A Multidrug Resistance Transporter in Magnaporthe Is Required for Host Penetration and for Survival during Oxidative Stress

In prokaryotes and eukaryotes, multidrug resistance (MDR) transporters use energy-dependent efflux action to regulate the intracellular levels of antibiotic or xenobiotic compounds. Using mutational analysis of ABC3, we define an important role for such MDR-based efflux during the host penetration step of Magnaporthe grisea pathogenesis. Mutants lacking ABC3 were completely nonpathogenic but were surprisingly capable of penetrating thin cellophane membranes to some extent. The inability of abc3Delta to penetrate the host surface was most likely a consequence of excessive buildup of peroxide and accumulation of an inhibitory metabolite(s) within the mutant appressoria. Treatment with antioxidants partially suppressed the host penetration defects in the abc3Delta mutant. abc3Delta was highly sensitive to oxidative stress and was unable to survive the host environment and invasive growth conditions. ABC3 transcript levels were redox-regulated, and on host surfaces, the activation of ABC3 occurred during initial stages of blast disease establishment. An Abc3-green fluorescent protein fusion localized to the plasma membrane in early appressoria (and in penetration hyphae) but became predominantly vacuolar during appressorial maturity. We propose that ABC3 function helps Magnaporthe to cope with cytotoxicity and oxidative stress within the appressoria during early stages of infection-related morphogenesis and likely imparts defense against certain antagonistic and xenobiotic conditions encountered during pathogenic development.