Molecular Pathogens 2024, Vol.15, No.3, 142-154 http://microbescipublisher.com/index.php/mp 145 Tissue culture techniques, such as somatic embryogenesis, are employed to clone resistant genotypes, ensuring the propagation of trees with desired resistance traits. This method is particularly useful for maintaining genetic fidelity and producing large quantities of planting material. Additionally, DNA and RNA samples from resistant trees are stored in biorepositories for future research and breeding efforts. These samples are often accompanied by detailed phenotypic data and genetic information to facilitate their use in marker-assisted selection (MAS) and other genetic improvement strategies (Liu et al., 2017). Moreover, genetic resources are often shared through collaborative networks and databases that allow researchers and breeders access to valuable genetic material. This collaborative approach ensures the wide distribution and utilization of resistance genes, contributing to the global effort to enhance disease resistance in pine forests. 4 Molecular Mechanisms of Disease Resistance 4.1 Gene expression and regulation The molecular mechanisms underlying disease resistance in pine trees involve complex interactions between various genes and regulatory pathways. Gene expression studies have revealed that resistance to pathogens like pine wood nematode (PWN) and white pine blister rust (WPBR) involves the differential expression of numerous genes. In Masson pine (Pinus massoniana), transcriptomic profiling identified key differentially expressed genes (DEGs) related to oxidative stress response, terpenoid biosynthesis, and syncytium formation, which are crucial for resistance against PWN (Liu et al., 2017). Additionally, proteomic analysis of resistant Masson pine clones highlighted significant upregulation of proteins involved in salicylic acid metabolism, antioxidant stress reaction, and polysaccharide degradation, which contribute to enhanced resistance (Gao et al., 2022). Another study on Pinus thunbergii demonstrated that resistant plants exhibited higher expression levels of genes associated with lignin synthesis and the oxidative stress pathway. Specifically, cinnamoyl-CoA reductase (CCR)-coding genes were upregulated in resistant phenotypes, indicating a role in reinforcing cell walls against pathogen invasion (Wang et al., 2023). These findings suggest that gene expression regulation in response to pathogen infection is critical for activating defense mechanisms in pine trees. 4.2 Pathogen recognition and defense response Pathogen recognition and the subsequent activation of defense responses are key aspects of disease resistance in pine trees. Pine species have evolved various receptor proteins that detect pathogen-associated molecular patterns (PAMPs) and initiate immune responses. For instance, nucleotide-binding site leucine-rich repeat (NBS-LRR) proteins play a pivotal role in recognizing specific pathogen effectors and triggering defense mechanisms. Studies have shown that these proteins are involved in both quantitative and qualitative resistance to WPBR in sugar pine (Pinus lambertiana) and southwestern white pine (Pinus strobiformis) (Weiss et al., 2020; Liu et al., 2021). Upon pathogen recognition, pine trees activate a cascade of defense responses, including the production of reactive oxygen species (ROS), synthesis of antimicrobial compounds, and reinforcement of cell walls. The role of ROS in pathogen defense is well-documented, with resistant pine varieties exhibiting enhanced ROS scavenging capabilities to mitigate oxidative damage and inhibit pathogen spread (Liu et al., 2017). Additionally, the biosynthesis of terpenoids and phenylpropanoids is upregulated in resistant pines, contributing to the production of antimicrobial compounds that deter pathogen growth (Modesto et al., 2022). 4.3 Signal transduction pathways Signal transduction pathways mediate the activation and regulation of defense responses in pine trees. The jasmonic acid (JA) and salicylic acid (SA) signaling pathways are particularly important in coordinating the plant's immune response. In resistant Pinus pinaster, the JA pathway is prominently induced, leading to the activation of secondary metabolism and lignin synthesis, which fortifies cell walls and enhances resistance to PWN (Modesto et al., 2021). Comparative transcriptome analysis of pine trees treated with resistance-inducing substances such as acibenzolar-S-methyl (ASM) and methyl salicylic acid (MeSA) revealed that these elicitors enhance the
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