Chinese scientists reveal molecular mechanism regulating grain length of rice

A team of Chinese scientists cracked the mystery of how copy number variants (CNVs) on Chromosome 7 (GL7) locus contributes to grain size diversity and quality in rice (Oryza sativa L.). 

The findings are published in online scientific journal Nature Genetics on July 6, 2015. The academic thesis describes the study jointly carried out by scientists from the China National Rice Research Institute of the Chinese Academy of Agricultural Sciences and the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences. The research findings have laid theoretic foundation for the molecular design of the grain size and the breeding of high-yield and high-quality novel varieties of rice.

The study on American long-grain japonica rice varieties show that the GL7 encodes a protein homologous to Arabidopsis thaliana Longifolia proteins which regulate longitudinal cell elongation. Further genetic study suggests that tandem duplication of a 17.1-kb segment at the GL7 locus leads to upregulation of GL7 and downregulation of its nearby negative regulator in gene expression levels, which results in an increase in grain length and the improvement of grain appearance quality due to decreased chalkiness and chalky kernel percentage. Thus, the complex genetic regulatory mechanism for grain length is revealed for American long-grain japonica rice.

In addition, the study on gene transfer indicates that the over-expression of GL7 in indica rice shows more visible changes compared with japanica rice, which shed new insight into significant improvement in appearance quality by insertion of GL7 locus into targeted rice chromosome with molecular marker technology.

Sequence analyses of 96 varieties confirm that allelic variants of GL7 and its negative regulator are associated with grain size diversity and show that the CNV at the GL7 locus was selected for and used in breeding. The research findings also explain the reason of high-quality of the indica rice growing in Guangdong Province such as Yuefeng rice variety lies in the pyramiding beneficial alleles of GL7 and GS3.　

The research is sponsored by the Ministry of Science and Technology and the National Natural Science Foundation of China. The co-first authors of the thesis are Yuexing Wang, Guosheng Xiong and Jiang Hu. 

Grain size and shape is a key indicator for appearance quality and an important trait related to rice yield. China grows both indica and japonica species of rice. The former is semi-late and late rice with long grain and cultivated in South China while the latter is short and roundish in grain and cultivated in North China and the Yangtze River basin for late rice.