Rpv3- and Rpv12-dependent defense mechanism against Plasmopara viticola

Viticulture relies on traditional European grape varieties (Vitis vinifera), which are highly susceptible to downy mildew (Plasmopara viticola). The high susceptibility of European grapevines to this pathogen requires the intensive use of fungicides to prevent significant losses in yield and quality. Building on naturally occurring resistant vines in North America and Asia, intensive breeding of new, fungus-resistant grape varieties has taken place over the past few decades. The goal was to reduce the number of plant protection treatments by leveraging the plants’ inherent resistance to harmful fungi. Despite the obvious economic and ecological advantages, as well as the now high quality of the products, resistant varieties account for only about 2.4% of the total vineyard area in Germany. Among the reasons for this very limited cultivation area of fungus-resistant grape varieties is a lack of knowledge regarding appropriate plant protection strategies for these new varieties.

To promote the cultivation of fungus-resistant grapevine varieties, detailed knowledge is needed regarding the variety-specific resistance traits and the associated number and timing of plant protection treatments under the respective climatic conditions and prevailing weather. Therefore, as part of this study, field trials were conducted with two fungus-resistant grape varieties (Cabernet Blanc (Rpv3) and VB Cal6-04 (Sauvignac) (Rpv3/12)). The results of these experiments show that a 50% to 75% reduction in fungicide use is possible without negative effects on quality or yield, depending on weather conditions, vine development, and resistance loci. Furthermore, it was demonstrated that the absence of plant protection treatments can have serious consequences for yield, quality, and plant health.

Understanding the resistance mechanisms mediated by different resistance loci is essential for breeding new varieties, as combining different resistance mechanisms would reduce the likelihood that the pathogen could overcome plant resistance. To obtain more information about the Rpv3- and Rpv12-mediatedresistance mechanisms, comparative microscopic studies as well as metabolome and transcriptome analyses were conducted. The Rpv3-mediated defense was analyzed in greater detail using a new approach applied in this study. This approach involved examining a successful and an unsuccessful defense response in an Rpv3 genotype by inoculation with a P. viticola isolate that was avirulent or virulent, respectively, toward the Rpv3 locus(Fig. 1). Since there is no rapid and routine method for generating mutants in grapevines, this approach offered a new way to gain further insights into Rpv3-mediated resistance against P. viticola. The results of this study suggest that the resistance mediated by the Rpv3 and Rpv12 lociis achieved through two distinct mechanisms. The resistance mediated by the Rpv12 locusis activated more rapidly and effectively than the Rpv3-mediated defense, leading to earlier cell death—which occurred 8 hours post-inoculation (hpi), and to severe growth inhibition that virtually halted pathogen development within 24 hpi. Successful Rpv3-mediated defense is associated with the detection of cell death at 24–32 hpi and the synthesis of toxic stilbenes, resulting in partial resistance with reduced, but not completely suppressed, growth of P. viticola.

To date, only two resistance genes involved in the recognition of pathogenic structures have been described in grapevines. To identify new genes involved in Rpv3-mediated resistance, an RNA sequencing experiment was conducted. Functional annotation of the encoded protein sequences of genes that were significantly upregulated during the Rpv3-mediated defense response suggests a putative role in the downstream Rpv3 resistance mechanism following pathogen recognition.

Publications

Eisenmann et al. (2019): Rpv3–1-mediated resistance to grapevine downy mildew is associated with specific host transcriptional responses and the accumulation of stilbenes. In: BMC Plant Biology 19

Eisenmann, Birgit (2019): Grapevine defense responses in Rpv3- and Rpv12-dependent resistance against Plasmopara viticola and their implications for crop protection management. Dissertation

Eisenmann (2018): Reducing Fungicide Use. In: Der Deutsche Weinbau

Eisenmann, Kortekamp, and Bogs (2017): Fighting Downy Mildew.