1 Introducion

The Triticeae tribe, a taxon within the Poaceae family, encompasses several of the world's most vital cereal crops, including wheat, barley, and rye, as well as numerous forage grasses (Yen and Yang, 2009). These species have been fundamental to human agriculture since the dawn of civilization, particularly in temperate regions where they serve as primary food sources (Mascher et al., 2017),. The tribe consists of approximately 325 species, with around 250 being perennials that are crucial for forage. The cultivated members of Triticeae, such as wheat and barley, are globally significant, while rye holds regional importance (Merker, 2008). The genetic diversity within this tribe is immense, providing a vast reservoir of traits that can be harnessed for crop improvement, particularly in response to environmental and biotic stresses (Feuillet et al., 2008).

Understanding the taxonomy and genetic resources of the Triticeae tribe is essential for several reasons. Firstly, taxonomic classification provides a framework for identifying and categorizing species, which is crucial for the effective utilization of germplasm in breeding programs (Yen and Yang, 2009). Traditional taxonomic methods based on morphology have limitations, often leading to misclassification due to environmental influences on phenotypic traits. Modern approaches incorporating cytogenetic and molecular genomic analyses offer more accurate classifications, reflecting true phylogenetic relationships (Yen and Yang, 2009). Secondly, the genetic resources within Triticeae, including wild and weedy taxa, represent a gigantic gene pool that is invaluable for crop improvement (Bothmer et al., 2008). These resources are particularly important for enhancing traits such as drought and salt tolerance, which are critical for adapting to climate change and ensuring food security (Nevo and Chen, 2010). The integration of genetic information through databases and advanced breeding techniques further accelerates the discovery and utilization of key loci involved in plant productivity (Mochida et al., 2008).

This study aims to comprehensively explore the taxonomy and genetic resources of the Triticeae family, particularly focusing on in-depth analysis of wild and cultivated species. By collecting, organizing, and analyzing a vast amount of Triticeae species information, the study intends to reveal the diversity, distribution patterns, evolutionary history, and the current status of distribution and utilization of genetic resources within the Triticeae family. Additionally, the study will explore protection strategies and utilization approaches for Triticeae genetic resources, providing scientific evidence and references for breeding improvement, genetic resource conservation, and sustainable agricultural production of Triticeae crops. This is not only significant for the research and application of Triticeae crops, but also plays an active role in promoting the progress and development of global agricultural production.

2 Taxonomy of Triticeae

2.1 Historical background and classification systems

The tribe Triticeae, a significant group within the Poaceae family, has been the subject of extensive taxonomic studies due to its inclusion of major cereal crops such as wheat, barley, and rye, as well as numerous forage grasses. Historically, taxonomic treatments of Triticeae were primarily based on comparative morphology and geography. Morphological characters, which are phenotypic expressions resulting from the interaction of dominant genes and environmental factors, were the main criteria for classification. However, this approach often led to misclassifications due to morphological convergence in distantly related taxa and divergence in closely related taxa under different environmental conditions (Yen and Yang, 2009).

With the emergence of cytogenetics and molecular genomic analysis, traditional classification systems have been developed, providing more accurate insights into the phylogenetic relationships within the wheat family. For example, Hyun et al. (2020) obtained single nucleotide polymorphism (SNP) markers covering all seven chromosomes from 283 wheat related genotypes using GBS technology. These SNP markers provide rich genetic information for the phylogenetic relationships between different species within the wheat genus. Based on these SNP data, researchers successfully constructed the first high-resolution phylogenetic tree of the wheat genus, providing new insights into the species classification and evolutionary relationships of the wheat genus (Figure 1).

Hyun et al. (2020) demonstrated the genetic relationships of 114 Triticum species and subspecies through a Bayesian phylogenetic tree. The color-coded and labeled branches help to understand the genetic grouping and chromosomal composition among different accessions. Additionally, the Bayesian posterior probabilities provide confidence information on these genetic relationships and offer valuable insights into the genetic relationships among Triticum species and subspecies, aiding in the further understanding of their evolutionary history and genetic diversity.

