1 Department of Plant Pathology, Citrus Research and Education Center, University of Florida, 700 Experiment Station Rd., Lake Alfred, FL 33850, U.S.A.; and.
2 Department of Agricultural Botany, Faculty of Agriculture, Tanta University, Tanta, Egypt.
Mol Plant Microbe Interact. 2019 Apr;32(4):413-427. doi: 10.1094/MPMI-09-18-0238-R. Epub 2019 Feb 15.
Huanglongbing (HLB), a destructive citrus disease, is associated with 'Candidatus Liberibacter asiaticus', which is transmitted by the Asian citrus psyllid Diaphorina citri. Both 'Ca. L. asiaticus' and its vector manipulate the host metabolism for their benefit, to meet their nutritional needs and neutralize the host defense responses. We used a targeted gas chromatography-mass spectrometry-based method to explore the connection between the tricarboxylic acid (TCA) cycle, γ-aminobutyric acid (GABA) shunt, and polyamines (PAs) pathways in citrus. 'Ca. L. asiaticus' and D. citri accelerated the conversion of α-ketoglutarate to glutamate, then to GABA, causing an accumulation of GABA in the cytosol. In silico analysis showed that the citrus genome possesses a putative GABA permease that connects the GABA shunt with the TCA cycle and supports the accumulation of succinate, fumarate, and citrate. Additionally, the PAs biosynthetic pathway might be connected directly to the TCA cycle, through the production of fumarate, or indirectly, via enhancement of GABA shunt. Taken together, we suggest that GABA shunt and PAs pathways are alternative pathways that contribute to the flux toward succinate rather than an intact TCA cycle in citrus. Both 'Ca. L. asiaticus' and its vector enhance these pathways. This study provides more insights into citrus responses to the HLB pathosystem and could be a further step toward clues for understanding the nutritional needs of 'Ca. L. asiaticus', which could help in culturing 'Ca. L. asiaticus'.
黄龙病（HLB）是一种破坏性的柑橘病害，与“亚洲韧皮部杆菌”（Ca. Liberibacter asiaticus）有关，该细菌通过亚洲柑橘木虱（Diaphorina citri）传播。“Ca. L. asiaticus”及其媒介都操纵宿主代谢以满足其营养需求并中和宿主防御反应。我们使用靶向气相色谱-质谱法研究了柑橘中三羧酸（TCA）循环、γ-氨基丁酸（GABA）支路和多胺（PAs）途径之间的联系。“Ca. L. asiaticus”和 D. citri加速了α-酮戊二酸向谷氨酸的转化，然后向 GABA 的转化，导致胞质中 GABA 积累。计算机分析表明，柑橘基因组具有一种推定的 GABA 渗透酶，它将 GABA 支路与 TCA 循环连接起来，并支持琥珀酸、富马酸和柠檬酸的积累。此外，PAs 生物合成途径可能通过产生富马酸直接连接到 TCA 循环，或者通过增强 GABA 支路间接连接到 TCA 循环。总之，我们认为 GABA 支路和 PAs 途径是替代途径，有助于琥珀酸而不是完整 TCA 循环的通量在柑橘中。“Ca. L. asiaticus”及其媒介都增强了这些途径。本研究为柑橘对 HLB 病理系统的反应提供了更多的见解，并可能为进一步了解“Ca. L. asiaticus”的营养需求提供线索，这有助于“Ca. L. asiaticus”的培养。
