Abstract
Victorin, the host-specific pathotoxin from the pathogen, Helminthosporium victoriae, is known to dramatically alter the structure and permeability of the plasma membrane of susceptible tissue and it has been reported that these effects are similar to symptoms associated with calcium deficiency. When a mixture of 45Ca 2+ and victorin is desalted on a Sephadex G-15 column, victorin and 45Ca 2+ elute together. This indicates that victorin binds 45Ca 2+ and provides a method of obtaining radioactive victorin. 45Ca 2+-labeled victorin prepared in this manner is found to bind reversibly to pelletable particles from homogenates of susceptible and resistant oat coleoptiles. Both victorin and Ca 2+ interfere with this binding, while 3-indoleacetic acid, an auxin reported to bind to the plasma membrane, does not. In addition, victorin does not influence the binding of 14C-labeled 3-indoleacetic acid to pelletable particles from homogenates of susceptible or resistant oat coleoptiles. Calcium specifically binds to pelletable particles from homogenates of susceptible and resistant oat coleoptiles. Scatchard plots of Ca 2+ binding data indicate at least two different classes of calcium binding site. The K m (2 × 10 −6 m) and the total number of binding sites (5 × 10 −9 mol/mg protein) for the high affinity binding site in susceptible tissue differ significantly ( P⩽0·01) from the K m (0·68 × 10 −6 m) and number (2 × 10 −9 mol/mg protein) of binding sites in resistant tissue. Furthermore, resistant tissue was found to contain more calcium, both bound and free, than susceptible tissue. These data suggest that the selective effects of victorin may be caused by a victorin-induced deficiency of calcium in susceptible but not in resistant tissue.
Published Version
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