携带不同原噬菌体的黄龙病菌在柑橘木虱体内的增殖及致病力

背景]在中国,柑橘黄龙病(citrus Huanglongbing,HLB)是由候选韧皮部杆菌亚洲种('Candidatus Liberibacter asiaticus',CLas)所引起的一种毁灭性病害,严重威胁着柑橘产业的可持续发展.前期研究表明CLas全基因组上鉴定出3种单独类型的原噬菌体基因序列,而广东黄龙...

Proliferation of Two Types Prophage of ‘Candidatus Liberibacter asiaticus’ in Diaphorina citri and their Pathogenicity

【Background】In China, citrus Huanglongbing is a destructive disease associated with ‘Candidatus Liberibacter asiaticus’ (CLas), which currently threatening the sustainable development of citrus industry. Previous studies had identified three types of prophage sequences in the genome of CLas strains. Prophage group typing analysis revealed that the Type 2 prophage strains and Type 1+3 prophage strains dominated in Guangdong Province. However, the difference in propagation capacity of these two strains of CLas in Asian citri psyllid (Diaphorina citri) and pathogenicity of these two strains leading by insect transmission remains unknown. 【Objective】The objective of this study is to assess the difference in proliferation capacity between Type-2-CLas and Type-1+3-CLas in D. citri and the pathogenicity variations between them on Citrus reticulata Blanco cv. Shatangju. 【Method】The buds with different strains of CLas were grafted to healthy Shatangju. Fifty nymphal and fifty adult D. citri were caged on the young shoots of infected Shatangju to acquire different strains of CLas for 6, 12, and 18 days, respectively. Real-time quantitative PCR (qPCR) was used to detect and analyze the difference in the acquisition rate and quantity of Type-2-CLas and Type-1+3-CLas between the nymphal stage and the adult stage of D. citri. Further, twenty adult D. citri infected with Type-2-CLas or Type-1+3-CLas were placed on the young shoots of healthy Shatangju to inoculate for two weeks, respectively. The D. citri were collected and detected by qPCR. The general information of infection rate and quantity of CLas was collected. Symptom development of Shatangju and titers of CLas in leaves were monitored each month. Morphological change of leaves phloem and parenchyma cell was visualized by light microscopy 360 days after inoculation. 【Result】There was no significant difference between acquisition rate and quantity of Type-2-CLas and Type-1+3-CLas detected in D. citri that fed on the infected plant at the nymphal stage. In contrast to fed as nymphs, the acquisition rate and quantity of Type-2-CLas were significantly higher than Type-1+3-CLas when D. citri fed on infected plant at the adult stage. The leaves presented more severe mottled symptom after inoculation with Type-2-CLas at 120 d after D. citri removal. The immature leaves at young shoots had an obstacle on turning green, presenting leathery, small size, and yellowing which was infected with Type-2-CLas. The symptoms on Type-1+3-CLas infected plants were presented as typical mottled leaves and leathery. Anatomical analyses indicated that Type-2-CLas caused more severe damage to phloem and accumulated more starch in parenchyma cell even if the titer of Type-1+3-CLas was closed to Type-2-CLas. 【Conclusion】Compared to Type-1+3-CLas, Type-2-CLas was more capable of invading and proliferating in adult D. citri, accompanied by a higher titer of CLas which indirectly proved the high contagious capacity of Type-2-CLas. The young leaves infected with Type-2-CLas presented symptoms of uniform yellowing, small size, while infected with Type-1+3-CLas showed lightly mottled and leathery. Type-2-CLas destroyed more phloem cells and caused severe starch accumulation, indicating that Type-2-CLas had stronger pathogenicity to citrus than Type-1+3-CLas.