2.2 Current taxonomic classification and key species

The current taxonomic classification of Triticeae integrates both morphological and genomic data to provide a more comprehensive understanding of the tribe's diversity. Recent genomic investigations have recognized approximately 30 genera within the Triticeae, reflecting a more refined and accurate classification system (Yen and Yang, 2009). Key species within this tribe include the major cereal crops wheat (Triticum spp.), barley (Hordeum spp.), and rye (Secale spp.), as well as important forage grasses such as elymus and agropyron (Bothmer et al., 2008; Knüpffer, 2009).

Chen et al. (2020) investigated the genetic relationships among different species and genomes in the Triticae family (Figure 2). They subdivided the Triticae family into wild-type, cultivated type, and hybrid type genomes based on the discovery that they originated from a common ancestor, and explained the genome naming method, emphasizing the differences between different genera, species, and strains within the same family, revealing the diversity of the wheat genome and its close relationship with species classification.

The genomic resources available for Triticeae have significantly advanced our understanding of these species. For example, the whole genomes of barley, wheat, Tausch’s goatgrass (Aegilops tauschii), and wild einkorn wheat (Triticum urartu) have been sequenced, providing valuable data for comparative genomics and crop improvement (Mochida and Shinozaki, 2013). These genomic tools are crucial for identifying new genes and understanding the evolutionary relationships within the tribe.

2.3 Challenges and controversies in Triticeae taxonomy

Despite advances in wheat taxonomy, there are still some challenges and controversies. A major issue is that the descriptions of genera and species are incomplete and sometimes inconsistent. For example, evaluations of some genome classification systems have shown that many operational taxonomic units (OTUs) cannot be resolved due to the presence of homoplasticity and parallel homology, indicating the need for a more comprehensive approach to treat all attributes equally (Rodriguez-R et al., 2018).

Another challenge is the morphological identification of genomic genera, particularly in perennial species with solitary spikelets. While it is possible to distinguish these groups based on morphology, it requires the examination of characters that have not been traditionally emphasized, such as the length of middle inflorescence internodes and the morphology of glumes (Barkworth et al., 2009). This highlights the ongoing need for detailed morphological studies to complement genomic data.

Moreover, the integration of wild and weedy taxa into the taxonomic framework poses additional difficulties. These taxa are often underrepresented in genetic studies, and there is limited knowledge about their seed physiology, genetic diversity, and seed handling techniques (Bothmer et al., 2008). Addressing these gaps is essential for a more comprehensive understanding of Triticeae taxonomy and for the effective utilization of these genetic resources in crop improvement.

In conclusion, the taxonomy of Triticeae is a complex and evolving field that requires the integration of morphological, cytogenetic, and genomic data. While significant progress has been made, ongoing research and refinement of classification systems are necessary to address the remaining challenges and controversies.

3 Wild Species of Triticeae

3.1 Overview of wild Triticeae species

The tribe Triticeae encompasses approximately 350 wild taxa, which form a significant gene pool for temperate cereals such as wheat, barley, and Rye, as well as several important forage grasses(Bothmer et al., 2008). Despite the vast number of species, the primary focus has traditionally been on the primary gene pools of cultivated species, with wild and weedy taxa receiving less attention until recent years(Bothmer et al., 2008). The wild species, which include around 250 perennial species, are crucial for their genetic diversity and potential to improve cultivated varieties.

3.2 Ecological roles and natural habitats

Wild Triticeae species occupy diverse ecological niches and play significant roles in their natural habitats. These species are found across various regions, often thriving in environments where cultivated species may not survive. For instance, many wild species are adapted to extreme conditions, contributing to the ecological stability and resilience of their habitats (Bothmer et al., 2008). The perennial species, which make up a substantial portion of the Triticeae tribe, are particularly important as forage grasses, supporting both natural ecosystems and agricultural systems.

