Multiple candidate effectors from the oomycete pathogen Hyaloperonospora arabidopsidis suppress host plant immunity

GEORGINA FABRO; JENS STEINBRENNER; MARY COATES; NAVEED ISHAQUE; LAURA BAXTER; DAVID J. STUDHOLME; EVELYN KÖRNER; REBECCA L. ALLEN; SOPHIE J.M. PIQUEREZ; ALEJANDRA ROUGON-CARDOSO; DAVID GREENSHIELDS; RITA LEI; JORGE L. BADEL; MARIE-CECILE CAILLAUD; KEE-HOON SOHN; GUIDO VAN DEN ACKERVEKEN; JANE E. PARKER; JIM BEYNON; JONATHAN D. G. JONES

PLOS PATHOGENS

PUBLIC LIBRARY SCIENCE

Lugar: Notre Dame Indiana USA; Año: 2011

1553-7366

Oomycete pathogens cause diverse plant diseases. To successfully colonize theirhosts, they deliver a suite of effector proteins that can attenuate plant defenses. Inthe oomycete downy mildews, effectors carry a signal peptide and an RxLR motif.Hyaloperonospora arabidopsidis (Hpa) causes downy mildew on the model plantArabidopsis thaliana (Arabidopsis). We investigated if candidate effectors predictedin the genome sequence of Hpa isolate Emoy2 (HaRxLs) were able to manipulatehost defenses in different Arabidopsis accessions. We developed a rapid andsensitive screening method to test HaRxLs by delivering them via the bacterial typethreesecretion system (TTSS) of Pseudomonas syringae pv tomato DC3000-LUX(Pst-LUX) and assessing changes in Pst-LUX growth in planta on 12 Arabidopsisaccessions, The majority (~ 70%) of the 64 candidates tested positively contributedto Pst-LUX growth on more than one accession indicating that Hpa virulence likelyinvolves multiple effectors with weak accession-specific effects. Further screeningwith a Pst mutant (ΔCEL) showed that HaRxLs that allow enhanced Pst-LUX growthusually suppress callose deposition, a hallmark of pathogen-associated molecularpattern (PAMP)-triggered immunity (PTI). We found that HaRxLs are rarely strongavirulence determinants. Although some decreased Pst-LUX growth in particularaccessions, none activated macroscopic cell death. Fewer HaRxLs conferredenhanced Pst growth on turnip, a non-host for Hpa, while several reduced itconsistent with the idea that turnip’s non-host resistance against Hpa could involve acombination of recognized HaRxLs and ineffective HaRxLs. We verified our resultsby constitutively expressing in Arabidopsis a sub-set of HaRxLs. Several transgeniclines showed increased susceptibility to Hpa and attenuation of Arabidopsis PTIresponses, confirming the HaRxLs’ role in Hpa virulence. This study shows TTSSscreening system provides a useful tool to test whether candidate effectors fromeukaryotic pathogens can suppress/trigger plant defense mechanisms and to ranktheir effectiveness prior to subsequent mechanistic investigation.