The B-box (BBX) family is a subgroup of zinc finger transcription factors that regulate flowering time, light-regulated morphogenesis, and abiotic stress in Arabidopsis. Overexpression of CmBBX24, a zinc finger transcription factor gene in chrysanthemum, results in abiotic stress tolerance. We have investigated and characterized the promoter of CmBBX24, isolating a 2.7-kb CmBBX24 promoter sequence and annotating a number of abiotic stress-related cis-regulatory elements, such as DRE, MYB, MYC, as well as cis-elements which respond to plant hormones, such as GARE, ABRE, and CARE. We also observed a number of cis-elements related to light, such as TBOX and GBOX, and some tissue-specific cis-elements, such as those for guard cells (TAAAG). Expression of the CmBBX24 promoter produced a clear response in leaves and a lower response in roots, based on β-glucuronidase histochemical staining and fluorometric analysis. The CmBBX24 promoter was induced by abiotic stresses (mannitol, cold temperature), hormones (gibberellic acid, abscisic acid), and different light treatments (white, blue, red); activation was measured by fluorometric analysis in the leaves and roots. The deletion of fragments from the 5'-end of the promoter led to different responses under various stress conditions. Some CmBBX24 promoter segments were found to be more important than others for regulating all stresses, while other segments were relatively more specific to stress type. D0-, D1-, D2-, D3-, and D4-proCmBBX24::CmBBX24 transgenic Arabidopsis lines developed for further study were found to be more tolerant to the low temperature and drought stresses than the controls. We therefore speculate that CmBBX24 is of prime importance in the regulation of abiotic stress in Arabidopsis and that the CmBBX24 promoter is inductive in abiotic stress conditions. Consequently, we suggest that CmBBX24 is a potential candidate for the use in breeding programs of important ornamental plants.
Keywords: BBX24; Chrysanthemum; Promoter activity; Stress tolerance.