Arabidopsis ARA4 modulates HY5-mediated seedling growth and ABA responsiveness.

HY5, a basic leucine zipper (bZIP) transcription factor, acts as a positive regulator of photomorphogenesis across various wavelengths of light. HY5 also mediates crosstalk between light and abscisic acid (ABA) signaling pathways. During transition from dark to light, HY5 regulates the transcription of about one third of genes in Arabidopsis, necessitating precise regulation of HY5 activity for proper seedling growth. On the other hand, ARA4 acts as a negative regulator of photomorphogenesis specifically in white light. Our study aims to understand how the developing seedlings integrate external cues with internal hormonal levels to maintain the homeostasis of key regulators like HY5 for optimal growth. Although HY5's role in integrating light and ABA signaling is well established, the regulation of HY5 itself during this process still needs to be explored. Here we report that hy5 is epistatic to ara4 in the regulation of hypocotyl length and light-responsive gene expression. Double mutant analyses further reveal that ARA4 and HY5 work additively to regulate ABA-mediated inhibition of seed germination. ARA4 physically interacts with HY5 and negatively regulates HY5 promoter activity. The ARA4-mediated negative regulation on HY5 expression is rescued by ABA. The transactivation and DNA-protein interaction studies reveal that ARA4 inhibits HY5 from binding to the promoter of its target, AtMYB4, and subsequent transcriptional activation. However, ABA enhances HY5 binding to the AtMYB4 promoter. Overall, this study highlights the functional interplay between ARA4 and HY5 on the regulation of light and ABA-mediated growth responses during Arabidopsis seedling development.