The identification of the gene Pp523, conferring downy mildew resistance to adult plants of broccoli (Brassica oleracea convar. italica), led to the construction of a genetic map that included this resistance locus, 301 amplified fragment length polymorphisms, 55 random amplified polymorphic DNAs, 46 inter-simple sequence repeats, three simple sequence repeats, four other PCR markers and a flower colour locus, all gathered into nine major linkage groups. Nineteen additional molecular markers were clustered into one group of four markers, one group of three markers and six pairs of markers. The map spans over 731.9 cM, corresponding to 89.5% of the 818 cM estimated to be the total genome length. A significant number of the mapped markers, 19.3%, showed distorted segregation. The average distance between mapped adjacent markers is 1.64 cM, which places this map among the densest published to date for this species. Using bulked segregant analysis, we identified a group of molecular markers flanking and closely linked in coupling to the resistance gene and included these in the map. Two markers linked in coupling, OPK17_980 and AT.CTA_133/134, are located at 3.1 cM and 3.6 cM, respectively, at each side from the resistance gene. These markers can be used for marker-assisted selection in breeding programs aiming at the introgression of this gene in susceptible B. oleracea genotypes. The fine mapping of the genomic region surrounding the Pp523 resistance gene is currently being carried out, a basic condition for its isolation via positional cloning.

We recently mapped the Pp523 locus that includes a single, dominant gene conferring resistance to downy mildew expressed in adult plants to a 75.1 cm long linkage group on a genetic linkage map of Brassica oleracea L. More recently, we identified a new AFLP marker 2.8 cm downstream from the resistance gene. The five DNA markers within an 8.5 cm region encompassing the Pp523 gene were cloned and sequenced. Three of these markers were transformed into SCARs (sequence characterised amplified regions), however, two among them were monomorphic and were analysed as CAPS (cleaved amplified polymorphic sequence) markers among the mapping population. Searched against genomic databases, the five B. oleracea DNA-marker sequences matched Arabidopsis thaliana L. gene sequences that delimit a conserved syntenic region in the top arm end of chromosome 1 of this last species. Considering the close genetic relatedness between both species, the information on this specific genomic region in A. thaliana is particularly useful for the construction of a fine-scale map of the corresponding genomic region in B. oleracea. The identified SCAR and CAPS markers can be used for marker assisted selection (MAS) in breeding programs aimed at the introgression of the Pp523 resistance locus, allowing the reliable indirect identification of plants harbouring the resistance gene with a margin of error of approximately six in ten-thousand selected plants.

No abstract

Interferência de plantas daninhas na cultura do quiabo ... ABSTRACT -An experiment was carried out under field conditions in Médio Vale do Rio Doce-MG, from May to October, 2007, to establish periods of weed interference in Abelmoschus esculentus crop. 'Santa Cruz-47' seeds were sown in a 0.25 x 1.0 m spacing, and weed control times varied from 0 to 120 days after emergence (DAE). Number of fruit per plant and yield as well as values in days for Period Previous to Interference (PPI), Critical Prevention Period of Interference (CPPI) and Total Period of Interference Prevention (TPIP) were determined, considering 5% loss. Plants from the species studied were placed in the plots, isolated or in competition with A. esculentus, aiming to to evaluate the competitive capacity of the main weeds. Area, number of leaves and plant height of A. esculentus were evaluated.Yield and number of fruit presented a similar behavior. The estimated PPI was 25 days, indicating when to start weed control. The CPPI observed was 75 days, indicating 100 days for TPIP. Among the weeds evaluated, Eleusine indica showed the highest competitive capacity against the the crop.

No abstract

We have previously constructed a genetic map of Brassica oleracea L. containing the Pp523 locus that confers downy mildew resistance to adult plants. In this work, 44 SSR markers of reference for the Brassica C genome chromosomes were added to the map, allowing the nine major linkage groups to be assigned to the nine chromosomes of B. oleracea. Locus Pp523 was located on chromosome C8, and a locus determining flower colour was mapped to chromosome C3. In comparison with the first version of the map, the new map is denser and more compact. The available genomic information on B. oleracea was enriched with the chromosome location of two phenotypic traits and 421 DNA markers (RAPD, ISSR, AFLP, SCAR, BAC-end derived STS, SSR and other PCR markers). Conversely, the genomic information on B. oleracea chromosome C8 is being used as an additional tool for the map-based cloning of Pp523, the first gene for adult plant resistance to downy mildew precisely located to a specific chromosome of this crop species.

Downy mildew caused by the oomycete Hyaloperonospora parasitica (formerly Peronospora parasitica) is a worldwide foliar disease of Brassica vegetables, which may cause seedling loss in the nurseries and damage to adult plants in the field. Disease symptoms start from the lower leaves and progress upwards. Three experiments were conducted under controlled environment conditions, using inoculated leaf discs, to determine the influence of leaf position, plant age, and leaf age on the expression of resistance to downy mildew in various Brassica oleracea genotypes. The upper leaves were more resistant than the lower leaves when 7-19 week-old plants of broccoli and Tronchuda cabbage were tested. Broccoli lines 'PCB21.32' and 'OL87123-2' were fully susceptible at the cotyledon stage, showed a clear resistance increase from lower to upper leaves at 6 weeks and 'PCB21.32' was fully resistant 16 weeks after sowing. Immature leaves were more resistant than adjacent fully expanded mature leaves. Susceptibility increased with leaf age when the same leaf was tested at two to 4-week intervals. Leaf age and upper-leaf position on the stem had opposite effects on disease score, since younger leaves collected from lower positions and older leaves collected from upper positions tended to score similarly in compatible interactions. The progression of downy mildew from the base of the plant upwards on B. oleracea in the field could be due to differences in leaf resistance in addition to environmental variation. To maximise the expression of a compatible reaction in adult plants lower leaves of Brassica plants that are at least 12 weeks-old should be used.

We describe the construction of a BAC contig and identification of a minimal tiling path that encompass the dominant and monogenically inherited downy mildew resistance locus Pp523 of Brassica oleracea L. The selection of BAC clones for construction of the physical map was carried out by screening gridded BAC libraries with DNA overgo probes derived from both genetically mapped DNA markers flanking the locus of interest and BAC-end sequences that align to Arabidopsis thaliana sequences within the previously identified syntenic region. The selected BAC clones consistently mapped to three different genomic regions of B. oleracea. Although 83 BAC clones were accurately mapped within a ∼4.6 cM region surrounding the downy mildew resistance locus Pp523, a subset of 33 BAC clones mapped to another region on chromosome C8 that was ∼60 cM away from the resistance gene, and a subset of 63 BAC clones mapped to chromosome C5. These results reflect the triplication of the Brassica genomes since their divergence from a common ancestor shared with A. thaliana, and they are consonant with recent analyses of the C genome of Brassica napus. The assembly of a minimal tiling path constituted by 13 (BoT01) BAC clones that span the Pp523 locus sets the stage for map-based cloning of this resistance gene.