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SUMMARY The butenolide molecule, karrikin (KAR), emerging in smoke of burned plant material, enhances light responses such as germination, inhibition of hypocotyl elongation, and anthocyanin accumulation in Arabidopsis. The KAR signaling pathway consists of KARRIKIN INSENSITIVE 2 (KAI2) and MORE AXILLARY GROWTH 2 (MAX2), which, upon activation, act in an SCF E3 ubiquitin ligase complex to target the downstream signaling components SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1‐LIKE 2 (SMXL2) for degradation. How degradation of SMAX1 and SMXL2 is translated into growth responses remains unknown. Although light clearly influences the activity of KAR, the molecular connection between the two pathways is still poorly understood. Here, we demonstrate that the KAR signaling pathway promotes the activity of a transcriptional module consisting of ELONGATED HYPOCOTYL 5 (HY5), B‐BOX DOMAIN PROTEIN 20 (BBX20), and BBX21. Thebbx20 bbx21mutant is largely insensitive to treatment with KAR2, similar to ahy5mutant, with regards to inhibition of hypocotyl elongation and anthocyanin accumulation. Detailed analysis of higher order mutants in combination with RNA‐sequencing analysis revealed that anthocyanin accumulation downstream of SMAX1 and SMXL2 is fully dependent on the HY5‐BBX module. However, the promotion of hypocotyl elongation by SMAX1 and SMXL2 is, in contrast to KAR2treatment, only partially dependent on BBX20, BBX21, and HY5. Taken together, these results suggest that light‐ and KAR‐dependent signaling intersect at the HY5‐BBX transcriptional module.
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KARRIKIN UP-REGULATED F-BOX 1 (KUF1) imposes negative feedback regulation of karrikin and KAI2 ligand metabolism in Arabidopsis thaliana
Karrikin (KAR) molecules found in smoke stimulate seed germination of many plant species that emerge after fire. Genetic studies in Arabidopsis thaliana have identified core components of the KAR signaling pathway, including an α/β-hydrolase, KARRIKIN INSENSITIVE2 (KAI2), that is required for KAR responses. Although KAI2 is often considered a KAR receptor, recent evidence suggests that KARs may require metabolism to become bioactive signals. In addition to sensing KARs or a KAR-derived signal, KAI2 is thought to recognize an unknown endogenous signal, KAI2 ligand (KL). We generated loss-of-function mutations in KARRIKIN-UP-REGULATED F-BOX1 ( KUF1 ), which is a transcriptional marker of KAR/KL signaling in A. thaliana and other plants. The kuf1 mutant in Arabidopsis shows several phenotypes that are consistent with enhanced activity of the KAI2 pathway, including reduced hypocotyl elongation, enhanced cotyledon expansion in light-grown seedlings, increased root hair density and elongation, and differential expression of KAR/KL-responsive transcriptional markers. Seedling phenotypes of kuf1 are dependent on KAI2 and its signaling partner MORE AXILLARY GROWTH2 (MAX2). Furthermore, kuf1 mutants are hypersensitive to KAR 1 , but not to other molecules that can signal through KAI2 such as GR24. This implies that kuf1 does not increase the overall responsiveness of the KAI2-dependent signaling pathway, but specifically affects the ability of KAI2 to detect certain signals. We hypothesize that KUF1 imposes feedback inhibition of KL biosynthesis and KAR 1 metabolism. As an F-box protein, KUF1 likely participates in an E3 ubiquitin ligase complex that imposes this regulation through polyubiquitylation of a protein target(s).
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- PAR ID:
- 10324284
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 119
- Issue:
- 11
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE (SMXL) proteins comprise a family of plant growth regulators that includes downstream targets of the karrikin (KAR)/KAI2 ligand (KL) and strigolactone (SL) signaling pathways. Following the perception of KAR/KL or SL signals by α/β hydrolases, some types of SMXL proteins are polyubiquitinated by an E3 ubiquitin ligase complex containing the F-box protein MORE AXILLARY GROWTH2 (MAX2)/DWARF3 (D3), and proteolyzed. Because SMXL proteins interact with TOPLESS (TPL) and TPL-related (TPR) transcriptional co-repressors, SMXL degradation initiates changes in gene expression. This simplified model of SMXL regulation and function in plants must now be revised in light of recent discoveries. It has become apparent that SMXL abundance is not regulated by KAR/KL or SL alone, and that some SMXL proteins are not regulated by MAX2/D3 at all. Therefore, SMXL proteins should be considered as signaling hubs that integrate multiple cues. Here we review the current knowledge of how SMXL proteins impose transcriptional regulation of plant development and environmental responses. SMXL proteins can bind DNA directly and interact with transcriptional regulators from several protein families. Multiple mechanisms of downstream genetic control by SMXL proteins have been identified recently that do not involve the recruitment of TPL/TPR, expanding the paradigm of SMXL function.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.2112820119
