B-box zinc finger transcription factor SlBBX6 negatively regulates drought and salt stress tolerance in tomato.

Abstract

Drought and salt stress severely constrain tomato growth and yield. The B-box (BBX) zinc finger transcription factor family is widely known for its roles in core developmental processes such as photomorphogenesis and flowering regulation, and its regulatory functions in abiotic stress responses have attracted increasing attention in recent years. However, the precise molecular mechanisms by which tomato BBX family members mediate drought and salt stress tolerance remain to be systematically elucidated. In this study, we functionally characterized SlBBX6, a tomato subfamily I BBX gene encoding a protein containing B-box and CCT domains. Functional validation using overexpression (OE) and CRISPR/Cas9-mediated knockout (KO) lines demonstrated that SlBBX6 negatively regulates drought and salt stress tolerance in tomato: under drought, OE lines exhibited reduced survival rates and exacerbated oxidative damage; under salt stress, they showed accelerated chlorophyll degradation and enhanced lipid peroxidation, whereas KO lines displayed enhanced tolerance to both stresses. Quantitative real-time PCR (qRT-PCR) analysis revealed that under drought stress, SlBBX6 transcriptionally represses SlAREB1, SlDREB2A, SlCAT2, and SlNCED1, attenuating abscisic acid (ABA) biosynthesis and signaling, stress responses, and antioxidant defense; under salt stress, it transcriptionally represses SlAREB1, SlDREB2A, SlP5CS1, and SlSOS1, attenuating ABA signaling, stress responses, osmotic adjustment, and the Na⁺ extrusion pathway. This negative regulatory effect exhibits developmental stage specificity. Collectively, these findings identify SlBBX6 as a negative regulator of drought and salt stress responses in tomato.