Abstract
The microbiome associated with crop plants has a strong impact on their health and productivity. Candidatus Liberibacter asiaticus (Las), the bacterial pathogen responsible for Huanglongbing (HLB) disease, lives inside the phloem of citrus plants including the root system. It has been suggested that Las negatively affects citrus microbiome. On the other hand, members of citrus microbiome also influence the interaction between Las and citrus. Here, we report the isolation and characterization of multiple putative beneficial bacteria from healthy citrus rhizosphere. Firstly, six bacterial strains showing antibacterial activity against two bacteria closely related to Las: Agrobacterium tumefaciens and Sinorhizobium meliloti were selected. Among them, Burkholderia metallica strain A53 and Burkholderia territorii strain A63 are within the β-proteobacteria class, whereas Pseudomonas granadensis strain 100 and Pseudomonas geniculata strain 95 are within the γ-proteobacteria class. Additionally, two gram-positive bacteria Rhodococcus jialingiae strain 108 and Bacillus pumilus strain 104 were also identified. Secondly, antimicrobial activity against three fungal pathogens: Alternaria alternata, Colletotrichum acutatum, Phyllosticta citricarpa, and two oomycetes: Phytophthora nicotianae and Phytophthora palmivora. Four bacterial strains Burkholderia territorii A63, Burkholderia metallica A53, Pseudomonas geniculata 95, and Bacillus pumilus 104 were shown to have antagonistic activity against the citrus root pathogen Phytophthora nicotianae based on dual culture antagonist assays and compartmentalized petri dish assays. The four selected bacteria were sequenced. Genes involved in phosphate solubilization, siderophore production and iron acquisition, volatile organic compound production, osmoprotection and osmotic tolerance, phytohormone production, antagonism, and nutrient competition were predicted and discussed related to the beneficial traits.
Highlights
Citrus Huanglongbing (HLB, known as citrus greening) is a devastating citrus disease (Bove et al, 1974; Wang et al, 2017a,b)
342 isolates were screened for antimicrobial activity against S. meliloti and Agrobacterium tumefaciens, which are closely related to Liberibacter asiaticus (Las) (Duan et al, 2009)
Four of the six strains belong to the proteobacteria phylum, the most abundant phylum in the citrus microbiome (Zhang et al, 2017), with Burkholderia metallica strain A53 and Burkholderia territorii strain A63 within the β-proteobacteria class, whereas Pseudomonas granadensis strain 100 and Pseudomonas geniculata strain 95 within the γ-proteobacteria class
Summary
Citrus Huanglongbing (HLB, known as citrus greening) is a devastating citrus disease (Bove et al, 1974; Wang et al, 2017a,b). HLB in Florida has been associated with the gramnegative α-proteobacteria Candidatus Liberibacter asiaticus (Las) which is fastidious and has not been cultured in vitro (Jagoueix et al, 1994; Bové, 2006). Once Las enters the phloem, it can multiply and spread throughout all phloem-containing tissues including leaf, bark, flowers, fruits, and roots (Tatineni et al, 2008). Huanglongbing negatively affects the root system and the root-associated microbial community changes as the disease progresses. At late stages of the disease, HLB affects carbohydrate metabolism in the plant and changes root physiology by highly decreasing starch content (Etxeberria et al, 2009). It has been shown that Las titer is often first detected in the root system and significantly reduces root density even in trees that remain asymptomatic in aerial tissues (Johnson et al, 2014). In addition to the direct effect of Las on root physiology, HLBaffected root systems are predisposed to secondary infection with Phytophthora nicotianae which results in a greater damage to fibrous roots (Graham et al, 2013)
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