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Rebecca Grumet, Mary Barczak and Chris Tabaka

An above ground screening method to study cucumber root growth was developed using the herb icicle banding technique of Robertson et al. (Crop Sci 25:1084, 1985). Those roots that grow deeper or faster, sooner reach the herbicide, and sooner exhibit herbicide damage symptoms. Greenhouse pot trials showed that 1/4-1/2 lb/A atrazine could be used to produce distinctive symptoms, differentiate between depths of banding, and among different genotypes. Based on root washing experiments of a few cultivars, root length and/or mass correlated with herbicide symptom expression. One hundred diverse cucumber genotypes were tested in the greenhouse. Time to symptom expression was normally distributed among the genotypes; analysis of variance indicated significant genotypic differences. The herbicide banding technique was also useful for monitoring cucumber root growth in the field. Response time and severity varied with herbicide concentration, depth, and distance from the seed row. The diverse cucumber genotypes are now being tested in the field to further determine if there are significant genotypic differences and to compare greenhouse and field behavior.

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Wayne Loescher, Zhulong Chan and Rebecca Grumet

Soil salinization is an increasing problem worldwide and is often intensified by irrigation. Unfortunately, few new crop cultivars have been developed resistant to saline soils, a consequence, in part, of the complexity of plant responses to salt stress. There are now, however, several non-traditional options to improving salt tolerance as a result of recent progress in better understanding the mechanisms involved. These mechanisms include 1) exclusion of Na+ and Cl from plant tissues; 2) inclusion of these ions in inert compartments or tissues; and/or 3) some means of osmotic adjustment with solutes that are compatible with the metabolic machinery of the cell. Although there are very few horticultural examples, several lines of evidence indicate that reductions in salt sensitivity through exclusion or inclusion can be achieved by single gene modifications of the ion transport system. Similarly, single genes resulting in osmotic adjustment with solutes compatible with the metabolic machinery of the cell have resulted in significant increases in salt tolerance. Recent advances in sequencing, use of quantitative trait loci, and marker-assisted selection promise to provide other options for improving salt tolerance.

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Kaori Ando, Sue Hammar and Rebecca Grumet

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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Laura Havenga, Kaori Ando and Rebecca Grumet

Cucumber PI308916 has a compact growth habit (short internodes and main stem length) due to a single recessive gene cp. We also observed that this PI has lower incidence of Phytophthora fruit rot, likely due to its upright fruit bearing habit. Previous researchers reported that compact cucumber lines derived from PI308916 exhibited the potential for higher yields due to increased planting density. Despite these potentially beneficial traits, efforts to develop PI308916-derived lines were terminated due to poor seedling establishment. The objective of this research is to determine the relationship between the compact phenotype and poor seedling establishment. Short internodes can be caused by deficiency in gibberellins or brassinosteroids that can also impact germination or apical hook formation, a trait important for seedling emergence from the soil. Germination rate and apical hook angle was recorded for `Wautoma' (control inbred line that forms normal apical hook), PI308916, and their F1, F2, and BC progeny. Germinating `Wautoma' seeds showed consistent, large apical hook angles (mean 147), while hook angles of PI308916 were broadly distributed from 0 to 180 (mean 96). F1 progeny for the reciprocal crosses had a similar angle (mean 134 and 133) to `Wautoma'. Segregation ratios for apical hook angle in the F2 and BC populations were consistent with a single recessive gene. Evaluation of the relationship between the apical hook and compact phenotype, showed an absence of the recombinant class of long internodes and small apical hook angle in the F2 population, suggesting that the two traits may be conferred by a single gene or two tightly linked genes.

