Strigolactone perception and D14-MAX2-SMXL signalling in shoot-branching control

Strigolactones (SLs) are carotenoid-derived hormones that inhibit axillary bud outgrowth. In Arabidopsis thaliana, SL is perceived by the alpha/beta-hydrolase receptor DWARF14 (D14), which binds and slowly hydrolyses the butenolide D-ring. SL binding changes the conformation of D14 so that it associates with the F-box protein MORE AXILLARY GROWTH2 (MAX2) in an SCF(MAX2) E3 ubiquitin ligase and with the SMAX1-LIKE co-repressors SMXL6, SMXL7 and SMXL8. The SMXLs are then polyubiquitinated and degraded by the proteasome, and D14 itself is degraded as negative feedback. In the absence of SL, the nuclear SMXL6/7/8 proteins repress target genes by recruiting TOPLESS-related co-repressors through an EAR motif, and SMXL6 can also bind target promoters itself. Their removal de-represses the bud-expressed TCP transcription factor BRANCHED1 (BRC1), which keeps buds dormant, in part by activating HD-ZIP genes that raise abscisic acid in the bud. SL-triggered SMXL degradation also reduces PIN1 accumulation and auxin transport in the stem, which inhibits bud activation in parallel with BRC1. The sister karrikin pathway uses the same F-box protein, MAX2, with the D14 paralog KAI2 and the SMAX1/SMXL2 repressors. That pathway, SL biosynthesis (D27, MAX3, MAX4, MAX1) and the auxin-transport branch are outside this module.

Concrete Arabidopsis instance; the rice orthologous pathway is D14-D3-D53 (PMID:24336200, PMID:24336215), and petunia/pea receptors are DAD2/RMS3. Annoton molecular functions match the core_functions of the D14, MAX2, SMXL6, SMXL7, SMXL8 and BRC1 gene reviews. Because GO has no strigolactone-specific receptor term, D14 is typed with signaling receptor activity (GO:0038023), and the D14 review proposes a new term, strigolactone receptor activity. GO also has no strigolactone signalling pathway term, so the module concept is cellular response to strigolactone (GO:1902348). SMXL6, SMXL7 and SMXL8 are modelled as three redundant annotons on one co-repressor node instead of a PANTHER family descriptor: their shared PANTHER family (PTHR43572, CLPD chaperone) is far broader than the SMXL6/7/8 clade. No GO-CAM in gocams/index.tsv contains these proteins. The ASK1 and CUL1 SCF scaffold subunits and the TOPLESS/TPR2 co-repressors are named in prose but not modelled as parts, because they are shared with many other hormone pathways. No Falcon deep research could be generated (all providers failed).