3.3 Genetic diversity and evolutionary significance

The genetic diversity within wild Triticeae species is immense and holds considerable evolutionary significance. This diversity is not only crucial for the adaptation and survival of these species in their natural habitats but also provides a valuable genetic reservoir for breeding programs aimed at improving cultivated cereals(Bothmer et al., 2008; Uauy, 2011). The structural polymorphisms observed in the chromosomes of wild species, such as those in the St, P, and Y genomes, highlight the evolutionary processes and genetic variability within the tribe (Wang et al., 2010). Understanding and utilizing this genetic diversity is essential for advancing our knowledge of Triticeae evolution and for the sustainable improvement of cereal crops (Uauy, 2011; Mochida and Shinozaki, 2013).

4 Cultivated Species of Triticeae

4.1 Major cultivated species

The Triticeae tribe includes several major cultivated species that are of significant agricultural importance globally. The primary cultivated species are wheat (Triticum spp.), barley (Hordeum vulgare), and rye (Secale cereale) (Bothmer et al., 2008; Merker, 2008). Wheat is one of the most widely grown crops worldwide, providing a staple food source for a large portion of the global population. Barley is primarily used for animal feed, brewing, and as a food grain, while rye is cultivated mainly in cooler climates and is used for bread, beer, and animal fodder (Merker, 2008).

4.2 Domestication history and agronomic traits

The domestication of these major Triticeae species has a rich history that dates back thousands of years. Wheat, for instance, was domesticated from wild emmer (Triticum dicoccoides) and other wild relatives in the Fertile Crescent around 10,000 years ago (Xie and Nevo, 2008). Barley was also domesticated in the same region and time period, while rye's domestication occurred later, primarily in Europe (Merker, 2008).

Agronomic traits of these species have been extensively studied and improved through breeding programs. Key traits include disease resistance, drought tolerance, and grain quality. For example, wild emmer harbors genes for abiotic stress tolerances (e.g., salt, drought, and heat) and biotic stress tolerances (e.g., powdery mildew, rusts) that have been transferred to cultivated wheat to enhance its resilience and productivity (Xie and Nevo, 2008). Similarly, the genetic diversity within Triticum urartu has been explored to identify alleles that can improve wheat agronomy and quality (Talini et al., 2019).

4.3 Genetic diversity within cultivated species

Genetic diversity within cultivated Triticeae species is crucial for their continued improvement and adaptation to changing environmental conditions. Wheat, barley, and rye have benefited from the genetic resources of their wild relatives, which provide a vast reservoir of alleles for various agronomic traits (Bothmer et al., 2008; Merker, 2008). For instance, the North American Triticale Genetic Resources Collection (NATGRC) has been established to conserve and evaluate the genetic diversity of triticale, a hybrid of wheat and rye, highlighting the importance of preserving unique gene combinations for future breeding efforts.

The genetic diversity within these species is also evident in the various genotypic and phenotypic traits observed in different accessions. For example, a study on Triticum urartu reported significant variation in phenology, plant architecture, and seed features(Figure 3), demonstrating the potential of this wild wheat relative to contribute valuable alleles for wheat improvement (Talini et al., 2019). Similarly, the diversity indices and principal component analyses of triticale accessions from different regions have shown considerable genetic variation, which is essential for breeding programs aimed at enhancing crop performance.

Talini et al. (2019) were able to reveal genetic and phenotypic differences among different varieties through PCA analysis, and further investigate how these differences are influenced by environmental factors such as geography and climate (Figure 3). These differences reflect genetic diversity, where different strains have undergone long-term adaptation and evolution in different geographical environments, forming their own unique genotype and phenotype characteristics. Through PCA analysis, it is clear that this diversity is reflected at the genetic and phenotypic levels, providing important data support for the study of genetic diversity, crop breeding, and ecological adaptation.

