General and species-specific transcriptional responses to downy mildew infection in a susceptible (Vitis vinifera) and a resistant (V. riparia) grapevine species

Abstract

Background: Downy mildew is a destructive grapevine disease caused by Plasmopara viticola (Berk. and Curt.) Berl. and de Toni, which can only be controlled by intensive fungicide treatments. Natural sources of resistance from wild grapevine (Vitis) species are used in conventional breeding approaches, but the signals and effectors involved in resistance in this important crop species are not well understood.

Results: Early transcriptional changes associated with P. viticola infection in susceptible V. vinifera and resistant V. riparia plants were analyzed using the Combimatrix microarray platform. Transcript levels were measured 12 and 24 h post-inoculation, reflecting the time points immediately preceding the onset of resistance in V. riparia, as determined by microscopic analysis. Our data indicate that resistance in V. riparia is induced after infection, and is not based on differences in basal gene expression between the two species. The strong and rapid transcriptional reprogramming involves the induction of pathogenesis-related proteins and enzymes required for the synthesis of phenylpropanoid-derived compounds, many of which are also induced, albeit to a lesser extent, in V. vinifera. More interestingly, resistance in V. riparia also involves the specific modulation of numerous transcripts encoding components of signal transduction cascades, hypersensitive reaction markers and genes involved in jasmonate biosynthesis. The limited transcriptional modulation in V. vinifera represents a weak attempted defense response rather than the activation of compatibility-specific pathways.

Conclusions: Several candidate resistance genes were identified that could be exploited in future biotechnological approaches to increase disease resistance in susceptible grapevine species. Measurements of jasmonic acid and methyl jasmonate in infected leaves suggest that this hormone may also be involved in V. riparia resistance to P. viticola.

Figure 1

Analysis of P. viticola infection steps. Infected leaf disks from V. vinifera (left panels) and V. riparia (right panels) were collected at 12, 24, 48 and 96 hpi, stained with 0.05% aniline blue and observed under an epifluorescence microscope. Panels A, B, C, D and F: magnification 200×; panels E, G and H: magnification 100×. Arrows indicate primary hyphae, arrowheads haustoria. Bars = 80 μm.

Figure 2

Transcriptional changes associated with P. viticola infection. Piled histograms represent the number of genes induced (gray bars) or repressed (black bars) in V. vinifera (Vv) and V. riparia (Vr), at 12 and 24 hpi with P. viticola.

Figure 3

Functional categories of transcripts modulated in V riparia and V. vinifera following infection with P. viticola. Transcripts modulated in V. riparia (A) and V. vinifera (B) after infection with P. viticola at 12 hpi (left panels) and 24 hpi (right panels) were manually grouped in functional categories on the basis of literature evaluation. Induced genes are represented in light gray, while repressed ones are in black. The total percentage of modulated transcripts within each category is shown next to each bar. The complete list of genes is available in Additional file 6.

Figure 4

Specificity of transcriptional changes in infected V. vinifera and V. riparia within selected functional categories. A. Proportion of transcripts modulated in V. vinifera (Vv) or V. riparia (Vr) or in both species at either 12 (upper panel) or 24 hpi (lower panel). B. Proportion of transcripts modulated in V. vinifera (Vv) or V. riparia (Vr) or in both species considering either time point collectively.

Figure 5

Common transcriptional changes in V. vinifera and V. riparia following infection with P. viticola. A. Intensity of the transcriptional changes of 'common' genes in V. riparia and V. vinifera at 12 hpi. Each functional category is shown in a different color. B. Distribution of the 147 'common' genes, modulated in both species at one or both time points, into functional categories.

Figure 6

Endogenous levels of jasmonic acid and MeJA in V. riparia (Vr) and V. vinifera (Vv). Measurements were taken using leaf samples collected at 12, 24, 28 and 96 hpi with P. viticola (Pv) or on the mock-inoculated control samples (w) at the corresponding time-points. Values are the average of three measurements, with standard errors.