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Rebecca Grumet*, Xiaofeng Wang, Mohamed Tawfik and Mitch McGrath

Genomics tools have become increasingly varied and valuable for crop improvement. While several species have been targeted for concerted genomic efforts, the majority of horticultural species have received limited attention. Despite the wide variety of important cucurbit crop species, the Cucurbitaceae family has had minimal effort. We have initiated projects to develop genomic tools for cucumber, Cucumis sativus L. Efforts include production of cDNA, yeast two-hybrid, and genomic libraries, and development of an EST database and website for cucumber genomics. Sequences of cucumber leaf ESTs so far indicate that the cDNA library is of high quality and has modest redundancy. Distribution of sequences, as nominally predicted from GeneBank BLAST analysis, indicates that expressed genes fall in the following general categories: photosynthesis (21%), DNA/RNA/protein synthesis (20%), metabolism (15%), signaling (5%), other (16%), and unknown proteins (23%). Cucumber sequence data have been deposited into GenBank and are available on the Michigan State Univ. website (http://genomics.msu.edu/cucumberdb). The yeast two-hybrid library has been successfully used to identify and characterize several genes based on interaction with key proteins of interest, including genes interacting with viral replicases and poly(A) binding protein. The genomic library has been verified to be of high quality and has been used to identify clones of interest.

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Rebecca Grumet, Mary Barczak, Chris Tabaka and Robert Duvall

A simple, aboveground method to study cucumber (Cucumis sativus L.) root growth was developed using a subsurface herbicide banding technique. Those plants with roots that grow deeper or faster reach the herbicide sooner and exhibit herbicide injury symptoms sooner. Greenhouse pot trials showed that 0.25 or 0.50 kg simazine/ha could be used to produce distinctive symptoms; time to symptom expression increased with the depth of the band from the soil surface. Root washing experiments verified that root length was associated with response time. In field trials, response time and severity of symptoms varied with herbicide concentration, depth, and distance from the seed row, thereby providing an indication of where the roots were in the soil. About 100 diverse cucumber genotypes were tested for differences in root growth rate in the greenhouse and in the field. Time to symptom expression was normally distributed among the genotypes; analysis of variance (ANOVA) indicated significant genotypic differences. This system can be used for cultural or physiological studies, or nondestructively for selection and breeding purposes. If the herbicide is placed sufficiently deep to prevent damage to the cotyledons, the plants are capable of flowering and producing fruit. Chemical name used: 6-chloro-N, N′-diethyl-1,3,5-triazine-2,4-diamine (simazine).

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Mohamed Tawfik, Alejandra Ferenczi, Daniel Enter and Rebecca Grumet

Abiotic stresses (e.g., salinity, drought, cold, oxidative stress) can be major factors limiting plant productivity worldwide. We sought to increase abiotic stress resistance in cucumber by expressing the A. thaliana transcription factors CBF1and CBF3, which regulate genes responsible for enhanced dehydration-stress resistance in Arabidopsis. Our previous studies in the greenhouse and field demonstrated increased salinity tolerance in CBF-expressing cucumber lines. In the current studies, we tested response of CBF-cucumber plants to drought, chilling, and oxidative stresses. Transgenic cucumber plants subjected to drought stress in the greenhouse showed elevated levels of the stress-inducible compatible solute, proline, compared to the nontransgenic controls. Preliminary results also indicate greater photochemical efficiency in CBF-expressing plants under drought stress conditions compared to the nontransgenic controls. Under nonstressed conditions, there were no significant differences in growth between the transgenic and the nontransgenic cucumber plants; however, after a cycle of drought stress, CBF-cucumber lines had less growth reduction compared to the nontransgenic counterparts. The advantage in growth was less pronounced after a second cycle of drought. We also evaluated the transgenic cucumber plants under chilling conditions (i.e., low, nonfreezing temperatures within the 0 to 12 °C range). Based on plant height and cotyledon and leaf damage measurements, transgenic cucumber seedlings did not show chilling tolerance compared to the wild-type control. The response of transgenic CBF-cucumber plants to oxidative stress using methyl viologen is also being evaluated.

