There is considerable variability in salt tolerance amongst members of the Triticeae, with the tribe even containing a number of halophytes. This is a review of what is known of the differences in salt tolerance of selected species in this tribe of grasses, and the potential to use wild species to improve salt tolerance in wheat. Most investigators have concentrated on differences in ion accumulation in leaves, describing a desirable phenotype with low leaf Na+ concentration and a high K+/Na+ ratio. Little information is available on other traits (such as "tissue tolerance" of accumulated Na+ and Cl-) that might also contribute to salt tolerance. The sources of Na+ "exclusion" amongst the various genomes that make up tetraploid (AABB) durum wheat (Triticum turgidum L. ssp. durum), hexaploid (AABBDD) bread wheat (Triticum aestivum L. ssp. aestivum), and wild relatives (e.g. Aegilops spp., Thinopyrum spp., Elytrigia elongata syn. Lophopyrum elongatum, Hordeum spp.) are described. The halophytes display a capacity for Na+ "exclusion", and in some cases Cl- "exclusion", even at relatively high salinity. Significantly, it is possible to hybridize several wild species in the Triticeae with durum and bread wheat. Progenitors have been used to make synthetic hexaploids. Halophytic relatives, such as tall wheatgrass spp., have been used to produce amphiploids, disomic chromosome addition and substitution lines, and recombinant lines in wheat. Examples of improved Na+ "exclusion" and enhanced salt tolerance in various derivatives from these various hybridization programmes are given. As several sources of improved Na+ "exclusion" are now known to reside on different chromosomes in various genomes of species in the Triticeae, further work to identify the underlying mechanisms and then to pyramid the controlling genes for the various traits, that could act additively or even synergistically, might enable substantial gains in salt tolerance to be achieved.

Use of wild relatives to improve salt tolerance in wheat

The origin of tetraploid wheat and the divergence of diploid ancestors of wheat A and D genomes were estimated to have occurred 0.36 and 2.7 million years ago, respectively. These estimates and the evolutionary history of 3159 gene loci were used to estimate the rates with which gene loci have been deleted and duplicated during the evolution of wheat diploid ancestors and during the evolution of polyploid wheat. During diploid evolution, the deletion rate was 2.1 × 10−3 locus−1 MY−1 for single-copy loci and 1.0 × 10−2 locus−1 MY−1 for loci in paralogous sets. Loci were duplicated with a rate of 2.9 × 10−3 locus−1 MY−1 during diploid evolution. During polyploid evolution, locus deletion and locus duplication rates were 1.8 × 10−2 and 1.8 × 10−3 locus−1 MY−1, respectively. Locus deletion and duplication rates correlated positively with the distance of the locus from the centromere and the recombination rate during diploid evolution. The functions of deleted and duplicated loci were inferred to gain insight into the surprisingly high rate of deletions of loci present apparently only once in a genome. The significance of these findings for genome evolution at the diploid and polyploid level is discussed.

https://www.genetics.org/content/genetics/171/1/323.full.pdf

Tempos of Gene Locus Deletions and Duplications and Their Relationship to Recombination Rate During Diploid and Polyploid Evolution in the Aegilops-Triticum Alliance

Loci detected by Southern blot hybridization of 3,977 expressed sequence tag unigenes were mapped into 159 chromosome bins delineated by breakpoints of a series of overlapping deletions. These data were used to assess synteny levels along homoeologous chromosomes of the wheat A, B, and D genomes, in relation to both bin position on the centromere-telomere axis and the gradient of recombination rates along chromosome arms. Synteny level decreased with the distance of a chromosome region from the centromere. It also decreased with an increase in recombination rates along the average chromosome arm. There were twice as many unique loci in the B genome than in the A and D genomes, and synteny levels between the B genome chromosomes and the A and D genome homoeologues were lower than those between the A and D genome homoeologues. These differences among the wheat genomes were attributed to differences in the mating systems of wheat diploid ancestors. Synteny perturbations were characterized in 31 paralogous sets of loci with perturbed synteny. Both insertions and deletions of loci were detected and both preferentially occurred in high recombination regions of chromosomes.

https://www.pnas.org/content/pnas/100/19/10836.full.pdf

Synteny perturbations between wheat homoeologous chromosomes caused by locus duplications and deletions correlate with recombination rates

Variation in restriction fragments in nullisomic–tetrasomic and ditelosomic lines of Triticum aestivum 'Chinese Spring' and disomic and ditelosomic substitutions of chromosomes of diploid species Lophopyrum elongatum, T. monococcum ssp. aegilopoides, T. tauschii, and T. umbellulatum for 'Chinese Spring' chromosomes were used to identify chromosomal loci of 5S rRNA genes (5S DNA) in wheat and related species. These loci are on wheat chromosome arms 1BS, 1DS, 5AS, 5BS, and tentatively 5DS, T. m. aegilopoides chromosomes 1A and 5A, T. tauschii chromosomes 1D and 5D, and T. umbellulatum chromosome 5U. In diploid L. elongatum a locus was detected on chromosome arm 1ES. In most genomes, the locus on chromosome 1 contains 5S DNA subfamily with short spacers and the locus on chromosome 5 contains 5S DNA subfamily with long spacers. Only a few genomes were found to be potential exceptions to this rule. Concerted evolution of the 5S DNA loci was examined in several genomes. It appeared that homogenization of spacer...

Organization and evolution of the 5S ribosomal RNA gene family in wheat and related species

Lophopyrum elongatum is a facultative halophyte related to wheat. Eleven unique clones corresponding to genes showing enhanced mRNA accumulation in the early stages of salt stress were previously isolated from a L. elongatum salt-stressed-root cDNA library. The chromosomal distribution of genes complementary to these clones in several genomes of the tribe Triticeae and their copy number in the L. elongatum and wheat genomes are reported. Genes complementary to clones pESI4, pESI14, pESI15, pESI28, and pESI32 were found in homoeologous group 5, those complementary to pESI18 and pESI35 in homoeologous group 6, and those complementary to pESI47, pESI48, pESI3, and pESI2 in homoeologous groups 1, 3, 4, and 7, respectively. The genes are present in a single copy per genome in L. elongatum with the exception of those complementary to pESI2 and pESI18 which are present in at least two and five copies, respectively. Since similar copy numbers per genome were found in wheat (except for pESI2), the ability of L. elongatum to accumulate higher mRNA levels than wheat in response to salt shock apears to have evolved by changes in the regulation of these genes.

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Comparison of the genetic organization of the early salt-stress-response gene system in salt-tolerant Lophopyrum elongatum and salt-sensitive wheat.

Isozyme markers were used to develop Triticum aestivum cv. Chinese Spring–Elytrigia elongata (= Agropyron elongatum, 2n = 14, genome E) disomic 3E and 5E addition lines. Subsequently, all possible ...

G. E. Hart

Papers

Isolation and characterization of wheat–Elytrigia elongata chromosome 3E and 5E addition and substitution lines