Premature and ectopic anthocyanin formation by silencing of anthocyanidin reductase in strawberry (Fragaria × ananassa)

Abstract

Summary Strawberry (Fragaria × ananassa) is a fruit crop with a distinct biphasic flavonoid biosynthesis. Whereas, in the immature receptacle, high levels of proanthocyanidins accumulate, which are associated with herbivore deterrence and pathogen defense, the prominent color‐giving anthocyanins are primarily produced in ripe ‘fruits’ helping to attract herbivores for seed dispersal. Here, constitutive experimental down‐regulation of one branch of proanthocyanidin biosynthesis was performed. As a result, the proportion of epicatechin monomeric units within the proanthocyanidin polymer chains was reduced, but this was not the case for the epicatechin starter unit. Shortened chain lengths of proanthocyanidins were also observed. All enzymatic activities for the production of color‐giving anthocyanins were already present in unripe fruits at levels allowing a striking red anthocyanin phenotype in unripe fruits of the RNAi silencing lines. An immediately recognizable phenotype was also observed for the stigmata of flowers, which is another epicatechin‐forming tissue. Thus, the down‐regulation of anthocyanidin reductase (ANR) induced a redirection of the proanthocyanidin pathway, leading to premature and ectopic anthocyanin biosynthesis via enzymatic glycosylation as the alternative pathway. This redirection is also seen in flavonol biosynthesis, which is paralleled by higher pollen viability in silencing lines. ANRi transgenic lines of strawberry provide a versatile tool for the study of the biological functions of proanthocyanidins.

Comparative transcriptomic analysis of key genes involved in flavonoid biosynthetic pathway and identification of a flavonol synthase from Artemisia annua L.

“…In strawberry, redirection of the proanthocyanidin pathway to anthocyanin was observed through down‐regulation of ANR leading to accumulation of anthocyanins, while down‐regulation of UPGT induced redirection of the anthocyanin pathway to the proanthocyanidin pathway (Fischer et al . ). The present transcriptomic analysis indicated that both ANR and LAR involved in the proanthocyanidin branch of the flavonoids did not differ significantly in expression between green and red calli, which suggests that proanthocyanidins might have no obvious effect on the biosynthesis of anthocyanins.…”

TERMINAL FLOWER1 is a breeding target for a novel everbearing trait and tailored flowering responses in cultivated strawberry (Fragaria × ananassa Duch.)

“…Plant transformation was performed using the Agrobacterium tumefaciens strain EHA105 complemented with the binary plasmid vector pK7GWIWG2D(II)_TFL1‐RNAi (Koskela et al ., ) as described by Fischer et al . (). Regenerated meristems were excised 9–12 weeks postinoculation.…”

The control of red colour by a family of MYB transcription factors in octoploid strawberry (Fragaria × ananassa) fruits

“…The key structural genes for flavonoid biosynthesis encode essential biosynthetic enzymes. Expression of early biosynthetic genes (EBG) (Figure ), including phenylalanine ammonia‐lyase ( PAL ), cinnamate 4‐hydroxylase ( C4H ), 4‐coumarate:coenzyme A ligase ( 4CL ), chalcone synthase ( CHS ), chalcone isomerase ( CHI ), and late biosynthetic genes (LBG) (Figure ), including flavonol synthase ( FLS ) for the formation of flavonols, dihydroflavonol‐4‐reductase/flavanone‐4‐reductase ( DFR ), anthocyanidin synthase ( ANS ) and flavonol‐O‐glucosyltransferases ( UFGT ), is required for the formation of anthocyanins; and expression of leucoanthocyanidin reductase ( LAR ) and anthocyanidin reductase ( ANR ) is required for the formation of proanthocyanin precursors and proanthocyanins (Figure ; Fischer et al , ; Griesser et al , ).…”