Transcriptome analysis in pea allows to distinguish chilling and acclimation mechanisms.

In order to distinguish chilling and freezing tolerance mechanisms in pea, responses to cold exposure were compared between the freezing tolerant line Champagne and the sensitive line Terese. Global gene expression was considered in the two lines and associated with morphological, histological and biochemical approaches. The chilling tolerance in both lines was related to responses of the CBF, COR and LEA genes belonging to the CBF regulon, with greater earliness of expression in the Champagne genotype. The freezing tolerance, only observed in Champagne, was associated with acclimation processes such as cellular osmotic stabilization, photosynthesis modifications, antioxidants production, modifications in hormone metabolism, cell wall composition and dynamics.

The flowering locus Hr colocalizes with a major QTL affecting winter frost tolerance in Pisum sativum L.

An understanding of the genetic determinism of frost tolerance is a prerequisite for the development of frost tolerant cultivars for cold northern areas. In legumes, it is not known to which extent vernalization requirement or photoperiod responsiveness are necessary for the development of frost tolerance. In pea (Pisum sativum L.) however, the flowering locus Hr is suspected to influence winter frost tolerance by delaying floral initiation until after the main winter freezing periods have passed. The objective of this study was to dissect the genetic determinism of frost tolerance in pea by QTL analysis and to assess the genetic linkage between winter frost tolerance and the Hr locus. A population of 164 recombinant inbred lines (RILs), derived from the cross Champagne x Terese was evaluated both in the greenhouse and in field conditions to characterize the photoperiod response from which the allele at the Hr locus was inferred. In addition, the population was also assessed for winter frost tolerance in 11 field conditions. Six QTL were detected, among which three were consistent among the different experimental conditions, confirming an oligogenic determinism of frost tolerance in pea. The Hr locus was found to be the peak marker for the highest explanatory QTL of this study. This result supports the hypothesis of the prominent part played by the photoperiod responsiveness in the determinism of frost tolerance for this species. The consistency of three QTL makes these positions interesting targets for marker-assisted selection.

Functional mapping in pea, as an aid to the candidate gene selection and for investigating synteny with the model legume Medicago truncatula.

The identification of the molecular polymorphisms giving rise to phenotypic trait variability-both quantitative and qualitative-is a major goal of the present agronomic research. Various approaches such as positional cloning or transposon tagging, as well as the candidate gene strategy have been used to discover the genes underlying this variation in plants. The construction of functional maps, i.e. composed of genes of known function, is an important component of the candidate gene approach. In the present paper we report the development of 63 single nucleotide polymorphism markers and 15 single-stranded conformation polymorphism markers for genes encoding enzymes mainly involved in primary metabolism, and their genetic mapping on a composite map using two pea recombinant inbred line populations. The complete genetic map covers 1,458 cM and comprises 363 loci, including a total of 111 gene-anchored markers: 77 gene-anchored markers described in this study, 7 microsatellites located in gene sequences, 16 flowering time genes, the Tri gene, 5 morphological markers, and 5 other genes. The mean spacing between adjacent markers is 4 cM and 90% of the markers are closer than 10 cM to their neighbours. We also report the genetic mapping of 21 of these genes in Medicago truncatula and add 41 new links between the pea and M. truncatula maps. We discuss the use of this new composite functional map for future candidate gene approaches in pea.

Microsatellite marker polymorphism and mapping in pea (Pisum sativum L.).

This paper aims at providing reliable and cost effective genotyping conditions, level of polymorphism in a range of genotypes and map position of newly developed microsatellite markers in order to promote broad application of these markers as a common set for genetic studies in pea. Optimal PCR conditions were determined for 340 microsatellite markers based on amplification in eight genotypes. Levels of polymorphism were determined for 309 of these markers. Compared to data obtained for other species, levels of polymorphism detected in a panel of eight genotypes were high with a mean number of 3.8 alleles per polymorphic locus and an average PIC value of 0.62, indicating that pea represents a rather polymorphic autogamous species. One of our main objectives was to locate a maximum number of microsatellite markers on the pea genetic map. Data obtained from three different crosses were used to build a composite genetic map of 1,430 cM (Haldane) comprising 239 microsatellite markers. These include 216 anonymous SSRs developed from enriched genomic libraries and 13 SSRs located in genes. The markers are quite evenly distributed throughout the seven linkage groups of the map, with 85% of intervals between the adjacent SSR markers being smaller than 10 cM. There was a good conservation of marker order and linkage group assignment across the three populations. In conclusion, we hope this report will promote wide application of these markers and will allow information obtained by different laboratories worldwide in diverse fields of pea genetics, such as QTL mapping studies and genetic resource surveys, to be easily aligned.

Genetic diversity within Pisum sativum using protein- and PCR-based markers.

A collection of 148 Pisum accessions, mostly from Western Europe, and including both primitive germplasm and cultivated types, was structured using 121 protein- and PCR-based markers. This molecular marker-based classification allowed us to trace back major lineages of pea breeding in Western Europe over the last decades, and to follow the main breeding objectives: increase of seed weight, introduction of the afila foliage type and white flowers, and improvement of frost tolerance for winter-sown peas. The classification was largely consistent with the available pedigree data, and clearly resolved the different main varietal types according to their end-uses (fodder, food and feed peas) from exotic types and wild forms. Fodder types were further separated into two sub-groups. Feed peas, corresponding to either spring-sown or winter-sown types, were also separated, with two apparently different gene pools for winter-sown peas. The garden pea group was the most difficult to structure, probably due to a continuum in breeding of feed peas from garden types. The classification also stressed the paradox between the narrowness of the genetic basis of recent cultivars and the very large diversity available within P. sativum. A sub-collection of 43 accessions representing 96% of the whole allelic variability is proposed as a starting point for the construction of a core collection.