Identification of Genes Responsible for Leaf Narrowing and Dwarfism in Rice.

Cultivated rice varieties show variation in terms of plant height and leaf size. These variations often show continuous phenotypic distribution and hence quantitative in nature. Because plant morphology has a direct implication to its economic value, weedy characteristics such as leaf narrowing and dwarfism were investigated. The aim of the study is to identify the genes responsible for weedy characteristics and to characterize the physiological changes associated to it.  Out of Cypress and Red Rice crosses, we got two near isogenic mutant lines that showed extreme leaf narrowing and dwarfism. The mutant line also showed increased leaf chlorophyll content and increased tiller number but with more than 50% yield reduction. Morphological cross section of the stem showed that the mutant line had reduced vascular bundle size and reduced number of sieve tube elements. Exogenous application gibberellic acid restored the plant height and the root length of the mutant lines into Cypress but the narrow leaves remained. Inheritance study revealed that the gene controlling the leaf narrowing is a single recessive gene.  Coarse mapping in 191 F2 populations for leaf narrowing shows that the gene is mapped to a 1cM distance between RM17486 and RM5473 in chromosome 4.  Dwarfism, increased chlorophyll content and flag leaf length phenotypes were mapped between markers RM5511 and RM3092 of chromosome 4.  Interestingly, among the list of genes between RM17486 and RM5473, an auxin-responsive SAUR21 gene was found and had increased gene expression in mutant line but not in Cypress. Taken together, our results suggest SAUR21 as a candidate gene for leaf narrowing.