Mutation to a cytochrome P450 -like gene alters the leaf color by affecting the heme and chlorophyll biosynthesis pathways in Brassica napus

Abstract

The regulated biosynthesis of chlorophyll is important because of its effects on plant photosynthesis and dry biomass production. In this study, a map-based cloning approach was used to isolate the cytochrome P₄₅₀ -like gene BnaC08g34840D (BnCDE1) from a chlorophyll-deficient mutant (cde1) of Brassica napus obtained by ethyl methanesulfonate (EMS) mutagenization. Sequence analyses revealed that BnaC08g34840D in the cde1 mutant (BnCDE1^I320T ) encodes a substitution at amino acid 320 (Ile320Thr) in the conserved region. The over-expression of BnCDE1^I320T in ZS11 (i.e., gene-mapping parent with green leaves) recapitulated a yellow-green leaf phenotype. The CRISPR/Cas9 genome-editing system was used to design two single-guide RNAs (sgRNAs) targeting BnCDE1^I320T in the cde1 mutant. The knockout of BnCDE1^I320T in the cde1 mutant via a gene-editing method restored normal leaf coloration (i.e., green leaves). These results indicate that the substitution in BnaC08g34840D alters the leaf color. Physiological analyses showed that the over-expression of BnCDE1^I320T leads to decreases in the number of chloroplasts per mesophyll cell and in the contents of the intermediates of the chlorophyll biosynthesis pathway in leaves, while it increases heme biosynthesis, thereby lowering the photosynthetic efficiency of the cde1 mutant. The Ile320Thr mutation in the highly conserved region of BnaC08g34840D inhibited chlorophyll biosynthesis and disrupted the balance between heme and chlorophyll biosynthesis. Our findings may further reveal how the proper balance between the chlorophyll and heme biosynthesis pathways is maintained.

Keywords: Brassica napus; chlorophyll precursor; cytochrome P450; tetrapyrrole metabolism; yellow-green leaf.