Rootstock-regulated gene expression patterns associated with fire blight resistance in apple

Abstract

BackgroundDesirable apple varieties are clonally propagated by grafting vegetative scions onto rootstocks. Rootstocks influence many phenotypic traits of the scion, including resistance to pathogens such as Erwinia amylovora, which causes fire blight, the most serious bacterial disease of apple. The purpose of the present study was to quantify rootstock-mediated differences in scion fire blight susceptibility and to identify transcripts in the scion whose expression levels correlated with this response.ResultsRootstock influence on scion fire blight resistance was quantified by inoculating three-year old, orchard-grown apple trees, consisting of 'Gala' scions grafted to a range of rootstocks, with E. amylovora. Disease severity was measured by the extent of shoot necrosis over time. 'Gala' scions grafted to G.30 or MM.111 rootstocks showed the lowest rates of necrosis, while 'Gala' on M.27 and B.9 showed the highest rates of necrosis. 'Gala' scions on M.7, S.4 or M.9F56 had intermediate necrosis rates. Using an apple DNA microarray representing 55,230 unique transcripts, gene expression patterns were compared in healthy, un-inoculated, greenhouse-grown 'Gala' scions on the same series of rootstocks. We identified 690 transcripts whose steady-state expression levels correlated with the degree of fire blight susceptibility of the scion/rootstock combinations. Transcripts known to be differentially expressed during E. amylovora infection were disproportionately represented among these transcripts. A second-generation apple microarray representing 26,000 transcripts was developed and was used to test these correlations in an orchard-grown population of trees segregating for fire blight resistance. Of the 690 transcripts originally identified using the first-generation array, 39 had expression levels that correlated with fire blight resistance in the breeding population.ConclusionsRootstocks had significant effects on the fire blight susceptibility of 'Gala' scions, and rootstock-regulated gene expression patterns could be correlated with differences in susceptibility. The results suggest a relationship between rootstock-regulated fire blight susceptibility and sorbitol dehydrogenase, phenylpropanoid metabolism, protein processing in the endoplasmic reticulum, and endocytosis, among others. This study illustrates the utility of our rootstock-regulated gene expression data sets for candidate trait-associated gene data mining.

Identification of apple miRNAs and their potential role in fire blight resistance

“…In addition to the abovementioned miRNAs, we also examined miRNAs targeting transcripts previously identified by us as up-or downregulated in FB-resistant trees (Jensen et al 2012). We identified three candidates, which might be targeted by miRNAs expressed in analyzed trees: NHX1, nodulin, and dinelactone hydrolase.…”

“…In our earlier studies, we identified 690 genes up-or downregulated in FB-resistant trees (Jensen et al 2012). This set of genes was obtained for the same scion/rootstock combinations as in this study.…”

“…The mechanism by which rootstock affects tree phenotype is not known, but it clearly influences gene expression pattern of their scion. In our previous studies, we identified genes demonstrating different expression patterns in scions from FBsusceptible and FB-resistant rootstocks (Jensen et al 2012(Jensen et al , 2010. Keeping in mind that microRNA (miRNA) molecules, being essential gene expression regulators, could be involved in this rootstock-scion regulation in apple, we assumed that miRNA expression profiles in scion should reflect rootstock characteristics, as is the case for protein coding genes.…”

Pathogenicity and infection strategies of the fire blight pathogen Erwinia amylovora in Rosaceae: State of the art

“…The overall result is an organ-and developmentally specific pattern of metabolites, characteristic for each plant species. Jensen et al (2012) suggested that the expression of the phenylpropanoid pathway as a whole might be one of the many predictors of fire blight resistance. Burse et al (2004) already showed that E. amylovora has the ability to protect itself against secondary metabolites in apple because of the internal efflux pump AcrAB, indicating the possible involvement of these metabolites in plant defence.…”

Phenolic profiles in apple leaves and the efficacy of selected phenols against fire blight (Erwinia amylovora)

“…In response to pathogen infection, the synthesis of defense-related phenolics such as flavonoids, phenolamines, flavonoid phytoalexins, lignin and salicylic acid (SA) is enhanced (Treutter 2001;Venisse et al 2002;Fischer et al 2007). Jensen et al (2012) suggested that the phenylpropanoid pathway mobilization and activity might be one of the hallmarks of fire blight resistance. However, it should be taken into account that although the stimulation of this pathway is a part of plant resistance response, the processes are dependent also on pathogen activity; E. amylovora belongs to the pathogens which in order to overcome plant defense can delay the expression of phenylpropanoid genes (Venisse et al 2002).…”