Rye (Secale cereale subsp. cereale L.) has long been exploited as a valuable alternative genetic resource in wheat (Triticum aestivum L.) breeding. Indeed, the introgression of rye genetic material led to significant breakthroughs in the improvement of disease and pest resistance of wheat, as well as a few agronomic traits.
1. Rye, an Ally to Wheat Breeding
The endeavour of combining the genetic material of rye (
S. cereale subsp.
cereale L.) and wheat (
T. aestivum L.) has a long history. The initial interest towards wheat–rye hybridisation resided in the possibility to combine the yield potential and technological qualities of wheat with the adaptability and resilience of rye. The first recorded attempts at wheat–rye hybridisation date back to the 19th century, and it was in 1875 that Scottish agronomist A. S. Wilson reported the very first viable, but infertile, wheat–rye hybrid plant [
1]. Eventually, these breeding efforts even gave rise to an entirely new crop: the man-made hybrid cereal triticale (×
Triticosecale Wittm.) [
1]. In triticale, the entire chromosome set of rye (RR) is combined with one (AA), two (AABB) or all three (AABBDD) chromosome sets of wheat, resulting in tetraploid, hexaploid, and octoploid triticale, respectively [
1]. Nevertheless, only hexaploid triticale (AABBRR) is a successfully established crop, whereas the other forms find application as breeding material [
1].
Of course, whole-genome introgression such as in the case of triticale has implications that go beyond the scope of improving a single or a few specific traits of interest in wheat itself. Thus, introgressions from rye to wheat at sub-genome scale, in the form of single-chromosome substitution, or translocation of chromosome segments have also been widely researched in wheat breeding since the 1920s [
2]. The main goal has mostly been the enhancement of wheat resistance to pests and diseases, and to this end, the 1RS, 2RL, 3RS, 4RS, 4RL, and 6RL rye chromosome segments have been introgressed in the wheat background [
3]. Among these, the introgression of the 1RS segment, in the form of 1RS.1BL or 1RS.1AL translocation, was the first to be achieved, and the one that undoubtedly had the greatest agricultural impact [
2,
3]. Loci for resistance to leaf, stem, and stripe rust, and to powdery mildew were introduced by means of this translocation in hundreds of wheat cultivars released between the 1960s and 1990s [
3]. The success of this translocation lasted even after these resistances were broken [
2]. This continued success is likely to be attributed to a locus for root biomass present on the 1RS segment, which positively affected the yield of the winter wheat varieties carrying the translocation [
2,
4].
Clearly then, it can be worthwhile to investigate rye chromatin not only for the presence of loci related to disease and pest resistance. For example, genetic studies on triticale already evidenced the contribution of its R genome to several other traits of agronomic interest, such as aluminium tolerance, biomass yield, and allelopathic effects [
5,
6,
7]. Nevertheless, wheat grain and end-use quality traits were never considered as traits that could profit from the introgression of rye chromatin. Rather, until now, rye chromatin has been viewed as detrimental to wheat quality. For example, a crucial issue that quickly emerged with the 1RS.1BL or 1RS.1AL translocations was the concomitant introgression in the wheat background of a major rye storage protein locus—
Sec-1—which encodes for γ-and ω-secalins [
8]. At the same time, gliadin loci and
Glu-3 loci encoding for low-molecular-weight glutenin subunits are lost together with the 1BS or 1AS arm, resulting in an overall reduction in end-use quality, with 1RS.1BL wheats suffering from the most severe quality drawbacks [
9,
10]. Nevertheless, “quality” does not refer only to the suitability for a certain end-use—such as, for example, baking, in the case of wheat—but also to nutritional quality. In recent years, nutritional aspects of wheat-based products have been gaining more and more popularity among consumers, and the wheat industry is adapting to this trend as well [
11,
12]. It is in the context of breeding for wheat nutritional qualities that rye chromatin can once more find a valuable and innovative application; thanks to the higher contents of dietary fibre (DF) components and bioactive compounds, such as folates or sterols, of rye [
13].
