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International audience; Hypocotyl elongation in the dark is a crucial process to ensure seedling emergence. It relies both on the cell number and cell length. The contribution of these two factors to the maximal hypocotyl length and the impact of environmental conditions on this contribution are not known. This is surprising considering the agronomic and economical importance of seedling emergence in crop establishment. Using 14 genotypes from a nested core collection representing Medicago truncatula (barrel medic) natural variation, we investigated how epidermal cell number and cell length contribute to hypocotyl length under optimal, low temperature (8°C) and water deficit (−0.50 MPa) conditions. Both cell number and length vary according to genotypes and contribute to maximal hypocotyl length differences between genotypes. This contribution, however, depends on growth conditions. Cell number is the major contributor under optimal conditions (60%) whereas cell length becomes the major determinant under stress. Maximal hypocotyl length is correlated with hypocotyl elongation rate under both stresses but not under optimal condition, revealing contrasted genotypes for cell elongation capacity under stress. To identify the genetic regulators determining cell number and cell length, quantitative trait loci (QTLs) were detected using a recombinant inbred lines population exhibiting segregation in maximal hypocotyl length. Two QTLs controlling cell number and three QTLs controlling cell length at low temperature were detected. One QTL for cell number and two for cell length were found to be associated with hypocotyl length under low temperature. This study provides new information to improve seedling emergence under abiotic stress.

Seed vigour is important for successful establishment and high yield, especially under suboptimal environmental conditions. In legumes, raffinose oligosaccharide family (RFO) sugars have been proposed as an easily available energy reserve for seedling establishment. In this study, we investigated whether the composition or amount of soluble sugars (sucrose and RFO) is part of the genetic determinants of seed vigour of Medicago truncatula using two recombinant inbred line (RIL) populations. Quantitative trait loci (QTL) mapping for germination rate, hypocotyl and radicle growth under water deficit and nutritional stress, seed weight and soluble sugar content was performed using RIL populations LR1 and LR4. Seven of the 12 chromosomal regions containing QTL for germination rate or post-germinative radicle growth under optimal or stress conditions co-located with Suc/RFO QTL. A significant negative correlation was also found between seed vigour traits and Suc/RFO. In addition, one QTL that explained 80% of the variation in the ratio stachyose/verbascose co-located with a stachyose synthase gene whose expression profile in the parental lines could explain the variation in oligosaccharide composition. The correlation and co-location of Suc/RFO ratio with germination and radicle growth QTL suggest that an increased Suc/RFO ratio in seeds of M. truncatula might negatively affect seed vigour.

DNA sequence polymorphism carries genealogical information and allows for testing hypotheses on selection and population history, especially through coalescent-based analysis. Understanding the evolutionary forces at work in plant domestication and subsequent selection is of critical importance for the management of genetic resources. In this study, we surveyed DNA sequence diversity at two assumed neutral nuclear loci in the wild-domesticated species complex of alfalfa (Medicago sativa L.). A high level of polymorphism was detected. The domesticated pool contains on average 31% less diversity than the wild pool, but with a high heterogeneity among loci. Coalescent simulations of the domestication process showed that this result cannot be explained by assuming a constant population size but is rather consistent with a demographic bottleneck during domestication. A very low level of divergence was detected between the wild and the domesticated forms as well as between the related subspecies of the M. sativa species complex. However, the originality of the Spanish wild populations, already observed based on mitochondrial DNA polymorphism, was confirmed. These results, together with patterns of intrapopulation polymorphism, suggest that nuclear sequence polymorphism could be a promising tool, complementary to mitochondrial DNA and phenotypic evaluations, to investigate historical demographic and evolutionary processes.

In order to clarify the relationships (genetic exchange and shared ancestry) between natural and cultivated populations of alfalfa (Medicago sativa L.) in Spain, we investigated the patterns of mitochondrial DNA variation (characterized through restriction fragment length polymorphism) for 248 individuals in seven natural and six cultivated populations of this species. Mitochondrial variation was evidenced in both natural and cultivated populations of M. sativa. Among the seven mitotypes idengified in the species, two were specific of the natural populations, a result attesting the fact that the Spanish wild form of M. sativa is an original genetic pool compared to the cultivated one. Other mitotypes were observed in both natural and cultivated populations, suggesting the occurrence of gene flow through seeds from cultivated towards natural populations. Comparisons with previously gathered nuclear and phenotypic data give insights into the different evolutionary forces acting on the different kinds of Spanish natural populations examined so far.

Genetic differentiation between co-occurring crops and their wild relatives will be greatly modified by crop-to-weed gene flow and variation between human and natural selective pressures. The maintenance of original morphological features in most natural populations of Medicago sativa in Spain questions the relative extent of these antagonistic forces. In this paper, we measured and compared the pattern of population differentiation within and among the wild and cultivated gene pool with respect to both allozymes and quantitative traits. Patterns of diversity defined three kinds of natural populations. First, some populations were intermediate with respect to both allozymes and quantitative traits. This suggests that crop-to-weed gene flow may have created hybrid populations in some locations. Second, some populations were different from all the cultivated landraces with respect to both allozymes and quantitative traits. This probably results from variable gene flow in space and in time, due to demographic stochasticity in either natural or cultivated populations. Third, differentiation from cultivated landraces was only achieved for the quantitative traits but not for allozymes in two populations. This suggests that natural selection in some locations may oppose gene flow to establish cultivated traits into the natural introgressed populations.