Downy mildew caused by the oomycete pathogen Pseudoperonospora cubensis is a devastating foliar disease of cucurbits worldwide. We previously demonstrated that the wild melon line PI 124111F (PI) is highly resistant to all pathotypes of P. cubensis . That resistance was controlled genetically by two partially dominant, complementary loci. Here, we show that unlike other plant disease resistance genes, which confer an ability to resist infection by pathogens expressing corresponding avirulence genes, the resistance of PI to P. cubensis is controlled by enhanced expression of the enzymatic resistance ( eR ) genes At1 and At2 . These constitutively expressed genes encode the photorespiratory peroxisomal enzyme proteins glyoxylate aminotransferases. The low expression of At1 and At2 in susceptible melon lines is regulated mainly at the transcriptional level. This regulation is independent of infection with the pathogen. Transgenic melon plants overexpressing either of these eR genes displayed enhanced activity of glyoxylate aminotransferases and remarkable resistance against P. cubensis . The cloned eR genes provide a new resource for developing downy mildew-resistant melon varieties.

Genetic diversity among Fusarium moniliforme isolates was analysed using vegetative compatibility group (VCG) and random amplified polymorphic DNA (RAPD) techniques. In the first experiment, RAPD was used to analyse a set of 43 isolates collected from different corn growing areas in Israel and the US. The isolates were assigned to 27 different VCGs. Thirty-two RAPD haplotypes were also detected by analysing 48 polymorphic bands. RAPD could differentiate all the VCGs, except in two cases where two VCGs were assigned a single RAPD haplotype. In six cases, however, molecular variation was detected among isolates belonging to the same VCG. Cluster analysis of the RAPD data showed a very good agreement with the VCG grouping, e.g. isolates of the same VCG were always closely clustered by the molecular data. In a second experiment, 63 isolates of Fusarium moniliforme were collected from six corn lines growing in a single corn field. Extensive genetic variation was observed among the isolates: 42 different VCGs and 37 RAPD haplotypes were identified. Once again, RAPD patterns could differentiate nearly all the VCGs. However, in four cases, two different VCGs were grouped into a single RAPD haplotype, while in another three cases, isolates of the same VCG could be differentiated by distinct molecular haplotypes. The variation data was used to gain insight on the population structure and the patterns of genetic variation among geographical locations and within a single field. Hierarchical gene diversity analysis of the RAPD data indicated that most of the genetic variability (81%) was distributed within corn lines in the same field, suggesting that RAPD haplotype, or VCG frequencies, were not significantly affected by the plant genotypes grown in this experiment. Most of the RAPD band combinations did not display significant gametic phase disequilibrium, suggesting that active recombination might be occurring in the field. Our results indicate that by using a small number of primers, similar resolution was obtained by RAPD and VCG analysis, respectively. RAPD analysis is however, simpler to perform and its sensitivity in genotyping individuals within Fusarium moniliforme can be increased by analysing more primers, enabling a more detailed population genetic analysis of this important pathogen.

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BU-21/3 is an inbred melon (Cucumis melo L.) genotype that exhibits superior in vitro capabilities for adventitious organogenesis. In order to characterize the inheritance of the regeneration competence in BU-21/3, this line was crossed with PMR45 or `Ananas-Yokneam'—two reticulatus cultivars virtually incompetent for organogenesis. F1, F2, and backcross generations were produced and evaluated for adventitious regeneration competence. Our results indicate that the regeneration trait in BU-21/3 is controlled by a single dominant locus, without cytoplasmic interactions. This locus may become a valuable tool for imperative improvements in future protocols for transgenic breeding of commercial melon genotypes.

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