5 Genetic Resources and Conservation

5.1 Importance of conserving triticeae genetic resources

The conservation of Triticeae genetic resources is crucial for several reasons. Firstly, Triticeae, which includes economically significant crops such as wheat, barley, and rye, forms a vital part of global food security. The genetic diversity within this tribe provides a reservoir of traits that can be harnessed for crop improvement, including resistance to diseases, tolerance to abiotic stresses, and enhanced nutritional qualities (Bothmer et al., 2008; Lu and Ellstrand, 2014). The loss of genetic diversity in these crops could severely impact agricultural productivity and sustainability, making conservation efforts essential (Uauy, 2011; Guzzon and Ardenghi, 2018).

5.2 Ex situ and in situ conservation strategies

Conservation strategies for Triticeae genetic resources can be broadly categorized into ex situ and in situ methods. Ex situ conservation involves the preservation of genetic material outside its natural habitat, typically in gene banks. This method allows for the long-term storage and easy accessibility of genetic resources for research and breeding purposes (Uauy, 2011). However, it is not without challenges, such as the need for accurate taxonomic identification to ensure the usability of the conserved material (Guzzon and Ardenghi, 2018).

In situ conservation, on the other hand, involves the protection of species within their natural habitats. This method helps maintain the evolutionary processes and ecological interactions that contribute to genetic diversity. Both strategies are complementary; while ex situ conservation provides a backup for genetic material that might be lost in nature, in situ conservation ensures the ongoing adaptation and evolution of species in their natural environments (Greene et al., 2014).

5.3 Role of gene banks and international collaborations

Gene banks play a pivotal role in the conservation of Triticeae genetic resources. They serve as repositories for a vast array of genetic material, ensuring its availability for future research and breeding programs. The effectiveness of gene banks depends on accurate documentation and regular updates to maintain the integrity of the conserved material. International collaborations are also crucial in this context. They facilitate the sharing of resources, knowledge, and technologies, thereby enhancing the global capacity for genetic conservation (Khoury et al., 2019).

Collaborative efforts, such as those under the Convention on Biological Diversity and the International Treaty on Plant Genetic Resources for Food and Agriculture, aim to create comprehensive conservation strategies and set ambitious targets for safeguarding genetic diversity (Khoury et al., 2019). These collaborations help address the gaps in current conservation efforts and promote the development of effective conservation indicators and methodologies (Khoury et al., 2019).

6 Genetic Improvement and Breeding

6.1 Traditional breeding methods in Triticeae

Traditional breeding methods in Triticeae have long relied on the utilization of both cultivated and wild relatives to enhance desirable traits in crops such as wheat, barley, and rye. These methods primarily involve the selection and cross-breeding of plants to combine favorable traits from different varieties. For instance, wheat breeders have successfully incorporated disease resistance traits from wild relatives into cultivated wheat varieties through traditional cross-breeding techniques (Merker, 2008). The primary gene pools, which include the most closely related species, have been the main focus of these efforts due to their higher compatibility and ease of gene transfer (Bothmer et al., 2008). However, the wild and weedy taxa have also gained attention for their potential to introduce novel traits into cultivated species, despite the challenges posed by their genetic diversity and the need for specialized seed handling techniques (Bothmer et al., 2008).

6.2 Modern genetic tools and biotechnological approaches

The advent of modern genetic tools and biotechnological approaches has revolutionized the breeding of Triticeae species. Techniques such as genome-wide association studies (GWAS), molecular markers, and next-generation sequencing have enabled more precise and efficient identification and incorporation of beneficial traits. For example, a genome-wide association study on Triticum urartu identified significant quantitative trait nucleotides (QTNs) for various agronomic and quality traits, highlighting the potential of this wild wheat relative as a valuable genetic resource for wheat improvement (Talini et al., 2019). Additionally, the sequencing of whole genomes of key Triticeae species, including wheat and barley, has provided comprehensive genomic resources that facilitate the discovery of new genes and the functional analysis of existing ones (Mochida and Shinozaki, 2013). These advancements have also enabled the integration of genomic data from model organisms like Brachypodium distachyon, further enhancing the understanding and manipulation of Triticeae genomes(Mochida and Shinozaki, 2013).

