DEFENSE-MECHANISM OF RICE PLANT TO RESPIRATORY INHIBITION BY A PROMISING CANDIDATE BLASTICIDE, SSF126

A new candidate systemic rice blasticide, SSF126, dose-dependently inh ibited NADH oxidation by submitochondrial particles from rice roots. H owever, oxidation by the root submitochondrial particles was much less susceptible to SSF126 compared to that by submitochondrial particles from mycelial cells of Pyricularia grisea, a pathogen causing rice bla st. interestingly SSF126 did not completely suppress the respiration b y intact rice roots, and the respiratory activity of the roots recover ed from inhibition time-dependently even in the presence of SSF126 at concentrations sufficient to fully block oxidation by the submitochond rial particles. This recovery was not due to selective extrusion of SS F126 from the roots, but to switching from the cytochrome pathway to t he alternative cyanide-resistant respiratory pathway. In immunoblots o f the alternative oxidase, high molecular mass species were detected i n the mitochondria from rice roots in addition to low molecular mass s pecies. Quantification of high and low molecular mass species revealed an increase in the amount of a protein corresponding to a 36-kDa spec ies equivalent to a decrease in the amount of a protein corresponding to a 72-kDa species following a 5-h incubation with SSF126. This conve rsion of the alternative oxidase to the low molecular mass species in the mitochondria was correlated with the respiratory recovery found in intact rice roots, suggesting that the low molecular mass species is the active form of the alternative oxidase and the high molecular mass species is the inactive form. These results suggest that rice plants can block the severe injury caused by limiting the cytochrome pathway by SSF126 through utilization of the alternative pathway promoted by t he interconversion of the alternative oxidase protein. (C) 1998 Academ ic Press.