Barley Genetic Resources: Advancing Conservation and Applications for Breeding—Series II

1. Introduction

The changing climate conditions and the growing world population require a constant increase in agricultural production. New crop cultivars with high and stable yield potential play an important role in reaching this goal. They often have durable and effective resistance to current biotic and abiotic stresses. Plant breeders cooperate with geneticists, gene bank curators, and scientists, working on resistance and tolerance to all kinds of stresses hampering the yield obtained by farmers. Agricultural products from such new cultivars also have to meet higher standards concerning their nutrition and technological value for consumers and the agroindustry. These intensive and complex breeding activities require the support of the broader and more precise exploitation of plant genetic resources (PGRs) [1,2].

The genetic uniformity of barley (Hordeum vulgare L.) modern cultivars is causing greater vulnerability to the negative effects of more unstable weather conditions caused by climate change [3,4,5]. Gene banks harbor a large collection of PGRs characterized and conserved for present and future generations. It is estimated that more than four hundred thousand barley accessions are stored in about fifty major barley collections worldwide [5,6]. Currently, many breeding programs use novel sources of genetic variation, which often originate directly from gene banks or are the result of pre-breeding efforts. Through broadening the genetic basis of their initial plant material, barley breeders attain gains in terms of the productivity and quality of new cultivars, and at the same time, they are increasingly responding efficiently to stresses caused by climate change [3,7].

Intensive research conducted by many teams of scientists concerning the advancement of conservation and applications in the breeding of barley genetic resources is very important from an economic point of view. Barley is one of the most important cereal crops in the world. It is predicted that this crop will be more important in the global food chain due to its adaptation to a wide range of altitudes and, in particular, its relatively high resistance or tolerance to biotic and abiotic stresses such as drought and soil salinity. In addition, there has been a growing interest in recent years in barley grain, not only as a form of food providing the necessary calory intake for humans, but also as a so-called “healthy food” with many health benefits in human diets. In several regions of the world, barley grains are economically important and used as livestock feed, malt, and foods, including roasted grains as a coffee substitute [2,3,8,9].

The importance of barley PGRs in breeding has increased because New Genomic Techniques (NGTs) are more commonly used to characterize available germplasm. In addition, high-throughput technologies are being developed to enable the analyses of extensive omics datasets for genomics and phenomics. These technologies significantly advance plant research and provide information about existing biodiversity [10,11,12,13,14,15]. It is predicted that in the next 10 years, plant breeders will have detailed information concerning the molecular characterization of PGRs available in gene banks for their specific breeding traits of interest. This molecular characterization is crucial for further improvement in plant genetics, especially concerning tolerance and resistance to biotic and abiotic stresses [10,11,12,13,14,15,16,17,18].

However, research activities related to the conservation and characterization of PGRs in gene banks remain costly and often have long-term returns [11,12,13,14,15]. There is therefore an urgent need to properly coordinate efforts concerning the efficient utilization of available resources in plant breeding programs in order to optimize these efforts. It is growing increasingly obvious to a rising number of gene bank managers that it is impossible to cover all aspects of the molecular characterization of their—often—significantly vast and diverse crop collections to satisfy all the needs of breeders. Highly specialized staff and equipment are frequently needed in connection with the necessary funds provided, which are often unavailable in gene banks. There is a need for specialized researchers to act as the necessary link between the curators of crop collection in gene banks and commercial breeders. Such researchers often conduct highly specialized studies to characterize in detail the traits identified by the breeder community as economically important, e.g., tolerance or resistance to specific pests and pathogens. These types of pre-breeding or germplasm enhancement are a very important part of crop improvement efforts, and many initiatives and projects on national and international levels were developed in this area [10,11,12,13,14,15,16,17,18]. This was one of the main rationales for the proposal of this Special Issue.

