Strigolactone and Karrikin Signaling Proteins
Strigolactones (SLs) are plant hormones that inhibit shoot branching. DWARF14 (D14), which inhibits rice tillering, is an SL receptor involved in the branching inhibition pathway. Close homologs, rice DWARF14-LIKE (D14L) or Arabidopsis KARRIKIN-INSENSITIVE 2 (KAI2), are the receptors of karrikins (KARs), which are derived from burnt vegetation and act as smoke stimulants of seed germination. Enormous progress has been made in the last decade with respect to molecular studies of the SL and KAR signaling pathways. The main axis of the signaling pathway is D14-D3/MAX2-D53/SMXL678 for SL and KAR-KAI2-D3/MAX2-SMAX1,2 for KAR. In these pathways, the main switch molecules are the hormone receptors, D14 and KAI2, which act as the input device of the pathway. The output device is the ubiquitylation substrate, being D53/SMXL678 for SL and SMAX1,2 for KAR signaling. Since D14 and KAI2 are not canonical hormone receptors but also are enzymes, an understanding of these proteins requires the acquisition and investigation of three-dimensional structures of higher-order complexes. The recent structure of the CLIM-D14-D3 complex has succeeded in shedding light on the mechanisms by which the chemical conversion of SL is able to switch on the signaling pathway. However, several fundamental issues remain to be solved in terms of molecular structures and chemistry.
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