Fine genetic mapping of a gene required for Rice yellow mottle virus cell-to-cell movement.

The very high resistance to Rice yellow mottle virus observed in the two rice varieties Gigante ( Oryza sativa) and Tog 5681 ( O. glaberrima) is monogenic and recessive. Bulked segregant analysis was carried out to identify AFLP markers linked to the resistance gene. Mapping of PCR-specific markers, CAPS and microsatellite markers on 429 individuals of an IR64 x Gigante F(2) population pinpointed this resistance gene on the long arm of chromosome 4 in a 3.7-cM interval spanned by PCR markers. These markers also flanked the resistance gene of the O. glaberrima accession Tog 5681 and confirmed previous allelism tests. The rarity of this recessive natural resistance was in line with a resistance mechanism model based on point mutations of a host component required for cell-to-cell movement of the virus. Preliminary data on the genetic divergence between the two cultivated rice species in the vicinity of the resistance locus suggested that two different resistance alleles are present in Gigante and Tog 5681. A large set of recombinants is now available to envisage physical mapping and cloning of the gene.

Analysis of a contiguous 211 kb sequence in diploid wheat (Triticum monococcum L.) reveals multiple mechanisms of genome evolution.

In plant species with large genomes such as wheat or barley, genome organization at the level of DNA sequence is largely unknown. The largest sequences that are publicly accessible so far from Triticeae genomes are two 60 kb and 66 kb intervals from barley. Here, we report on the analysis of a 211 kb contiguous DNA sequence from diploid wheat (Triticum monococcum L.). Five putative genes were identified, two of which show similarity to disease resistance genes. Three of the five genes are clustered in a 31 kb gene-enriched island while the two others are separated from the cluster and from each other by large stretches of repetitive DNA. About 70% of the contig is comprised of several classes of transposable elements. Ten different types of retrotransposons were identified, most of them forming a pattern of nested insertions similar to those found in maize and barley. Evidence was found for major deletion, insertion and duplication events within the analysed region, suggesting multiple mechanisms of genome evolution in addition to retrotransposon amplification. Seven types of foldback transposons, an element class previously not described for wheat genomes, were characterized. One such element was found to be closely associated with genes in several Triticeae species and may therefore be of use for the identification of gene-rich regions in these species.

The Genetic Basis of High Resistance to Rice Yellow Mottle Virus (RYMV) in Cultivars of Two Cultivated Rice Species.

Three cultivars of Oryza sativa (IR64, Azucena, and Gigante) and four cultivars of O. glaberrima (Tog5681, Tog5673, CG14, and SG329) were evaluated for their resistance to two isolates of rice yellow mottle virus (RYMV) by enzyme-linked immunosorbent assay (ELISA) and symptomatology. Cultivars Tog5681 and Gigante were highly resistant, and no symptoms were observed when either virus isolate was inoculated at 10 or 20 days postgermination and assayed by ELISA at 7, 14, 22, 35, 50, or 64 days postinoculation. Azucena showed a partial resistance, whereas the other cultivars were susceptible. Symptom appearance was associated with increase in ELISA absorbance in the systemically infected leaves. The best discrimination among the cultivars occurred when the plants were inoculated at 10 days postgermination. Crosses were made between the highly resistant (Gigante and Tog5681) and the susceptible (IR64) cultivars to determine the genetic basis of resistance to RYMV. Evaluation of F1 hybrids and interspecific progenies, as well as the segregation of resistance in F2 and F3 lines of the IR64 × Gigante cross, provided results consistent with the presence of a single recessive resistance gene common to Tog5681 and Gigante.

A major quantitative trait locus for rice yellow mottle virus resistance maps to a cluster of blast resistance genes on chromosome 12.

ABSTRACT Two doubled-haploid rice populations, IR64/Azucena and IRAT177/ Apura, were used to identify markers linked to rice yellow mottle virus (RYMV) resistance using core restriction fragment length polymorphism (RFLP) maps. Resistance was measured by mechanical inoculation of 19-day-old seedlings followed by assessment of virus content by enzyme-linked immunosorbent assay tests 15 days after inoculation. IR64/Azucena and IRAT177/Apura populations, 72 and 43 lines, respectively, were evaluated, and resistance was found to be polygenic. Resistance was expressed as a slower virus multiplication, low symptom expression, and limited yield loss when assessed at the field level. Bulked segregant analysis using the IR64/Azucena population identified a single random amplified polymorphic DNA marker that mapped on chromosome 12 and corresponded to a major quantitative trait locus (QTL) evidenced by interval mapping. When pooling RFLP data, integrated mapping of this chromosome revealed that the QTL was common to the two populations and corresponded to a small chromosomal segment known to contain a cluster of major blast resistance genes. This region of the genome also reflected the differentiation observed at the RFLP level between the subspecies indica and japonica of Oryza sativa. This is consistent with the observation that most sources of RYMV resistance used in rice breeding are found in upland rice varieties that typically belong to the japonica subspecies.

Bacteriocin Typing of Burkholderia (Pseudomonas) solanacearum Race 1 of the French West Indies and Correlation with Genomic Variation of the Pathogen.

Burkholderia solanacearum race 1 isolates indigenous to the French West Indies were characterized by bacteriocin typing and two genomic fingerprinting methods: pulsed-field gel electrophoresis of genomic DNA digested by rare-cutting restriction endonucleases (RC-PFGE) and PCR with primers corresponding to repetitive extragenic palindromic (REP), enterobacterial repetitive intergenic consensus (ERIC), and BOX elements (collectively known as rep-PCR). The survey comprised 24 reference strains and 65 isolates obtained from a field trial in Guadeloupe in 1993. Comparison of the data identified RC-PFGE as the most discriminatory method, delineating 17 pulsed-field gel profile types. rep-PCR and bacteriocin typing identified nine rep-PCR profile types and nine bacteriocin groups. Independent determination of similarity coefficients and clustering of RC-PFGE and rep-PCR data identified six groups common to both sets of data that correlated to biovar and bacteriocin groups. Further study of bacteriocin production in planta gave results consistent with in vitro bacteriocin typing. It was observed that spontaneous bacteriocin-resistant mutants exhibited a cross-resistance to other bacteriocins as identified by the typing scheme and that such mutants possessed a selective advantage for growth over isogenic nonmutants in the presence of a bacteriocin. The results are significant in the search for biological control of disease by nonpathogenic mutants of the wild-type organism.