Moonlighting PPKL1 reveals a role of cytokinin in regulating rice grain size

Rice is the staple food of more than half of the world's population,and major efforts are being made to improve rice yield.Rice yield is mainly determined by three components:number of panicles per plant,number of grains per panicle,and grain weight,which is positively associated with grain size.Therefore,grain size is an important agricultural trait for yield improvement in rice,and it is controlled by multiple genetic factors and environmental cues(Li et al.,2018).The final size of rice grains is largely determined by cell proliferation and cell expansion in the spikelet hull(husk),which sets the storage capacity of the grain and limits grain filling(Li et al.,2018).A number of quantitative trait loci for grain size have been identified and their underlying molecular functions unraveled.These include,for example,protein ubiquitination,chromatin modifications,transcription factors,RNA-mediated regulatory networks,and multiple plant hormones(Li and Li,2016).Among the plant hormones,brassinosteroids and cytokinin play pivotal roles in grain size control.Recently,Liu et al.(2021)reported the discovery of an unexpected novel link between the brassinosteroid and the cytokinin signaling pathways with a major impact on rice grain size.

Increased branching independent of strigolactone in cytokinin oxidase 2-overexpressing tomato is mediated by reduced auxin transport

Tomato production is influenced by shoot branching,which is controlled by different hormones.Here we produced tomato plants overexpressing the cytokinin-deactivating gene CYTOKININ OXYDASE 2(CKX2).CKX2-overexpressing(CKX2-OE)plants showed an excessive growth of axillary shoots,the opposite phenotype expected for plants with reduced cytokinin content,as evidenced by LC-MS analysis and ARR5-GUS staining.The TCP transcription factor SlBRC1b was downregulated in the axillary buds of CKX2-OE and its excessive branching was dependent on a functional version of the GRAS-family gene LATERAL SUPPRESSOR(LS).Grafting experiments indicated that increased branching in CKX2-OE plants is unlikely to be mediated by root-derived signals.Crossing CKX2-OE plants with transgenic antisense plants for the strigolactone biosynthesis gene CAROTENOID CLEAVAGE DIOXYGENASE(CCD7-AS)produced an additive phenotype,indicating independent effects of cytokinin and strigolactones on increased branching.On the other hand,CKX2-OE plants showed reduced polar auxin transport and their bud outgrowth was reduced when combined with auxin mutants.Accordingly,CKX2-OE basal buds did not respond to auxin applied in the decapitated apex.Our results suggest that tomato shoot branching depends on a fine-tuning of different hormonal balances and that perturbations in the auxin status could compensate for the reduced cytokinin levels in CKX2-OE plants.