Salinity Adaptation and the Contribution of Parental Environmental Effects in Medicago truncatula.

High soil salinity negatively influences plant growth and yield. Some taxa have evolved mechanisms for avoiding or tolerating elevated soil salinity, which can be modulated by the environment experienced by parents or offspring. We tested the contribution of the parental and offspring environments on salinity adaptation and their potential underlying mechanisms. In a two-generation greenhouse experiment, we factorially manipulated salinity concentrations for genotypes of Medicago truncatula that were originally collected from natural populations that differed in soil salinity. To compare population level adaptation to soil salinity and to test the potential mechanisms involved we measured two aspects of plant performance, reproduction and vegetative biomass, and phenological and physiological traits associated with salinity avoidance and tolerance. Saline-origin populations had greater biomass and reproduction under saline conditions than non-saline populations, consistent with local adaptation to saline soils. Additionally, parental environmental exposure to salt increased this difference in performance. In terms of environmental effects on mechanisms of salinity adaptation, parental exposure to salt spurred phenological differences that facilitated salt avoidance, while offspring exposure to salt resulted in traits associated with greater salt tolerance. Non-saline origin populations expressed traits associated with greater growth in the absence of salt while, for saline adapted populations, the ability to maintain greater performance in saline environments was also associated with lower growth potential in the absence of salt. Plastic responses induced by parental and offspring environments in phenology, leaf traits, and gas exchange contribute to salinity adaptation in M. truncatula. The ability of plants to tolerate environmental stress, such as high soil salinity, is likely modulated by a combination of parental effects and within-generation phenotypic plasticity, which are likely to vary in populations from contrasting environments.

Alternative oxidase 1 (Aox1) gene expression in roots of Medicago truncatula is a genotype-specific component of salt stress tolerance.

Alternative oxidase (AOX) is the central component of the non-phosphorylating alternative respiratory pathway in plants and may be important for mitochondrial function during environmental stresses. Recently it has been proposed that Aox can be used as a functional marker for breeding stress tolerant plant varieties. This requires characterization of Aox alleles in plants with different degree of tolerance in a certain stress, affecting plant phenotype in a recognizable way. In this study we examined Aox1 gene expression levels in Medicago truncatula genotypes differing in salt stress tolerance, in order to uncover any correlation between Aox expression and tolerance to salt stress. Results demonstrated a specific induction of Aox1 gene expression in roots of the tolerant genotype that presented the lowest modulation in phenotypic and biochemical stress indices such as morphologic changes, protein level, lipid peroxidation and ROS generation. Similarly, in a previous study we reported that induction of antioxidant gene expression in the tolerant genotype contributed to the support of the antioxidant cellular machinery and stress tolerance. Correlation between expression patterns of the two groups of genes was revealed mainly in 48 h treated roots. Taken together, results from both experiments suggest that M. truncatula tolerance to salt stress may in part due to an efficient control of oxidative balance thanks to (i) induction of antioxidant systems and (ii) involvement of the AOX pathway. This reinforces the conclusion that differences in antioxidant mechanisms can be essential for salt stress tolerance in M. truncatula and possibly the corresponding genes, especially Aox, could be utilized as functional marker.

QTL mapping of physiological traits associated with salt tolerance in Medicago truncatula Recombinant Inbred Lines.

In this study, QTL mapping of physiological traits in the model Legume (Medicago truncatula) was performed using a set of RILs derived from LR5. Twelve parameters associated with Na+ and K+ content in leaves, stems and roots were measured. Broad-sense heritability of these traits was ranged from 0.15 to 0.83 in control and from 0.14 to 0.61 in salt stress. Variation among RILs was dependent on line, treatment and line by treatment effect. We mapped 6 QTLs in control, 2 in salt stress and 5 for sensitivity index. No major QTL was identified indicating that tolerance to salt stress is governed by several genes with low effects. Detected QTL for leaf, stem and root traits did not share the same map locations, suggesting that genes controlling transport of Na+ and K+ may be different. The maximum of QTL was observed on chromosome 1, no QTL was detected on chromosomes 5 and 6.

Evidence that the exoH gene of Sinorhizobium meliloti does not appear to influence symbiotic effectiveness with Medicago truncatula 'Jemalong A17'.

