Defect in Brnym1, a magnesium-dechelatase protein, causes a stay-green phenotype in an EMS-mutagenized Chinese cabbage (Brassica campestris L. ssp. pekinensis) line

Abstract

Leaf color is an important target trait in Chinese cabbage breeding programs. Leaf yellowing may reduce crop commercial and nutritional values. Some plants with the "stay-green" trait maintain leaf greenness during senescence and even after death. Stay-green Chinese cabbage may be a focal point of future breeding projects because it could improve crop quality and yield and prolong shelf life. A new stay-green mutant, <i>non-yellowing mutant 1</i> (<i>nym1</i>), was identified in Chinese cabbage derived from an ethyl methane sulfonate (EMS)-mutagenized population. The mutant had stay-green characteristics and a higher chlorophyll content than the wild-type during leaf senescence. The stay-green trait in the mutant Chinese cabbage was controlled by the recessive gene <i>Brnym1</i>. MutMap and KASP analyses showed that <i>Brnym1</i> (<i>BraA03g050600.3C</i>) encodes an mg-dechelatase (SGR protein), which might be the causal gene of the mutation in Chinese cabbage. A nonsynonymous single nucleotide base substitution (G to A) in the third exon of <i>Brnym1</i> caused an amino acid substitution from L to F in the highly conserved domain of the magnesium-dechelatase. Ectopic overexpression showed that the <i>BrNYM1</i> gene of wild-type Chinese cabbage complemented the SGR-defective stay-green mutant <i>nye1-1</i> of <i>Arabidopsis</i>. The magnesium-dechelatase activity in the <i>nym1</i> mutant was significantly downregulated compared to that in the wild type. <i>Brnym1</i> was relatively upregulated in the mutant during late senescence, and BrNYM1 was localized to the chloroplasts. These results indicate that <i>Brnym1</i> (<i>BraA03g050600.3C</i>) is the causal gene of the stay-green mutation and could be of particular significance in the genetic improvement of Chinese cabbage.