Abstract
Red leaf color is an attractive trait of Malus families, including crabapple (Malus spp.); however, little is known about the molecular mechanisms that regulate the coloration. Dihydroflavonols are intermediates in the production of both colored anthocyanins and colorless flavonols, and this current study focused on the gene expression balance involved in the relative accumulation of these compounds in crabapple leaves. Levels of anthocyanins and the transcript abundances of the anthocyanin biosynthetic gene, dihydroflavonol 4-reductase (McDFR) and the flavonol biosynthetic gene, flavonol synthase (McFLS), were assessed during the leaf development in two crabapple cultivars, ‘Royalty’ and ‘Flame’. The concentrations of anthocyanins and flavonols correlated with leaf color and we propose that the expression of McDFR and McFLS influences their accumulation. Further studies showed that overexpression of McDFR, or silencing of McFLS, increased anthocyanin production, resulting in red-leaf and red fruit peel phenotypes. Conversely, elevated flavonol production and green phenotypes in crabapple leaves and apple peel were observed when McFLS was overexpressed or McDFR was silenced. These results suggest that the relative activities of McDFR and McFLS are important determinants of the red color of crabapple leaves, via the regulation of the metabolic fate of substrates that these enzymes have in common.
Highlights
Flavonoids are a class of plant secondary metabolites that collectively have diverse functions, including providing protection against abiotic stresses, UV-irradiation, and biotic factors, such as phytophagous insects, as well as attracting pollinators[1,2]
We evaluated the abundance of anthocyanins and flavonols in the leaves of these cultivars at 5 development stages of the crabapple leaf growing season by high-performance liquid chromatography (HPLC) (Fig. 2)
The chromatography results showed that cyanidin 3-O-glucoside was the predominant anthocyanin, and we found that that the major flavonols were quercetin derived compounds, such as quercetin 3-O-diglucoside, quercetin 3-O-glucoside and quercetin 3-O-glycosidase isomer (Fig. 2B)
Summary
Flavonoids are a class of plant secondary metabolites that collectively have diverse functions, including providing protection against abiotic stresses, UV-irradiation, and biotic factors, such as phytophagous insects, as well as attracting pollinators[1,2]. Chalcone synthase (CHS; EC 2.3.1.74) uses beta-coumaroyl-CoA and 3 Malonyl-CoA as substrates to form naringenin chalcone[15] This condensation reaction is a key step in the pathway leading to the formation of flavonoids[16]. It has been reported that the expression level of DFR positively correlates with the abundance of anthocyanins in peanut (Arachis hypogaea)[29] Flavonols provide another important co-pigment in the colorful organs of terrestrial plants, such as the yellow petals of Lathyrus chrysanthus[30], and they influence pollen tube growth[31]. An indirect effect of a camellia FLS gene (CnFLS1) on anthocyanin accumulation during floral coloration was suggested following an experiment where its overexpression in transgenic tobacco (N. benthamiana) plants resulted in an increase in flavonol content, but a reduction in anthocyanin levels in petals[37]
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