Organ identity and environmental conditions determine the effectiveness of nonhost resistance in the interaction between Arabidopsis thaliana and Magnaporthe oryzae.

Mechanisms leading to nonhost resistance of plants against nonadapted pathogens are thought to have great potential for the future management of agriculturally important diseases. In this article, we report an investigation of nonhost resistance motivated by the advantages of studying an interaction between two model organisms, namely Arabidopsis thaliana and Magnaporthe oryzae. During the course of our studies, however, we discovered an unexpected plasticity in the responses of Arabidopsis against this ostensibly nonhost pathogen. Thus, we elucidated that certain experimental conditions, such as the growth of plants under long days at constantly high humidity and the use of high inoculum concentrations of M. oryzae conidia, forced the interaction in leaves of some Arabidopsis ecotypes towards increased compatibility. However, sporulation was never observed. Furthermore, we observed that roots were generally susceptible to M. oryzae, whereas leaves, stems and hypocotyls were not infected. It must be concluded, therefore, that Arabidopsis roots lack an effective defence repertoire against M. oryzae, whereas its leaves possess such nonhost defence mechanisms. In summary, our findings point to organ-specific determinants and environmental conditions influencing the effectiveness of nonhost resistance in plants.