OsMAPK6, a mitogen‐activated protein kinase, influences rice grain size and biomass production

Summary Grain size is an important agronomic trait in determining grain yield. However, the molecular mechanisms that determine the final grain size are not well understood. Here, we report the functional analysis of a rice ( Oryza sativa L.) mutant, dwarf and small grain1 ( dsg1 ), which displays pleiotropic phenotypes, including small grains, dwarfism and erect leaves. Cytological observations revealed that the small grain and dwarfism of dsg1 were mainly caused by the inhibition of cell proliferation. Map‐based cloning revealed that DSG 1 encoded a mitogen‐activated protein kinase ( MAPK ), Os MAPK 6. Os MAPK 6 was mainly located in the nucleus and cytoplasm, and was ubiquitously distributed in various organs, predominately in spikelets and spikelet hulls, consistent with its role in grain size and biomass production. As a functional kinase, Os MAPK 6 interacts strongly with Os MKK 4, indicating that Os MKK 4 is likely to be the upstream MAPK kinase of Os MAPK 6 in rice. In addition, hormone sensitivity tests indicated that the dsg1 mutant was less sensitive to brassinosteroids ( BR s). The endogenous BR levels were reduced in dsg1 , and the expression of several BR signaling pathway genes and feedback‐inhibited genes was altered in the dsg1 mutant, with or without exogenous BR s, indicating that Os MAPK 6 may contribute to influence BR homeostasis and signaling. Thus, Os MAPK 6, a MAPK , plays a pivotal role in grain size in rice, via cell proliferation, and BR signaling and homeostasis.