Abstract
Citrus canker is a quarantined disease, severely harming citrus plants. Soil beneficial arbuscular mycorrhizal fungi (AMF) can provide a biological control pathway to resist pathogens. This work was to test changes of signal substances including hydrogen peroxide (H2O2), nitric oxide (NO), calmodulin (CaM), salicylic acid (SA) and jasmonic acid (JA) and the pathogen defense gene expression in roots of AMF (Paraglomus occultum) and non-AMF trifoliate orange (Poncirus trifoliata) seedlings after infected by a expressions citrus canker pathogen (Xanthomonas axonopodis pv. Citri, Xac). AMF inoculation significantly improved plant height, stem diameter and leaf number. Xac infection dramatically decreased root H2O2, NO, and SA levels, but increased root CaM and JA concentrations in non-AMF seedlings. There were higher H2O2 and CaM levels and lower JA levels in Xac-infected seedlings than in non-Xac-infected seedlings under mycorrhization. Under non-Xac infection, mycorrhizal treatment reduced root H2O2, NO, and SA but increased CaM and JA levels. However, under Xac infection, mycorrhizal inoculation distinctly accelerated root H2O2, NO, CaM, and SA accumulation, accompanied with up-regulated expression levels of root PtEPS1 (enhance pseudomonas susceptibility 1) and PtPR4 (pathogenesis related gene 4), indicating that Xac stimulated mycorrhizal roles in enhancing resistance of citrus canker. Such results imply that citrus plants with pre-inoculated AMF had stronger resistance to Xac infection through increasing signal substrate accumulation and pathogen defense gene expressions.
 
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 In press - Online First. Article has been peer reviewed, accepted for publication and published online without pagination. It will receive pagination when the issue will be ready for publishing as a complete number (Volume 47, Issue 4, 2019). The article is searchable and citable by Digital Object Identifier (DOI). DOI link will become active after the article will be included in the complete issue.
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Highlights
Citrus is the most popular fruit in China, where citrus planted area is the biggest and the yield is account for 15% of total fruit yield in 2017
Citrus plants are often subjected to various diseases and insect pests, whilst citrus canker caused by Xanthomonas axonopodis pv. citri (Xac) is a quarantined disease in the world, severely decreasing tree growth and fruit production (Das, 2003)
The present study indicated a positive effect of arbuscular mycorrhizal fungi (AMF) inoculation on plant growth of trifoliate orange, which had been proved in various host plants such as Metrosideros laurifolia (Amie et al, 2019), maize (Ghorchiani et al, 2018), tea (Shao et al, 2018) and tomato (Vani et al, 2018)
Summary
Citrus is the most popular fruit in China, where citrus planted area is the biggest (about 22% of total fruit areas) and the yield is account for 15% of total fruit yield in 2017. Citrus plants are often subjected to various diseases and insect pests, whilst citrus canker caused by Xanthomonas axonopodis pv. Citri (Xac) is a quarantined disease in the world, severely decreasing tree growth and fruit production (Das, 2003). Arbuscular mycorrhizal (AM) fungi (AMF), a kind of soil beneficial microorganisms, establish the symbiotic association with roots to enhance biotic resistance of host plants including root-knot nematode, bacterioses and fungal disease (Liu et al, 2007; Nair et al, 2015; Sharma and Sharma, 2017; Wu et al, 2017). Oyewole et al (2017) reported that inoculation with Glomus deserticola and Gigaspora gigantea exhibited inhibitive effects on charcoal rot disease of cowpea. Mycorrhizal cyclamen plants had higher resistance to Fusarium wilt and anthracnose disease (Maya and Matsubara, 2013). MUCL 41833 strongly decreased leaf infection index of Phytophthora infestans, and induced expressions of pathogenesis related genes (PR) PR1 and PR2 in potato, indicating systemic resistance appeared in mycorrhizal seedlings (Gallou et al, 2011)
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