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Key message
RsMYB1a
was the crucial MYB, and
RsbHLH4
is the essential partner in regulating the anthocyanin biosynthesis in radish.
There are four color types of radish according to whether or not the anthocyanin accumulates in the skin and flesh of taproot. Red radishes accumulate a substantial amount of anthocyanins in both the skin and flesh. It is well known that the MYB-bHLH-WD40 transcription factor(s) complex regulates the biosynthesis of anthocyanin in plants. Here in, four candidate
MYB
and
bHLH
genes,
RsMYB1a
,
RsMYB1b
,
RsbHLH2
and
RsbHLH4
, were isolated from red radish ‘Hongxin 1’. The expression of
RsbHLH4
and the two structural genes
RsANS
and
RsUFGT
was significantly positively correlated with anthocyanin contents. The expression of
RsMYB1a
was also highly correlated with anthocyanin accumulation, particularly when the white flesh sample of ‘Hongxin 1-1’ was excluded. The transient expression of
RsMYB1a
in the radish cotyledon and leaf induced anthocyanin accumulation with even stronger promoting role when expression in combination with
RsbHLH4
. These results suggested that
RsMYB1a
was the crucial MYB, and that
RsbHLH4
is an essential partner in regulating the biosynthesis of anthocyanins in radish. The low or undetectable
RsbHLH4
expression paralleled the lack of anthocyanin accumulation in the white flesh of ‘Hongxin 1-1’ and ‘Shaguan 1’. Assays demonstrated that RsMYB1a interacted with RsbHLH4 and activated the expression of
RsbHLH4
. Notably, all the dark red radish cultivars have a longer
RsMYB1a
genomic DNA sequence, while the short and nonfunctional
RsMYB1a
is present in non-red cultivars. The length of the first intron and the presence of an early stop codon of
RsMYB1
might underlie the differential anthocyanin accumulation in the radish taproot.