Fulvic Acid Suppresses Excessive Cytokinin Biosynthesis to Reduce ROS Content and thus Decrease Na+ Accumulation and Toxicity, Enhancing Salt Tolerance in Rice

Rice (Oryza sativa L.), a major global food crop, faces serious threats from salt stress. Although fulvic acid (FA) is known to positively modulate plant stress responses, its specific role in enhancing salt tolerance in rice remains unclear. Likewise, the involvement of cytokinin (CK) homeostasis in this process is not fully understood. This study demonstrates that salt stress induces excessive CK accumulation by upregulating OsIPT genes, which promote CK biosynthesis. The elevated CK levels subsequently increase reactive oxygen species (ROS) by upregulating ROS-generating genes (OsTHT1, OsTHT2, and Osrboh1) and downregulating ROS-scavenging genes (OsAPX1, OsMT2b, and ALM1). Accumulated ROS further downregulates Na+ transport genes (OsHKT1;1, OsSOS1, and OsNHX1), leading to Na+ over-accumulation and toxicity and reduced survival rate in rice. Conversely, FA counteracts salt stress by downregulating OsIPTs to suppress excessive CK accumulation. The reduced CK levels decrease ROS through suppressed generation and enhanced scavenging, achieved by downregulating biosynthetic genes, upregulating scavenging genes, and boosting activities of antioxidant enzymes (SOD, POD, and CAT). The subsequent ROS reduction upregulates the expression of Na+ transport genes, thereby limiting Na+ accumulation and alleviating Na+ toxicity, and consequently improving salt tolerance. These findings offer a theoretical foundation and technical support for utilizing FA as a salt tolerance regulator in rice production.