A B-Box Transcription Factor CoBBX24 from Camellia oleifera Delays Leaf Senescence and Enhances Drought Tolerance in Arabidopsis

Plants face various biotic and abiotic stress factors during their growth and development, among which, drought is a serious adverse factor that affects yield and quality in agriculture and forestry. Several transcription factors are involved in regulating plant responses to drought stress. In this study, the B-box (BBX) transcription factor CoBBX24 was cloned from Camellia oleifera. This gene encodes a 241-amino-acid polypeptide containing two B-box domains at the N-terminus. A phylogenetic analysis revealed that CoBBX24 and CsBBX24 from Camellia sinensis are in the same branch, with their amino acid sequences being identical by 96.96%. CoBBX24 was localized to the nucleus and acted as a transcriptional activator. The overexpression of CoBBX24 in Arabidopsis heightened its drought tolerance along with a relatively high survival rate, and the rate of water loss in the OX-CoBBX24 lines was observably lower than that of the wild-type. Compared to the wild-type, the root lengths of the OX-CoBBX24 lines were significantly inhibited with abscisic acid. Leaf senescence was delayed in the OX-CoBBX24 lines treated with abscisic acid. The expression of genes related to leaf senescence and chlorophyll breakdown (e.g., SAG12, SAG29, NYC1, NYE1, and NYE2) was downregulated in the OX-CoBBX24 lines. This study indicated that CoBBX24 positively regulates the drought tolerance in Arabidopsis through delayed leaf senescence.