Strigolactones (SLs) are a new class of carotenoid-derived phytohormones essential for developmental processes shaping plant architecture and interactions with parasitic weeds and symbiotic arbuscular mycorrhizal fungi. Despite the rapid progress in elucidating the SL biosynthetic pathway, the perception and signaling mechanisms of SL remain poorly understood. Here we show that DWARF53 (D53) acts as a repressor of SL signaling and SLs induce its degradation. We found that the rice d53 mutant, which produces an exaggerated number of tillers compared to wild type plants, is caused by a gain-of-function mutation and is insensitive to exogenous SL treatment. The D53 gene product shares predicted features with the class I Clp ATPase proteins and can form a complex with the α/β hydrolase protein DWARF14 (D14) and the F-box protein DWARF3 (D3), two previously identified signaling components potentially responsible for SL perception. We demonstrate that, in a D14- and D3-dependent manner, SLs induce D53 degradation by the proteasome and abrogate its activity in promoting axillary bud outgrowth. Our combined genetic and biochemical data reveal that D53 acts as a repressor of the SL signaling pathway, whose hormone-induced degradation represents a key molecular link between SL perception and responses.
Grain size is a major determinant of grain yield in cereal crops. qSW5/GW5, which exerts the greatest effect on rice grain width and weight, was fine-mapped to a 2,263-bp/21-kb genomic region containing a 1,212-bp deletion, respectively. Here, we show that a gene encoding a calmodulin binding protein, located ∼5 kb downstream of the 1,212-bp deletion, corresponds to qSW5/GW5. GW5 is expressed in various rice organs, with highest expression level detected in young panicles. We provide evidence that the 1,212-bp deletion affects grain width most likely through influencing the expression levels of GW5. GW5 protein is localized to the plasma membrane and can physically interact with and repress the kinase activity of rice GSK2 (glycogen synthase kinase 2), a homologue of Arabidopsis BIN2 (BRASSINOSTEROID INSENSITIVE2) kinase, resulting in accumulation of unphosphorylated OsBZR1 (Oryza sativa BRASSINAZOLE RESISTANT1) and DLT (DWARF AND LOW-TILLERING) proteins in the nucleus to mediate brassinosteroid (BR)-responsive gene expression and growth responses (including grain width and weight). Our results suggest that GW5 is a novel positive regulator of BR signalling and a viable target for genetic manipulation to improve grain yield in rice and perhaps in other cereal crops as well.
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