Role of a chalcone isomerase-like protein in flavonoid biosynthesis in Arabidopsis thaliana

Flavonoids are important natural products for plant defence and human health. Although almost all the flavonoid pathway genes have been well-documented by biochemical and/or genetic approaches, the role of the Arabidopsis chalcone isomerase-like (CHIL) gene remains unclear. Two chil mutants with a seed colour similar to that of wild-type Arabidopsis have been identified here, but in sharp contrast to the characteristic transparent testa seed phenotype associated with other known flavonoid pathway genes. CHIL loss-of-function mutations led to a strong reduction in the proanthocyanidin and flavonol levels in seeds, but not in the anthocyanin levels in leaves. CHIL over-expression could partially recover the mutant phenotype of the chil mutant and increased both proanthocyanidin and flavonol accumulation in wild-type Arabidopsis. However, the CHIL gene could not rescue the mutant phenotype of TT5 that encodes the intrinsic chalcone isomerase in Arabidopsis. Parallel phenotypical and metabolic analyses of the chil, tt5, chs, and f3h mutants revealed that, genetically, CHIL functions at the same step as TT5. Moreover, it is demonstrated that CHIL co-expresses, co-localizes, and interacts with TT5 in Arabidopsis for flavonoid production. Based on these genetic and metabolic studies, it is concluded that CHIL functions with TT5 to promote flavonoid production, which is a unique enhancer in the flavonoid pathway.