MATH/BTB CRL3 Receptors Target the Homeodomain-Leucine Zipper ATHB6 to Modulate Abscisic Acid Signaling

Being sessile organisms, plants need rapid and finely tuned signaling pathways to adapt their growth and survival over their immediate and often adverse environment. Abscisic acid (ABA) is a plant hormone crucial for both biotic and abiotic stress responses. In this study, we highlight a function of six Arabidopsis MATH-BTB proteins in ABA signaling. MATH-BIB proteins act as substrate-binding adaptors for the Cullin3-based ubiquitin E3 ligase. Our genetic and biochemical experiments demonstrate that the MATH-BIB proteins directly interact with and target for proteasomal degradation the class I homeobox-leucine zipper (HD-ZIP) transcription factor ATHB6, which was previously identified as a negative regulator of ABA responses. Reducing CUL3(BPM) function leads to higher ATHB6 protein accumulation, reducing plant growth and fertility, and affects stomatal behavior and responses to ABA. We further demonstrate that ABA negatively regulates ATHB6 protein turnover, a situation reminiscent to ABI5, another transcription factor involved in ABA signaling.