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Light is one of the most important environmental cues regulating multiple aspects of plant growth and development, and abscisic acid (ABA) is a plant hormone that plays important roles during many phases of the plant life cycle and in plants' responses to various environmental stresses. How plants integrate the external light signal with endogenous ABA pathway for better adaptation and survival remains poorly understood. Here, we show that BBX21 (also known as SALT TOLERANCE HOMOLOG 2), a B-box (BBX) protein previously shown to positively regulate seedling photomorphogenesis, is also involved in ABA signaling. Our genetic data show that BBX21 may act upstream of several  ABA INSENSITIVE (ABI)  genes and ELONGATED HYPOCOTYL 5 (HY5) in ABA control of seed germination. Previous studies showed that HY5 acts as a direct activator of  ABI5  expression, and that BBX21 interacts with HY5. We further demonstrate that BBX21 negatively regulates  ABI5  expression by interfering with HY5 binding to the  ABI5  promoter. In addition, ABI5 was shown to directly activate its own expression, whereas BBX21 negatively regulates this activity by directly interacting with ABI5. Together, our study indicates that BBX21 coordinates with HY5 and ABI5 on the  ABI5  promoter and that these transcriptional regulators work in concert to integrate light and ABA signaling in  Arabidopsis thaliana .

Convergence of Light and ABA Signaling on the ABI5 Promoter