Ethylene-activated PpERF105 induces the expression of the repressor-type R2R3-MYB gene PpMYB140 to inhibit anthocyanin biosynthesis in red pear fruit.

Ethylene induces anthocyanin biosynthesis in most fruits, including apple (Malus domestica), strawberry (Fragaria × ananassa), and plum (Prunus spp.). However, ethylene inhibits anthocyanin biosynthesis in pear (Pyrus spp.), but the underlying molecular mechanism has not been characterized. In this study, ethylene induced the expression of PpERF105, which encodes a transcription factor. PpERF105 functioned as a transcriptional activator, but it inhibited anthocyanin biosynthesis in pear. A transcriptome analysis revealed that PpERF105 activated the expression of PpMYB140, which encodes an R2R3-MYB transcriptional repressor. Moreover, PpMYB140 directly inhibited the expression of anthocyanin-related structural genes. It also competed with PpMYB114 for the binding to bHLH3, ultimately resulting in the formation of the MYB140-bHLH-WDR complex rather than the conventional MBW complex, thereby further inhibiting anthocyanin biosynthesis. Furthermore, PpMYB140 prevented the overaccumulation of anthocyanins in the absence of ethylene. Collectively, our study data indicate that ethylene-induced PpERF105 inhibits anthocyanin biosynthesis by upregulating PpMYB140 expression. Our findings may be useful for elucidating the molecular basis of the ethylene-mediated inhibition of anthocyanin biosynthesis in fruit.

Transcriptome analysis provides new insights into the transcriptional regulation of methyl jasmonate-induced flavonoid biosynthesis in pear calli.

BACKGROUND: Flavonoid biosynthesis is strongly influenced by phytohormones. For example, methyl jasmonate (MeJA) enhances the flavonoid accumulation in pear. However, the molecular mechanism underlying the MeJA-induced flavonoid biosynthesis in pear is largely uncharacterized. Therefore, the transcriptome of pear calli treated with MeJA was analyzed to elucidate the mechanism regulating MeJA-mediated flavonoid biosynthesis. RESULTS: The application of exogenous MeJA significantly enhanced flavonoid accumulation, especially anthocyanin, in pear calli. A weighted gene co-expression network analysis identified the differentially expressed genes associated with MeJA-induced flavonoid biosynthesis. The MeJA treatment upregulated the expression of the flavonoid biosynthesis pathway structural genes (PcCHS, PcCHI, PcF3H, PcDFR, PcANS, PcANR2a, and PcLAR1). The MYB family members were the main transcription factors regulating the MeJA-induced flavonoid biosynthesis, but the bHLH, AP2-EREBP, NAC, WRKY, and TIFY families were also involved. In addition to PcMYB10, which is a known positive regulator of anthocyanin biosynthesis in pear, several novel MYB candidates that may regulate flavonol and proanthocyanidin biosynthesis were revealed. Yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated that PcMYB10 and PcMYC2 can directly interact with each other and bind to JAZ repressors (PcJAZ1 and PcJAZ2). CONCLUSIONS: The PcMYB10-PcMYC2 molecular complex is likely involved in the regulation of jasmonate-mediated flavonoid biosynthesis at the transcript level. The data generated in this study may clarify the transcriptional regulatory network associated with the MeJA-induced flavonoid accumulation in pear calli and provide a solid foundation for future studies.

R2R3-MYB transcription factor PpMYB17 positively regulates flavonoid biosynthesis in pear fruit.

MAIN CONCLUSION: PpMYB17 positively regulates flavonoid biosynthesis in pear fruit by activating PpCHS, PpCHI, PpF3H, and PpFLS in the flavonoid biosynthesis pathway independently of bHLH or WD40 cofactors in the MBW complex. Flavonoids are important secondary metabolites in plants. The flavonoid biosynthesis pathway is regulated by various transcription factors, with MYB transcription factors considered to be the key regulators. However, the regulation of flavonoid biosynthesis in the pear fruit has not been fully characterized. The R2R3-MYB transcription factor PpMYB17 was isolated from 'Red Zaosu' pear fruit and functionally characterized. An exposure to light upregulated PpMYB17 expression in the pear fruit. A phylogenetic analysis indicated PpMYB17 is related to the flavonol regulators. A subcellular localization assay suggested that PpMYB17 is a nuclear protein. Overexpression of PpMYB17 increased the flavonoid content of pear calli and Arabidopsis via the upregulated expression of structural genes in the flavonoid biosynthesis pathway, especially FLS. The LC-MS/MS analysis revealed most of the differentially accumulated flavonols, flavanones, flavones, isoflavones, and anthocyanins were significantly more abundant in PpMYB17-overexpressing calli than in wild-type calli. Moreover, PpMYB17 did not interact with PpbHLH3, PpbHLH33, or PpWD40 in a yeast system. Dual-luciferase assays demonstrated that PpMYB17 strongly activates the promoters of PpCHS, PpCHI, PpF3H, PpFLS, and PpUFGT which are key downstream genes in the flavonoid biosynthesis pathway, independently of the PpbHLH3 cofactor. These gene expression changes may enhance flavonoid biosynthesis in pear fruit. The data presented may be useful for further elucidating the flavonoid biosynthesis regulatory network, potentially leading to the development of new pear cultivars that produce fruits with increased flavonoid contents.