Abstract
BackgroundRust caused by Melampsora larici-populina is one of the most damaging diseases of poplars. Rust is considered to be a model pathogen for genetic studies because both pathogen and host genomes are available. The poplar ‘Robusta’, whose general rust resistance is defeated by virulent rust E4, provides suitable host material for studies of the gene regulation involved in rust resistance/susceptibility. In this study, we investigated the microRNA-mediated susceptible poplar gene expression regulation associated with the infection of virulent rust. We were particularly interested in delineating the host-pathogen interactions with a specific focus on microRNAs (miRNAs).ResultsTo study the susceptibility of poplar to M. larici-populina, small RNA (sRNA) libraries, a degradome cDNA library and digital gene expression libraries were constructed for rust-inoculated and rust-free susceptible poplar ‘Robusta’ leaves through high-throughput sequencing. Altogether, 12,722 regulating interactions were identified. The results delineated the framework of post-transcriptional regulation of gene expression in the susceptible poplar, which was infected by the virulent rust. The results indicated that pathogen-associated molecular patterns (PAMPs) and PAMP-triggered immunity were induced by the infection of virulent rust E4 and that miRNAs still functioned at this stage. After this stage, miRNA-regulated R genes, such as TIR-NBS-LRR and CC-NBS-LRR, were not fully functional. Additionally, the rust-responsive miRNAs did not regulate the signaling component genes related to the salicylic acid pathway or the hypersensitive response.ConclusionsWe found that the defense-related post-transcriptional regulation of the susceptible poplar ‘Robusta’ functions normally only at the stage of PAMPs and PAMP-triggered immunity (PTI). More importantly, the miRNA-mediated post-transcriptional regulation of defense signal pathway genes were inactivated by the infection of virulent rust at the stage of effector-triggered susceptibility and during the following stages of salicylic acid and hypersensitive responses. This inactivation was the major characteristic of ‘Robusta’ susceptibility.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2286-6) contains supplementary material, which is available to authorized users.
Highlights
Rust caused by Melampsora larici-populina is one of the most damaging diseases of poplars
Small RNA sequencing profile To study the post-transcriptional regulation associated with poplar susceptibility to rust, miRNA accumulation was investigated by small RNA sequencing
Based on miRNA, degradome cDNA and digital gene expression (DGE) library analyses, we investigated the regulatory functions of miRNA in a susceptible poplar under rust attack
Summary
Rust caused by Melampsora larici-populina is one of the most damaging diseases of poplars. In the Far East, M. larici-populina infection occurs in a wide range of poplar species and hybrids and often causes severe damages [3, 4]. It has a very complex life cycle, producing 5 spore stages during its life cycle and alternating on larch (Larix spp.). Poplars (Populus spp.) occur naturally in most parts of the northern hemisphere from subarctic to subtropical regions [5] and are widely planted beyond their natural range in both the southern and northern hemispheres [6] Poplars and their hybrids vary in rust resistance. Great advances have been made in researching the genetic mechanisms of plant disease resistance, the mechanisms underlying plant disease susceptibility remain unclear
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