MAX2 Affects Multiple Hormones to Promote Photomorphogenesis

Ubiquitin-26S proteasome system (UPS) has been shown to play central roles in light and hormone-regulated plant growth and development. Previously, we have shown that MAX2, an F-box protein, positively regulates facets of photomorphogenic development in response to light. However, how MAX2 controls these responses is still unknown. Here, we show that MAX2 oppositely regulates GA and ABA biosynthesis to optimize seed germination in response to light. Dose response curves showed that max2 seeds are hyposensitive to GA and hypersensitive to ABA in seed germination responses. RT-PCR assays demonstrated that the expression of GA biosynthetic genes is down-regulated, while the expression of GA catabolic genes is up-regulated in the max2 seeds compared to wild-type. Interestingly, expression of both ABA biosynthetic and catabolic genes is up-regulated in the max2 seeds compared to wild-type. Treatment with an auxin transport inhibitor, NPA, showed that increased auxin transport in max2 seedlings contributes to the long hypocotyl phenotype under light. Moreover, light-signaling phenotypes are restricted to max2, as the biosynthetic mutants in the strigolactone pathway, max1, max3, and max4, did not display any defects in seed germination and seedling de-etiolation compared to wild-type. Taken together, these data suggest that MAX2 modulates multiple hormone pathways to affect photomorphogenesis.