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Manjul Dutt, Dennis J. Gray, Zhijian T. Li, Sadanand Dhekney and Marilyn M. Van Aman

A major drawback to the use of embryogenic cultures for transformation of grapevine is that their ability to undergo genetic transformation is cultivar-dependent. Also, depending on cultivar, embryogenic cultures are difficult to impossible to maintain over time, reducing their utility for use in genetic transformation. An alternative to the use of embryogenic cultures for transformation of grapevine is the use of micropropagation cultures, which are easier to initiate from a wide range of grapevine cultivars and can be maintained over time without loss of function. Vitis vinifera `Thompson Seedless' was used as a model for genetic transformation using micropropagation cultures. In vitro cultures were initiated from apical meristems of actively growing vines and maintained in C2D medium containing 4 μM of 6-benzylaminopurine (C2D4B). Shoot tips and nodes were collected from proliferating in vitro cultures for transformation studies. A variety of wounding techniques, including nicking, sonication, and fragmenting of meristematic tissues was employed in order to enable Agrobacterium infection. We used a construct containing a bidirectional 35S promoter complex with a marker gene composed of a bifunctional fusion between an enhanced green fluorescent protein (EGFP) gene and a neomycin phosphotransferase (NPTII) gene in one direction and a hybrid lytic peptide gene in the other. Transgenic shoots growing in C2D4B medium containing 200 mg·L-1 each of carbenicillin and cefotaxime and 20 mg·L-1 of kanamycin were selected based on GFP fluorescence. Transgenic shoots were rooted and transferred to a greenhouse. To date, 18 transgenic lines have been generated. Details on the transformation procedure will be discussed.

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D. J. Gray, Z. T. Li, D. L. Hopkins, M. Dutt, S. A. Dhekney, M. M. Van Aman, J. Tattersall and K. T. Kelley

Pierce's disease (PD), caused by the xylem-limited bacterium Xylella fastidiosa, is endemic to the coastal plain of the southeastern United States. Although native southern grapevines are tolerant to X. fastidiosa, all varieties of Vitisvinifera grown in the region will succumb to PD. Genetic transformation to add disease resistance genes, while not disturbing desirable phenotypic characters, holds promise for expanding the southeastern U.S. grape industry by allowing use of established fruit and wine varieties. We utilize embryogenic cell cultures and Agrobacterium strain EHA105 to refine transformation systems for Vitis species and hybrids. V. vinifera`Thompson Seedless' is employed as a model variety to test various transgenes for disease resistance, since as many as 150 independent transgenic plant lines routinely are produced from 1 g of embryogenic culture material. Transgenic plants are stringently screened for PD resistance in greenhouses by mechanical inoculation with X. fastidiosa. Transgenic plants are compared with both susceptible and resistant control plants by assessing typical PD symptom development and by assaying bacterial populations in xylem sap over time. Using these procedures, nine putative PD resistance genes have been inserted into grapevine and over 900 unique transgenic lines have been evaluated. A range of susceptible-to-resistant responses has been catalogued. Thus far, the best construct for PD resistance contains a grape codon-optimized hybrid lytic peptide gene in a high-performance bi-directional 35S promoter complex. Certain transgenic plant lines containing this construct exhibit better resistance than that of resistant control vines.