Fruit initiation in Arabidopsis (Arabidopsis thaliana) is generally repressed until fertilization occurs. However, mutations in AUXIN RESPONSE FACTOR8 (ARF8) uncouple fruit initiation from fertilization, resulting in the formation of seedless, parthenocarpic fruit. Here we induced parthenocarpy in wild-type Arabidopsis by introducing either the mutant genomic (g) Atarf8-4 sequence or gAtARF8:b-glucuronidase translational fusion constructs by plant transformation. Silencing of endogenous AtARF8 transcription was not observed, indicating that the introduced, aberrant ARF8 transcripts were compromising the function of endogenous ARF8 and/or associated factors involved in suppressing fruit initiation. To analyze the role of ARF8 in tomato (Solanum lycopersicum) we initially emasculated 23 tomato cultivars to test for background parthenocarpy. Surprisingly, all had a predisposition to initiate fertilization-independent fruit growth. Expression of gAtarf8-4 in transgenic tomato ('Monalbo') resulted in a significant increase in the number and size of parthenocarpic fruit. Isolation of tomato ARF8 cDNA indicated significant sequence conservation with AtARF8. SlARF8 may therefore control tomato fruit initiation in a similar manner as AtARF8 does in Arabidopsis. Two SlARF8 cDNAs differing in size by 5 bp were found, both arising from the same gene. The smaller cDNA is a splice variant and is also present in Arabidopsis. We propose that low endogenous levels of the splice variant products might interfere with efficient formation/function of a complex repressing fruit initiation, thereby providing an explanation for the observed ovary expansion in tomato and also Arabidopsis after emasculation. Increasing the levels of aberrant Atarf8-4 transcripts may further destabilize formation/function of the complex in a dosage-dependent manner enhancing tomato parthenocarpic fruit initiation frequency and size and mimicking the parthenocarpic dehiscent silique phenotype found in homozygous Atarf8-4 mutants. Collectively these data suggest that similar mechanisms involving auxin signaling exist to inhibit parthenocarpic fruit set in tomato and Arabidopsis.

Anthocyanins are flavonoid compounds responsible for red/purple colors in the leaves, fruit, and flowers of many plant species. They are produced through a multistep pathway that is controlled by MYB transcription factors. VvMYBA1 and VvMYBA2 activate anthocyanin biosynthesis in grapevine (Vitis vinifera) and are nonfunctional in white grapevine cultivars. In this study, transgenic grapevines with altered VvMYBA gene expression were developed, and transcript analysis was carried out on berries using a microarray technique. The results showed that VvMYBA is a positive regulator of the later stages of anthocyanin synthesis, modification, and transport in cv Shiraz. show that Vv3AT has a broad anthocyanin substrate specificity and can also utilize both aliphatic and aromatic acyl donors, a novel activity for this enzyme family found in nature. In cv Pinot Noir, a red-berried grapevine mutant lacking acylated anthocyanins, Vv3AT contains a nonsense mutation encoding a truncated protein that lacks two motifs required for BAHD protein activity. Promoter activation assays confirm that Vv3AT transcription is activated by VvMYBA1, which adds to the current understanding of the regulation of the BAHD gene family. The flexibility of Vv3AT to use both classes of acyl donors will be useful in the engineering of anthocyanins in planta or in vitro.

Background and Aims Grape flavonoids impart colour and mouthfeel to wines. This work aimed to understand how genetic manipulation of the hydroxylation of flavonoids affected grape flavonoid composition and wine properties. Methods and Results We produced transgenic grapevines with decreased expression of flavonoid 3′‐hydroxylase (F3′H) and flavonoid 3′5′‐hydroxylase (F3′5′H) to investigate how this altered the composition of flavonoids in grapes and wine. Grapes from transgenic grapevines with decreased expression of F3′5′H genes had a concentration of anthocyanin and skin tannin similar to that of Shiraz Controls but had increased seed tannin and a greatly decreased proportion of trihydroxylated anthocyanin and tannin. Lines with decreased expression of F3′H had a decreased concentration of seed tannin and an increased proportion of trihydroxylated flavonoids. When expression of both F3′H and F3′5′H was decreased, concentration of anthocyanin and tannin in the grapes was decreased to 20–30% of the Shiraz Controls. Wines made from grapes with decreased expression of F3′5′H had lower wine colour, and a lower concentration of anthocyanin, tannin and phenolic substances. Conclusions In grapes, the flavonoid pathway does not effectively use monohydroxylated intermediates so F3′H and F3′5′H are required to produce dihydroxylated and trihydroxylated intermediates. The hydroxylation status of flavonoids in grapes determines grape and wine colour and tannin composition. Significance of the Study The composition as well as the total amount of anthocyanin and tannin in grapes is important for wine colour and quality. The expression of genes encoding F3′H and F3′5′H in grapes is dependent on viticultural factors including temperature, light exposure and vine water stress, providing the potential to manipulate flavonoid composition of existing cultivars in the vineyard.

Background and Aims: The flavonoid composition of grapes and wine was determined in transgenic grapevines with altered expression of the tannin synthesis genes anthocyanidin reductase (ANR) and leucoanthocyanidin reductase (LAR). Methods and Results: Shiraz grapevines were transformed with constructs aimed at decreasing expression of genes encoding VvANR and VvLAR1. Many of the ANRsi (silenced) transgenic lines had an altered appearance and a low concentration of leaf tannin but produced no fruit. Those that did produce fruit had a higher concentration of flavonol and a lower concentration of tannin in the skin but no change in seed or overall berry tannin. The ANRsi wines had a higher concentration of anthocyanin, flavonol, tannin and phenolic substances and higher colour. The LARsi lines had a lower concentration of tannin in the leaves and seeds but not in berry skins and produced wines with a lower concentration of anthocyanin, tannin and phenolic substances and lower colour. Conclusions: Manipulation of tannin genes altered flavonoid composition in grapevines and suggests that ANR is more important for tannin synthesis in leaves and berry skins whereas LAR1 is involved more in tannin production in seeds. Significance of the Study: There is potential to alter wine colour and tannin properties by manipulating the flavonoid pathway in grapevines.