Genetic and Epigenetic Effects of Plant–Pathogen Interactions: An Evolutionary Perspective  Boyko Alex , Kovalchuk Igor   Molecular Plant  Volume 4, Issue 6, Pages 1014-1023 (November 2011) DOI: 10.1093/mp/ssr022 Copyright © 2011 The Authors. All rights reserved. Terms and Conditions

Figure 1 The Hypothetical Scenario for the Acquisition of New Plant R-Gene Recognition Specificities during Infection with a Compatible Virus. Infection with a compatible virus (r) result activates various signals such as small RNAs. The signal (e.g. smRNAs) spreads systemically from the place of infection (leaf) to non-infected tissues, including those that form the gametes (dark grey box). Here, the signal results in a loss or gain of DNA methylation at the specific loci such as R-gene and R-gene-like loci. Thus, the newly established methylation patterns can be transmitted to the immediate progeny (F1). Heritable changes in DNA methylation represent epimutations. Hypermethylation can potentially lead to an increased frequency of C→T mutations, whereas hypomethylation can lead to an increased frequency of homologous recombination (HR) (Ossowski et al., 2010). Dashed line shows that recombination occurs between homologous allelic loci. Both genetic mechanisms (point mutations and gene recombination) may lead to the formation of new functional alleles over multiple (F1-Fx) plant generations. The selection of individuals from a population of variants of epimutations and mutations will result in an increased chance of appearance of epialleles and alleles conferring new avirulence gene recognition specificities, and thus allowing for the incompatible interaction and resistance to the same virus in the future. Molecular Plant 2011 4, 1014-1023DOI: (10.1093/mp/ssr022) Copyright © 2011 The Authors. All rights reserved. Terms and Conditions

Figure 2 Possible Mechanisms of Generating SRS during Virus Infection. Compatible or incompatible infection of tobacco plants results in the generation of various signaling molecules including reactive oxygen species (ROS). This initial production of radicals in the first 1 h of exposure leads to the production of a systemic recombination signal (SRS). Both conditions result in the generation of salicylic acid (SA), although the amount of SA produced is significantly higher upon an incompatible interaction (Love et al., 2005). We hypothesized that this second SA- and NPR1-dependent wave of radicals (Grant et al., 2000; Love et al., 2005) inactivates SRS. High amounts of SA upon the incompatible interaction trigger a massive production of ROS as a part of the hypersensitive response and generate another type of a signal, the systemic acquired resistance signal (SARS) that establishes the systemic state of protection against the same and other pathogens—a state known as systemic acquired resistance (SAR). Upon the compatible interaction, SRS results in a systemic increase in homologous recombination frequency (HRF). Molecular Plant 2011 4, 1014-1023DOI: (10.1093/mp/ssr022) Copyright © 2011 The Authors. All rights reserved. Terms and Conditions