ULTRASTRUCTURAL AND CYTOCHEMICAL INVESTIGATIONS OF PATHOGEN DEVELOPMENT AND HOST REACTIONS IN SUSCEPTIBLE AND PARTIALLY-RESISTANT CARROT ROOTS INFECTED BY PYTHIUM-VIOLAE, THE MAJOR CAUSAL AGENT FOR CAVITY SPOT

Two carrot genotypes, cultivar Nanco and line 24, susceptible and part ially- resistant respectively to cavity spot, were compared ultrastruc turally and cytochemically 24 h, 48 h and 72 h after root inoculation with a virulent Pythium violae isolate. The extent of pathogen ingress and the response of the host differed markedly with the two genotypes . In cv Nanco, growth of fungal hyphae was predominantly intracellular and was accompanied by pronounced damage; by 48 h after inoculation, pericycle and the first cell layers of the phloem parenchyma were inva ded, resulting in host wall dissolution and cytoplasm aggregation. The growth of P. violae in Line 24 was limited to the pericycle, even up to 72 h after inoculation; fungal colonization was accompanied by retr action of cytoplasm and in the appearance of granular or fibrillar mat erial in the host cell lumen. Some affected host cells were filled wit h structureless osmophilic material. In cultivar Nanco, invading funga l hyphae were unaffected; by contrast in line 24, the cytoplasm of inv ading hyphae, particularly those inside the cell host, was disorganise d and structureless. Infection and host response in the two cultivars were studied with two specific labels: Aplysia gonad lectin (AGL), a p olygalacturonic acid-binding lectin, and an exoglucanase complexed to colloidal gold were used to locate pectin and cellulosic beta-(1,4)-gl ucans respectively in infected tissues. The decrease of cytochemical l abeling beyong fungal penetration showed clearly hydrolysis of pectin and cellulose in cell walls of the cv Nanco. By contrast, the cell wal l of line 24 remained largely intact, although unlabeled amorphous and electron-dense material was observed inside the wall. Fibrillar or el ectron dense material commonly observed in infected tissue of Line 24 apparently did not contain pectic or cellulosic substances. Moreover, material observed in host cells or fungal hyphae was also free of labe ling. The origin and the chemical composition of these compounds as we ll as their possible role in the defence mechanisms of carrot against P violae are discussed.