Gene bank managers and curators of crop germplasm collections often have to make difficult choices concerning using their staff and funds for specific gene bank activities. On one hand, they would prefer their accessions to be widely and effectively used by breeders, but at the same time, the primary goal of a gene bank is to conserve and maintain the genetic diversity of their crop collections. This is why pre-breeding activities are so crucial for the improvement of all major global crops, not only those in the frame of gene banks but also in other specialized institutions and labs. It should be noted that the pre-breeding approach consists not only of the detailed characterization of available germplasm for the specific traits of interest of breeders, but also usually of carrying out a wide cross between an elite and a donor parent [13,14,15,16,17,18]. This cross has to be followed by phenotypic observation if the trait of interest is expressed. To obtain plants with general high-yielding parameters, there is a need to repeat crosses (backcrossing) to the elite parent. This kind of work is often tedious and slow. In addition, it can be performed only for one or very few traits at a time and is conducted frequently by specialists with specific skills, e.g., plant pathologists, plant physiologists, and molecular biologists. The marker-assisted selection can speed up this process and the number of backcrosses needed is reduced. However, it has to be performed with caution, and there is a need to conduct foreground selection (for the gene of interest) and background selection (for the high amount of elite genome) at the same time [15,16,17,18,19,20,21].

Pre-breeding studies and germplasm enhancement are especially necessary for the characterization of crop wild relatives (CWRs) and the introduction of their specific economically important traits into the context of modern breeding lines or elite cultivars. This is often the only means of making the germplasm traits of CWRs available to plant breeders so as to address their major breeding aims, e.g., resistance or tolerance to stresses [15,16,17,18,19,20,21,22]. Based on my experience in cooperating with commercial plant breeders, breeders are very reluctant to directly use CWRs in an effort to produce new barley cultivars that outperform the present ones. The practical reason is simple: plant breeders almost always prefer to create genetically diverse initial materials via crosses among the currently highest yielding elite cultivars and breeding lines. Their major focus in breeding is the high and stable yielding of new commercial cultivars. The strategy frequently used by breeders is to “cross the best with the best” currently available cultivars or breeding lines from their point of view, and carefully select the best variants in the obtained pedigree in the proper breeding process.

To conclude, as part of the efforts in constantly improving barley, there has to be close cooperation and exchange of information between three major groups of actors: gene bank staff, pre-breeding researchers, and commercial breeders [17,18,19,20,21,22,23]. The present Special Issue—Series II focuses on “Barley Genetic Resources: Advancing Conservation and Applications for Breeding”. Four papers have been selected, which cover research topics that are divided into three sections: (1) characteristics of barley genetic resources for breeding to improve important agronomic traits (contribution 1), (2) barley genetic resources as sources of tolerance or resistance to abiotic stresses (contribution 2), and (3) barley genetic resources as sources of resistance to biotic stresses (contributions 3 and 4). The papers presented herein significantly contribute to broadening our knowledge concerning the characterization of barley germplasm. This knowledge can be used by scientists and breeders to improve the characteristics of future cultivars. These cultivars should have better agronomic characteristics, including their phenotype (e.g., plant height and leaf morphology) and tolerance or resistance to abiotic (e.g., cadmium contaminations) and biotic stresses (e.g., fungal diseases), and they will provide farmers with stable and high income, making their farms resilient to more unpredictable weather conditions caused by climate change. In the following paragraphs, the papers in this Special Issue will be briefly described. The purpose of this Editorial is not to elaborate on each of the texts but rather to encourage the reader to explore them and to make an effort to use the data and conclusions presented for their present and/or future research.

2. Characteristics of Barley Genetic Resources for Breeding to Improve Important Agronomic Traits

The research conducted by Xinyao Hong, Hui Deng, Yuxuan Zhao, Jiang Qi, Xinyu Huang, Chao Lv, FeifeiWang, Juan Zhu, Rugen Xu, and Baojian Guo (contribution 1) from Yangzhou University, China, resulted in the identification and characterization of the m-876 mutant in barley (Hordeum vulgare L.), which exhibited an extreme reduction in leaf width and plant height. Their genetic analysis revealed that the m-876 mutant was controlled by a single recessive gene. In the paper, it was described as a map-based cloning strategy used to narrow down the m-876 mutant to an 11.4 Mb genomic interval on the long arm of chromosome 5. Through analyzing the gene annotation information and nucleotide sequences, it was found that HvWOX3A (HORVU.MOREX.r3.5HG0467090) had a G-to-A substitution at the second exon in the m-876 mutant, resulting in a change in the coding amino acid from Tryptophan to a premature stop codon at the 200th amino acid position. It was concluded that the mutation of the HvWOX3A gene leads to changes in gene expression in the m-876 mutant.

