Co-mapping studies of QTLs for fruit acidity and candidate genes of organic acid metabolism and proton transport in sweet melon (Cucumis melo L.).

Sweet melon cultivars contain a low level of organic acids and, therefore, the quality and flavor of sweet melon fruit is determined almost exclusively by fruit sugar content. However, genetic variability for fruit acid levels in the Cucumis melo species exists and sour fruit accessions are characterized by acidic fruit pH of <5, compared to the sweet cultivars that are generally characterized by mature fruit pH values of >6. In this paper, we report results from a mapping population based on recombinant inbred lines (RILs) derived from the cross between the non-sour 'Dulce' variety and the sour PI 414323 accession. Results show that a single major QTL for pH co-localizes with major QTLs for the two predominant organic acids in melon fruit, citric and malic, together with an additional metabolite which we identified as uridine. While the acidic recombinants were characterized by higher citric and malic acid levels, the non-acidic recombinants had a higher uridine content than did the acidic recombinants. Additional minor QTLs for pH, citric acid and malic acid were also identified and for these the increased acidity was unexpectedly contributed by the non-sour parent. To test for co-localization of these QTLs with genes encoding organic acid metabolism and transport, we mapped the genes encoding structural enzymes and proteins involved in organic acid metabolism, transport and vacuolar H+ pumps. None of these genes co-localized with the major pH QTL, indicating that the gene determining melon fruit pH is not one of the candidate genes encoding this primary metabolic pathway. Linked markers were tested in two additional inter-varietal populations and shown to be linked to the pH trait. The presence of the same QTL in such diverse segregating populations suggests that the trait is determined throughout the species by variability in the same gene and is indicative of a major role of the evolution of this gene in determining the important domestication trait of fruit acidity within the species.

Acetyl-coa: alcohol acetyltransferase activity and aroma formation in ripening melon fruits.

Melon varieties (Cucumis melo L.) differ in a range of physical and chemical attributes. Sweetness and aroma are two of the most important factors in fruit quality and consumer preference. Volatile acetates are major components of the headspace of ripening cv. Arava fruits, a commercially important climacteric melon. In contrast, volatile aldehydes and alcohols are most abundant in cv. Rochet fruits, a nonclimacteric melon. The formation of volatile acetates is catalyzed by alcohol acetyltransferases (AAT), which utilize acetyl-CoA to acetylate several alcohols. Cell-free extract derived from Arava ripe melons exhibited substantial levels of AAT activity with a variety of alcohol substrates, whereas similar extracts derived from Rochet ripe melons had negligible activity. The levels of AAT activity in unripe Arava melons were also low but steadily increased during ripening. In contrast, similar extracts from Rochet fruits displayed low AAT activity during all stages of maturation. In addition, the benzyl- and 2-phenylethyl-dependent AAT activity levels seem well correlated with the total soluble solid content in Arava fruits.

Integrated Management of Sudden Wilt in Melons, Caused by Monosporascus cannonballus, Using Grafting and Reduced Rates of Methyl Bromide.

Sudden wilt of melons induced by Monosporascus cannonballus is a worldwide problem, mainly in arid and semi-arid regions. Soil disinfestation by fumigation with methyl bromide is a common and very effective treatment for disease control. However, methyl bromide is expected to be banned from use within the next 5 to 10 years; therefore, alternative measures for disease management are urgently needed. In this study, the efficacy of methyl bromide at a reduced rate (15 g/m2 instead of 50 g/m2) and grafted plants, alone or in combination, was examined. Experiments were conducted in the 'Arava region in southern Israel in naturally infested fields. First, various rootstock accessions were tested in three experiments. Of the eight rootstocks evaluated, cv. Brava (Cucurbita maxima) was selected to serve as a rootstock in the subsequent experiments due to its effects on wilt reduction (63 to 100% reduction in two experiments and 35% reduction in one experiment) and its horticultural performance. Next, integration of grafting and a reduced rate of methyl bromide was tested in two experiments. The reduced methyl bromide rate suppressed disease, but results were variable (32 and 91% reduction in the two experiments). Grafting, when used alone, significantly reduced wilt incidence by 84 to 87% compared to untreated plots. Integration of the two control measures provided adequate and reliable results and wilt incidence was always significantly lower (75 to 100% reduction) than incidence in untreated plots. Because control by each measure alone varies, integrated application is preferable.