The purpose of this study was to identify strains of Sinorhizobium meliloti that formed either an effective or completely ineffective symbiosis with Medicago truncatula L. 'Jemalong A17' and, subsequently, to determine whether differences existed between their exoH genes. Sinorhizobium meliloti TII7 and A5 formed an effective and ineffective symbiosis with M. truncatula 'Jemalong A17', respectively. Using a multilocus sequence typing method, both strains were shown to have chromosomes identical with S. meliloti Rm1021 and RCR2011. The 2260-bp segments of DNA stretching from the 3' end of exoI through open reading frames of hypothetical proteins SM_b20952 and SM_b20953 through exoH into the 5' end of exoK in strains TII7 and Rm1021 differed by a single nucleotide at base 127 of the hypothetical protein SM_b20953. However, the derived amino acid sequences of the exoH genes of effective TII7, ineffective A5, and strain Rm1021 were shown to be identical with each other. Therefore, it would seem unlikely that the gene product of exoH is directly involved with the low efficiency of a symbiosis of strain Rm1021 with M. truncatula 'Jemalong A17'. Complementation or complete genome sequence analyses involving strains TII7 and A5 might be useful approaches to investigate the molecular bases for the differential symbiotic response with M. truncatula 'Jemalong A17'.

Partial resistance of Medicago truncatula to Aphanomyces euteiches is associated with protection of the root stele and is controlled by a major QTL rich in proteasome-related genes.

A pathosystem between Aphanomyces euteiches, the causal agent of pea root rot disease, and the model legume Medicago truncatula was developed to gain insights into mechanisms involved in resistance to this oomycete. The F83005.5 French accession and the A17-Jemalong reference line, susceptible and partially resistant, respectively, to A. euteiches, were selected for further cytological and genetic analyses. Microscopy analyses of thin root sections revealed that a major difference between the two inoculated lines occurred in the root stele, which remained pathogen free in A17. Striking features were observed in A17 roots only, including i) frequent pericycle cell divisions, ii) lignin deposition around the pericycle, and iii) accumulation of soluble phenolic compounds. Genetic analysis of resistance was performed on an F7 population of 139 recombinant inbred lines and identified a major quantitative trait locus (QTL) near the top of chromosome 3. A second study, with near-isogenic line responses to A. euteiches confirmed the role of this QTL in expression of resistance. Fine-mapping allowed the identification of a 135-kb sequenced genomic DNA region rich in proteasome-related genes. Most of these genes were shown to be induced only in inoculated A17. Novel mechanisms possibly involved in the observed partial resistance are proposed.

Morphological and microsatellite diversity associated with ecological factors in natural populations of Medicago laciniata Mill. (Fabaceae).

Genetic variability in 10 natural Tunisian populations of Medicago laciniata were analysed using 19 quantitative traits and 12 polymorphic microsatellite loci. A large degree of genetic variability within-populations and among-populations was detected for both quantitative characters and molecular markers. High genetic differentiation among populations for quantitative traits was seen, with Q(ST) = 0.47, and F(ST) = 0.47 for microsatellite markers. Several quantitative traits displayed no statistical difference in the levels of Q(ST) and F(ST). Further, significant correlations between quantitative traits and eco-geographical factors suggest that divergence in the traits among populations may track environmental differences. There was no significant correlation between genetic variability at quantitative traits and microsatellite markers within populations. The site-of-origin of eco-geographical factors explain between 18.13% and 23.40% of genetic variance among populations at quantitative traits and microsatellite markers, respectively. The environmental factors that most influence variation in measured traits among populations are assimilated phosphorus (P(2)0(5)) and mean annual rainfall, followed by climate and soil texture, altitude and organic matter. Significant associations between eco-geographical factors and gene diversity, He, were established in five microsatellite loci suggesting that these simple sequence repeats (SSRs) are not necessarily biologically neutral.

Quantitative and molecular genetic variation in sympatric populations of Medicago laciniata and M. truncatula (Fabaceae): relationships with eco-geographical factors.

Medicago laciniata is restricted to south of the Mediterranean basin and it extends in Tunisia from the inferior semi-arid to Saharan stages, whereas M. truncatula is a widespread species in such areas. The genetic variability in four Tunisian sympatric populations of M. laciniata and M. truncatula was analysed using 19 quantitative traits and 20 microsatellites. We investigated the amplification transferability of 52 microsatellites developed in M. truncatula to M. laciniata. Results indicate that about 78.85% of used markers are valuable genetic markers for M. laciniata. M. laciniata displayed significantly lower quantitative differentiation among populations (QST=0.12) than did M. truncatula (QST=0.45). However, high molecular differentiations, with no significant difference, were observed in M. laciniata (FST=0.48) and M. truncatula (FST=0.47). Several quantitative traits exhibited significantly smaller QST than FST for M. laciniata, consistent with constraining selection. For M. truncatula, the majority of traits displayed no statistical difference in the level of QST and FST. Furthermore, these traits are significantly associated with eco-geographical factors, consistent with selection for local adaptation rather than genetic drift. In both species, there was no significant correlation between genetic variation at quantitative traits and molecular markers. The site-of-origin explains about 5.85% and 11.27% of total quantitative genetic variability among populations of M. laciniata and M. truncatula, respectively. Established correlations between quantitative traits and eco-geographical factors were generally more moderate for M. laciniata than for M. truncatula, suggesting that the two species exhibit different genetic bases of local adaptation to varying environmental conditions. Nevertheless, no consistent patterns of associations were found between gene diversity (He) and environmental factors in either species.

