Ultrastructure of in vivo interactions of the antagonist bacteria Bacillus cereus X16 and B. thuringiensis 55T with Fusarium roseum var. sambucinum, the casual agent of potato dry rot

Abstract

The interaction of Fusarium roseum var. sambucinum, the causal agent of potato dry rot, with two antagonistic
bacteria, Bacillus cereus X16 and B. thuringiensis 55T, was studied on wounded potato tubers using light
and electron microscopy. Application of B. cereus X16 or B. thuringiensis 55T to potato wounds before challenge with
the pathogen suppressed dry rot and restricted fungal growth in plant tissues to the first few cell layers beneath the
site of inoculation. Both bacterial antagonists penetrated into potato tissues and established themselves through
intercellular and intracellular proliferation. The extent of Fusarium colonization was appreciably reduced in the
bacterized tubers, and most fungal cells in these tubers were severely damaged, with appreciable morphological and
structural changes. In potato tubers bacterized by B. thuringiensis 55T, Fusarium invasion of the host tissues did not
stimulate structural host reactions, and direct parasitism, which operates by degradation of the fungal cell walls and
disintegration of the fungal cytoplasm, seemed to play a key role in the antagonism against Fusarium hyphae. In
potato tubers inoculated with B. cereus X16 and challenged with the pathogen, on the other hand, a set of defense
reactions, were triggered, including modifications of the primary cell walls and the occlusion of some cells and vascular
tissues with different types of electron-opaque materials. Fungal hyphae in the vicinity of these barriers, apparently
containing higher than usual levels of phenol-like compounds, usually showed advanced stages of disorganization,
suggesting the existence of a fungitoxic environment. The results presented here show that the two antagonistic
bacilli use different biocontrol strategies to suppress Fusarium dry rot development.