Journal of Energy Bioscience 2024, Vol.15, No.3, 135-146 http://bioscipublisher.com/index.php/jeb 136 Furthermore, the review will examine the advancements in metabolic engineering to increase starch production in cassava roots, which is pivotal for bioenergy applications (Ihemere et al., 2006). The potential of cassava biomass for biogas production and the technologies applicable for its optimization will also be critically assessed (Okudoh et al., 2014). By integrating insights from life cycle assessments and biogeochemical models, the review will evaluate the energy saving and carbon emission mitigation potential of cassava-based bioenergy (Jiang et al., 2019). The objectives of this review are to synthesize current knowledge on the genetic and biochemical optimization of cassava for bioenergy, identify bottlenecks in root yield and starch production, and propose strategies for crop improvement through metabolic engineering (Sonnewald et al., 2020; Obata et al., 2020). By doing so, the review expects to provide a comprehensive understanding of the opportunities and challenges in transforming cassava into an optimized bioenergy crop, thereby contributing to global efforts in sustainable energy production. 2 Genetic Foundations for Biomass Production in Cassava The genetic foundation for biomass production in cassava is being strengthened through the identification of key genetic traits, molecular genetic advancements, and strategic breeding approaches. These efforts are paving the way for optimizing cassava as a sustainable and efficient source of bioenergy. 2.1 Genetic traits Cassava (Manihot esculenta Crantz) is known for its high rates of CO2 fixation and sucrose synthesis, which are foundational for its potential as a bioenergy crop. However, the actual field yields often fall short of their potential. Research has identified that genetic traits such as enhanced tuberous root ADP-glucose pyrophosphorylase (AGPase) activity can lead to substantial increases in starch production and biomass yield. Transgenic cassava plants expressing a modified bacterial glgCgene, which encodes for AGPase, have demonstrated up to a 2.6-fold increase in total tuberous root biomass under controlled conditions (Ihemere et al., 2006). Additionally, traits like improved leaf retention have been associated with increased total fresh biomass and root dry matter yield, suggesting that leaf longevity is a valuable trait for enhancing cassava productivity (Lenis et al., 2006). 2.2 Molecular genetics Genetic modifications have been employed to optimize cassava for bioenergy production. For instance, the expression of a more active bacterial form of AGPase in cassava roots has been shown to increase starch production, which is a key component for bioenergy applications (Ihemere et al., 2006). Moreover, genetic engineering approaches have been used to overcome the limitations of traditional breeding in cassava, such as high heterozygosity and trait separation, allowing for the rapid improvement of target traits like pest and disease resistance, biofortification, and starch quality (Liu et al., 2011). The engineering of bioenergy crops has also focused on improving the polysaccharide properties and composition of biomass to reduce recalcitrance to enzymatic deconstruction (Table 1, adopted from Brandon and Scheller, 2020), which is crucial for efficient bioenergy conversion (Brandon and Scheller, 2020). 2.3 Breeding strategies Traditional breeding techniques, coupled with modern genetic tools, have been utilized to enhance bioenergy traits in cassava. Selection for traits such as leaf retention, which is highly heritable and positively correlated with root yield, has been suggested as an effective breeding strategy (Lenis et al., 2006). Additionally, the development of short-duration cassava genotypes allows for the effective utilization of resources and diversification of income for smallholder farmers, which is beneficial for bioenergy crop production systems (Suja et al., 2010). Genetic transformation technologies have matured over the years, enabling the transition from model cultivars to farmer-preferred varieties, thus facilitating the breeding of cassava with improved traits for bioenergy (Liu et al., 2011).
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