Salt-tolerant rhizobia isolated from a Tunisian oasis that are highly effective for symbiotic N2-fixation with Phaseolus vulgaris constitute a novel biovar (bv. mediterranense) of Sinorhizobium meliloti.

Nodulation of common bean was explored in six oases in the south of Tunisia. Nineteen isolates were characterized by PCR-RFLP of 16S rDNA. Three species of rhizobia were identified, Rhizobium etli, Rhizobium gallicum and Sinorhizobium meliloti. The diversity of the symbiotic genes was then assessed by PCR-RFLP of nodC and nifH genes. The majority of the symbiotic genotypes were conserved between oases and other soils of the north of the country. Sinorhizobia isolated from bean were then compared with isolates from Medicago truncatula plants grown in the oases soils. All the nodC types except for nodC type p that was specific to common bean isolates were shared by both hosts. The four isolates with nodC type p induced N(2)-fixing effective nodules on common bean but did not nodulate M. truncatula and Medicago sativa. The phylogenetic analysis of nifH and nodC genes showed that these isolates carry symbiotic genes different from those previously characterized among Medicago and bean symbionts, but closely related to those of S. fredii Spanish and Tunisian isolates effective in symbiosis with common bean but unable to nodulate soybean. The creation of a novel biovar shared by S. meliloti and S. fredii, bv. mediterranense, was proposed.

Agrobacterium strains isolated from root nodules of common bean specifically reduce nodulation by Rhizobium gallicum.

In a previous work, we showed that non-nodulating agrobacteria strains were able to colonize root nodules of common bean. Both rhizobia and agrobacteria co-existed in the infected nodules. No impact on symbiosis was found in laboratory conditions when using sterile gravel as a support for growth. In this study, soil samples originating from different geographic and agronomic regions in Tunisia were inoculated with a mixture of agrobacteria strains isolated previously from root nodules of common bean. A significant effect on nodulation and vegetal growth of common bean was observed. Characterization of nodulating rhizobia and comparison with non-inoculated controls showed a biased genetic structure. It seemed that Rhizobium gallicum was highly inhibited, whereas nodulation by Sinorhizobium medicae was favored. Co-inoculation of non-sterile soils with R. gallicum and agrobacteria confirmed these findings. In vitro antibiosis assays indicated that agrobacteria exercised a significant antagonism against R. gallicum.

Changes in ascorbate peroxidase, catalase, guaiacol peroxidase and superoxide dismutase activities in common bean (Phaseolus vulgaris) nodules under salt stress.

To analyse nodular antioxidant enzyme expression in response to salt stress, Phaseolus vulgaris genotype BAT477 was inoculated with reference strain CIAT899, and treated with 50 mM NaCl. Plant growth, nodulation and nitrogen fixing activity were analysed. Results showed that: (1) all parameters, particularly in nodules, were affected by salt treatments, and (2) confirmed preferential growth allocation to roots. The ARA was significantly decreased by salt treatments. Protein dosage confirmed that nodules were more affected by salt treatment than were roots. We analysed superoxide dismutase, catalase, ascorbate peroxidase and peroxidase in nodules, roots and a free rhizobial strain. Our results indicated that SOD and CAT nodular isozymes had bacterial and root origins. The SOD expressed the same CuZn, Fe and Mn SOD isoforms in nodules and roots, whereas in free rhizobia we found only one Fe and Mn SOD. APX and POX nodule and root profiles had only root origins, as no rhizobial band was detected. Under salt stress, plant growth, nitrogen fixation and activities of antioxidant defense enzymes in nodules were affected. Thus, these enzymes appear to preserve symbiosis from stress turned out that NaCl salinity lead to a differential regulation of distinct SOD and POX isoenzyme. So their levels in nodules appeared to be consistent with a symbiotic nitrogen fixing efficiency hypothesis, and they seem to function as the molecular mechanisms underlying the nodule response to salinity.

Colonization of Phaseolus vulgaris nodules by Agrobacterium-like strains.

