Genetic analysis of physiological components of salt tolerance conferred by Solanum rootstocks. What is the rootstock doing for the scion?

Grafting desirable crop varieties on stress-tolerant rootstocks provides an opportunity to increase crop salt tolerance. Here, a commercial hybrid tomato variety was grafted on two populations of recombinant inbred lines developed from a salt-sensitive genotype of Solanum lycopersicum var. cerasiforme, as female parent, and two salt-tolerant lines, as male parents, from S. pimpinellifolium, the P population, and S. cheesmaniae, the C population, to identify an easy screening method for identifying rootstocks conferring salt tolerance in terms of fruit yield. Potential physiological components of salt tolerance were assessed in the scion: leaf biomass, [Na(+)], nutrition, water relations and xylem ABA concentration. A significant correlation between scion fruit yield and scion leaf fresh weight, water potential or the ABA concentration was found in the C population under salinity, but the only detected QTL did not support this relationship. The rootstocks of the P population clearly affected seven traits related to the sodium, phosphorous and copper concentrations and water content of the scion leaf, showing heritability estimates around 0.4 or higher. According to heritability estimates in the P population, up to five QTLs were detected per trait. QTLs contributing over 15% to the total variance were found for P and Cu concentrations and water content of the scion leaf, and the proportion of fresh root weight. Correlation and QTL analysis suggests that rootstock-mediated improvement of fruit yield in the P population under salinity is mainly explained by the rootstock's ability to minimise perturbations in scion water status.

Identification of fruit yield loci controlling the salt tolerance conferred by solanum rootstocks.

The rootstock effect on the fruit yield of a grafted tomato variety was genetically analyzed under salinity using as rootstock two populations of F(9) lines developed from a salt sensitive genotype of Solanum lycopersicum var. cerasiforme, as female parent, and two salt tolerant lines, as male parents, from S. pimpinellifolium, the P population (123 lines), and S. cheesmaniae, the C population (100 lines). There were rootstock lines from the two populations (up to 65% in the P population) that raised the fruit yield of the commercial hybrid under saline conditions. It is shown that this salt tolerance rootstock effect is a heritable trait (h (2) near 0.3), governed by at least eight QTLs. The most relevant component was the number of fruits. Thus most detected QTLs correspond to this component. In general, QTL gene effects are medium-sized, with contributions from 8.5 up to 15.9% at most, and the advantageous allele comes from the wild, salt tolerant species. Only two fruit yield QTLs on chromosomes P9 and C11 might correspond to fruit yield QTLs of the non-grafted lines indicating their root system dependence. A fruit yield QTL on chromosome 3 is acting epistatically in both populations. The epistatic interactions found were dominant and they were unveiled using the associated marker as cofactor in the composite interval mapping methodology. Therefore, an efficient and profitable utilization of wild germplasm can be carried out through the improvement of rootstocks that confer salt tolerance in terms of fruit yield to the grafted variety.

Genetic analysis of Na(+) and K (+) concentrations in leaf and stem as physiological components of salt tolerance in Tomato.

The sodium and potassium concentrations in leaf and stem have been genetically studied as physiological components of the vegetative and reproductive development in two populations of F(8) lines, derived from a salt sensitive genotype of Solanum lycopersicum cv. Cerasiforme, as female parent, and two salt tolerant lines, as male parents, from S. pimpinellifolium, the P population (142 lines), and S. cheesmaniae, the C population (116 lines). Genetic parameters of ten traits under salinity and five of them under control conditions were studied by ANOVA, correlation, principal component and QTL analysis to understand the global response of the plant. Two linkage maps including some tomato flowering time and salt tolerance candidate genes encoding for SlSOS1, SlSOS2, SlSOS3, LeNHX1, LeNHX3, were used for the QTL detection. Thirteen and 20 QTLs were detected under salinity in the P and C populations, respectively, and four under control conditions. Highly significant and contributing QTLs (over 40%) for the concentrations of Na(+) and K(+) in stems and leaves have been detected on chromosome 7 in both the populations. This is the only genomic position where the concentration QTLs for both the cations locate together. The proportion of QTLs significantly affected by salinity was larger in the P population (64.3%, including all QTLs detected under control) than in the C population (21.4%), where the estimated genetic component of variance was larger for most traits. A highly significant association between the leaf area and fruit yield under salinity was found only in the C population, which is supported by the location of QTLs for these traits in a common region of chromososome C1. As far as breeding for salt tolerance is concerned, only two sodium QTLs (lnc1.1 and lnc8.1) map in genomic regions of C1 and C8 where fruit yield QTLs are also located but in both the cases the profitable allele corresponds to the salt sensitive, cultivated species. One of those QTLs, lnc1.1 might involve LeNHX3.

