The rice PLATZ protein SHORT GRAIN6 determines grain size by regulating spikelet hull cell division

Grain size is a major determinant of cereal grain yields; however, the relevant regulatory mechanisms controlling this trait have not been fully elucidated. The rice (Oryza sativa) mutant short grain6 (sg6) was identified based on its reduced grain length and weight. Here, we functionally characterized the role of SG6 in determining grain size through the regulation of spikelet hull cell division. SG6 encodes a previously uncharacterized plant AT-rich sequence and zinc-binding (PLATZ) protein that is ubiquitously localized throughout the cell and is preferentially expressed in the early developing panicles but not in the endosperm. The overexpression of SG6 resulted in significantly larger and heavier grains, as well as increased plant heights, which is consistent with its elevated spikelet hull cell division rate. Yeast two-hybrid analyses revealed that SG6 interacts with the core cell cycle machinery DP protein and several other putative cell division regulators, consistent with our transcriptomic analysis, which showed that SG6 activates the expression of many DNA replication and cell-cycle-related genes. These results confirm the crucial role of SG6 in determining grain size by regulating spikelet hull cell division and provide clues for understanding the functions of PLATZ family proteins and the network regulating cereal grain size.