International Journal of Horticulture, 2024, Vol.14, No.3, 142-155 http://hortherbpublisher.com/index.php/ijh 143 This study aims to provide a comprehensive overview of recent advances in using CRISPR/Cas9 technology to develop citrus germplasm resistant to ACP. We will examine the current state of research, highlight successful case studies, and discuss the technical and regulatory challenges associated with deploying gene-edited citrus in the field. By synthesizing the latest findings and insights, we seeks to offer a forward-looking perspective on the potential of CRISPR/Cas9 in citrus breeding and its role in ensuring the sustainability of the citrus industry. This study lies in its potential to guide future research directions, inform policy decisions, and ultimately contribute to the development of robust, disease-resistant citrus varieties. 1 Biology and Impact of Asian Citrus Psyllid 1.1 Life cycle and behavior of ACP The Asian citrus psyllid (ACP), Diaphorina citri, is an insect belonging to the family Liviidae. It is mainly distributed in Guangdong, Guangxi, Fujian, and other provinces and regions of China, and it also occurs in some citrus-producing areas of Zhejiang, Jiangxi, and Yunnan. It is also found in countries such as Indonesia, Vietnam, Malaysia, and Nigeria (Oke et al., 2020). ACP primarily harms plants of the Rutaceae family, with Citrus genus being the most severely affected. The life cycle of ACP consists of three main stages: egg, nymph, and adult (Oke et al., 2020). Females lay yellow-orange eggs on the young shoots and leaves of citrus plants. Upon hatching, nymphs go through five instar stages, feeding on the phloem sap of the plant. Nymphs are yellowish-orange with red eyes and produce a waxy substance that helps them adhere to the plant surface (Qasim et al., 2021). Adults are small, measuring about 3-4 millimeters in length, with mottled brown wings. They exhibit a characteristic head-down, tail-up feeding position on young leaves and stems. ACP adults are highly mobile, capable of flying long distances, which aids in their rapid spread across citrus groves. They feed by inserting their needle-like mouthparts into the plant tissue to consume phloem sap (Figure 1), causing direct damage to the plant and creating entry points for pathogens (Patt et al., 2018; Alba-Tercedor et al., 2021). 1.2 Transmission of Huanglongbing (HLB) disease by ACP ACP is notorious for its role as the primary vector of Candidatus Liberibacter asiaticus, the bacterium responsible for Huanglongbing (HLB) or citrus greening disease (Ammar et al., 2018; Shi et al., 2018). The transmission of HLB occurs when ACP feeds on an infected plant and subsequently transfers the bacterium to healthy plants through its saliva during feeding (Ajene et al., 2020). Once inside the plant, the bacterium moves through the phloem, causing systemic infection. Infected trees display symptoms such as leaf mottling, yellow shoots, and asymmetrical fruit development. The disease disrupts nutrient flow within the tree, leading to reduced fruit quality, premature fruit drop, and eventual tree decline (Shi et al., 2018; Ajene et al., 2020). There is currently no cure for HLB, making vector control and resistant plant varieties critical components of disease management strategies. 1.3 Economic impact of ACP and HLB on the citrus industry The economic impact of ACP and HLB on the citrus industry is profound. HLB has led to substantial yield losses, increased production costs, and reduced fruit quality, severely affecting the profitability of citrus farming (Beloti et al., 2018; Shi et al., 2019). In regions heavily infested by ACP, the costs associated with pest management, including chemical control measures and biological control agents, have escalated significantly. Moreover, the decline in tree health and productivity necessitates the replanting of orchards with healthy trees, further increasing operational costs. In some areas, HLB has resulted in the abandonment of citrus groves, leading to job losses and economic instability in communities reliant on citrus production (Stelinski et al., 2019). The cumulative economic burden underscores the urgent need for sustainable solutions, such as the development of ACP-resistant citrus germplasm through advanced biotechnological approaches like CRISPR/Cas9 gene editing (Chaverra-Rodriguez et al., 2023). By understanding the biology and behavior of ACP, its role in transmitting HLB, and the significant economic impact on the citrus industry, researchers and growers can better strategize and implement effective control measures. The integration of CRISPR/Cas9 technology into citrus breeding programs holds promise for creating
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