Strengthening Grapevine Resistance by Pseudomonas fluorescens PTA-CT2 Relies on Distinct Defense Pathways in Susceptible and Partially Resistant Genotypes to Downy Mildew and Gray Mold Diseases.

Sara Lakkis,Aziz Aziz,Patricia Trotel-Aziz,Adrian Schwarzenberg,Fanja Rabenoelina,Christophe Clément,Eric Nguema-Ona

doi:10.3389/fpls.2019.01112

Abstract

Downy mildew caused by the oomycete Plasmopara viticola and gray mold caused by the fungus Botrytis cinerea are among the highly threatening diseases in vineyards. The current strategy to control these diseases relies totally on the application of fungicides. The use of beneficial microbes is arising as a sustainable strategy in controlling various diseases. This can be achieved through the activation of the plants’ own immune system, known as induced systemic resistance (ISR). We previously showed that bacteria-mediated ISR in grapevine involves activation of both immune response and priming state upon B. cinerea challenge. However, the effectiveness of beneficial bacteria against the oomycete P. viticola remains unknown, and mechanisms underpinning ISR against pathogens with different lifestyles need to be deciphered. In this study, we focused on the capacity of Pseudomonas fluorescens PTA-CT2 to induce ISR in grapevine against P. viticola and B. cinerea by using two grafted cultivars differing in their susceptibility to downy mildew, Pinot noir as susceptible and Solaris as partially resistant. On the basis of their contrasting phenotypes, we explored mechanisms underlying ISR before and upon pathogen infection. Our results provide evidence that in the absence of pathogen infection, PTA-CT2 does not elicit any consistent change of basal defenses, while it affects hormonal status and enhances photosynthetic efficiency in both genotypes. PTA-CT2 also induces ISR against P. viticola and B. cinerea by priming common and distinct defensive pathways. After P. viticola challenge, PTA-CT2 primes salicylic acid (SA)- and hypersensitive response (HR)-related genes in Solaris, but SA and abscisic acid (ABA) accumulation in Pinot noir. However, ISR against B. cinerea was associated with potentiated ethylene signaling in Pinot noir, but with primed expression of jasmonic acid (JA)- and SA-responsive genes in Solaris, together with downregulation of HR-related gene and accumulation of ABA and phytoalexins.

Highlights

Grapevine (Vitis vinifera L.) is a perennial crop species that is sensitive to a large spectrum of pathogens
We evaluated by related genes by quantitative reverse-transcription PCR (RT-qPCR) the expression of eight selected defense genes from the different functional categories: genes encoding a glutathione-S-transferase (GST1), a pathogenesis-related protein 1 (PR1), and a β-1,3-glucanase (PR2) as markers of the salicylic acid (SA) signaling pathway; a 9-lipoxygenase (LOX9) involved in oxylipin synthesis as a marker of the jasmonic acid (JA) pathway; a ACC oxidase (ACO) that catalyzes the conversion of ACC to ET; a hypersensitive response (HR)-related (HSR) gene as a marker of cell death; a phenylalanine ammonia-lyase (PAL) catalyzing the first step in the phenylpropanoid pathway; and a stilbene synthase (STS) encoding a key enzyme of the resveratrol biosynthesis
We examined the immune responses triggered by P. fluorescens PTA-CT2 against biotrophic oomycete P. viticola and the necrotrophic fungus B. cinerea and compared these responses in two grafted grapevine cultivars with contrasting susceptibility to downy mildew, Pinot noir and Solaris, the former being susceptible and the latter partially resistant to infection

Summary

Introduction

Grapevine (Vitis vinifera L.) is a perennial crop species that is sensitive to a large spectrum of pathogens. P. viticola is a biotrophic oomycete that attacks all green parts of the grapevine, while B. cinerea is a necrotrophic fungus that induces cell death to consume the host nutrients to spread (Dangl and Jones, 2001; Lenzi et al, 2016) The control of both diseases requires frequent fungicide applications to prevent significant crop losses. The development of resistant cultivars is among the environmentally friendly alternatives to the use of chemicals In this context, several resistance loci conferring resistance to P. viticola have been identified in Muscadinia rotundifolia (Bellin et al, 2009; Bouquet, 2011; Vezzulli et al, 2019), Vitis riparia (Marguerit et al, 2009), Vitis amurensis (Blasi et al, 2011; Schwander et al, 2012), and Vitis cinerea (Ochssner et al, 2016). P. viticola isolates are able to bypass or overcome resistance genes (Peressotti et al, 2010), thereby compromising the resistance sustainability in vineyards, while no source of resistance has been characterized against B. cinerea so far (Töpfer et al, 2011; Schwander et al, 2012)

Methods

Results

Conclusion