B. Quilot

QTL analysis of quality traits in an advanced backcross between Prunus persica cultivars and the wild relative species P. davidiana

Genetic control of the different attributes involved in peach quality has been investigated in an advanced backcross population derived from a cross between Prunus davidiana clone P1908, a wild parent with poor agronomic performance, and a commercial variety, Summergrand. A total of 24 physical and biochemical traits were investigated. Quantitative trait loci (QTLs) were detected for all the traits studied. We identified alleles from P. davidiana with agronomically favorable effects regarding fruit and stone sizes, sugar and acid concentrations and red flesh coloration, in clear contrast to its phenotype. We identified three main regions of the genome where alleles from P. davidiana had negative effects on multiple traits. In other regions, co-locations of QTLs with opposite effects on quality traits were also detected. We discuss the nature of these co-locations in the light of the probable physiological mechanisms involved. Strategies to cope with negative correlations between favorable traits and co-locations of P. davidiana alleles with negative effects on quality traits and positive effects regarding resistance to powdery mildew are discussed from a breeding point of view.

Under what circumstances can process-based simulation models link genotype to phenotype for complex traits? Case-study of fruit and grain quality traits

Detailed information has arisen from research at gene and cell levels, but it is still incomplete in the context of a quantitative understanding of whole plant physiology. Because of their integrative nature, process-based simulation models can help to bridge the gap between genotype and phenotype and assist in deconvoluting genotype-by-environment (GxE) interactions for complex traits. Indeed, GxE interactions are emergent properties of simulation models, i.e. unexpected properties generated by complex interconnections between subsystem components and biological processes. They co-occur in the system with synergistic or antagonistic effects. In this work, different kinds of GxE interactions are illustrated. Approaches to link model parameters to genes or quantitative trait loci (QTL) are briefly reviewed. Then the analysis of GxE interactions through simulation models is illustrated with an integrated model simulation of peach (Prunus persica (L.) Batsch) fruit mass and sweetness, and with a model of wheat (Triticum aestivum L.) grain yield and protein concentration. This paper suggests that the management of complex traits such as fruit and grain quality may become possible, thanks to the increasing knowledge concerning the genetic and environmental regulation of organ size and composition and to the development of models simulating the complex aspects of metabolism and biophysical behaviours at the plant and organ levels.

Local adaptation in European populations of Arabidopsis lyrata (Brassicaceae)

We studied local adaptation to contrasting environments using an organism that is emerging as a model for evolutionary plant biology-the outcrossing, perennial herb Arabidopsis lyrata subsp. petraea (Brassicaceae). With reciprocal transplant experiments, we found variation in cumulative fitness, indicating adaptive differentiation among populations. Nonlocal populations did not have significantly higher fitness than the local population. Experimental sites were located in Norway (alpine), Sweden (coastal), and Germany (continental). At all sites after one year, the local population had higher cumulative fitness, as quantified by survival combined with rosette area, than at least one of the nonlocal populations. At the Norwegian site, measurements were done for two additional years, and fitness differences persisted. The fitness components that contributed most to differences in cumulative fitness varied among sites. Relatively small rosette area combined with a large number of inflorescences produced by German plants may reflect differentiation in life history. The results of the current study demonstrate adaptive population differentiation in A. lyrata along a climatic gradient in Europe. The studied populations harbor considerable variation in several characters contributing to adaptive population differentiation. The wealth of genetic information available makes A. lyrata a highly attractive system also for examining the functional and genetic basis of local adaptation in plants.

Analysis of genotypic variation of sugar and acid contents in peaches and nectarines through the Principle Component Analysis

Genotypic variations and correlations were estimated for sugar and acid contents on afresh flesh weight basis in peach and nectarine genotypes derived from a clone of a wild peach (Prunus davidiana) by three generations of crosses with commercial nectarine varieties. 107genotypes were studied in Avignon (France), 40 in Gotheron (France), and 18 of them were common to both locations. Considerable variations in sugar and acid contents were found among genotypes in both locations. Though location significantly affected sucrose, sorbitol and acid contents of the18 common genotypes, principal component analysis (PCA) indicated that genotypic correlations among sugar and acid contents were stable in both locations. Almost all sugars and acids analysed exhibited positive loadings for the first principal component (PC). Variations of dry matter content among genotypes only partially explained this general trend as shown by the PCA on a dry flesh weight basis. Glucose and fructose contents were closely correlated and were similar in amount inmost genotypes, while fructose content was lower than glucose content in about 12% of the genotypes. A positive but loose relationship was found between malic and citric acid contents in both locations. Likewise, sucrose, sorbitol and quinic acid contents were positively associated. The first PC could be used as a general flavour component. In addition, three groups of closely associated variables (fructose and glucose; malic and citric acids; sucrose, sorbitol and quinic acid) were detected through PCA.

Analysis of genotypic variation in fruit flesh total sugar content via an ecophysiological model applied to peach.

A simulation model of the evolution of total sugar content (CTS) in fruit was developed in order to describe the within- and between-genotype variation of CTS observed in a peach (Prunus persica (L.) Batsch) breeding population. The parameter k defines the ratio of carbon used for synthesizing compounds other than sugars for each genotype. Model input variables are dry flesh growth rate and fresh flesh mass of fruit. We estimated k for 137 peach and nectarine genotypes derived from a clone of a wild peach (Prunus davidiana) by three generations of crosses with commercial nectarine varieties. We tested the predictive quality of the model on independent datasets. Despite an underestimation of the observed CTS, the correlation between observations and predictions was suitable (0.72). Spearman correlation coefficients between 2001 and 2002 for model input variables and parameter k were higher than for CTS. None of the three components assimilation supply to the fruit, metabolism, or dilution, seemed to have a greater relative effect on CTS variation than the others. Indeed, CTS variation seemed to result from the balance between the three components. The interest of this approach, which consists of dissecting traits into components via an ecophysiological model, for breeding strategy and for sugar accumulation studies are discussed.

