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Hormone-activated proteolysis is a recurring theme of plant hormone signaling mechanisms. In strigolactone signaling, the enzyme-receptor DWARF14 (D14) and an F-box protein, MORE AXILLARY GROWTH2 (MAX2), mark SUPPRESSOR OF MAX2 1- LIKE (SMXL) family proteins SMXL6, SMXL7, and SMXL8 for rapid degradation. Removal of these transcriptional corepressors initiates downstream growth responses. The homologous proteins SMXL3, SMXL4, and SMXL5, however, are resistant to MAX2- mediated degradation. We discovered that the smxl4 smxl5 mutant has enhanced responses to strigolactone. SMXL5 attenuates strigolactone signaling by interfering with AtD14-SMXL7 interactions. SMXL5 interacts with AtD14 and SMXL7, providing two possible ways to inhibit SMXL7 degradation. SMXL5 function is partially dependent on an EAR motif that typically mediates interactions with the TOPLESS family of transcriptional corepressors. However, we find that loss of the EAR motif reduces SMXL5-SMXL7 interactions and the attenuation of strigolactone signaling by SMXL5. We hypothesize that integration of SMXL5 into heteromeric SMXL complexes reduces the susceptibility of SMXL6/7/8 proteins to strigolactone-activated degradation, and that the EAR motif promotes the formation or stability of these complexes. This mechanism may provide a way to spatially or temporally fine-tune strigolactone signaling through the regulation of SMXL5 expression or translation. 

Summary Quantitative information on the spatiotemporal distribution of polarised proteins is central for understanding cell‐fate determination, yet collecting sufficient data for statistical analysis is difficult to accomplish with manual measurements.Here we present Polarity Measurement (Pome), a semi‐automated pipeline for the quantification of cell polarity and demonstrate its application to a variety of developmental contexts.Pomeanalysis reveals that, during asymmetric cell divisions in theArabidopsis thalianastomatal lineage, polarity proteins BASL and BRXL2 are more asynchronous and less mutually dependent than previously thought. A similar analysis of the linearly arrayed stomatal lineage ofBrachypodium distachyonrevealed that the MAPKKK BdYDA1 is segregated and polarised following asymmetrical divisions.Our results demonstrate that Pomeis a versatile tool, which by itself or combined with tissue‐level studies and advanced microscopy techniques can help to uncover new mechanisms of cell polarity.