CONFERENCE 10 MARCH 2006
LATEST TECHNICS TO IMPROVE THE MONITORING OF GRAPEBERRY
RIPENING
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I – Modern approaches of ripening physiology |
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Transcriptomics is the study
of the transcriptome, the complete set of messenger RNA transcripts that
can be translated into proteins and enzymes. It is a very powerful
approach towards understanding gene function and can lead to important
strategies for plant improvement. A good example of how this approach has been used for gene discovery is a global transcript analysis of ripening strawberries (Aharoni, Keizer et al. 2000). Strawberry cDNA microarrays were used for several stages of ripening strawberries and a number of genes induced by ripening were identified. One cDNA clone had sequence similarity to other known genes encoding enzymes with acyltransferase activity. The strawberry alcohol acyltransferase (SAAT) was highly expressed in semi-ripe to very ripe strawberries only. Functional expression of the SAAT cDNA in E. coli confirmed that SAAT produced volatile esters from alcohols as measured by gas chromatography-mass spectrometry (GC-MS). These same volatile esters also contributed to important flavor components in ripe strawberries. Working in close collaboration with the International Grape Genome Program, (www.vitaceae.org) and Affymetrix, we have taken a leading role in designing and developing a first-generation oligonucleotide microarray for grapes called the GeneChip® Vitis vinifera Genome Array (http://www.affymetrix.com/products/arrays/specific/vitis.affx) that was made publicly available in June 2004. The current design contains probe sets for 14,650 Vitis vinifera genes and an additional 1,944 probe sets for various non-vinifera species and hybrids. The Vitis array is the first array to provide wide coverage of the V. vinifera genome and was developed as part of the GeneChip® Consortia Design Program, a collaborative initiative that has allowed Affymetrix and international plant and animal research communities together to create novel whole-genome arrays. The availability of the GeneChip® Vitis vinifera Genome Array permits detailed gene expression profiling studies to be conducted on a wide range of research topics related to grape biology, viticulture and enology. Ultimately, the above GeneChip® has the potential to become a powerful diagnostic tool that will afford growers the ability to assess the results of viticulture practices on wine composition and flavor. This avenue of research will enable improvements to be made in both production efficiency and wine quality under both favorable and environmentally adverse growing conditions. We are using the GeneChip® Vitis vinifera Genome Array to survey tissue-specific mRNA expression patterns within grape berries and other tissues, such as leaves and roots, and to examine changes in gene expression in response to water-deficit stress treatment. Our results indicate that there are pronounced differences in transcript patterns among berry tissues of Cabernet Sauvignon. Skin tissue, which is involved in pathogen defense and pigment production, showed an increase in the relative transcript abundance of genes concerned with flavonoid biosynthesis, pathogen resistance, and cell wall modification. Pulp, which is considered a nutritive tissue, exhibited increased transcript abundance of genes involved with cell wall function and transport processes. Seeds, which supply essential resources for embryo development, showed increased transcript abundance of phenylpropanoid biosynthetic genes, seed storage protein genes and late embryogenesis abundant protein genes. We also observe pronounced gene expression differences between berries harvested from well-watered and water-deficit-treated grapevines including increased transcript expression patterns of genes involved in ion and water transport, phenylpropanoid biosynthesis, and stress responses. These results reveal novel insights into the molecular mechanisms governing tissue differentiation within berries and show that water deficit stress profoundly affects berry gene products that impact wine quality. In another study of berry development for two different cultivars, Cabernet Sauvignon and Chardonnay, we found pronounced differences in gene expression patterns not only for each of the developmental stages examined, but also depending on water status changes and the cultivar examined. Between 27 to 34% of the 14,650 gene probes represented on the microarray show differential expression across berry development in both cultivars. Our results indicated that transcript abundance for the phenylpropanoid pathway (affecting berry phenolics and color) was significantly different between the two cultivars and affected by water stress. Detailed analysis of the transcript expression patterns of transcription factor gene families indicated a divergence in transcriptional responses to water stress and between cultivars throughout berry development. In addition, we have linked the effects of water-deficit stress to changes in the expression of several genes and aromas in Chardonnay berries and wines. These studies provide new insights into the complex interactions between fruit development and environmental stress and the relationship between water deficit stress and wine quality. One long-term goal of our research is to develop diagnostic tools that will allow producers to more efficiently monitor the effects of viticultural practices on gene expression networks that influence aroma, color, flavor and health-promoting components of wine. Aharoni, A., L. C. Keizer, et al. (2000). "Identification of the SAAT gene involved in strawberry flavor biogenesis by use of DNA microarrays." Plant Cell 12(5): 647-62. |
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