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Holly A. Little, Rebecca Grumet and James F. Hancock

The next wave of genetically engineered crops will use genes that modify gene regulation, plant metabolism, or signal transduction. The potential for these genes to have cascading effects on metabolism, physiology, and development increases the possibility for unintended effects that influence crop performance or environmental impact. This review examines altered ethylene signaling as an example of a complex trait with many horticultural applications. Genes for modified ethylene production or perception intended to regulate ripening, senescence, or stress or disease resistance have been observed to cause a broad range of secondary effects, including modified growth and development and increased severity to biotic and abiotic stresses. Successful use of complex traits in crop varieties will frequently require methods to reduce secondary effects, including the use of targeted gene expression. Risk assessment will need to consider observed pleiotropic effects on fitness within the context of potential environmental impacts.

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Stan C. Hokanson, James F. Hancock and Rebecca Grumet

A further characterization of the cucumber (Cucumis sativus L.) mutant `Wisconsin SMR-18' bla (blunt leaf apex) revealed a new character associated with the previously described leaf phenotype. The attachment of the blade to the petiole of bla plants is flat across, as opposed to the cordate or indented attachment in the wild-type `Wisconsin SMR-18' plants. The new character (truncate leaf base) was easier to score and becomes distinctive earlier in development than previously described leaf apex characters. It was expressed consistently in homozygous bla plants. Segregation analysis of 1159 F2 seedlings arising from self-pollinated `Wisconsin SMR-18' × `Wisconsin SMR-18' bla F1 plants suggested that the leaf base and leaf apex character were controlled by a single locus or two tightly linked ones with a maximum distance between the two of 0.03 cM. In a field study of growth and fitness characteristics, the two genotypes did not differ significantly for flower or fruit count. The similar flowering and fruiting characteristics, along with the reliable early occurring truncate character, likely will make the two genotypes useful for pollination and gene movement studies.

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Adam D. Call, Adam D. Criswell, Todd C. Wehner, Kaori Ando and Rebecca Grumet

Downy mildew, a foliar disease caused by the oomycete Pseudoperonospora cubensis (Berk. and Curt.) Rostow, is one of the most destructive pathogens of cucurbits. From 1961 to 2003, resistant cucumber cultivars in the United States had sufficient resistance to grow a successful crop without the use of fungicides. The pathogen resurged as a major problem in 2004. Since then, the dm-1 gene has not been effective against the new strain of downy mildew, and yield losses are high without the use of fungicides. The objective of this experiment was to identify cultivars having high yield and resistance to the new downy mildew. The experiment had 86 cultivars and breeding lines (hereafter collectively referred to as cultigens) and was conducted in Clinton, NC, in 2007 and 2009, in Castle Hayne, NC, in 2008 and 2009, and in Bath, MI, in 2007 to 2009. Plots were rated weekly on a 0 to 9 scale (0 = none, 1–2 = trace, 3–4 = slight, 5–6 = moderate, 7–8 = severe, and 9 = dead). Mean ratings for downy mildew leaf damage ranged from 2.9 to 5.7 in Michigan in 2008 and 2009 and from 3.8 to 6.8 in North Carolina in 2007 to 2009. None of the cultigens tested in this study showed a high level of resistance, although differences in resistance were detected. Lines WI 2757 and M 21 and cultivar Picklet were consistently among the top resistant lines in North Carolina and Michigan. The cultivars Coolgreen, Wis. SMR 18, and Straight 8 were identified as moderately to highly susceptible. An unreleased hybrid, ‘Nun 5053 F1’, and the cultivar Cates were the top yielding lines overall. The highest yield in a single year and location was from the cultivar Cates in Clinton, NC, in 2009, with 25.6 Mg·ha−1. The best cultivars in this study were only moderately resistant and would likely require fungicide applications to achieve high yield and quality in the presence of downy mildew. Until high resistance becomes available, growers would benefit by using fungicides in combination with tolerant and moderately resistant cultigens.