Genomics and Applied Biology 2024, Vol.15, No.1, 47-53 http://bioscipublisher.com/index.php/gab 48 1 The Main Diseases of Cassava and Traditional Disease Resistant Breeding 1.1 Main diseases of cassava As an important food crop widely cultivated in tropical and subtropical regions, cassava is not only the staple food on the dining table of many people in many countries, but also deeply affects the economic structure and social well-being of these regions. However, the production of this crop faces many challenges, among which the most severe are diseases caused by various viruses, especially cassava mosaic disease (CMD) and African cassava mosaic virus (ACMDV), which belong to the dicotyledonoviridae family. Cassava mosaic disease is a viral disease that can cause symptoms such as mosaic mottled leaves, reduced leaf size, and twisted deformation. In severe cases, it can also cause plant dwarfing, ultimately significantly reducing the yield and quality of root tubers. CMD is transmitted through mediating insects such as white flies, and is more easily spread in warm and humid environments, making cassava plantations in tropical regions particularly vulnerable. African cassava mosaic virus is a specific variant of CMD, with particularly significant destructive power on cassava crops. It can cause similar but often more severe symptoms, leading to widespread crop death and a more fatal impact on yield. The virus also relies on specific mediators for transmission, and in some areas, its impact is particularly prominent due to the lack of effective disease resistant varieties and management measures (Zhu et al., 2020). 1.2 Application and limitations of traditional breeding techniques in improving cassava disease resistance Traditional breeding techniques have been employed to improve disease resistance in cassava. However, these methods face several challenges and limitations. Conventional breeding is often a lengthy process that may not keep pace with the rapid evolution of plant pathogens. Additionally, it relies on the availability of resistance genes within the gene pool of cassava, which may be limited or come with undesirable traits that are co-inherited during the breeding process. The precision of these traditional methods is also less than ideal, leading to a need for more advanced and targeted approaches to develop disease-resistant cassava varieties (Ahmad et al., 2020). 1.3 Breeding process and effectiveness of disease resistant varieties The breeding process for disease-resistant cassava varieties involves the selection of parent plants that exhibit traits of resistance to specific diseases and then cross-breeding them to combine these traits in the progeny. The effectiveness of these disease-resistant varieties can be variable, as resistance may not be absolute and can be overcome by the pathogen over time. Moreover, the resistance bred into cassava must be carefully balanced with other agronomic traits to ensure that the resulting varieties are acceptable to farmers and consumers. While traditional breeding has provided some disease-resistant cassava varieties, the limitations of this approach highlight the need for more innovative and precise genetic tools. The advent of CRISPR/Cas genome editing technology offers a promising alternative to overcome these challenges by enabling precise modifications to the plant genome, potentially leading to durable and robust disease resistance in cassava (Mehta et al., 2019). 2 Genetic Basis of Cassava Disease Resistance 2.1 Known cassava disease resistance related genes and genetic markers Recent advancements in genetic research have identified specific regions and genetic markers associated with disease resistance in cassava. A genome-wide association mapping (GWAS) study has pinpointed two regions that are significantly associated with cassava brown streak disease (CBSD) resistance. One region on chromosome 4 coincides with a segment introgressed from Manihot glaziovii, while another on chromosome 11 contains a cluster of nucleotide-binding site-leucine-rich repeat (NBS-LRR) genes, which are known for their role in plant immune responses (Wheatley and Yang, 2020). 2.2 How disease resistance related genes affect the disease resistance of cassava The NBS-LRR gene cluster identified on chromosome 11 is particularly noteworthy as these genes are a common type of resistance (R) gene in plants. They function by recognizing specific pathogen effectors and triggering defense mechanisms. The presence of these genes in cassava suggests a genetic basis for the plant's ability to recognize and respond to pathogens, thereby conferring resistance to diseases such as CBSD (Chavez-Granados et al., 2022).
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