REPRESSOR OF UV-B PHOTOMORPHOGENESIS proteins target ABSCISIC ACID INSENSITIVE 5 for degradation to promote early plant development.

REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and REPRESSOR OF UV-B PHOTOMORPHOGENESIS 2 (RUP2) are WD-40 domain-containing proteins that have been extensively characterized for their role in UV-B signaling. However, the roles of the RUP proteins outside the canonical UV-signaling pathway are less known. Here, we identify that RUP1 and RUP2 play important roles in ABA signaling to regulate seed germination and early seedling development in Arabidopsis thaliana. Our protein interaction studies confirmed that RUP1 and RUP2 physically interact with ABA INSENSITIVE 5 (ABI5). In the presence of abscisic acid, rup1, rup2, and rup1rup2 exhibited reduced germination and seedling establishment compared with the wild type. Germination and seedling establishment in rup1rup2abi5-8 were similar to abi5-8, suggesting that RUP1 and RUP2 suppress ABA-mediated inhibition of germination and early seedling development in an ABI5-dependent manner. The DDB1-binding WD40 protein RUP2 promoted the ubiquitination of ABI5 to regulate its degradation. ABI5, in turn, establishes a negative feedback loop to inhibit the expression of RUP1/RUP2. ABI5 also inhibited the direct binding of ELONGATED HYPOCOTYL 5 (HY5) to the promoters of RUP1 and RUP2 under ABA. This study highlights the coordinated action of RUP1, RUP2, ABI5, and HY5 in regulating early plant development.