Molecular and functional characterization of rice MAP kinases involved in biotic and abiotic stress tolerance

Ethylene (ET) biosynthesis and signal transduction are early, active responses of the rice blast resistance pathway. Rice cells amplify the response to rice blast invasion by relaying signals to components of the mitogen activated protein kinase cascade (MAPK). The rice genome encodes 17 MPK genes. Four of which are inducible by rice blast infection and ET. In this study, we have characterized the role of OsMPK17, OsMPK13 and OsMPK5 in the rice blast resistance response.;To elucidate the biological role of OsMPK17 in high salinity, drought and the rice blast resistance response, expression of the OsMPK17 gene was constitutively increased or suppressed in transgenic rice. Overexpression of OsMPK17 leads to an increase in rice blast resistance and drought tolerance in transgenic rice. Suppression of the OsMPK17 gene led to a hypersensitivity to ABA. Both the overexpression and suppression of the OsMPK17 gene resulted in ET insensitivity. OsMPK17 appears to acts as a point of convergence for rice abiotic and biotic stress signal transduction.;The transcripts of OsMPK13 are induced by rice blast, ET and salicylic acid. To determine the biological function of OsMPK13 in the rice blast defense response, transgenic rice with constitutive suppression of the OsMPK13 gene were generated. Suppression of OsMPK13 leads to an increase of resistance to the rice blast fungus and ET insensitivity.;Previously OsMPK5 was shown to inversely modulate disease resistance and abiotic stress tolerance. However, the mechanism of this inverse modulation remained to be elucidated. In this study we have determined the role of OsMPK5 in mediating ABA and ET levels during rice defense responses. Transgenic analysis demonstrated that suppression of OsMPK5 expression via RNA interference resulted in decreased levels of endogenous ABA and elevated levels of endogenous ET. In addition, OsMPK5 RNAi suppression lines exhibited decreased sensitivity to ABA during seed germination, constitutive activation of defense genes and enhancement of disease resistance. Treatment of these transgenic plants with ABA leads to a more drastic decrease of ET levels than wildtype plants, suggesting that OsMPK5 may negatively regulate ET levels during defense responses. Additionally OsMPK5 is a key signaling component mediating the antagonistic interaction of ABA and ET pathways during rice defense response. The three studies suggest that the ET-inducible OsMPK17, -13 and -5 are key signaling components of the ET-mediated rice blast resistance.;The OsMPK17 transcript and protein are induced by virulent and avirulent isolates of the rice blast fungus, ET and abscisic acid (ABA). Kinase activity of OsMPK17 is enhanced specifically by the avirulent isolate of the rice blast fungus in addition to ET and abscisic acid (ABA), high salinity and drought stress. The molecular characterization of OsMPK17 suggests that the kinase is involved in rice abiotic stress signal transduction and the rice blast resistance response.