2. A Case Study for Wheat Quality Breeding: Dietary Fibre Content
DF content is a nutritional aspect that has been receiving an ever-increasing attention in the cereal food industry, thanks to the rich variety of health benefits that are associated with an adequate DF intake [11,14]. Cereal-based products constitute the major source of DF in the diets of many European countries for example, providing about 33% of DF in countries such as Belgium and Spain, and up to 49% of total DF in Sweden [14]. Wheat white flour is often a main ingredient of such products. Conventional milling to yield white flour separates the starchy endosperm from the fibre-rich bran layers, thus reducing considerably the DF provision of white flour-based foods.
The principal constituent of wheat white flour DF is the cell wall polysaccharide arabinoxylan (AX), where it accounts for 60-70% of total DF [30–32]. Enhancing wheat AX content to improve white flour DF provision is becoming an established goal in the European wheat breeding work frame [34]. In the context of this breeding effort, rye has considerable potential as an alternative genetic resource. In fact, AX is the principal cell wall polysaccharide also in the rye endosperm and therefore, the main source of DF in rye flour as well [13,39]. In terms of AX content, rye is known to be richer in AX compared to wheat – with rye flour AX content ranging from 3.11% to 4.31% of dry matter [13]; whereas AX content of wheat white flour ranges from 1.35% to 2.75% of dry matter [35].
A proof of concept of the increasing effect of rye genetic material on wheat flour AX content can be readily found in triticale – the hybrid progeny of wheat and rye [42]. Although no comprehensive genetic study is yet available on the specific contribution of triticale’s R genome to its AX content, the AX content of triticale was shown to be intermediate to that of wheat and rye, and to also show inter-cultivar variation [42]. Furthermore, recent works on wheat-rye single chromosome addition lines yielded promising results for this novel application of rye chromatin in wheat quality breeding [43–45]. Namely, addition of chromosomes 1R, 4R, and 6R lead to a significant increase in AX content in the disomic addition lines, compared to the wheat parent [43–45].
3. Challenges With the Introgression of Rye Chromatin
As discussed in Section 1, there are multiple successful examples of introgression of target rye chromatin in wheat [2–4]. This endeavour is mostly achieved by means of traditional breeding methods and relies on spontaneous recombination between wheat and rye chromosomes [2]. Thus, it presents two key challenges. First, crossability of wheat with rye is under genetic control. Three major genes, Kr1 on 5BL, Kr2 on 5AL, and SKr on 5BS, are known to affect this trait in wheat [90–92]. In their dominant state, these genes actively repress crossability, whereas in their recessive state they are “null” alleles [93]. The crossable phenotype occurs at high frequency in wheat landraces and varieties from Asia (i.e., China, Japan, East Siberia, and Iran), whereas West European wheat varieties are mostly poorly crossable with rye [93].
Second, translocation of rye chromosome segments onto wheat chromosomes depends on the degree of homoeology between the wheat and rye chromosomes involved [2]. Comparison of the genetic maps of wheat and rye revealed overall extensive conserved collinearity [94]. These new results are in accordance with previous studies on wheat-rye homoeology as well as practical breeding experience [3,95,96]. Complete synteny is realised only between chromosome 1R and wheat chromosome group 1 [94–96]. Next, chromosomes 2R, 3R, 5R, and 6R are mostly syntenic with their respective wheat chromosomes groups. Nevertheless, in chromosome 6R there is evidence for an important translocation event between the distal segment of its long arm and the corresponding distal segment in wheat chromosome group 3L [94,95]. Lastly, synteny between chromosomes 4R and 7R and wheat chromosome groups 4 and 7 is limited to narrow stretches of the chromosomes and there is evidence for several translocation events [94,95]. Most notably, an extensive translocation occurred between a consistent segment of the long arm of chromosome 4R and the short arm of wheat chromosome group 7; however, the short arm of chromosome 7R shows homoeology with the long arm of only wheat chromosome 4A [94,95]. These results on wheat-rye homoeology are important to consider when approaching the introgression of AX-related rye chromatin, as the complex homoeology relationship between the rye chromosomes and the wheat genome can determine additional challenges for the introgression of rye chromatin in wheat.
This entry is adapted from the peer-reviewed paper 10.3390/plants12040737