6.3 Case studies of successful genetic improvement

Several case studies illustrate the successful application of both traditional and modern breeding methods in the genetic improvement of Triticeae species. One notable example is the North American Triticale Genetic Resources Collection, which assembled over 3 000 accessions of triticale from various breeding programs. This collection has been extensively characterized and evaluated, revealing significant genetic diversity and providing a valuable resource for future breeding efforts. Another example is the use of wild relatives in wheat breeding, where traits such as disease resistance have been successfully transferred from wild species to cultivated wheat, demonstrating the practical benefits of utilizing the genetic diversity within the Triticeae tribe (Merker, 2008). Furthermore, the development of genomic tools and resources, such as those provided by the transplant project, has facilitated the integration and analysis of complex genomic data, thereby accelerating the breeding and improvement of Triticeae crops (Spannagl et al., 2016).

7 Challenges in Triticeae Genetic Research

7.1 Genetic complexity and genome organization

The genetic complexity and genome organization of Triticeae species present significant challenges in genetic research. The tribe includes both diploid and polyploid species, with polyploids arising from hybridization events and genome duplications, leading to intricate genome structures (Maestra and Naranjo, 2000). The large and complex genomes of Triticeae species, such as wheat, barley, and rye, complicate genetic mapping and the identification of specific genes responsible for desirable traits (Uauy, 2011). Additionally, the presence of repetitive DNA sequences and transposable elements further complicates genome assembly and annotation (Uauy, 2011). Despite these challenges, advances in molecular markers, chromosome genomics, and comparative genomics have facilitated the study of these complex genomes (Uauy, 2011).

7.2 Abiotic and biotic stress resistance

Abiotic and biotic stress resistance is a critical area of research in Triticeae genetics due to the significant impact of environmental stresses on crop yield and quality. Wild relatives of Triticeae species, such as Triticum dicoccoides and Hordeum spontaneum, possess valuable genetic resources for drought and salt tolerance, which have been identified and transferred to cultivated wheat and barley (Nevo and Chen, 2010). Similarly, genes conferring resistance to diseases like powdery mildew and leaf rust have been tracked in wild relatives such as Triticum boeoticum and T. urartu, providing valuable resources for breeding programs (Hovhannisyan et al., 2018). However, the introgression of these resistance genes into cultivated varieties remains challenging due to the genetic complexity and potential linkage drag associated with wild germplasm (Merker, 2008).

7.3 Socio-economic and policy-related challenges

Socio-economic and policy-related challenges also play a significant role in Triticeae genetic research. The conservation and utilization of genetic resources from wild and weedy taxa are often hindered by limited knowledge of seed physiology, seed handling techniques, and genetic diversity. Additionally, the collection and evaluation of these resources are constrained by socio-political factors, such as access to germplasm and international regulations on genetic resource exchange (Bothmer et al., 2008). Furthermore, the integration of advanced genetic research into practical breeding programs requires substantial investment and collaboration between public and private sectors, which can be challenging to achieve. Addressing these socio-economic and policy-related challenges is crucial for the effective utilization of Triticeae genetic resources in crop improvement and ensuring global food security (Lu and Ellstrand, 2014).

In summary, the genetic complexity and genome organization of Triticeae species, the need for abiotic and biotic stress resistance, and socio-economic and policy-related challenges are significant hurdles in Triticeae genetic research. Overcoming these challenges requires a multidisciplinary approach, combining advances in genomics, breeding techniques, and international collaboration to harness the full potential of Triticeae genetic resources for crop improvement.

8 Future Directions and Research Priorities

8.1. Emerging trends and technologies in Triticeae research

Recent advancements in Triticeae research have been significantly influenced by the integration of genomic technologies and bioinformatics tools. The development of high-throughput genotyping platforms, such as SNP-based marker systems, has facilitated the screening of genetic diversity and the introgression of desirable traits from wild relatives into cultivated species (Przewieslik-Allen et al., 2019). Additionally, the creation of extensive EST libraries and SSR markers for perennial Triticeae species has expanded the genomic resources available for genetic mapping and diversity studies (Bushman et al., 2008). These technologies are crucial for advancing our understanding of the genetic basis of important traits and for improving breeding programs.