Comparative QTL analysis of salinity tolerance in terms of fruit yield using two Solanum populations of F7 lines.

Salt tolerance has been analysed in two populations of F(7) lines developed from a salt sensitive genotype of Solanum lycopersicum var. cerasiforme, as female parent, and two salt tolerant lines, as male parents, from S. pimpinellifolium, the P population (142 lines), and S. cheesmaniae, the C population (116 lines). Salinity effects on 19 quantitative traits including fruit yield were investigated by correlation, principal component analysis, ANOVA and QTL analysis. A total of 153 and 124 markers were genotyped in the P and C populations, respectively. Some flowering time and salt tolerance candidate genes were included. Since most traits deviated from a normal distribution, results based on the Kruskal-Wallis non-parametric test were preferred. Interval mapping methodology and ANOVA were also used for QTL detection. Eight out of 15 QTLs at each population were detected for the target traits under both control and high salinity conditions, and among them, only average fruit weight (FW) and fruit number (FN) QTLs (fw1.1, fw2.1 and fn1.2) were detected in both populations. The individual contribution of QTLs were, in general, low. After leaf chloride concentration, flowering time is the trait most affected by salinity because different QTLs are detected and some of their QTLxE interactions have been found significant. Also reinforcing the interest on information provided by QTL analysis, it has been found that non-correlated traits may present QTL(s) that are associated with the same marker. A few salinity specific QTLs for fruit yield, not associated with detrimental effects, might be used to increase tomato salt tolerance. The beneficial allele at two of them, fw8.1 (in C) and tw8.1 (for total fruit weight in P) corresponds to the salt sensitive parent, suggesting that the effect of the genetic background is crucial to breed for wide adaptation using wild germplasm.

Comparative microsatellite linkage analysis and genetic structure of two populations of F6 lines derived from Lycopersicon pimpinellifolium and L. cheesmanii.

A population of recombinant inbred lines (RILs) has several advantages over its F2 population counterpart with respect to quantitative trait loci (QTLs) and genomic studies. The objective of the investigation reported here was the comparative characterization by simple sequence repeat (SSR) and sequence characterized amplified region (SCAR) markers of two populations of F6 lines derived from Lycopersicon pimpinellifolium (P population, consisting of 142 lines) and L. cheesmanii (C population, consisting of 115 lines) and sharing the female parent, L. esculentum var. cerasiforme. Almost the same percentage of polymorphic markers was found for each population although a different set of markers was involved. The proportion of SSR primer pairs (93 in total) that resulted in polymorphism for the main band was larger (55-56%) than for SCAR ones (13-16%). The C population showed the largest proportion of markers with zygotic and gametic segregation distortion, which is in agreement with the larger genetic distance reported between L. esculentum and L. cheesmanii than with the former and L. pimpinellifolium. Zygotic distortion corresponded primarily to an excess of heterozygotes in both populations, suggesting that the increment of homozygosity was the main factor limiting viability/self-fertility of the lines. Despite both populations sharing the female parent, P alleles were slightly favored in the P population while E alleles were the most frequently fixed in the C population. A linkage map for each population was obtained, with the average distances between consecutive markers being 3.8 cM or 3.4 cM depending on the population. Discrepancy between the maps for the location of only four markers on chromosomes 3, 6 and 10 was observed. Two possible causes of this discrepancy were investigated and can not be discarded: (1) the presence of duplicated markers and (2) segregation distortion caused by the selective advantage of gametes carrying one of the two alleles. This marker characterization of both populations will continue and will enable the comparative QTLs and candidate gene analysis of complex traits towards a more efficient utilization of genetic resources and breeding strategies.