Unique resistance traits against downy mildew from the domestication center of grapevine

Silvia Laura Toffolatti^1*, Piero Attilio Bianco¹, Osvaldo Failla¹ and Gabriella De Lorenzis^1*

¹ Department of Agricultural and Environmental Sciences, University of Milan, Via Celoria 2, 20133 Milano (Italy)

^*Corresponding authors: silvia.toffolatti@unimi.it; gabriella.delorenzis@unimi.it

Article extracted from Silvia Toffolatti's presentation in occasion of Enoforum Web Conference (23-25 February 2021)

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Plasmopara viticola: an enemy of grapevine

Downy mildew is one of the most severe diseases of Vitis vinifera, the grapevine species cultivated worldwide for wine fresh fruit and raisin production. It is caused by the oomycete Plasmopara viticola, a polycyclic pathogen able to grow only on grapevine causing damages to all green tissues of the host: leaves, shoots, inflorescences and bunches (Gessler et al., 2011). Due to the high susceptibility of V. vinifera, fungicide application is unavoidable to obtain an adequate yield, in terms of quality and quantity. Fungicide application is however associated with risks for human health and the environment. To achieve a sustainable grapevine production, there is the need to develop alternative and complementary disease control measures. The cultivation of grapevine varieties resistant to P. viticola could help reducing the fungicide use.

Resistance traits from the grapevine domestication center

Traditionally, resistance is found in American and/or Asian grapevine species that co-evolved with the pathogen or with a close pathogen species. The hybrids obtained by crossing V. vinifera with those grapevine species, however, do not meet the standard criteria for the production of high quality wines, which can be obtained only with pure V. vinifera cultivars. With the aim of finding sources of resistance in V. vinifera, our research group screened a large collection of Georgian grapevine accessions, that are characterized by a high genetic variability (Sargolzaei et al., 2021), and found a black cultivar, named Mgaloblishvili, that showed mild (or absent) symptoms in field and reduced disease severity and sporulation in laboratory trials (Toffolatti et al., 2016). Aim of our research was, then, the characterization of the resistance mechanism of Mgaloblishvili, before exploiting it for breeding.

Putative resistance mechanism of Mgaloblishvili against P. viticola infection

Transcriptome analysis of grapevine genes allowed us to draw the putative resistance mechanism of Mgaloblishvili (Toffolatti et al., 2018), that is based on: i) recognition of the pathogen through PAMP (Pathogen Associated Microbial Pattern) receptors and effector receptor (rust resistance kinase LR10); ii) recognition of the damages caused to the host by the pathogen  during infection through DAMP (Damage Associated Microbial Pattern) receptors; and iii) synthesis of antimicrobial compounds, mainly terpenes (Ricciardi  et al., 2021), and structural barriers through cell wall reinforcement.

Apart from the detection of the first resistance genes of V. vinifera, for the first time the existence of a susceptibility gene in the grapevine species was discovered (Toffolatti et al., 2020). Susceptibility genes facilitate the compatibility between plant and pathogen and are essential for their interaction and the disease development. Therefore, their silencing results in pathogen resistance (Zaidi et al., 2018; Thatcher et al., 2012).

GWA (Genome-wide association) analysis allowed us to identify three resistance loci in Georgian cultivars located on chromosomes 14 (locus Rpv29), chromosome 3 (locus Rpv30) and chromosome 16 (locus Rpv31). These loci include genes that are involved in defenses from biotic stress and in particular in pathogen recognition (rust resistance kinase LR10), cell wall reinforcement and signal transduction (Sargolzaei et al., 2020).

What is happening from the pathogen point of view?

Based on the analysis of P. viticola transcriptome, we moreover identified the pathogenicity genes involved in the modulation of the plant immune system (Toffolatti et al., 2020). In particular, genes encoding for four cytoplasmic effectors, twenty apoplastic effectors and forty proteins with signal peptide for secretion were overexpressed by P. viticola in the interaction with the V. vinifera cultivars Mgaloblishvili and Pinot noir.

New prospective for the modern viticulture

In conclusion, putative grapevine resistance and susceptibility genes were identified. These genes could be exploited for achieving V. vinifera varieties that are resistant to P. viticola and that can be, at the same time, cultivated also for producing Protected Designation of Origin (PDO) wines. Genetic improvement for resistance could be performed through traditional or, better, innovative breeding techniques, such as the transgene-free system CRISPR, that could allow us to transform already existing cultivars from susceptible to resistant. The loci associated with resistance will be used to screen collections of V. vinifera to identify other cultivated or wild V. vinifera accessions possessing it and will be used for Marker Assisted Selection (MAS) breeding. Finally, to avoid the selection of P. viticola strains that are able to overcome resistance, we aim at developing new and safe target-specific fungicides, such as peptide aptamers, acting against P. viticola genes with no effect on beneficial off-target organisms. The aptamers, that are short chains of amino acids, will contribute to control the pathogen, that possesses a great evolutionary potential due to its polycyclic behavior and the occurrence of sexual reproduction, and reduce the selection pressure exerted by the resistant plants on the pathogen.

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