Molecular Pathogens 2024, Vol.15, No.3, 142-154 http://microbescipublisher.com/index.php/mp 148 6.3 Breeding strategies for disease resistance Breeding strategies for disease resistance in pine trees involve both traditional and advanced methods to enhance the genetic resistance of pine populations. Traditional breeding relies on selecting and cross-breeding individuals with desirable traits, but this process is time-consuming. Advanced breeding strategies, such as genomic selection and genomic-wide association studies (GWAS), have revolutionized pine breeding by providing insights into the genetic basis of disease resistance and allowing for more targeted breeding efforts (Sniezko and Koch, 2017). Genomic selection involves using genome-wide markers to predict the breeding value of individuals for disease resistance traits. This approach has been successfully applied in Norway spruce to select for resistance against the pine weevil (Hylobius abietis), demonstrating the potential for similar strategies in pine species (Lenz et al., 2019). GWAS has been used to identify SNPs associated with resistance to WPBR in sugar pine, providing valuable information for breeding programs (Weiss et al., 2020). In addition to these techniques, breeding programs are incorporating knowledge from proteomic and transcriptomic studies to understand the molecular mechanisms underlying disease resistance. This comprehensive approach allows for the development of pines with enhanced resistance to multiple pathogens, ensuring the sustainability and health of pine forests in the face of increasing biotic stressors (Mukrimin et al., 2019). 7 Case Studies of Disease Resistance in Pines 7.1 Successful gene integration and expression A significant case study involves the integration and expression of the Cr1 resistance gene in sugar pine (Pinus lambertiana), which confers resistance to white pine blister rust (WPBR) caused by Cronartium ribicola. This gene was successfully mapped and linked to specific SNP markers, which facilitated the development of a PCR-based genotyping assay to identify and propagate resistant individuals. Field trials have shown that trees carrying the Cr1 gene exhibit strong resistance to WPBR, demonstrating the successful integration and functional expression of this gene in natural populations (Wright et al., 2022). In another study, the PR10 gene family from western white pine (Pinus monticola) was investigated for its role in resistance to WPBR. The gene PmPR10-3.1 was cloned and expressed in Escherichia coli, and the recombinant protein exhibited antifungal activity, indicating its functional role in disease resistance. Subsequent expression in transgenic pines confirmed its contribution to enhanced resistance, showcasing a successful case of gene integration and expression (Figure 2) (Liu et al., 2021). 7.2 Field trials and disease resistance Field trials play a critical role in validating the efficacy of resistance genes under natural conditions. For example, southwestern white pine (Pinus strobiformis) has been evaluated for resistance to WPBR through extensive field trials. Progeny from families with known resistance genes were inoculated with Cronartium ribicola and monitored for disease symptoms. The trials revealed significant variation in resistance levels, with some families exhibiting high survival rates and reduced disease severity, demonstrating the practical benefits of selecting for resistance genes in breeding programs (Johnson and Sniezko, 2021). Another notable example is the resistance of maritime pine (Pinus pinaster) to pine wilt disease (PWD) caused by Bursaphelenchus xylophilus. Field trials conducted on half-sib families identified several resistant lines. These trials included inoculation with the nematode and monitoring for survival and growth. The resistant families showed significantly higher survival rates and lower nematode populations, confirming the effectiveness of selecting for genetic resistance in field conditions (Carrasquinho et al., 2018). 7.3 Lessons learned and challenges Several key lessons have been learned from these case studies. The importance of integrating molecular and field data to select and propagate disease-resistant pines is evident. Molecular markers linked to resistance genes, when validated through field trials, can significantly enhance the efficiency and accuracy of breeding programs.
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