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  • Author or Editor: Sue Hammar x
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We sought to develop efficient regeneratio nand transformation procedures for cucumber. Factors tested for regeneration included: hormone types and levels, genotype, explant source, and environmental conditions. Optimum regeneration was obtained using cotyledon pieces from 4 day old GY14A seedlings and culturing for 3 weeks under cool white lights (30-40 uE-2 s -1) on MS medium supplemented with 1.0 mg/l 2,4-D, 0.5 mg/l BA, 0.3 mg/l ABA, 30 g/l sucrose, 1 g/l MES, and 3.07 g/l Scott gelrite. Shoots developed via somatic embryogenesis ca. 2 wk after explants were transferred to MS supplemented with 20 g/l sucrose, 1 g/l MES, and 4.37 g/l gelrite. Ca. 80% of the explants produce shoots, 1/3-1/2 of which produce rooted plantlets; total time from explant to rooted plantlet is ca. 8 wks. Transformation experiments utilized Agrobacterium tumefaciens strains LBA4404 bearing the CIBA-GEIGY pCIB10 vector with a selectable marker gene for kanamycin resistance. Optimal conditions include 45 mg/l kan, 10 min inoculation and 3 day co-cultivation. Preliminary evidence suggests that tobacco nurse cultures increase transformation efficiency. Transgenic plants were confirmed by Southern or dot blot analysis.

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We sought to develop efficient regeneratio nand transformation procedures for cucumber. Factors tested for regeneration included: hormone types and levels, genotype, explant source, and environmental conditions. Optimum regeneration was obtained using cotyledon pieces from 4 day old GY14A seedlings and culturing for 3 weeks under cool white lights (30-40 uE-2 s -1) on MS medium supplemented with 1.0 mg/l 2,4-D, 0.5 mg/l BA, 0.3 mg/l ABA, 30 g/l sucrose, 1 g/l MES, and 3.07 g/l Scott gelrite. Shoots developed via somatic embryogenesis ca. 2 wk after explants were transferred to MS supplemented with 20 g/l sucrose, 1 g/l MES, and 4.37 g/l gelrite. Ca. 80% of the explants produce shoots, 1/3-1/2 of which produce rooted plantlets; total time from explant to rooted plantlet is ca. 8 wks. Transformation experiments utilized Agrobacterium tumefaciens strains LBA4404 bearing the CIBA-GEIGY pCIB10 vector with a selectable marker gene for kanamycin resistance. Optimal conditions include 45 mg/l kan, 10 min inoculation and 3 day co-cultivation. Preliminary evidence suggests that tobacco nurse cultures increase transformation efficiency. Transgenic plants were confirmed by Southern or dot blot analysis.

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Phytophthora capsici causes severe losses in vegetable production, including many cucurbit crops. Our previous work showed that cucumber (Cucumis sativus) fruit are most susceptible to P. capsici when they are very young and rapidly elongating, but develop resistance as they approach full length at 10 to 12 days postpollination (DPP). In this study, fruit from seven additional cucurbit crops representing four species, melon (Cucumis melo), butternut squash (Cucurbita moschata), watermelon (Citrullus lanatus), and zucchini, yellow summer squash, acorn squash, and pumpkin (Cucurbita pepo), were tested for the effect of fruit development on susceptibility to P. capsici. Field-grown fruit of the different crops varied in overall susceptibility. Zucchini and yellow summer squash were the most susceptible, with the majority of fruit exhibiting water-soaking symptoms within 24 hours postinoculation. Fruit from all of the crops exhibited size-related decrease in susceptibility, but to varying degrees. Cucumber had the most pronounced effect. In infested fields, cucumber fruit were found to be most frequently infected at the blossom end. Comparison of the peduncle and blossom end showed a difference in susceptibility along the length of the fruit for cucumber, butternut squash, and zucchini. Greenhouse-grown, hand-pollinated pumpkin, acorn squash, and butternut squash showed an age-related decrease in susceptibility similar to field-grown fruit. For all of these fruit, a pronounced reduction in susceptibility accompanied the transition from the waxy green to green stage at ≈3 to 8 DPP.

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Fruit rot caused by Phytophthora capsici is a major constraint in cucumber (Cucumis sativus) production. In an effort to identify a source of resistance, we developed a more streamlined detached fruit method for high-throughput screening and tested the U.S. cucumber PI collection for fruit rot resistance. A total of 1076 PI collections, from 54 geographic locations around the world, along with the susceptible commercial cultivar, Vlaspik, were grown in the field and tested for resistance to P. capsici. Using the knowledge gained from our prior studies regarding greater susceptibility of young fruits compared with older fruits, very young fruits (≈3 to 4 days post-pollination) were collected and inoculated with zoospore suspensions of P. capsici isolate OP97. From the screens performed in 2011 and 2012, 99% of the tested PIs were rated as moderately or highly susceptible based on symptom development and pathogen growth at 5 days post-inoculation. The cv. Vlaspik control showed consistent high susceptibility to P. capsici with a mean symptom rating of 8.0 on a 9-point scale. A set of 28 PIs was chosen for further testing in the greenhouse or field in 2013. The disease ratings of PIs rescreened in 2013 were much lower compared with that of the full collection of PIs. Three accessions, PI109483, PI178884, and PI214049, showed consistent low mean disease ratings and may be considered as possible sources of resistance to young cucumber fruit infection by P. capsici. Evaluation of the S1 progeny of PI109483 suggests that the resistance is heritable and should allow for development of useful breeding materials that can be used for developing P. capsici-resistant cucumber cultivars.

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