BolMYB122-BolTGG1 module enhances salt stress tolerance via accelerating the hydrolysis of glucosinolates to produce sulforaphane in broccoli.

Glucosinolates (GSLs), one type of secondary metabolites mainly enriched in cruciferous plants, play important roles in environmental stress responses. However, the function and regulatory mechanism of GSLs in combating salt stress in broccoli are largely unclear. In this study, BolTGG1, a member of β-thioglucoside glucohydrolase (TGG) genes mainly functioning to hydrolyze GSLs, was identified in broccoli. BolTGG1 expression was upregulated under salt stress. Overexpression of BolTGG1 significantly improved the tolerance to salt stress in both broccoli and Arabidopsis. Subsequently, BolTGG1 was heterologously expressed in yeast and the BolTGG1 protein was isolated to evaluate its ability to hydrolyze GSLs. The results confirmed that exogenous addition of BolTGG1 could obviously increase the yield of sulforaphane (SFN), a major hydrolysis product of GSLs, in vitro. Meanwhile, the content of SFN was significantly increased in BolTGG1-overexpression transgenic broccoli plants. The role of SFN in the salt stress response was then explored. The results indicated that external application of SFN could directly enhance the salt resistance with the elevated activities of superoxide dismutase (SOD) and peroxidase (POD), and reduced malondialdehyde (MDA) levels in broccoli. Furthermore, BolMYB122 was confirmed to be the upstream regulatory transcription factor of BolTGG1. BolMYB122 positively regulated BolTGG1 transcription. In summary, these results indicated that BolMYB122-BolTGG1 module confers salt stress tolerance mainly by accelerating the generation of SFN. These findings provide new insights into the role and regulatory mechanism of GSL metabolites in plant salt stress response, and suggest a novel breeding strategy for stress resistance from the perspective of regulating plant secondary metabolism.