VIEW ARTICLE

Resistance

Role of Flavanolic Wall Infusions in the Resistance Induced by Laccaria bicolor to Fusarium oxysporum in Primary Roots of Douglas-fir. N. E. Strobel, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5909, Present address: Department of Plant Pathology, University of Kentucky, Lexington 40546-0091; W. A. Sinclair, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5909. Phytopathology 81:420-425. Accepted for publication 25 October 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-420.

When primary roots of newly germinated seedlings of Douglas-fir (Pseudotsuga menziesii) were inoculated with Laccaria bicolor, yellow-to-brown infusions were detected in many cortical cell walls beginning 13 days after initial contact with the fungus. Control roots developed only faint pigmentation in a few cortical walls. When control roots were challenged with a nonaggressive isolate of Fusarium oxysporum, the pathogen colonized intercellular spaces of the cortex and then usually penetrated cell walls to initiate intracellular colonization and lesion formation. Control roots that resisted F. oxysporum developed intense brown infusions in cortical walls adjacent to intercellular hyphae. Histo-chemical tests indicated that these wall infusions were phenolic and were probably derived from flavanols. Peroxidase activity, independent of exogenous H₂O₂, was also detected in walls undergoing infusion. The pathogen also colonized roots exposed to L. bicolor, but intracellular colonization and lesion formation usually were prevented. The pathogen was restricted to intercellular spaces in protected roots. This restriction was associated with the stimulation of flavanolic wall infusion by L. bicolor. When in one test L. bicolor did not stimulate wall infusion, it also did not induce resistance to F. oxysporum. Flavanolic wall infusions also were associated with resistance of intact root tips to degradation by cellulase. Thus, in this system, the prevention of lesion formation in living roots appeared to be a consequence of flavanolic wall infusions, induced by L. bicolor, which restricted F. oxysporum to intercellular spaces by inhibiting degradation of host cell walls. Variable stimulation of flavanolic wall infusion may contribute to inconsistent root protection by L. bicolor.