Genetic control of sodium exclusion in tetraploid wheat.

Abstract

Worldwide, soil salinity is one of the major abiotic stress factors limiting crop production. Durum wheat (Triticum turgidum L. ssp. durum [Desf.]) is a relatively salt sensitive species. Its sensitivity to salt is thought to be due to its poor ability in limiting accumulation of toxic Na⁺ in leaf tissues. The present research was undertaken to investigate the genetic control of sodium exclusion in a tetraploid wheat population of recombinant inbred lines developed from a durum wheat cultivar Atil and a cultivated emmer wheat (Triticum turgidum L. ssp. dicoccum [Schrank].Thell.) accession PI94628. In order to estimate the nature and extent of variation for sodium exclusion within the progeny, two experiments were carried out using a supported hydroponic system. In both cases, Na⁺ and K⁺ concentration were determined from the fourth leaf of plants which had imposed a 100 mM NaCl stress for 10 days. In the first experiment, screening of replicated parental lines and a subset of inbred lines (24 RILs) indicated the existence of significant genetic variation for sodium exclusion within the population. It was also found that the spatial variation within the experimental equipment contributed only 8-9% of the total observed phenotypic variation. In the second experiment, screening of the entire population indicated transgressive segregation for both of the Na⁺ and K⁺ accumulation traits, with the durum wheat line Atil found to be the better sodium excluder. A significant negative correlation (r = - 0.7) was found between leaf Na⁺ and K⁺ concentration, however, neither of these traits was found to be correlated with the shoot biomass of 30-day-old seedlings (21 days under salt stress). To construct a genetic linkage map, 1057 markers (916 DArT and 141 SSRs) were used. Of these, 467 markers were eliminated from the linkage analysis due the segregation distortion. The remaining 495 DArTs and 95 SSR markers were used in map construction. They provided reasonable genome coverage (2136.5 cM), with a marker density of 3.6 cM/marker. The markers were distributed on 34 linkage groups representing parts of 14 chromosomes, with gaps of greater than 15 cM still remaining in 12 of the chromosomes. The majority of these markers showed conserved locations and orientation when compared to those described in previous genetic linkage maps of tetraploid and hexaploid wheat. Marker polymorphisms in two regions on chromosome 5A and 5B were significantly associated phenotypic variation for both Na⁺ and K⁺ accumulation in the leaf tissues. In both of these QTL regions, the durum wheat parent was contributed the alleles for lower Na⁺ concentration and higher K⁺ concentration. The two QTLs explained 28% and 19% of total phenotypic variation for Na⁺ and K⁺ accumulation, respectively. Both QTLs were mapped in the centromeric regions of the chromosomes. No QTLs for these traits have previously been reported in these regions. A highly significant QTL for shoot biomass was also mapped on chromosome 5A; explaining 16% of the phenotypic variation but this was over 32.4 cM from the leaf Na⁺ or K⁺ concentration QTLs. Furthermore, the QTLs were not found to be associated with traits related to vigour and/or vernalisation requirement. The molecular markers in the QTL regions detected here could serve as starting points for further characterisation of these genomic regions to elucidate the physiological and molecular bases of these QTLs.