Plant Pathogens Affecting the Establishment of Plant-Symbiont Interaction

Hundreds of different microorganisms are attached to the surface of roots. Therefore, it is not surprising that plants have the ability to distinguish threatening intruders from beneficial microbiota (Tóth and Stacey, 2015). Pathogens can be discriminated by plant cells through amyriad of plasma membrane and intracellular receptors that recognize molecules released by microbes, in a process called innate immune system. In spite of the immune ability of plants to prevent pathogen infection, symbiotic signaling molecules are perceived by the host plant, triggering signaling cascades that lead to symbiont infection and accommodation (Tóth and Stacey, 2015). However, some symbiotic signaling molecules can induce responses that are normally associated with plant innate immunity (Pauly et al., 2006), and several observations that are consistent with a rapid, defense-like response occurring in legumes when infected by rhizobia have been obtained, mainly involving programmed cell death, cell wall thickening, reactive oxygen species (ROS) generation, defense phytohormones and salicylic acid (SA) production (Jones and Dangl, 2006; Stacey et al., 2006; Dodds and Rathjen, 2010; Montiel et al., 2012). Similar to bacterial pathogens, symbionts alone also have the ability to actively suppress innate immune response, as previously shown (Liang et al., 2013). Plant-symbiont-pathogen interaction is an emerging topic, and several questions in this field have been elucidated in the recent years. However, the main focus of these studies is commonly limited to the effects of symbiont microorganisms on the activation of plant defense responses and elicitation of induced systemic resistance to pathogens (Pieterse et al., 2001; de Vleeschauwer and Höfte, 2009), which usually does not come with the normal costs of reduced growth rates and reproductive outcomes in resistance-expressing plants (Spaepen et al., 2009). Studies considering plant pathogens as limiting factors to the symbiosis establishment are still scarce (Faessel et al., 2010; de Román et al., 2011; van Dam and Heil, 2011; Ballhorn et al., 2014). However, such studies are highly relevant for the use of symbiotic inoculum in particular in monocultures of pathogen-susceptible crops. In this opinion article, we focus on rhizobial and mycorrhizal symbiosis inhibition mediated by plant pathogens. We present the current state-of-the-art through the compilation and comparison of available information that can help to elucidate intriguing questions, as the sensing and signaling of plant-symbiont-pathogen interaction.