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  • Author or Editor: Rebecca Grumet x
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One of the first major successes in the genetic engineering of useful traits into plants has been the engineering of virus resistance. The first example of genetically-engineered virus resistance was published in 1986, since then there have been more than 50 reports of genetically engineered plant virus resistance. These examples span a range of virus types, a variety of plant species, and have utilized several different types of genes. A unique feature of the genetically-engineered virus resistance is that the resistance genes came from the virus itself, rather than the host plant. Most examples have utilized coat protein genes, but more recently, replicase-derived genes have proved highly effective. Other strategies include the use of antisense or sense-defective sequences, and satellite or defective interfering RNAs. This talk will provide an overview of the different approaches, possible mechanisms, the crops and viruses to which they have been applied, and progress toward commercial applications.

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The Cucurbitaceae family is noted for a diversity of sex expression phenotypes. Typically, a phase of male flowers precedes either female or bisexual flower production. Sex determination of individual flowers is regulated by a combination of genetic, environmental, and hormonal factors. Ethylene, auxins, and gibberellins have all been shown to influence flower sex expression in cucurbits. Ethylene, which promotes femaleness, plays a predominant role. In this study, we tested whether brassinosteroids (BR), a more recently identified class of plant hormones, also influences cucurbit sex expression. Applied epi-brassinolide (epi-BL) caused a significant decrease in time of appearance of the first female flower on monoecious cucumber plants, and increased total female flowers on the main stem. Increasing concentrations had a stronger effect. Of the three species tested, cucumber, melon and zucchini, cucumber was the most responsive to BR. Application of epi-BL also caused an increase in ethylene production by cucumber and zucchini seedlings, suggesting that the BR effect may be mediated by ethylene. To investigate the possible relationship between BR and ethylene on sex expression, we identified the concentration of ethephon (5 ppm) that caused an increase in ethylene production comparable to that induced by 10 μm epi-BL (approximately two-fold). Treatment with 5 ppm ethephon was sufficient to increase femaleness of cucumber plants, but not zucchini plants, suggesting that the difference in response to epi-BL treatment may reflect differences in sensitivity to ethylene. Collectively, our results indicate that application of brassinosteroids to cucumber cause earlier and increased female flower production, and that the effects may be mediated, at least in part, by brassinosteroid-induced production of ethylene.

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Phytophthoracapsici, a soil-borne oomycete pathogen causing fruit rot in cucumber, has become a limiting factor for cucumber production in the Midwest. In the process of screening plant introductions (PIs) for resistance to P. capsici, it appeared that degree of susceptibility might decrease as fruits develop. To examine this more carefully, detached, greenhouse-grown, hand-pollinated `Vlaspik' fruits aged 2–18 days post-pollination (dpp) were inoculated with P. capsici mycelium and evaluated for symptoms. There was a reproducible decrease in susceptibility with increasing fruit age. The fruits that sporulated were usually younger and smaller (2–9 dpp), 10-3 dpp fruit tended to develop water-soaked symptoms, while the fruits that remained symptom-free were usually older (>14 dpp) and oversized for pickling cucumber. The transition from susceptible to more resistant appeared to occur at the end of the period of rapid fruit elongation. Detached field-grown `Straight Eight' fruits showed similar size-related trends. Candidate resistant genotypes identified from the PI screening were re-screened using 7 and 14 dpp fruits. Again an age-dependent difference in response was observed, indicating that the increase in resistance is not genotype-specific. Furthermore, field observations suggest a gradation of susceptibility within the fruits as the blossom end was most frequently infected. Preliminary tests of detached greenhouse-grown, hand-pollinated fruits suggested that as the fruits grew older, the blossom end remained susceptible longer than the stem end. These findings could have implications for appropriate screening methods, the stage of fruit likely to become infected in the field, and appropriate spray practices.

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The determinate allele (de) of cucumber (Cucumis sativus L.) causes shorter vine length and fewer and shorter internodes and lateral branches than the indeterminate allele (De). Four sets of cucumber inbreds carrying determinate or indeterminate alleles were compared with respect to root growth rate in the field and greenhouse using an herbicide banding method. Although the lines exhibited the expected differences in shoot growth, differences in root growth were not correlated with the differences in shoot growth. These results indicate that root growth was independent of the determinate shoot growth allele.

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Potyviruses cause severe loss in cucurbit crops. Inbred lines derived from the Chinese cucumber cultivar, Taichung Mau Gua (TMG), have been identified to be resistant to several potyviruses including zucchini yellow mosaic virus (ZYMV), zucchini yellow fleck virus, watermelon mosaic virus, and the watermelon strain of papaya ringspot virus. Recently, an additional virus that infects cucurbits, the Moroccan watermelon mosaic virus (MWMV), has been identified to be a distinct member of the potyvirus group. In this study, we sought to determine if TMG-1 is resistant to MWMV and, if so, examine whether a relationship exists between resistance to MWMV and resistance to ZYMV. Progeny analyses show that TMG-1 is resistant to MWMV and, like resistance to ZYMV, MWMV resistance is conferred by a single recessive gene. Sequential inoculation of progeny possessing resistance to ZYMV followed by MWMV (or MWMV followed by ZYMV) suggests that both resistances are conferred by the same gene, or two tightly linked genes. Additionally, all F3 families derived from F2 individuals selected for resistance to ZYMV, were resistant to MWMV. A second source of resistance to ZYMV, allelic to the TMG-1 source, has been incorporated into the Dutch hybrid Dina. Progeny analyses show Dina to posses a single recessive gene for MWMV resistance. As with TMG-1, no segregation of resistances was observed when ZYMV resistant progeny were inoculated with MWMV (or MWMV followed by ZYMV). Collectively, these results suggest that a single gene, or two tightly linked genes, control resistance to the potyviruses ZYMV and MWMV.

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