Non-nodulating Agrobacterium-like strains identified among root nodule isolates of common bean were labeled with gusA, a reporter gene encoding beta-glucuronidase (GUS). Bean plants were then co-inoculated with an infective Rhizobium strain and labeled transconjugants of Agrobacterium-like strains. Blue staining of nodules showed that Agrobacterium-like strains were able to colonize these symbiotic organs. Isolation and characterization by restriction fragment length polymorphism analysis of PCR-amplified 16S rRNA genes revealed a mixed population of Rhizobium and Agrobacterium-like strains in all nodules showing GUS activity. PCR amplification of the nifH gene and nodulation tests did not show any evidence of acquisition of symbiotic gene by lateral transfer from Rhizobium to Agrobacterium-like strains. Moreover, these strains were able to invade mature nodules. Based on sequencing of the 16S rRNA gene, one of these Agrobacterium-like strains showed 99.4% sequence similarity with Agrobacterium bv. 1 reference strains and 99% similarity with an Agrobacterium bv. 1 strain isolated from Acacia mollisima in Senegal. Agrobacterium tumefaciens C58 and the disarmed variant AT123 did not show any ability to colonize nodules. Co-inoculation of bean seeds with Agrobacterium and Rhizobium strains did not enhance nodulation and plant yield under controlled conditions.

Nodule conductance varied among common bean (Phaseolus vulgaris) genotypes under phosphorus deficiency.

Common bean genotypes BAT477, COCOT, DOR364, Flamingo, and NAG310 were inoculated with Rhizobium tropici CIAT899 and grown under phosphorus deficiency. This treatment induced a significant decrease in shoot and nodule growth that varied among genotypes from 35% to 57% and from 45% to 61%, respectively, whereas root biomass was less affected. Phosphorus deficiency affected differently the genotypes for nodule number and size, and the responses of nodulated-root O2 uptake (Conr) to raising rhizospheric PO2. From the later data, nodule conductance could be computed by dividing the slope of the regression of Conr as a function of external pO2 by nodule surface area. It is concluded that differences in nodule conductance are related to genotypic tolerance to P deficiency.

Rhizobial strain involvement in plant growth, nodule protein composition and antioxidant enzyme activities of chickpea-rhizobia symbioses: modulation by salt stress.

Mesorhizobium ciceri, Mesorhizobium mediterraneum and Sinorhizobium medicae strains showed different symbiotic performances when inoculated to chickpea (Cicer arietinum L., cv. chetoui) at unstressed conditions and under salt stress. The analysis of nodular proteic composition and antioxidant enzyme activities revealed a polymorphism of patterns on SDS and native PAGE suggesting a potential dependence on the bacterial partner. Salt effect was analysed on plant growth, nitrogen fixation and antioxidant enzymes. M. ciceri, the most efficient strain, seemed to allow a best tolerance to chickpea plants under salt stress. This constraint did not affect the nodular superoxide dismutase (SOD, E.C. 1.15.1.1) activity of the symbiosis implicating the latter strain. This symbiosis showed the least decrease for the nodule protein level and the catalase (CAT, E.C. 1.11.1.6) activity, and the highest increase of peroxidase (POX, E.C. 1.11.1.7) activity that seemed to be related with the tolerance to salt.

Different species and symbiotic genotypes of field rhizobia can nodulate Phaseolus vulgaris in Tunisian soils.

Abstract A collection of 160 isolates of rhizobia nodulating Phaseolus vulgaris in three geographical regions in Tunisia was characterized by restriction fragment length polymorphism analysis of polymerase chain reaction (PCR)-amplified 16S rDNA, nifH and nodC genes. Nine groups of rhizobia were delineated: Rhizobium gallicum biovar (bv.) gallicum, Rhizobium leguminosarum bv. phaseoli and bv. viciae, Rhizobium etli bv. phaseoli, Rhizobium giardinii bv. giardinii, and four groups related to species of the genus Sinorhizobium, Sinorhizobium meliloti, Sinorhizobium medicae and Sinorhizobium fredii. The most abundant rhizobial species were R. gallicum, R. etli, and R. leguminosarum encompassing 29-20% of the isolates each. Among the isolates assigned to R. leguminosarum, two-thirds were ineffective in nitrogen fixation with P. vulgaris and harbored a symbiotic gene typical of the biovar viciae. The S. fredii-like isolates did not nodulate soybean plants but formed numerous effective nodules on P. vulgaris. Comparison of nodC gene sequences showed that their symbiotic genotype was not related to that of S. fredii, but to that of the S. fredii-like reference strain GR-06, which was isolated from a bean plant grown in a Spanish soil. An additional genotype including 16% of isolates was found to be closely related to species of the genus Agrobacterium. However, when re-examined, these isolates did not nodulate their original host.