Abstract
BackgroundErwinia chrysanthemi (Ec) is a destructive pathogen which causes soft-rot diseases in diverse plant species including orchids. We investigated whether colonization of Oncidium roots by the endophytic fungus Piriformospora indica (Pi) restricts Ec-induced disease development in leaves, and whether this might be related to the regulation of nucleotide binding site-leucine rich repeat (NBS-LRR) Resistance (R) genes.ResultsRoot colonization of Oncidium stackings by Pi restricts progression of Ec-induced disease development in the leaves. Since Pi does not inhibit Ec growth on agar plates, we tested whether NBS-LRR R gene transcripts and the levels of their potential target miRNAs in Oncidium leaves might be regulated by Pi. Using bioinformatic tools, we first identified NBS-LRR R gene sequences from Oncidium, which are predicted to be targets of miRNAs. Among them, the expression of two R genes was repressed and the accumulation of several regulatory miRNA stimulated by Ec in the leaves of Oncidium plants. This correlated with the progression of disease development, jasmonic and salicylic acid accumulation, ethylene synthesis and H2O2 production after Ec infection of Oncidium leaves. Interestingly, root colonization by Pi restricted disease development in the leaves, and this was accompanied by higher expression levels of several defense-related R genes and lower expression level of their target miRNA.ConclusionBased on these data we propose that Pi controls the levels of NBS-LRR R mRNAs and their target miRNAs in leaves. This regulatory circuit correlates with the protection of Oncidium plants against Ec infection, and molecular and biochemical investigations will demonstrate in the future whether, and if so, to what extent these two observations are related to each other.
Highlights
Erwinia chrysanthemi (Ec) is a destructive pathogen which causes soft-rot diseases in diverse plant species including orchids
We reported that colonization of Oncidium roots by Piriformospora indica (Pi) regulates a group of miRNAs and related target R genes [30]. 1083 miRNAs belonging to 56 families were detected in a transcriptomic library from Pi-colonized roots, but not from uncolonized roots
Our work revealed that root colonization by Pi activates the expression of nucleotide binding site-leucine rich repeat (NBS-LRR) R genes in the leaves which correlated with an increase resistance against Ec infection
Summary
Erwinia chrysanthemi (Ec) is a destructive pathogen which causes soft-rot diseases in diverse plant species including orchids. We investigated whether colonization of Oncidium roots by the endophytic fungus Piriformospora indica (Pi) restricts Ec-induced disease development in leaves, and whether this might be related to the regulation of nucleotide binding site-leucine rich repeat (NBS-LRR) Resistance (R) genes. Orchids such as Oncidium, Phalaenopsis and Dendrobium have high commercial value and are becoming globally important for the agro-industry. Erwinia chrysanthemi (Ec, known as Dickeya dadantii or Pectobacterium chrysanthemi) is one of the soft rot pathogens [1, 2] which causes economic losses in a wide variety of crops and orchids, including Oncidium Identification and characterization of R genes from the existing orchid germplasm resources would be helpful in breeding high-resistance orchid cultivars and in genetic engineering programs
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