Abstract
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.
Highlights
Flavonoids are a large group of natural products that are widely present in the leaves, flowers, fruits, and seeds of various plants
Based on these genetic and metabolic studies, it is concluded that chalcone isomerase-like (CHIL) functions with TT5 to promote flavonoid production, which is a unique enhancer in the flavonoid pathway
In order to screen for genes involved in PA biosynthesis, a tt2 mutant line (CS83) with a pale yellow seed colour phenotype was used for global gene expression analysis using the Arabidopsis Affymetrix microarray
Summary
Flavonoids are a large group of natural products that are widely present in the leaves, flowers, fruits, and seeds of various plants Based on their basic chemical structures, flavonoids can be further divided into different subgroups, including the three major subgroups of flavonols, anthocyanins, and proanthocyanidins (PAs) (Winkel-Shirley, 2001; Lepiniec et al, 2006). Naringenin can be hydroxylated by flavanone 3-hydroxylase (F3H) to form dihydrokaempferol, and dihydrokaempferol can be further hydroxylated by flavanone 3′-hydroxylase (F3′H) to form dihydroquercetin These intermediates are further converted to flavonols, anthocyanins, and PAs through distinct branches of the flavonoid pathway (Fig. 1). Mutations in the genes encoding the enzymes of the core flavonoid pathway usually lead to transparent testa seed phenotypes in Arabidopsis due to the lack of oxidized flavonoid compounds (mainly PAs)
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