Moreover, the Triticeae Toolbox (T3) database has emerged as a pivotal resource, enabling researchers to combine, visualize, and analyze phenotype and genotype data. This tool supports various applications, including genome-wide association studies and genomic prediction, which are essential for modern plant breeding (Blake et al., 2016). The integration of these technologies is expected to accelerate the development of improved Triticeae cultivars with enhanced traits such as stress tolerance, disease resistance, and yield.

8.2 Integration of genomic and phenotypic data

The integration of genomic and phenotypic data is a critical area of focus in Triticeae research. The comprehensive characterization of genetic resources, including both cultivated and wild taxa, provides a vast gene pool for crop improvement (Bothmer et al., 2008; Lu and Ellstrand, 2014). The use of genomic tools to analyze these resources has revealed significant insights into the evolutionary relationships and genetic diversity within the tribe (Yen and Yang, 2009; Kawahara, 2009).

For instance, the Triticeae Toolbox (T3) facilitates the integration of phenotype and genotype data, allowing researchers to define specific datasets for various analyses (Blake et al., 2016). This integration is essential for identifying genetic markers associated with desirable traits and for understanding the genetic architecture of complex traits. Additionally, the development of genomic resources for perennial Triticeae species has enabled comparative mapping and the identification of syntenous regions across different species, further enhancing our understanding of their genetic relationships (Bushman et al., 2008).

8.3 Collaborative efforts and funding opportunities

Collaborative efforts and funding opportunities are vital for advancing Triticeae research. The establishment of large-scale genetic resource collections, such as the North American Triticale Genetic Resources Collection (NATGRC), highlights the importance of international collaboration in conserving and utilizing genetic diversity. These collections provide valuable resources for researchers worldwide and facilitate the exchange of germplasm and knowledge.

Funding opportunities from governmental and non-governmental organizations are essential for supporting research initiatives and fostering collaboration. Programs like the Triticeae Coordinated Agricultural Project (TCAP) have played a significant role in advancing Triticeae research by providing financial support and promoting collaborative efforts among researchers (Blake et al., 2016). Continued investment in such programs is crucial for addressing the challenges of food security and sustainable agriculture.

In conclusion, the future of Triticeae research lies in the continued development and integration of advanced genomic technologies, the comprehensive characterization of genetic resources, and the promotion of collaborative efforts and funding opportunities. These strategies will enable researchers to harness the full potential of Triticeae species for crop improvement and contribute to global food security.

9 Concluding Remarks

Through the aforementioned research, it is evident that the Triticeae family exhibits remarkable species diversity and genetic resource abundance, which are indispensable for agricultural production and crop improvement. Wild and cultivated species differ significantly in genetic traits and adaptability, providing a valuable gene pool for crop breeding. Furthermore, in-depth studies in Triticeae taxonomy not only aid in understanding the genetic relationships and evolutionary history among species, but also provide crucial references for the protection and rational utilization of genetic resources.

For researchers, a comprehensive understanding of Triticeae taxonomy and genetic resources will facilitate breakthroughs in crop breeding, genetic improvement, and biotechnology research. For farmers, utilizing these genetic resources can significantly enhance crop yield, quality, and stress resistance, thereby strengthening the stability and sustainability of agricultural production. For policymakers, emphasizing the importance of genetic resource protection and adopting appropriate strategies for their conservation and management is crucial to ensure their sustainable utilization.

Despite significant progress in Triticeae taxonomy and genetic resource research, further exploration is still needed. Therefore, future researchers can continue to delve into the genetic characteristics and evolutionary mechanisms of Triticeae, discovering more valuable genetic resources. At the same time, governments and various sectors of society should also strengthen the protection and management of genetic resources to ensure their sustainable utilization. Through global cooperation and joint efforts, it is expected to make greater contributions to the development of global agricultural production and the maintenance of food security.

Acknowledgments

Heartfelt thanks to the peer reviewers for their invaluable feedback on the initial draft of this manuscript. 

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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