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

The effects of purified Helminthosporium maydis T (HmT) toxin on active Ca(2+) transport into isolated mitochondria and microsomal vesicles were compared for a susceptible (T) and a resistant (N) strain of corn (Zea mays). ATP, malate, NADH, or succinate could drive (45)Ca(2+) transport into mitochondria of corn roots. Ca(2+) uptake was dependent on the proton electrochemical gradient generated by the redox substrates or the reversible ATP synthetase, as oligomycin inhibited ATP-driven Ca(2+) uptake while KCN inhibited transport driven by the redox substrates. Purified native HmT toxin completely inhibited Ca(2+) transport into T mitochondria at 5 to 10 nanograms per milliliter while transport into N mitochondria was decreased slightly by 100 nanograms per milliliter toxin. Malate-driven Ca(2+) transport in T mitochondria was frequently more inhibited by 5 nanograms per milliliter toxin than succinate or ATP-driven Ca(2+) uptake. However, ATP-dependent Ca(2+) uptake into microsomal vesicles from either N or T corn was not inhibited by 100 nanograms per milliliter toxin. Similarly, toxin had no effect on proton gradient formation ([(14)C]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.