The Downy Mildew Effector Proteins ATR1 and ATR13 Promote Disease Susceptibility in Arabidopsis thaliana

Abstract

The downy mildew (Hyaloperonospora parasitica) effector proteins ATR1 and ATR13 trigger RPP1-Nd/WsB-and RPP13-Nddependent resistance, respectively, in Arabidopsis thaliana. To better understand the functions of these effectors during compatible and incompatible interactions of H. parasitica isolates on Arabidopsis accessions, we developed a novel delivery system using Pseudomonas syringae type III secretion via fusions of ATRs to the N terminus of the P. syringae effector protein, AvrRPS4. ATR1 and ATR13 both triggered the hypersensitive response (HR) and resistance to bacterial pathogens in Arabidopsis carrying RPP1-Nd/WsB or RPP13-Nd, respectively, when delivered from P. syringae pv tomato (Pst) DC3000. In addition, multiple alleles of ATR1 and ATR13 confer enhanced virulence to Pst DC3000 on susceptible Arabidopsis accessions. We conclude that ATR1 and ATR13 positively contribute to pathogen virulence inside host cells. Two ATR13 alleles suppressed bacterial PAMP (for Pathogen-Associated Molecular Patterns)-triggered callose deposition in susceptible Arabidopsis when delivered by DC3000 DCEL mutants. Furthermore, expression of another allele of ATR13 in plant cells suppressed PAMP-triggered reactive oxygen species production in addition to callose deposition. Intriguingly, although Wassilewskija (Ws-0) is highly susceptible to H. parasitica isolate Emco5, ATR13 Emco5 when delivered by Pst DC3000 triggered localized immunity, including HR, on Ws-0. We suggest that an additional H. parasitica Emco5 effector might suppress ATR13-triggered immunity.

Genetic disassembly and combinatorial reassembly identify a minimal functional repertoire of type III effectors in Pseudomonas syringae

“…delivery of effectors from other strains, or even from oomycetes (20), can increase the virulence of WT strains. Here, we have learned additional properties of the DC3000 T3E repertoire in the context of interactions with N. benthamiana: (i) no single T3E is sufficient for significant virulence, (ii) some T3Es seem to interfere with an early phase of the plant immune response (i.e., by disrupting PAMP perception) such that other T3Es make a contribution to virulence only in their presence, (iii) early-acting effectors also seem to suppress defenses elicited by the T3SS machinery, (iv) T3Es in small groups with reduced redundancy can be observed to contribute in a hierarchical fashion to growth and symptom production, and (v) a minimal functional repertoire seems to require several effectors and members of at least two REGs.…”

Mechanisms of Nuclear Suppression of Host Immunity by Effectors from the Arabidopsis Downy Mildew Pathogen Hyaloperonospora arabidopsidis (Hpa)

“…Stable effector expression in planta allows us to test whether the ectopic expression of a given effector affects plant susceptibility toward the corresponding pathogen (Bozkurt et al 2011;Fabro et al 2011;Caillaud et al 2012) or even other pathogens (van Esse et al 2008;Hewezi et al 2010;Bouwmeester et al 2011;Fabro et al 2011). Using this approach, effectors from Hpa, including ATR13, were shown to suppress plant defenses, notably PAMP-induced ROS burst and callose deposition, which are typical indicators of plant immune response activation (Sohn et al 2007;Fabro et al 2011).…”

“…Using this EDV system, Sohn et al (2007) have shown that Hpa effectors ATR1 and ATR13 confer enhanced virulence to the bacterial pathogen Pst DC3000. Kemen et al (2011) showed with this method that several classes of Albugo laibachii effectors conferred enhanced virulence to Pst DC3000.…”

“…The effector detector vector (EDV) system is based on the delivery by the bacterial T3SS of Pst of an effector of interest fused to the amino-terminal part of the bacterial effector AvrRPS4 (Sohn et al 2007). Using this EDV system, Sohn et al (2007) have shown that Hpa effectors ATR1 and ATR13 confer enhanced virulence to the bacterial pathogen Pst DC3000.…”

“…Little is known about biotrophic effector functions, mainly because working with obligate pathogens is difficult. This difficulty drove researchers to develop heterologous systems to indirectly assess effector functions (Sohn et al 2007;Rentel et al 2008;Schornack et al 2010). Recognition of the avirulent effector by the corresponding R gene generally initiates a localized celldeath response and should, therefore, be amenable for investigation in a cobombardment assay.…”