Transcriptional regulation of flavonoid biosynthesis in nectarine (Prunus persica) by a set of R2R3 MYB transcription factors.

BACKGROUND: Flavonoids such as anthocyanins, flavonols and proanthocyanidins, play a central role in fruit colour, flavour and health attributes. In peach and nectarine (Prunus persica) these compounds vary during fruit growth and ripening. Flavonoids are produced by a well studied pathway which is transcriptionally regulated by members of the MYB and bHLH transcription factor families. We have isolated nectarine flavonoid regulating genes and examined their expression patterns, which suggests a critical role in the regulation of flavonoid biosynthesis. RESULTS: In nectarine, expression of the genes encoding enzymes of the flavonoid pathway correlated with the concentration of proanthocyanidins, which strongly increases at mid-development. In contrast, the only gene which showed a similar pattern to anthocyanin concentration was UDP-glucose-flavonoid-3-O-glucosyltransferase (UFGT), which was high at the beginning and end of fruit growth, remaining low during the other developmental stages. Expression of flavonol synthase (FLS1) correlated with flavonol levels, both temporally and in a tissue specific manner. The pattern of UFGT gene expression may be explained by the involvement of different transcription factors, which up-regulate flavonoid biosynthesis (MYB10, MYB123, and bHLH3), or repress (MYB111 and MYB16) the transcription of the biosynthetic genes. The expression of a potential proanthocyanidin-regulating transcription factor, MYBPA1, corresponded with proanthocyanidin levels. Functional assays of these transcription factors were used to test the specificity for flavonoid regulation. CONCLUSIONS: MYB10 positively regulates the promoters of UFGT and dihydroflavonol 4-reductase (DFR) but not leucoanthocyanidin reductase (LAR). In contrast, MYBPA1 trans-activates the promoters of DFR and LAR, but not UFGT. This suggests exclusive roles of anthocyanin regulation by MYB10 and proanthocyanidin regulation by MYBPA1. Further, these transcription factors appeared to be responsive to both developmental and environmental stimuli.

ABA may promote or delay peach fruit ripening through modulation of ripening- and hormone-related gene expression depending on the developmental stage.

Peach (Prunus persica laevis L. Batsch) was chosen as a model to further clarify the physiological role of ABA during fruit ripening. To this aim, branches bearing one fruit at mid-S3, S3/S4 and S4 stages of fruit development and characterized by a different ripening index (I(AD)), as revealed by a non-destructive device called a DA-meter, were treated with ABA (0.02 mM) for 1 and 5 days. Exogenously applied ABA interfered with the progression of ripening leading to less ripe or riper fruit depending on the physiological stage. To better understand the molecular basis of ABA interference with ripening, the time-course changes in the expression of ethylene-, cell wall-, and auxin-related genes as well as other genes (NCED, PIP, LOX, AOS and SOT) was evaluated in the fruit mesocarp. Real-time PCR analyses revealed that in mid-S3 fruit transcript levels of ethylene biosynthesis and signaling (ACS1, ACO1, ETR2, ERF2), cell wall softening-related (PG, PMEI, EXP1, EXP2) and auxin biosynthesis, conjugation, transport and perception (TRPB, IGPS, Aux/IAA, GH3, PIN1 and TIR1) genes were substantially down-regulated on day 5 indicating a ripening delay. On the contrary, in more advanced stages (S3/S4 and S4) the same genes were early (day 1) up-regulated suggesting an acceleration of ripening. Transcript profiling of other ripening-related genes revealed changes that were in accord with a ripening delay (mid-S3) or acceleration (S3/S4 and S4). Thus, in peach fruit, ABA appears to modulate ripening through interference not only with ethylene and cell wall but also with auxin-related genes.