CsMYB60 is a key regulator of flavonols and proanthocyanidans that determine the colour of fruit spines in cucumber.

Mengyu Liu,Xiaoquan Qi,Jialin Li,Lixin Duan,Zhonghai Ren,Cunjia Zhang,Aigang Yang,Qianqian Luan

doi:10.1093/jxb/ery336

Abstract

Spine colour is an important fruit quality trait that influences the commercial value of cucumber (Cucumis sativus). However, little is known about the metabolites and the regulatory mechanisms of their biosynthesis in black spine varieties. In this study, we determined that the pigments of black spines are flavonoids, including flavonols and proanthocyanidins (PAs). We identified CsMYB60 as the best candidate for the previously identified B (Black spine) locus. Expression levels of CsMYB60 and the key genes involved in flavonoid biosynthesis were higher in black-spine inbred lines than that in white-spine lines at different developmental stages. The insertion of a Mutator-like element (CsMULE) in the second intron of CsMYB60 decreased its expression in a white-spine line. Transient overexpression assays indicated that CsMYB60 is a key regulatory gene and Cs4CL is a key structural gene in the pigmentation of black spines. In addition, the DNA methylation level in the CsMYB60 promoter was much lower in the black-spine line compared with white-spine line. The CsMULE insert may decrease the expression level of CsMYB60, causing hindered synthesis of flavonols and PAs in cucumber fruit spines.

Highlights

Cucumber (Cucumis sativus) is an economically important global vegetable crop (Xu et al, 2015b)
We revealed the key roles of CsMYB60, the best candidate for the B gene, and Cs4CL in the colouration of black spines, and we found that the decrease in expression of CsMYB60 in white-spine plants is caused by an insertion of a Mutator-like element, which might function in an epigenetic manner
Phylogenetic analysis of CsMudrA homologues from several species revealed that CsMudrA shares the highest similarity with CuMudrA (Supplementary Fig. S11).These results indicated that this insertion in CsMYB60 from whitespined RNS8 is a Mutator-like element (MULE), which we named CsMULE (GenBank: MG558001)

Summary

Introduction

Cucumber (Cucumis sativus) is an economically important global vegetable crop (Xu et al, 2015b). Li et al (2013) mapped the B gene to a 50-kb region of the short arm of cucumber chromosome 4 (chr.4) based on linkage analysis using 2001 F2 plants from a cross between two inbred lines, WI7200 (black-spine, orange mature fruit) and WI7201 (white-spine, cream-coloured mature fruit). In this 50-kb region, a gene encoding a R2R3-MYB transcription factor, later named CsMYB60 (Zhao et al, 2014), was considered as the best candidate for B, because of the presence of a 1-bp deletion in the third intron and a lower expression level of this R2R3-MYB gene in both immature and mature fruit in WI7201 compared with WI7200 (Li et al, 2013). The composition of pigments and the mechanisms regulating their biosynthesis in black-spine plants need to be clarified

Methods

Results

Conclusion