Abstract
BackgroundAccumulating evidence suggests that genome plasticity allows filamentous plant pathogens to adapt to changing environments. Recently, the generalist plant pathogen Phytophthora ramorum has been documented to undergo irreversible phenotypic alterations accompanied by chromosomal aberrations when infecting trunks of mature oak trees (genus Quercus). In contrast, genomes and phenotypes of the pathogen derived from the foliage of California bay (Umbellularia californica) are usually stable. We define this phenomenon as host-induced phenotypic diversification (HIPD). P. ramorum also causes a severe foliar blight in some ornamental plants such as Rhododendron spp. and Viburnum spp., and isolates from these hosts occasionally show phenotypes resembling those from oak trunks that carry chromosomal aberrations. The aim of this study was to investigate variations in phenotypes and genomes of P. ramorum isolates from non-oak hosts and substrates to determine whether HIPD changes may be equivalent to those among isolates from oaks.ResultsWe analyzed genomes of diverse non-oak isolates including those taken from foliage of Rhododendron and other ornamental plants, as well as from natural host species, soil, and water. Isolates recovered from artificially inoculated oak logs were also examined. We identified diverse chromosomal aberrations including copy neutral loss of heterozygosity (cnLOH) and aneuploidy in isolates from non-oak hosts. Most identified aberrations in non-oak hosts were also common among oak isolates; however, trisomy, a frequent type of chromosomal aberration in oak isolates was not observed in isolates from Rhododendron.ConclusionThis work cross-examined phenotypic variation and chromosomal aberrations in P. ramorum isolates from oak and non-oak hosts and substrates. The results suggest that HIPD comparable to that occurring in oak hosts occurs in non-oak environments such as in Rhododendron leaves. Rhododendron leaves are more easily available than mature oak stems and thus can potentially serve as a model host for the investigation of HIPD, the newly described plant-pathogen interaction.
Highlights
Accumulating evidence suggests that genome plasticity allows filamentous plant pathogens to adapt to changing environments
Phenotypic analysis of P. ramorum isolates from foliar hosts A collection of 110 isolates of Phytophthora ramorum all belonging to the NA1 lineage was collected between 2006 and 2015 in Washington State
Nwt colony morphology observed among isolates from Rhododendron and a stream in Washington State is indistinguishable from those observed among isolates from oaks in California (Fig. 1 and Additional file 2)
Summary
Accumulating evidence suggests that genome plasticity allows filamentous plant pathogens to adapt to changing environments. Genomes and phenotypes of the pathogen derived from the foliage of California bay (Umbellularia californica) are usually stable. We define this phenomenon as host-induced phenotypic diversification (HIPD). Phytophthora ramorum is a recent example of an invasive pathogen attacking more than 130 host plant species across 75 genera [4]. It causes Sudden Oak Death (SOD) or Sudden Larch Death (SLD) [5] in forests, and Ramorum blight on most of its other hosts including ornamentals in nurseries. Epidemiological and regulatory consequences of the disease have caused millions of dollars in losses to ornamental nurseries, larch plantations, and native vegetation [6, 7]
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