Redesigning green revolution trait with increased grain yield and nitrogen utilization efficiency by reducing brassinosteroid signaling in semidwarf wheat

Securing food and feed for the increasing world population and decreasing arable land is one of the major challenges of our time.Green revolution or agriculture revolution was propelled by the use of semidwarf rice and wheat varieties around the world since the 1960s(Peng et al.,1999;Sasaki et al.,2002).The semi-dwarfness leads to lodging resistance and thus allows planting of crops in high densities。

A cryptic inhibitor of cytokinin phosphorelay controls rice grain size

Plant hormone cytokinin signals through histidine-aspartic acid(H-D)phosphorelay to regulate plant growth and development.While it is well known that the phosphorelay involves histidine kinases,histidine phosphotransfer proteins(HPs),and response regulators(RRs),how this process is regulated by external components remains unknown.Here we demonstrate that protein phosphatase with kelch-like domains(PPKL1),known as a signaling component of steroid hormone brassinosteroid,is actually a cryptic inhibitor of cytokinin phosphorelay in rice(Oryza sativa).Mutation at a specific amino acid D364 of PPKL1 activates cytokinin response and thus enlarges grain size in a semi-dominant mutant named s48.Overexpression of PPKL1 containing D364,either with the deletion of the phosphatase domain or not,rescues the s48 mutant phenotype.PPKL1 interacts with OsAHP2,one of authentic HPs,and D364 resides in a region resembling the receiver domain of RRs.Accordingly,PPKL1 can utilize D364 to suppress OsAHP2-to-RR phosphorelay,whereas mutation of D364 abolishes the effect.This function of PPKL1 is independent of the phosphatase domain that is required for brassinosteroid signaling.Importantly,editing of the D364-residential region produces a diversity of semi-dominant mutations associated with variously increased grain sizes.Further screening of the edited plants enables the identification of two genotypes that confer significantly improved grain yield.Collectively,our study uncovers a noncanonical cytokinin signaling suppressor and provides a robust tool for seed rational design.

Histone Lysine Methyltransferase SDG8 Is Involved in Brassinosteroid-Regulated Gene Expression in Arabidopsis thaliana

The plant steroid hormones, brassinosteroids （BRs）, play important roles in plant growth, development, and responses to environmental stresses. BRs signal through receptors localized to the plasma membrane and other signaling components to regulate the BES1/BZR1 family of transcription factors, which modulates the expression of thousands of genes. How BESl/BZR1 and their interacting proteins function to regulate the large number of genes are not com- pletely understood. Here we report that histone lysine methyltransferase SDG8, implicated in histone 3 lysine 36 diand trimethylation （H3K36me2 and me3）, is involved in BR-regulated gene expression. BES1 interacts with SDG8, directly or indirectly through IWSl, a transcription elongation factor involved in BR-regulated gene expression. The knockout mutant sdg8 displays a reduced growth phenotype with compromised BR responses. Global gene expression studies demonstrated that, while BR regulates about 5000 genes in wild-type plants, the hormone regulates fewer than 700 genes in sdg8 mutant. In addition, more than half of BR-regulated genes are differentially affected in sdg8 mutant. A Chromatin Immunoprecipitation （CHIP） experiment showed that H3K36me3 is reduced in BR-regulated genes in the sdg8 mutant. Based on these results, we propose that SDG8 plays an essential role in mediating BR-regulated gene expression. Our results thus reveal a major mechanism by which histone modifications dictate hormonal regulation of gene expression.