In summary, their results indicated that the loss function of the HORVU.MOREX.r3.5HG0467090 gene might be responsible for the phenotypic variation in barley mutants. Their research is interesting and has practical potential for breeding programs because the application of high-yielding semi-dwarf varieties plays an important role in food production and the grain yield of cereal crops. A decrease in barley plant height was their main strategy for increasing the grain yield and the harvest index through reduced crop lodging.

3. Barley Genetic Resources as Sources of Tolerance or Resistance to Abiotic Stresses

The article by Nawroz Abdul-razzak Tahir, Djshwar Dhahir Lateef, Kamil Mahmu Mustafa, Kamaran Salh Rasul, and Fawzy Faidhullah Khurshid from the University of Sulaimani, Iraq, addresses the issue of barley tolerance to cadmium. In their work, they identified characteristics linked to cadmium tolerance by examining the phenotypic, physiological, and biochemical responses of fifty-nine barley accessions from all regions of Iraq to three different cadmium concentrations at the seedling stage. Moreover, in the study, the biomarker parameters were identified, which would aid in the early-growth-stage selection of the best-performing accession. As a result of testing three barley accessions, Acsad-14, ABN, and Arabi Aswad, were found to be the most tolerant accessions under all cadmium exposure. Their OMIC analysis identified that the biomarker parameters for differentiating the high-, moderate-, and low-tolerant groups were as follows: WU for Cd-125 stress; GPA, WU, CAT, and total phenolic content for Cd-250 stress; and all physiochemical traits, except the CAT feature, for Cd-500 treatment. A majority of the trait pairings showed significant correlations. The important contribution of the study for breeding programs lies in determining soil contamination caused by heavy metals such as cadmium, which is present in many countries around the world and often negatively results in crop performance during the seedling stage, as well as their performance and growth. In addition, heavy metals can cause long-term toxic effects on the health of the ecosystem and humans. In summary, Acsad-14, ABN, and Arabi Aswad barley accessions, which showed great performance under cadmium conditions, are potential candidates for selection in a breeding program to improve the growth of plants and crops in lands contaminated by cadmium. An important conclusion from the study is that seed water uptake, guaiacol peroxidase, and proline content were identified as biomarker traits that would aid in the early-growth-stage selection of the best-performing accession under cadmium stress conditions.

4. Barley Genetic Resources as Sources of Resistance to Biotic Stresses

Two papers are grouped in this section that deal with barley genetic resources as sources of resistance to biotic stresses. The research conducted by Jerzy H. Czembor and Elzbieta Czembor (contribution 3) presents an investigation to detect new sources of barley powdery mildew (Blumeria hordei) resistance in 81 accessions of wild barley (H. vulgare subsp. spontaneum) collected in Jordan, Lebanon, and Libya. European differential isolates of the fungus were used with major virulences present, and resistant single plant lines were selected. After an analysis of the obtained results, it was concluded that all 31 tested single plant lines of wild barley had genes for resistance that are not represented in the barley differential set for powdery mildew resistance genes. Twenty-six tested lines selected from accessions originating from Jordan and Libya showed resistance reactions to all the isolates used.

The importance of the study lies in the fact that B. hordei is a fungus that is considered to be one of the most economically important pathogens on barley and can cause economically significant yield losses. Many studies have shown that it is rapidly developing many new races and that its spores are dispersed by wind over long distances. It occurs in many barley-growing regions of the world, but it is especially common in Europe. Moreover, the grain yield obtained from barley fields where this disease was present is very often characterized by lower-quality characteristics that are important in malt production, such as a higher content of grain protein and a lack of proper grain size uniformity.

