Background
As a class of natural antioxidants in plants, fruit flavonol metabolites are beneficial to human health. However, the regulatory networks for flavonol biosynthesis in most fruits are largely unknown. Previously, we reported a spontaneous pear bud sport ‘Red Zaosu’ (
Pyrus bretschneideri
Rehd.) with a high flavonoid content in its fruit. The identification of the flavonol biosynthetic regulatory network in this mutant pear fruit is crucial for elucidating the flavonol biosynthetic mechanism in fruit.
Results
Here, we demonstrated the
PbMYB12b
positively regulated flavonols biosynthesis in ‘Red Zaosu’ fruit. Initially, we investigated the accumulation patterns of four major quercetin glycosides and two major isorhamnetin glycosides in the fruit of ‘Red Zaosu’ and its wild-type ‘Zaosu’. A PRODUCTION OF FLAVONOL GLYCOSIDES (PFG)-type MYB transcription factor
PbMYB12b
was also screened for because of its correlation with flavonol accumulation in pear fruit. The biofunction of
PbMYB12b
was verified by transient overexpression and RNAi assays in pear fruit and young leaves. Overexpression of
PbMYB12b
enhanced the biosynthesis of quercetin glycosides and isorhamnetin glycosides by positively regulating a general flavonoids biosynthesis gene
PbCHSb
and a flavonol biosynthesis gene
PbFLS
. This finding was also supported by dual-luciferase transient expression assay and transient β-glucuronidase (GUS) reporter assay.
Conclusions
Our study indicated that
PbMYB12b
positively regulated flavonol biosynthesis, including four major quercetin glycosides and two major isorhamnetin glycosides, by promoting the expression of
PbCHSb
and
PbFLS
in pear fruit.
Electronic supplementary material
The online version of this article (10.1186/s12870-019-1687-0) contains supplementary material, which is available to authorized users.
Parthenocarpy, the production of seedless fruit without fertilization, has a variety of valuable qualities, especially for self-incompatible species, such as pear. To explore whether melatonin (MT) induces parthenocarpy, we used ‘Starkrimson’ pear as a material for morphological observations. According to our results, exogenous MT promoted the expansion and division of the mesocarp cells in a manner similar to hand pollination. However, the seeds of exogenous MT-treated fruit were undeveloped and aborted later in the fruit-setting stage. To further investigate how MT induced parthenocarpy, we studied changes of related hormones in the ovaries and found that MT significantly increased the contents of the gibberellins (GAs) GA3 and GA4. Thus, paclobutrazol (PAC), a GA-biosynthesis inhibitor, was used to study the relationship between GAs and MT. In addition, spraying MT after treatment with PAC did not increase GA content nor lead to parthenocarpy. Through a transcriptome analysis, we discovered that MT can cause significant upregulation of PbGA20ox and downregulation of PbGA2ox. However, no significant difference was observed in PbGA2ox compared with the control after PAC and MT applications. Thus, MT induces parthenocarpy by promoting GA biosynthesis along with cell division and mesocarp expansion in pear.
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