Abstract: Methods and compositions to overproduce L-leucine in plants, thereby increasing the nutritional value of plants use mutant forms of plant IPMS genes. Isopropylmalate synthase (IPMS) genes of the leucine biosynthetic pathway in a higher plant have been identified and isolated. Methods to engineer mutant forms of IPMS based on the wild-type sequence information are disclosed. Full length cDNAs of three loci in Arabidopsis namely IMS1, IMS2, and IMS3 were analyzed and their expression patterns characterized. Mutant forms of IPMS genes of the present invention, in particular IMS2, are selected after mutagenesis, and transformed into plants. The plants transformed with a desired mutant form overproduce L-leucine and thus have a better nutritional value.