The identified lines of wild barley with highly effective resistance to powdery mildew will be further tested as new sources of effective resistance are used in barley pre-breeding programs. The obtained results confirm that the wild barley accessions stored in gene banks may offer added value for the preservation and use of barley biodiversity. The authors discussed the importance of incorporating new sources of resistance to breeding programs and their importance for agricultural production in longer periods and climate change conditions.

The next article by Kadir Akan, Ahmet Cat, Onur Hocaoglu, and Mehmet Tekin (contribution 4) from Türkiye (Turkey) describes the diversity of 40 Turkish barley varieties and their reactions to scald disease, caused by the fungus Rhynchosporium commune (formerly known as Rhynchosporium secalis). The three objectives of this study were (i) to determine the reactions of some Turkish barley varieties against scald, (ii) to identify the disease reaction groups such as immune, resistant, moderately resistant, moderately susceptible, and susceptible, and (iii) to evaluate the identified reaction groups by performing genotype plus genotype-by-environment (GGE) Biplot analysis. The 40 Turkish barley varieties were evaluated under natural conditions and the reactions of barley varieties were assessed using a newly designed two-digit scale ranging from 11 to 99. In addition, genotype plus genotype-by-environment (GGE) interactions of scale values were analyzed through GGE Biplot and explained 97.65% of the total variation. The ranking of genotypes based on scale groups generally showed consistency with GGE Biplot results, but GGE Biplot offered a more detailed classification, especially for moderately susceptible varieties. The relationship between the two methods indicated the relative stability of variety reactions, as GGE Biplot analysis also considered genotype stability.

The practical importance of this research for barley breeding is due to the fact that seven barley varieties (Yesilköy 387, Zafer 160, Avcı-2002, Kıral-97, Erginel 90, Çetin 2000, and Akhisar 98) were determined as resistant to scald. This result shows the important potential of these varieties for breeding for resistance to this disease. The identified resistant varieties can serve as valuable genetic resources for further genetic studies concerning the resistance of barley to scald. In addition, the assessment of genotype stability is a requirement for the field trials conducted in multiple years and locations. In this study, the results of GGE Biplot reveal the insights of the multi-environmental data as a whole, complementing the scale assessment which does not relate to the genotype–environment interaction. The results suggest that using the GGE Biplot to determine scald reactions of Turkish barley varieties is convenient for the characterization of barley varieties grown under varying environments over the years. In conclusion, the use of the newly developed scale for evaluating scald reactions in barley provides reliable results.

5. Conclusions

In summary, this Special Issue brought together the recent findings and the literature on the characterization of barley germplasm. The research presented in this Special Issue confirms that gene banks harbor a large “green treasure” of PGRs. There is an urgent need to use them effectively in barley improvement, and often, pre-breeding work is needed in cooperation with barley breeders to introduce traits of interest into elite breeding materials. Plant breeders realize that the concentration on the use of elite breeding lines and/or cultivars as initial materials leads to the limitation of genetic diversity (genetic erosion) in their breeding materials. As a consequence, the breeding materials and new cultivars are often vulnerable to adapting to emerging threats connected with emerging abiotic and biotic stresses [23,24]. In more unstable agrometeorological conditions in many regions of the world, such characteristics as yield stability based on resistance and tolerance to often-novel stresses is a key to the commercial success of newly registered cultivars. Pre-breeding programs must be based on information and the collaboration of gene bank curators and plant breeders to be able to provide novel and diverse genetic variability to meet current challenges in agricultural production. Enhanced barley germplasm lines resulting from pre-breeding activities with backgrounds of known cultivars with high and stable yields can be used directly by breeders in their breeding programs. Breeders frequently aim to gather complementary physiological traits to raise yield potential and stability in new cultivars [23,25]. The published articles in the current Special Issue do not constitute a fully comprehensive collection of this interdisciplinary research topic. This Special Issue lacks highly important topics such as the improvement in grain quality for food and malt production. However, we hope that it provides a stimulus for more research on the characterization of barley genetic resources for breeding.