Compatible Puccinia hordei infection in barley induces basal defense to subsequent infection by Blumeria graminis Reza Aghnoum 1 , Rients E. Niks * Laboratory of Plant Breeding, Graduate School for Experimental Plant Sciences, Wageningen University, Droevendaalsesteeg 1, 6708 PB Wageningen, The Netherlands article info Article history: Accepted 31 October 2011 Keywords: Biotrophic pathogens Haustorium formation Induced resistance Papilla formation Resistance mechanisms abstract Rusts and powdery mildews employ different strategies to suppress defense during penetration. We observed that a compatible interaction of barley-Puccinia hordei induced increased penetration resis- tance to a challenge infection by powdery mildew. This induced resistance is local and its level is not determined by the virulence spectrum of the challenger isolate. Our data suggest that the inducer effect is due to rust-stoma communication during penetration, to the presence of the rust hyphae in the apoplast, or to penetration resistance mounted by the rust attacked mesophyll cells. We hypothesized that the rust “primes” the basal defense prior to the mildew infection. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Infection by a pathogen or parasite may condition plants to activate or suppress defense reactions against subsequent infection attempts by other pathogen strains or species. Induction of plant defense systems as a result of prior exposure to pathogens [1], non- pathogenic root-colonizing bacteria [2], feeding by herbivores [3] or treatment with chemicals [4] is termed induced resistance. Three types of induced resistance have been described, viz. systemic acquired resistance (SAR) [5], induced systemic resistance (ISR) [2] and induced localized resistance [6]. Suppression of plant defense systems against a non-adapted or incompatible race of a pathogen as a result of prior exposure to a compatible pathogen is called induced susceptibility. Induced localized resistance and induced susceptibility have been reported in many plantepathogen combinations. In cereals induction of localized resistance or susceptibility has been reported as a result of a prior attack by both necrotrophic and biotrophic fungi (e.g [7e10]). In cereals, most research on localized induction of resistance and susceptibility has concentrated on interaction of compatible and incompatible isolates of the powdery mildew pathogen, Blumeria graminis (hereafter Bg). Failed establishment by an adapted or non-adapted powdery mildew pathogen induces penetration resistance to subsequent challenge inoculation with a compatible isolate (e.g [6,9,11e 13]). Conversely, successful penetration and haustorium formation by an adapted, virulent isolate of Bg induces susceptibility at cell level to haustorium formation by subsequent challenge with adapted virulent or avir- ulent isolates and also non-adapted formae speciales and species (e.g [13e17]).“Susceptibility” at cell level is often called accessibility, because the avirulent or non-adapted challenger pathogen typically will not complete its life cycle, due to defense mechanisms that back-up the penetration resistance. For similar reasons “resistance” at cell level is often called inaccessibility. Most of the reports indi- cate that this induction of accessibility and inaccessibility is local- ized and limited to the challenged cell and its adjacent cells. Induced resistance to rust pathogens as a result of prior inocu- lation with the same or different rust pathogens has been reported by several workers (e.g [18e21]). In the barley leaf rust (Puccinia hordei) system, failed haustorium formation by a non-adapted rust fungus, however, does not compromise haustorium formation by a compatible challenger rust but a compatible interaction suppresses the defense system of the same cell at challenge with a non-adapted rust species [10]. Powdery mildew and rust fungi both have biotrophic lifestyles and require living plant tissues to survive and reproduce. Successful establishment of specialized feeding structures of the pathogen, the so-called haustoria, is a prerequisite for a compatible interaction. In barley, non-host resistance against non-adapted powdery mildews, the quantitatively inherited basal resistance and mlo-mediated resistance to the adapted powdery mildew all act on the basis of papilla formation at the point of attempted ingress into the cell, which results in impaired haustorium formation. Papilla formation is involved in induced inaccessibility in the barley-powdery mildew [22]. A similar papilla formation is involved in basal host and non- host resistance against rust fungi [10,23]. * Corresponding author. Tel.: þ31 317 482 508; fax: þ31 317 483 457. E-mail address: rients.niks@wur.nl (R.E. Niks). 1 Present address: Agricultural and Natural Resources Research Center of Khor- asan Razavi, P.O. Box: 91735-488, Mashhad, Iran. Contents lists available at SciVerse ScienceDirect Physiological and Molecular Plant Pathology journal homepage: www.elsevier.com/locate/pmpp 0885-5765/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pmpp.2011.10.003 Physiological and Molecular Plant Pathology 77 (2012) 17e22