Identification and characterization of MYB-bHLH-WD40 regulatory complexes controlling proanthocyanidin biosynthesis in strawberry (Fragaria × ananassa) fruits.

Strawberry (Fragaria × ananassa) fruits contain high concentrations of flavonoids. In unripe strawberries, the flavonoids are mainly represented by proanthocyanidins (PAs), while in ripe fruits the red-coloured anthocyanins also accumulate. Most of the structural genes leading to PA biosynthesis in strawberry have been characterized, but no information is available on their transcriptional regulation. In Arabidopsis thaliana the expression of the PA biosynthetic genes is specifically induced by a ternary protein complex, composed of AtTT2 (AtMYB123), AtTT8 (AtbHLH042) and AtTTG1 (WD40-repeat protein). A strategy combining yeast-two-hybrid screening and agglomerative hierarchical clustering of transcriptomic and metabolomic data was undertaken to identify strawberry PA regulators. Among the candidate genes isolated, four were similar to AtTT2, AtTT8 and AtTTG1 (FaMYB9/FaMYB11, FabHLH3 and FaTTG1, respectively) and two encode putative negative regulators (FaMYB5 and FabHLH3∆). Interestingly, FaMYB9/FaMYB11, FabHLH3 and FaTTG1 were found to complement the tt2-1, tt8-3 and ttg1-1 transparent testa mutants, respectively. In addition, they interacted in yeast and activated the Arabidopsis BANYULS (anthocyanidin reductase) gene promoter when coexpressed in Physcomitrella patens protoplasts. Taken together, these results demonstrated that FaMYB9/FaMYB11, FabHLH3 and FaTTG1 are the respective functional homologues of AtTT2, AtTT8 and AtTTG1, providing new tools for modifying PA content and strawberry fruit quality.

RNA interference silencing of chalcone synthase, the first step in the flavonoid biosynthesis pathway, leads to parthenocarpic tomato fruits.

Parthenocarpy, the formation of seedless fruits in the absence of functional fertilization, is a desirable trait for several important crop plants, including tomato (Solanum lycopersicum). Seedless fruits can be of great value for consumers, the processing industry, and breeding companies. In this article, we propose a novel strategy to obtain parthenocarpic tomatoes by down-regulation of the flavonoid biosynthesis pathway using RNA interference (RNAi)-mediated suppression of chalcone synthase (CHS), the first gene in the flavonoid pathway. In CHS RNAi plants, total flavonoid levels, transcript levels of both Chs1 and Chs2, as well as CHS enzyme activity were reduced by up to a few percent of the corresponding wild-type values. Surprisingly, all strong Chs-silenced tomato lines developed parthenocarpic fruits. Although a relation between flavonoids and parthenocarpic fruit development has never been described, it is well known that flavonoids are essential for pollen development and pollen tube growth and, hence, play an essential role in plant reproduction. The observed parthenocarpic fruit development appeared to be pollination dependent, and Chs RNAi fruits displayed impaired pollen tube growth. Our results lead to novel insight in the mechanisms underlying parthenocarpic fruit development. The potential of this technology for applications in plant breeding and biotechnology will be discussed.

The light-hyperresponsive high pigment-2dg mutation of tomato: alterations in the fruit metabolome.

Overall metabolic modifications between fruit of light-hyperresponsive high-pigment (hp) tomato (Lycopersicon esculentum) mutant plants and isogenic nonmutant (wt) control plants were compared. Targeted metabolite analyses, as well as large-scale nontargeted mass spectrometry (MS)-based metabolite profiling, were used to phenotype the differences in fruit metabolite composition. Targeted high-performance liquid chromatography with photodiode array detection (HPLC-PDA) metabolite analyses showed higher levels of isoprenoids and phenolic compounds in hp-2dg fruit. Nontargeted GC-MS profiling of red fruits produced 25 volatile compounds that showed a 1.5-fold difference between the genotypes. Analyses of red fruits using HPLC coupled to high-resolution quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) in both ESI-positive and ESI-negative mode generated, respectively, 6168 and 5401 mass signals, of which 142 and 303 showed a twofold difference between the genotypes. hp-2dg fruits are characterized by overproduction of many metabolites, several of which are known for their antioxidant or photoprotective activities. These metabolites may now be more closely implicated as resources recruited by plants to respond to and manage light stress. The similarity in metabolic alterations in fruits of hp-1 and hp-2 mutant plants helps us to understand how hp mutations affect cellular processes.