Location: Corn Host Plant Resistance ResearchTitle: Cloning and Characterization of a Putative GS3 Ortholog Involved in Maize Kernel Development Author
Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2009
Publication Date: 1/1/2010
Citation: Li, Q., Yang, X., Bai, G., Warburton, M.L., Mahuku, G., Gore, M., Dai, J., Li, J., Yan, J. 2010. Cloning and Characterization of a Putative GS3 Ortholog Involved in Maize Kernel Development. Theoretical and Applied Genetics. 120:753-763. Interpretive Summary: The GS3 gene in rice encodes a protein which is important for kernel development. When this protein is not expressed, as when a single nucleotide change inserts a premature stop codon, rice grains are significantly longer and larger than when it is expressed. It has been the target for selection during rice improvement for increased yield. The genetic sequence in rice was used to find a homolog in maize to test if the maize gene is also associated with bigger maize kernels and thus a potential target for maize yield increases in future breeding programs. However, maize GS3 was not found to have been a target for selection during maize evolution, and is only nominally associated with increased grain size. Although a function is implied for kernel development in this paper, no allele was found that completely turns off transcription of this gene in maize, and thus, it may not have had a large enough phenotypic effect for farmer and breeder selection to work on for yield improvement.
Technical Abstract: A single nucleotide polymorphism (SNP) in the second exon of GS3 is the causal mutation for grain size and weight variation in rice. This gene has been cloned and is hypothesized to be involved in the evolution of grain size during domestication. In this study, we isolated the maize homolog, ZmGS3, and investigated its role in the evolution of maize grain size. Maize ZmGS3 has five exons and encodes a 198-amino acid long protein, and has domains in common with the rice GS3 protein. Compared with its progenitor, teosinte, nucleotide diversity at this gene is reduced in maize, comparable to the reduction found in non-selected maize genes that evolve only via mutation, migration, recombination, or bottlenecks. No positive selection was detected anywhere along the length of the gene using either the Hudson-Kreitman-Aguadé (HKA) test or the Tajima’ D test. Phylogenetic analysis revealed a dispersed distribution of maize sequences among different clades, including related teosinte sequences. The nucleotide polymorphism analysis, selection test and phylogenetic analysis show that ZmGS3 has not been subjected to selection. In maize, ZmGS3 is primarily expressed in immature ears and kernels, implying a role in maize kernel development. Our association mapping analysis revealed that four polymorphisms in the fifth exon and 3'UTR were significantly associated with kernel length, kernel width and kernel thickness. Collectively, these results imply that ZmGS3 is involved in maize kernel development, but with different alleles and thus, possibly different mechanisms than rice GS3.