Legume Genomics and Genetics 2024, Vol.15, No.3, 105-117 http://cropscipublisher.com/index.php/lgg 111 populations is further enriched by the presence of heritable symbionts, such as those found in pea aphids, which contribute to the overall genetic variation and adaptability of the host plants (Russell et al., 2013). 7.2 Genomic insights into nitrogen fixation The ability of peas to form symbiotic nodules with Rhizobium leguminosarumsv. viciae (Rlv) is a key trait that enhances nitrogen fixation. Studies have shown that the efficiency of nitrogen fixation and the competitiveness for nodulation are distinct traits influenced by both plant and bacterial genetic determinants (Bourion et al., 2018). The genomic analysis of pea nodules has revealed a variable gene family encoding nodule-specific cysteine-rich (NCR) peptides, which play a critical role in bacteroid differentiation and nitrogen fixation (Zorin et al., 2022). Furthermore, the identification of nearly 200 genes required for symbiotic nitrogen fixation in legumes, including peas, has advanced our understanding of the molecular mechanisms underlying this complex trait (Roy et al., 2019). These genes are involved in various processes such as microbial infection, nodule development, and the regulation of nitrogen fixation, highlighting the intricate genetic network that supports this symbiotic relationship (Kouchi et al., 2010). 7.3 Pathogen resistance and plant immunity The interaction between peas and their symbiotic partners also influences their resistance to pathogens and overall plant immunity. The genetic and molecular mechanisms that regulate symbiotic specificity in legume-rhizobium interactions are diverse and involve a wide range of host and bacterial genes and signals (Wang et al., 2018). These mechanisms not only ensure successful symbiosis but also contribute to the plant's ability to fend off pathogenic attacks. Comparative genomic analyses have provided insights into the genetic determinants of infection susceptibility and the evolutionary pressures exerted by host-pathogen interactions (Sironi et al., 2015). Additionally, the presence of symbiotic genes and their allelic variations can impact the effectiveness of symbiotic interactions and the plant's overall health and productivity (Zhukov et al., 2021). Understanding these interactions is crucial for improving the resilience and yield of pea crops in agricultural settings. In summary, the evolutionary genomics of peas reveals the significant impact of symbiotic relationships on their domestication and diversity. The intricate genetic networks governing nitrogen fixation, pathogen resistance, and plant immunity underscore the importance of these interactions in shaping the evolutionary trajectory of peas. 8 Applications in Breeding and Crop Improvement 8.1 Modern breeding techniques for peas Modern breeding techniques for peas have evolved significantly with the advent of genomic selection (GS) and high-density genotyping. GS has emerged as a promising technique to enhance the accuracy and efficiency of marker-based selection. By utilizing genome-wide molecular marker data, GS predicts the breeding values of candidate lines, thereby accelerating the breeding cycle and improving selection gains. For instance, a study involving a collection of 339 genetic resource accessions demonstrated high genomic prediction accuracies for traits such as thousand seed weight (TSW), number of seeds per plant (NSeed), and date of flowering (Tayeh et al., 2015). The study highlighted the importance of the size and composition of the training population in maximizing prediction accuracies, which is crucial for developing effective GS strategies in pea breeding (Tayeh et al., 2015). 8.2 Genomic selection and CRISPR/Cas9 in pea improvement The integration of genomic selection and CRISPR/Cas9 technology offers a powerful approach for pea improvement. Genomic selection leverages high-density marker scores to predict the breeding values of lines, incorporating all marker information to avoid biased estimates and capture more variation due to small-effect quantitative trait loci (QTL). This method has shown promise in accelerating the breeding cycle and enhancing gains per unit time (Varshney et al., 2017). CRISPR/Cas9, a versatile genome editing tool, has revolutionized crop improvement by enabling precise and efficient gene editing. This technology allows for the rapid introduction of beneficial traits, such as disease resistance, abiotic stress tolerance, and improved nutritional quality, into pea varieties (Figure 2) (Arora and
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