Helminthosporium maydis T Toxin Decreased Calcium Transport into Mitochondria of Susceptible Corn

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Abstract
The effects of purified H. maydis T (HmT) toxin on active Ca2+ transport into isolated mitochondria and microsomal vesicles were compared for a susceptible (T) and a resistant (N) strain of corn. ATP, malate, NADH or succinate could drive 45Ca2+ transport into mitochondria of corn roots. Ca2+ uptake was dependent on the proton electrochemical gradient generated by the redox substrates or the reversible ATP synthetase, as oligomycin inhibited ATP-driven Ca2+ uptake while KCN inhibited transport drive by the redox substrates. Purified native HmT toxin completely inhibited Ca2+ transport into T mitochondria at 5-10 ng/ml while transport into N mitochondria was decreased slightly by 100 ng/ml toxin. Malate-driven Ca2+ transport in T mitochondria was frequently more inhibited by 5 ng/ml per ml toxin than succinate or ATP-driven Ca2+ uptake. However, ATP-dependent Ca2+ uptake into microsomal vesicles from either N or T corn was not inhibited by 100 ng/ml toxin. Toxin had no effect on proton gradient formation ([14C]methylamine accumulation) in microsomal vesicles. These results show that mitochondrial and not microsomal membrane is a primary site of HmT toxin action. HmT toxin may inhibit formation of or dissipate the electrochemical proton gradient generated by substrate-driven electron transport or the mitochondrial ATPase, after interacting with a component(s) of the mitochondrial membrane in susceptible corn.