Characterization of the NGP4A Gene in Regulating Grain Number Per Panicle of Rice (Oryza sativa L.)

Abstract

Grain number per panicle (GNPP) is a major factor influencing rice yield (Oryza sativa L.). However, the molecular mechanisms of GNPP determination are not well understood. A rice GNPP mutant, ngp4a, was isolated from an ethyl methanesulfonate-mutagenized rice library of japonica Nipponbare. ngp4a produced fewer grains than wild-type plants at maturity as the number of secondary branches decreased significantly. The mutant phenotype of ngp4a was controlled by a recessive nuclear gene, which was fine-mapped into a 155.2 kb region on chromosome 4. One GNPP-related gene, Gnp4/LAX2 (LOC_Os04g32510), was found in the mapped region. The deletion of 3-bp nucleotides in the first exon of NGP4A resulted in a threonine residue loss. The mutation in NGP4A was responsible for the mutant phenotype of ngp4a. These results suggest that NGP4A is a new allele for Gnp4 and LAX2, while the mutant phenotype and underlying causation differed. Notably, transcriptome analysis revealed that NGP4A could regulate GNPP determination through the phenylpropanoid biosynthesis and mitogen-activated protein kinase signaling pathways. Our results further elucidated the vital roles of Gnp4/LAX2 in GNPP determination, providing a new genetic resource and theoretical basis to further explore the molecular mechanisms of GNPP in rice.