Abstract: Seeds of the Arabidopsis thaliana ecotype Landsberg erceta were chemically mutagenized with ethyl-methylsulfonate (EMS) and the M₂ progeny screened for seedlings with chloroplast-related mutations affecting leaf coloration.An independent mutant line of reticula controlled by a novel nuclear gene,designated E-210,was generated and identified.Genetics analysis indicates that the mutant was controlled by a single recessive gene E-210.Phenotypic analysis indicates that the reticula mutant grows up more slowly than the wild-type plant and typically has a green vasculature on yellow lamina.Transmission electron microscopic examination shows that the chloroplasts in the mutant are not obviously different from ones in the wild-type plant.We suppose that the gene for E-210 mutant is not directly related to chloroplast development,but probably related to chlorophyll or chloroplast biosynthesis.We have mapped E-210 to a region of 87.130 kb between the molecular markers MRB17 and MBG8-5 on chromosome 5 using map-based cloning technique.We propose At5g54770 encoding TH1,the thizaole biosynthesis enzyme,is responsible for the phyenotype of E-210 mutant by analysis of 87.130 kb.