Transcriptome analysis of Pinus monticola primary needles by RNA-seq provides novel insight into host resistance to Cronartium ribicola

Jun-Jun Liu,Rona N Sturrock,Ross Benton

doi:10.1186/1471-2164-14-884

Abstract

BackgroundFive-needle pines are important forest species that have been devastated by white pine blister rust (WPBR, caused by Cronartium ribicola) across North America. Currently little transcriptomic and genomic data are available to understand molecular interactions in the WPBR pathosystem.ResultsWe report here RNA-seq analysis results using Illumina deep sequencing of primary needles of western white pine (Pinus monticola) infected with WPBR. De novo gene assembly was used to generate the first P. monticola consensus transcriptome, which contained 39,439 unique transcripts with an average length of 1,303 bp and a total length of 51.4 Mb. About 23,000 P. monticola unigenes produced orthologous hits in the Pinus gene index (PGI) database (BLASTn with E values < e-100) and 6,300 genes were expressed actively (at RPKM ≥ 10) in the healthy tissues. Comparison of transcriptomes from WPBR-susceptible and -resistant genotypes revealed a total of 979 differentially expressed genes (DEGs) with a significant fold change > 1.5 during P. monticola- C. ribicola interactions. Three hundred and ten DEGs were regulated similarly in both susceptible and resistant seedlings and 275 DEGs showed regulatory differences between susceptible and resistant seedlings post infection by C. ribicola. The DEGs up-regulated in resistant seedlings included a set of putative signal receptor genes encoding disease resistance protein homologs, calcineurin B-like (CBL)-interacting protein kinases (CIPK), F-box family proteins (FBP), and abscisic acid (ABA) receptor; transcriptional factor (TF) genes of multiple families; genes homologous to apoptosis-inducing factor (AIF), flowering locus T-like protein (FT), and subtilisin-like protease. DEGs up-regulated in resistant seedlings also included a wide diversity of down-stream genes (encoding enzymes involved in different metabolic pathways, pathogenesis-related -PR proteins of multiple families, and anti-microbial proteins). A large proportion of the down-regulated DEGs were related to photosystems, the metabolic pathways of carbon fixation and flavonoid biosynthesis.ConclusionsThe novel P. monticola transcriptome data provide a basis for future studies of genetic resistance in a non-model, coniferous species. Our global gene expression profiling presents a comprehensive view of transcriptomic regulation in the WPBR pathosystem and yields novel insights on molecular and biochemical mechanisms of disease resistance in conifers.

Highlights

Five-needle pines are important forest species that have been devastated by white pine blister rust (WPBR, caused by Cronartium ribicola) across North America
Transcriptome sequencing by RNA sequencing (RNA-seq) and de novo assembly Three RNA-seq 76-bp paired end read libraries were prepared from total RNA extracted from primary needles of uninfected seedlings and infected seedlings with resistant (Cr2/-) and susceptible genotypes
We performed a single assembly of the total reads from all three cDNA libraries, which generated a common transcriptome data set of 36,923 contigs having a minimum length of 300-bp and a minimum mapped read of 50

Summary

Introduction

Five-needle pines are important forest species that have been devastated by white pine blister rust (WPBR, caused by Cronartium ribicola) across North America. Histochemical analysis revealed that the resistance response to systemic C. ribicola spread is localized internally in needle and stem tissues and that the build-up of physical barriers and deposition of cell wall-bound phenolic compounds play a crucial role in the defense reaction [8,9]. Despite these important results, there is still much to learn about the genetic basis of host resistance to C. ribicola in WWP and other five-needle pines (such as whitebark pine and limber pine)

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