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      Nonhost resistance to rust pathogens – a continuation of continua

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          Abstract

          The rust fungi (order: Pucciniales) are a group of widely distributed fungal plant pathogens, which can infect representatives of all vascular plant groups. Rust diseases significantly impact several crop species and considerable research focuses on understanding the basis of host specificity and nonhost resistance. Like many pathogens, rust fungi vary considerably in the number of hosts they can infect, such as wheat leaf rust ( Puccinia triticina), which can only infect species in the genera Triticum and Aegilops, whereas Asian soybean rust ( Phakopsora pachyrhizi) is known to infect over 95 species from over 42 genera. A greater understanding of the genetic basis determining host range has the potential to identify sources of durable resistance for agronomically important crops. Delimiting the boundary between host and nonhost has been complicated by the quantitative nature of phenotypes in the transition between these two states. Plant–pathogen interactions in this intermediate state are characterized either by (1) the majority of accessions of a species being resistant to the rust or (2) the rust only being able to partially complete key components of its life cycle. This leads to a continuum of disease phenotypes in the interaction with different plant species, observed as a range from compatibility (host) to complete immunity within a species (nonhost). In this review we will highlight how the quantitative nature of disease resistance in these intermediate interactions is caused by a continuum of defense barriers, which a pathogen needs to overcome for successfully establishing itself in the host. To illustrate continua as this underlying principle, we will discuss the advances that have been made in studying nonhost resistance towards rust pathogens, particularly cereal rust pathogens.

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          Perception of the bacterial PAMP EF-Tu by the receptor EFR restricts Agrobacterium-mediated transformation.

          Higher eukaryotes sense microbes through the perception of pathogen-associated molecular patterns (PAMPs). Arabidopsis plants detect a variety of PAMPs including conserved domains of bacterial flagellin and of bacterial EF-Tu. Here, we show that flagellin and EF-Tu activate a common set of signaling events and defense responses but without clear synergistic effects. Treatment with either PAMP results in increased binding sites for both PAMPs. We used this finding in a targeted reverse-genetic approach to identify a receptor kinase essential for EF-Tu perception, which we called EFR. Nicotiana benthamiana, a plant unable to perceive EF-Tu, acquires EF-Tu binding sites and responsiveness upon transient expression of EFR. Arabidopsis efr mutants show enhanced susceptibility to the bacterium Agrobacterium tumefaciens, as revealed by a higher efficiency of T-DNA transformation. These results demonstrate that EFR is the EF-Tu receptor and that plant defense responses induced by PAMPs such as EF-Tu reduce transformation by Agrobacterium.
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            A putative ABC transporter confers durable resistance to multiple fungal pathogens in wheat.

            Agricultural crops benefit from resistance to pathogens that endures over years and generations of both pest and crop. Durable disease resistance, which may be partial or complete, can be controlled by several genes. Some of the most devastating fungal pathogens in wheat are leaf rust, stripe rust, and powdery mildew. The wheat gene Lr34 has supported resistance to these pathogens for more than 50 years. Lr34 is now shared by wheat cultivars around the world. Here, we show that the LR34 protein resembles adenosine triphosphate-binding cassette transporters of the pleiotropic drug resistance subfamily. Alleles of Lr34 conferring resistance or susceptibility differ by three genetic polymorphisms. The Lr34 gene, which functions in the adult plant, stimulates senescence-like processes in the flag leaf tips and edges.
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              A receptor kinase-like protein encoded by the rice disease resistance gene, Xa21.

              The rice Xa21 gene, which confers resistance to Xanthomonas oryzae pv. oryzae race 6, was isolated by positional cloning. Fifty transgenic rice plants carrying the cloned Xa21 gene display high levels of resistance to the pathogen. The sequence of the predicted protein, which carries both a leucine-rich repeat motif and a serine-threonine kinase-like domain, suggests a role in cell surface recognition of a pathogen ligand and subsequent activation of an intracellular defense response. Characterization of Xa21 should facilitate understanding of plant disease resistance and lead to engineered resistance in rice.
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                Author and article information

                Contributors
                Journal
                Front Plant Sci
                Front Plant Sci
                Front. Plant Sci.
                Frontiers in Plant Science
                Frontiers Media S.A.
                1664-462X
                11 December 2014
                2014
                : 5
                : 664
                Affiliations
                [1] 1The Sainsbury Laboratory, Norwich Research Park Norwich, UK
                [2] 2Commonwealth Scientific and Industrial Research Organisation, Agriculture Flagship Canberra, ACT, Australia
                Author notes

                Edited by: Gitta Coaker, University of California, Davis, USA

                Reviewed by: Ralph Panstruga, RWTH Aachen University, Germany; Guus Bakkeren, Agriculture and Agri-Food Canada, Canada

                *Correspondence: Michael Ayliffe, Commonwealth Scientific and Industrial Research Organisation, Agriculture Flagship, P.O. Box 1600, Clunies Ross Street, Acton, Canberra, ACT 2601, Australia e-mail: michael.ayliffe@ 123456csiro.au ; Matthew J. Moscou, The Sainsbury Laboratory, Norwich Research Park, Norfolk, Norwich NR4 7UH, UK e-mail: matthew.moscou@ 123456tsl.ac.uk

                This article was submitted to Plant-Microbe Interaction, a section of the journal Frontiers in Plant Science.

                Article
                10.3389/fpls.2014.00664
                4263244
                25566270
                32165949-ee4f-4f97-a482-e011b24debe0
                Copyright © 2014 Bettgenhaeuser, Gilbert, Ayliffe and Moscou.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                History
                : 29 September 2014
                : 07 November 2014
                Page count
                Figures: 3, Tables: 0, Equations: 0, References: 118, Pages: 15, Words: 0
                Categories
                Plant Science
                Review Article

                Plant science & Botany
                host range,disease resistance,nonadapted pathogen,nhr,formae speciales,pucciniales

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