Plant Gene and Traits 2024, Vol.15, No.3, 108-117 http://genbreedpublisher.com/index.php/pgt 112 4 Case Studies 4.1 Successful breeding programs Cassava breeding programs have made significant strides in improving starch yield and quality by leveraging genetic insights. A comprehensive review highlighted the identification of 45 genes involved in starch biosynthesis in cassava, including key enzymes like ADPG pyrophosphorylase (AGPase) and granule bound starch synthase (GBSS). These genetic discoveries have facilitated the development of 110 quantitative trait loci (QTLs) for starch content and pasting properties, which are instrumental for breeders to enhance cassava starch through biotechnologies such as transgenic breeding and molecular marker-assisted selection (Tappiban et al., 2019). 4.2 Genetic modification successes Genetic modification has been a powerful tool in the enhancement of starch synthesis in cassava. CRISPR-Cas9 mediated targeted mutagenesis of genes involved in amylose biosynthesis, such as PROTEIN TARGETING TO STARCH (PTST1) and GRANULE BOUND STARCH SYNTHASE (GBSS), has been shown to reduce or eliminate amylose content in root starch. This modification not only impacts the physicochemical properties of starch but also accelerates flowering, which is beneficial for breeding programs. The resulting transgene-free progeny inherits these edited genes, demonstrating the potential of genome editing in creating novel cassava varieties with modified starch suitable for both food and industrial applications (Figure 3) (Bull et al., 2018). Figure 3 provides a comparative overview of traditional breeding versus New Plant Breeding Techniques (NPBTs) for trait improvement in crops. It illustrates the lengthy and complex process required for breeding recessive traits using conventional methods, which often involves multiple generations of crossing and selection to introgress desired traits from wild relatives or mutagenized plants into preferred crop genotypes. In contrast, the diagram depicts how NPBTs, such as genome editing with CRISPR/Cas9, can significantly accelerate this process. By using Agrobacterium-mediated transformation to introduce specific mutations, breeders can achieve faster flowering, simplify the segregation of edited lines, and potentially bypass some of the more time-consuming steps associated with traditional breeding, such as repeated backcrossing and selfing. 4.3 Field trials and outcomes Field trials are critical for assessing the practical impacts of genetic and biochemical knowledge on cassava cultivation. Genetic modification of cassava to enhance starch production has been tested with promising results. Transgenic plants expressing a modified bacterial glgC gene, which encodes a more active form of AGPase, showed up to a 2.6-fold increase in total tuberous root biomass. This increase in sink strength for carbohydrate led to significant increases in both tuberous and above-ground biomass, suggesting that modifying enzymes that regulate source-sink relationships can be an effective strategy for increasing carbohydrate yields in cassava (Ihemere et al., 2006). Additionally, the integration of omics data into the metabolic pathway of starch biosynthesis has provided insights into the distinct activities of the pathway during different stages of root development, which is crucial for the genetic improvement of cassava (Saithong et al., 2013). 5 Breeding Implications 5.1 Breeding objectives: traits related to starch content and quality that are targeted in breeding programs Breeding programs for cassava (Manihot esculenta Crantz) have been primarily focused on improving starch content and quality due to its significance as a staple food and its industrial applications. The starch in cassava is composed of amylose and amylopectin, which determine its unique properties for food processing and industrial uses (Tappiban et al., 2019). The identification of 110 quantitative trait loci (QTLs) for starch content and pasting properties has provided breeders with valuable genetic markers for selecting desirable traits (Tappiban et al., 2019). Additionally, the discovery of genes specific to wild and domesticated varieties of cassava, such as those involved in photosynthesis and starch accumulation, has highlighted the potential for selecting traits that enhance carbohydrate production (Wang et al., 2014).
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