Application of Forward and Reverse Genetics to Identify Rice Mutants with Altered Starch Traits

Rice (Oryza sativa L.) is unique among major cereal crops as the vast majority is used directly for human consumption as whole milled kernels. In this form, rice starch provides the bulk of the daily calorie intake for billions of people worldwide. Major differences in local, eco-regional, or social-cultural preferences require the development of a wide array of grain quality characteristics to meet the needs of diverse consumers. Forward and reverse genetic approaches were applied to identify mutants of interest from populations derived from chemical mutagenesis of the varieties Nipponbare and Kitaake. Forward screens based on visual evaluation of grains for altered translucence resulted in the identification of about 30 mutants which are currently being evaluated for changes in starch properties. Targeting of Induced Local Lesions in Genomes (TILLING) and targeted sequencing by exon capture have been employed to identify mutations in starch synthesis genes. A total of 63 putative mutations in eight target genes were identified by TILLING. Of these mutations, 29 in six genes were confirmed by Sanger sequencing and lines carrying homozygous mutations were isolated. Most of these mutations are novel based on comparison to the natural variation present in the O. sativa germplasm of the 3,000 rice genomes project. Scanning electron microscopy and physicochemical tests are being performed to evaluate the effect of the mutations on grain quality. Genetic studies are underway to identify the mutations underlying the mutants identified in forward screens and to examine the effects of combining mutant alleles of starch synthesis genes.