Virtual profiling: a new way to analyse phenotypes

Simulation models can be used to perform virtual profiling in order to analyse eco-physiological processes controlling plant phenotype. To illustrate this, an eco-physiological model has been used to compare and contrast the status of a virtual fruit system under two situations of carbon supply. The model simulates fruit growth, accumulation of sugar, citric acid and water, transpiration, respiration and ethylene emission, and was successfully tested on peach (Prunus persica L. Batsch) for two leaf-to-fruit ratios (6 and 18 leaves per fruit). The development stage and the variation in leaf number had large effects of the fruit model variables dealing with growth, metabolism and fruit quality. A sensitivity analysis showed that changing a single parameter value, which could correspond to a genotypic change induced by a mutation, either strongly affects most of the processes, or affects a specific process or none. Correlation analysis showed that, in a complex system such as fruit, the intensity of many physiological processes and quality traits co-varies. It also showed unexpected co-variations resulting from emergent properties of the system. This virtual profiling approach opens a new route to explore the impact of mutations, or naturally occurring genetic variations, under differing environmental conditions.

Leaf light-saturated photosynthesis for wild and cultivated peach genotypes and their hybrids: A simple mathematical modelling analysis

Summary We measured leaf photosynthetic activity, stomatal conductance and leaf carbohydrate reserves (soluble sugars and starch) for seven peach genotypes of different introgression level, i.e. commercial varieties, Prunus davidiana and genotypes derived from Prunus davidiana. In early May, branches of each genotype were girdled and leaf to fruit ratio treatments were applied to obtain contrasting source-sink ratios. Photosynthetic activity was significantly different between treatments. We adjusted an asymptotic curve (monomolecular) to the strong relationship between light-saturated photosynthesis (Asat) and stomatal conductance (gs). This monomolecular curve appeared to be independent of genotype and year. Thus, one unique value for the maximum of Asat (Amax) and one unique value for the initial increase rate of Asat per unit of gs (q) were estimated (Amax=20.68 µmol m–2 s–1 and q=83.73). Leaf reserves (soluble sugars and starch) became significantly larger for low crop loads than for high crop loads, whatever the genotype. Genotype effect on soluble sugars and starch concentrations was limited. Low Asat appeared to be correlated with a high accumulation of leaf reserves. A three-parameter curve was used to describe the relationship between photosynthesis activity and leaf reserves. Values of the parameters of this relation appeared to be independent of genotype and year. Practical considerations on source-sink conditions and experimental treatments are recommended.

Is competition between mesocarp cells of peach fruits affected by the percentage of wild species genome

The number of cells and the mean cell volume in the mesocarps of fruits from peach genotypes with different percentages of the genome of Prunus davidiana, a wild, related, species, were evaluated. The mesocarp mass varied greatly between the four groups of genotypes. The mean cell volume and the number of cells were negatively correlated within each group. This correlation can be interpreted as a relationship of competition between cells. In order to describe the type of competition in the different groups, we tried to adjust a model of competition for resources proposed by Lescourret and Génard (Ecoscience 10:334–341, 2003). To estimate the values of the three parameters of the model for the different groups, we applied model selection. Within nested models, we identified a single best model with six parameter values. This model was roughly accurate, but it allowed us to describe the general relationship for each group. The parameter values revealed a strong and under-compensating density-dependence effect for all groups. The percentage of P. davidiana genome appeared to influence the maximal number of cells and the strength of the competition, but no effect was found on the maximal mean volume of cells.

SHAPE, MASS AND DRY MATTER CONTENT OF PEACHES OF VARIETIES WITH DIFFERENT DOMESTICATION LEVELS

We studied the variations in fruit and stone shapes and compared the relative mass of the different compartments of peaches between varieties with different domestication levels. Fruit shape appeared stable and round whatever the level of domestication of the varieties, whereas stone shape was oval and more varying from one variety to another. Variations in mesocarp thickness seemed to balance the oval shape of the stone. Fruit fresh mass varied greatly among the varieties. Variations in mesocarp fresh mass explained most of the variations in fruit mass, since stone fresh mass was roughly similar among varieties. No link between level of domestication and dry matter content of mesocarp was observed. Hypotheses to explain mesocarp mass differences are discussed.

Application of a SUGAR model to analyse sugar accumulation in peach cultivars that differ in glucose–fructose ratio

A SUGAR model, which was established to predict the partitioning of carbon into sucrose, glucose, fructose and sorbitol in fruit mesocarp of peach cultivars ( Prunus persica (L.) Batch) with normal glucose: fructose ratio (G:F) of 0·8–1·5, was evaluated and extended for peach cultivars with a high G:F ratio of 1·5–7·8. The extended model (SUGARb) is more generic and assumes a high G:F ratio to be due to preferential transformation of sorbitol into glucose, preferential utilization of fructose or preferential conversion of fructose into glucose. The simulated seasonal variations in sugars via the SUGARb-model-matched experimental data for three normal and three high G:F cultivars well, and accurately exhibited G:F ratio characteristics. The relative rates of sucrose transformation into glucose and fructose differed according to cultivar but not according to G:F status. Compared with hexosephosphate interconversion, a lower production rate of fructose than glucose from sorbitol, and/or a higher utilization rate of fructose than that of glucose might be preferential alternatives for forming high G:F ratios in the high G:F cultivars studied in the present study, which is discussed in the light of recent results on enzyme activities.