Arabidopsis (rockcress) is a genus in the family Brassicaceae. Representatives of Brassicaceae family are small-flowering plants, some of them
are important crops such as cabbage, cauliflower, radish, and canola.
Arabidopsis genus includes nine species and eight subspecies. The
subspecies delimitation is quite recent, and is based on morphological
and molecular phylogenetics (Elizabeth 2000). Arabidopsis thalianacommonly known as thale cress, has 10 chromosomes in diploid stage
and was the first plant to have its entire genome sequenced. Most of the
species in Arabidopsis are indigenous to Europe and only two species are
found in North America and Asia.

Arabidopsis Genome Initiative

Applications of genome mapping and marker-assisted selection (MAS) in crop improvement are reviewed. The following aspects are considered: a comparison of the choice of markers available for the generation of linkage maps (including amplified fragment length polymorphisms (AFLP); restriction fragment length polymorphisms (RFLP); randomly amplified polymorphic DNA (RAPD) and simple sequence repeats (SSR)); quantitative trait loci (QTL) analysis; use of molecular markers in the exploitation of hybrid vigour; physical genome mapping; map-based cloning and transposon tagging of agriculturally important genes; synteny in cereal genomes; and the use of MAS in breeding for disease and pest resistance.

Genome mapping, molecular markers and marker-assisted selection in crop plants

The candidate gene (CG) approach has been applied in plant genetics in the past decade for the characterisation and cloning of Mendelian and quantitative trait loci (QTLs). It constitutes a complementary strategy to map-based cloning and insertional mutagenesis. The goal of this paper is to present an overview of CG analyses in plant genetics. CG analysis is based on the hypothesis that known-function genes (the candidate genes) could correspond to loci controlling traits of interest. CGs refer either to cloned genes presumed to affect a given trait (`functional CGs') or to genes suggested by their close proximity on linkage maps to loci controlling the trait (`positional CGs'). In plant genetics, the most common way to identify a CG is to look for map co-segregation between CGs and loci affecting the trait. Statistical association analyses between molecular polymorphisms of the CG and variation in the trait of interest have also been carried out in a few studies. The final validation of a CG will be provided through physiological analyses, genetic transformation and/or sexual complementation. Theoretical and practical applications of validated CGs in plant genetics and breeding are discussed.

The candidate gene approach in plant genetics: a review

DNA markers in plant improvement: An overview

One stop mycology

Leptosphaeria maculans, the causal agent of blackleg of crucifers, is a major threat to rapeseed (Brassica napus) production throughout the world. Genes controlling blackleg resistance in B. napus were mapped using an F 1 -derived doubled haploid (DH) population of 105 lines and 138 restriction fragment length polymorphism (RFLP) markers. The host-pathogen interaction phenotype was assessed qualitatively and by several quantitative measurements using different environments and plant developmental stages. A single major locus controlling blackleg resistance (LEM1) was mapped to linkage group 6 based on qualitative scores of the interaction phenotype on inoculated cotyledons. This resistance locus was also identified by interval mapping using quantitative measurements of the interaction phenotype on cotyledon- and stem-inoculated plants. Four other genomic regions were significantly associated with quantitative measurements of resistance on cotyledon and stem, among them a marker locus interval in linkage group 17 that included a pathogenesis related gene (PR2). Two genomic regions associated with resistance in field-evaluated plants were different from those identified in cotyledon- and stem-evaluated plants. The use of different environments and plant developmental stages for mapping disease resistance loci is discussed

Mapping loci controlling Brassica napus resistance to Leptosphaeria maculans under different screening conditions.

Genes for resistance to white rust (Albugo candida) in oilseed Brassica rapa were mapped using a recombinant inbred (RI) population and a genetic linkage map consisting of 144 restriction fragment length polymorphism (RFLP) markers and 3 phenotypic markers Young seedlings were evaluated by inoculating cotyledons with A candida race 2 (AC2) and race 7 (AC7) and scoring the interaction phenotype (IP) on a 0-9 scale The IP of each line was nearly identical for the two races and the population showed bimodal distributions, suggesting that a single major gene (or tightly linked genes) controlled resistance to the two races The IP scores were converted to categorical resistant and susceptible scores, and these data were used to map a single Mendelian gene controlling resistance to both races on linkage group 4 where resistance to race 2 had been mapped previously A quantitative trait loci (QTL) mapping approach using the IP scores detected the same major resistance locus for both races, plus a second minor QTL effect for AC2 on linkage group 2 These results indicate that either a dominant allele at a single locus (Acal) or two tightly linked loci control seedling resistance to both races of white rust in the biennial turnip rape cultivar Per The map positions of white rust resistance genes in B rapa and Brassica napus were compared and the results indicate where additional loci that have not been mapped may be located Alignment of these maps to the physical map of the Arabidopsis genome identified regions to target for comparative fine mapping using this model organism

Linkage mapping of genes controlling resistance to white rust (Albugo candida) in Brassica rapa (syn. campestris) and comparative mapping to Brassica napus and Arabidopsis thaliana

S. R. Rimmer

Papers

Mapping loci controlling Brassica napus resistance to Leptosphaeria maculans under different screening conditions.

Linkage mapping of genes controlling resistance to white rust (Albugo candida) in Brassica rapa (syn. campestris) and comparative mapping to Brassica napus and Arabidopsis thaliana