Fig. 6.
Proposed mechanism of SL perception by rice D14. (A) Schematic diagram of a proposed rice D14-mediated hydrolysis process of 5DS. The hydrolysis of 5DS (1) is proposed to involve a nucleophilic attack by S147, which produces ABC-OH (2) and compound 3, and the generation of D-OH (6). The Nε2 atom of H297 attacks the aldehyde carbon atom of the S147-linked compound 3 to form the H297- and S147-linked linear compound 4, referred to as the covalently linked intermediate molecule (CLIM). Compound 4 initiates an intramolecular nucleophilic attack to generate the H297-linked circular compound 5, which appears as a C5H5O2 modification on H297 detected by MS/MS and denotes the existence of CLIM inside the D3-bound D14. Compound 5 would be further hydrolyzed from H297 to produce D-OH (6). A similar deduction of the AtD14-mediated (+)-GR24 hydrolysis process can be found in detail in our recent study (Yao et al., 2016). (B) A simplified model of SL perception. D14/AtD14 docks SL in the catalytic cavity, hydrolyzes SL into a D-ring-derived intermediate (CLIM), which is covalently sealed inside the catalytic center of D14/AtD14 to promote the interaction with the D3/MAX2-based SCF complex and the repressor D53/SMXLs for triggering SL-regulated plant branching (Yao et al., 2016).