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-BTB proteins act as substrate-binding adaptors for the Cullin3-based ubiquitin E3 ligase. Our genetic and biochemical experiments demonstrate that the MATH-BTB 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.

Regulation of plant height, one of the most important agronomic traits, is the focus of intensive research for improving crop performance. Stem elongation takes place as a result of repeated cell divisions and subsequent elongation of cells produced by apical and intercalary meristems. The gibberellin (GA) phytohormones have long been known to control stem and internodal elongation by stimulating the degradation of nuclear growth-repressing DELLA proteins; however, the mechanism allowing GA-responsive growth is only slowly emerging. Here, we show that DELLAs directly regulate the activity of the plant-specific class I TCP transcription factor family, key regulators of cell proliferation. Our results demonstrate that class I TCP factors directly bind the promoters of core cell-cycle genes in Arabidopsis inflorescence shoot apices while DELLAs block TCP function by binding to their DNA-recognition domain. GAs antagonize such repression by promoting DELLA destruction and therefore cause a concomitant accumulation of TCP factors on promoters of cell-cycle genes. Consistent with this model, the quadruple mutant tcp8 tcp14 tcp15 tcp22 exhibits severe dwarfism and reduced responsiveness to GA action. Altogether, we conclude that GA-regulated DELLA-TCP interactions in inflorescence shoot apex provide a novel mechanism to control plant height.

In fungi and metazoans, the SCF-type Ubiquitin protein ligases (E3s) play a critical role in cell cycle regulation by degrading negative regulators, such as cell cycle-dependent kinase inhibitors (CKIs) at the G1-to-S-phase checkpoint. Here we report that FBL17, an Arabidopsis thaliana F-box protein, is involved in cell cycle regulation during male gametogenesis. FBL17 expression is strongly enhanced in plants co-expressing E2Fa and DPa, transcription factors that promote S-phase entry. FBL17 loss-of-function mutants fail to undergo pollen mitosis II, which generates the two sperm cells in mature A. thaliana pollen. Nonetheless, the single sperm cell-like cell in fbl17 mutants is functional but will exclusively fertilize the egg cell of the female gametophyte, giving rise to an embryo that will later abort, most likely due to the lack of functional endosperm. Seed abortion can, however, be overcome by mutations in FIE, a component of the Polycomb group complex, overall resembling loss-of-function mutations in the A. thaliana cyclin-dependent kinase CDKA;1. Finally we identified ASK11, as an SKP1-like partner protein of FBL17 and discuss a possible mechanism how SCFFBL17 may regulate cell division during male gametogenesis.