Ultrastructural and developmental alterations induced by Periconia circinata toxin in the root tip of sorghum

Abstract

Cytological and developmental effects induced by Periconia circinata toxin were examined to better understand the mechanism of action for this toxin. Roots of sorghum seedlings susceptible and resistant to P. circinata were incubated in 500 ng toxin/mL (treated) or water (controls). Root cap cells of resistant seedlings treated with the toxin were cytologically similar to those of controls, although the toxin caused a transient inhibition of mitosis in cells of the primary root tip. In outer root cap cells of susceptible seedlings treated for 0.25 h, hypersecretory activity was lost, secretory vesicles were fewer, and secretory product accumulated between the plasma membrane and cell wall. Also, inner root cap cells showed increased vacuolation. Longer treatments caused increased vacuolation, loss of starch, increased numbers of lipid bodies, pleomorphic amyloplasts, regularly stacked endoplasmic reticulum, apparent changes in the amounts of cytomembranes, dispersion of heterochromatin, and autolysis. Mitochondrial morphology was normal, but lesions in the tonoplast occurred before autolysis. The toxin also inhibited expansion and sloughing off of root cap cells and mitotic activity in the root tip. Stacked endoplasmic reticulum, nonhypersecretory dictyosomes, fewer secretory vesicles, increased vacuolation, reorganization of heterochromatin, and increased secretory product outside the protoplast were induced by P. circinata toxin and by cyanide. These data suggested that a cyanogenic compound is biologically active in cells treated with P. circinata toxin. Our results suggest that the toxin transiently affects resistant seedlings and in susceptible seedlings alters vacuolar expansion, secretory activity, and endomembrane flow, although other processes may also be affected.