Genome-wide identification and expression analysis of B-box genes during abiotic stress responses in the tea plant

BBX proteins are zinc-finger transcription factors that regulate light-dependent processes including photomorphogenesis, flowering, and circadian rhythms. While BBX families have been characterized in model plants, their evolutionary constraints and stress-response mechanisms in perennial woody crops remain poorly understood, limiting crop improvement strategies for climate resilience. Here, we show that the tea plant (Camellia sinensis) maintains precisely 32 CsBBX members despite its 20-fold larger genome compared to Arabidopsis, revealing remarkable evolutionary conservation on BBX family expansion. Through comprehensive genome-wide identification, phylogenetic analysis, and expression profiling across eight tissues and four abiotic stresses, we uncovered specialized regulatory modules within the CsBBX family. Tissue-specific analysis revealed distinct expression patterns, including root-specific CsBBX21 and reproductive tissue-enriched CsBBX6-1/2 and CsBBX19-3, indicating functional diversification beyond light responses. Cold stress emerged as the most potent inducer, with CsBBX7-1 showing 40-fold rapid induction, while CsBBX30-1 demonstrated unique drought-specific accumulation (14-fold). Functional characterization revealed that CsBBX19-3 and CsBBX24 activate the promoter of the stress-responsive transcription factor CsDREB2A, establishing a novel regulatory hierarchy upstream of both DREB2A and DREB1/CBFs-dependent cold tolerance pathways. Our findings establish CsBBX proteins as master regulators integrating light and stress signals, expanding beyond canonical photoperiodic functions. The identification of stress-specialized CsBBX members provides immediate targets for developing climate-resilient tea cultivars through molecular breeding approaches.