The F-box protein CHALK10 mediates SEMIDWARF-1 ubiquitination and degradation to negatively regulate grain chalkiness in rice

In rice (Oryza sativa), defects in endosperm development can give grains a chalky texture, which decreases grain quality and is therefore undesirable for breeding and marketing. However, the molecular basis of chalkiness remains largely unknown. Here, we identified CHALK10, which encodes an F-box protein that negatively regulates rice chalkiness. The chalk10 mutant exhibited abnormal starch granule development, decreased starch content and alteration of starch physicochemical properties when compared to the wild type. CHALK10 interacts with the gibberellin (GA) oxidase SEMIDWARF-1 (SD1) and promotes the ubiquitination and degradation of SD1 through the 26S proteasome pathway. The grains of SD1-overexpressing plants had increased chalkiness, and introducing the sd1 mutant into the chalk10 background largely suppressed the higher chalkiness seen in the chalk10 mutant. GA levels were higher in the chalk10 mutant compared to the wild type, and application of the bioactive GA form GA3 increased the chalkiness of grains. The expression of genes related to starch degradation or biosynthesis was altered in the chalk10 mutant, resulting in diminished starch production and higher metabolizable sugar contents in the endosperm. In summary, our findings reveal a novel mechanism that regulates chalkiness and provide potential targets for improving rice quality.