Abstract
Flavonols and proanthocyanidins (PAs) are the main pigments in the black spines of cucumber (Cucumis sativus) fruit, and CsMYB60 is a key regulator of the biosynthesis of flavonols and PAs. However, in cucumber, the tissue distribution pattern of flavonols and PAs and the mechanism of their biosynthesis regulated by CsMYB60 remain unclear. In this study, we clarified the tissue-specific distribution of flavonoids and the unique transcriptional regulation of flavonoid biosynthesis in cucumber. CsMYB60 activated CsFLS and CsLAR by binding to their promoters and directly or indirectly promoted the expression of CsbHLH42, CsMYC1, CsWD40, and CsTATA-box binding protein, resulting in the formation of complexes of these four proteins to increase the expression of Cs4CL and interact with CsTATA-box binding protein to regulate the expression of CsCHS, thereby regulating the biosynthesis of flavonols and PAs in cucumber. Our data provide new insights into the molecular mechanism of flavonoid biosynthesis, which will facilitate molecular breeding to improve fruit quality in cucumber.
Highlights
Flavonols and proanthocyanidins (PAs), which are polyphenolic compounds synthesized through the flavonoid biosynthetic pathway, are widely distributed in plants and have a variety of biological functions, including UV light protection, pigmentation, and anti-herbivore and phytopathogen resistance activities[1,2,3,4,5,6], in addition to presenting many benefits for human health[7,8,9]
The results indicated that large amounts of PAs accumulated in black spines, and there were detectable amounts in the pericarp, pulp, seed coat, and male flowers of the black-spined cucumber line, while there were detectable amounts of PAs only in the pericarp, pulp, and seed coat of the whitespined line (Fig. 1a, b)
Among the 12 putative interacting proteins, we focused on the four transcription factors CsbHLH42 (CsaV3_6G037080.1), CsMYC1 (CsaV3_6G000530.1), CsWD40 (CsaV3_1G031140.1), Fig. 2 Phenotypic analysis of 35S:CsMYB60 transgenic cucumber lines. a Cucumber ovaries at 0 DBA. b Cucumber fruits at 4 days after anthesis (DAA). c The spine color of cucumber fruits at 4 DAA from the Xintaimici and OX-1 lines observed under a stereoscopic microscope. d Cross-section of cucumber fruits at 4 DAA from the WT and OX-1 lines observed under a stereoscopic microscope. e The trichome color of male flower receptacles at 0 days before anthesis (DBA) from the WT and OX-1 lines observed under a stereoscopic microscope. f Relative expression levels of CsMYB60 in the WT, RNS9, and 35S:CsMYB60 transgenic lines
Summary
Flavonols and proanthocyanidins (PAs), which are polyphenolic compounds synthesized through the flavonoid biosynthetic pathway, are widely distributed in plants and have a variety of biological functions, including UV light protection, pigmentation, and anti-herbivore and phytopathogen resistance activities[1,2,3,4,5,6], in addition to presenting many benefits for human health[7,8,9]. Flavonols play pivotal roles in modulating reactive oxygen species induced by abscisic acid to control the stomatal aperture[10] and protect plants against UV light[11,12]. PAs play roles in the coloration of plant. The accumulation of flavonols and PAs is restricted to specific cell types and organs.
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