Molecular Plant Breeding 2024, Vol.15, No.3, 144-154 http://genbreedpublisher.com/index.php/mpb 145 2 Molecular Mechanisms of Disease Resistance in Pines 2.1 Pathogen recognition and signal transduction pathways Pathogen recognition in pines involves a complex interplay of molecular mechanisms that enable the plant to detect and respond to pathogenic threats. Pines utilize pattern recognition receptors (PRRs) to identify pathogen-associated molecular patterns (PAMPs), which are conserved microbial signatures. Upon recognition, these PRRs initiate a cascade of signal transduction pathways that activate defense responses. For instance, the study on limber pine (Pinus flexilis) identified numerous receptor-like protein kinase genes (RLKs) that play a crucial role in pathogen recognition and subsequent signal transduction (Liu et al., 2019). These RLKs are essential for the activation of downstream defense mechanisms, highlighting their importance in the early stages of pathogen detection. 2.2 Role of disease resistance (R) genes in pines Disease resistance (R) genes are pivotal in the defense against pathogens in pines. These genes encode proteins that can recognize specific pathogen effectors and trigger robust immune responses. The genetic mapping of limber pine revealed the presence of numerous nucleotide-binding site leucine-rich repeat (NBS-LRR) genes, which are a major class of Rgenes involved in pathogen recognition and resistance (Liu et al., 2019). Additionally, the PmPR10-3.1 gene in western white pine (Pinus monticola) has been identified as a significant contributor to quantitative disease resistance (QDR) against white pine blister rust, demonstrating the critical role of Rgenes in conferring resistance to specific pathogens (Liu et al., 2021). 2.3 Defense mechanisms activated by Rgenes Upon activation by R genes, pines deploy a variety of defense mechanisms to combat pathogen invasion. These mechanisms include the production of pathogenesis-related (PR) proteins, which have antimicrobial properties. For example, the PmPR10-3.1 gene in western white pine encodes a PR10 protein that exhibits inhibitory effects on the growth of fungal pathogens, thereby contributing to the plant's defense (Figure 1) (Liu et al., 2021). Furthermore, the disruption of susceptibility (S) genes using genome editing tools like CRISPR/Cas9 has been shown to enhance disease resistance by interfering with pathogen compatibility, providing a transgene-free approach to developing durable disease-resistant pine varieties (Zaidi et al., 2018). These defense responses are crucial for maintaining the health and survival of pine species in the face of pathogenic threats. Figure 1 Micrographs showing the effects of PmPR10-3.1 on spore germination of fungal pathogens (Adopted from Liu et al., 2021) Image caption: Conidiospores of Phoma exigua (isolate PFC 2705) were treated by pure PmPR10-3.1 protein for 18 h and photographs were taken at 63× magnification DIC. (a) Desalt buffer control; (b) 10 μg/mL PR10-3.1; (c) 42 μg/mL PR10; (d) 75 μg/mL PR10; (e) 100 μg/mL PR10-3.1. Urediniospores of Cronartium ribicola were treated with pure PmPR10-3.1 protein for 24 h and photographs were taken at 200× using Nimarsky filter: (f) desalt buffer control; (g) 100 μg/mL PR10-3.1. Bars in insets represent 100 μm. Arrows indicate reduced hyphal growth and swelling at hyphal tips due to PmPR10-3.1 (Adopted from Liu et al., 2021)
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