRISPR/Cas9 technology presented increased sensitivity to Al stress, which was associated with reduced citrate secretion from the roots. Together, our findings demonstrate that SlSTOP1 directly interacts with cis-acting elements located in the promoters of target genes involved in diverse pathways contributing to Al resistance in tomato.