Ultrastructural and Immunocytochemical Studies on Effects of Barley Yellow Dwarf Virus – Infection on Fusarium Head Blight, Caused by Fusarium graminearum, in Wheat Plants

The interactions between barley yellow dwarf virus (BYDV) and Fusarium head blight (FHB), caused by Fusarium graminearum, were studied in the two winter wheat cultivars (cvs.), Agent (susceptible to FHB) and Petrus (moderately resistant to FHB), using ultrastructural and immunocytochemical methods. Infections of wheat plants of both cvs. by BYDV increased susceptibility to FHB. BYDV infection caused numerous cytological changes in lemma tissue of both cvs. such as formation of vesicles in the cytoplasm, degradation of fine structures of chloroplasts of both cvs. and accumulation of large starch grains in the chloroplasts. Electron microscopical studies showed that the development of F. graminearum on spike surfaces was not affected in BYDV‐infected plants. After penetration and intercellular growth in lemma tissue, defence responses to Fusarium infections were markedly reduced in BYDV‐diseased plants compared to the tissue of virus‐free plants. At sites of contact of fungal cells with host tissue, depositions of cell wall material were distinctly less pronounced than in tissues of virus‐free plants of cv. Petrus. Detection of β‐1,3‐glucanases and chitinases in lemma tissue of cv. Agent revealed no appreciably increased accumulation of both defence enzymes in F. graminearum‐infected virus‐free and BYDV‐infected tissues compared to the non‐infected control tissue. On the other hand, in cv. Petrus, infection with F. graminearum induced a markedly enhanced activity of both enzymes 3 days after inoculation. The increase of both enzyme activities was less pronounced in BYDV‐infected plants than in tissue exclusively infected with F. graminearum. Cytological studies suggest that in contrast to the susceptible cv. Agent postinfectional defence responses may play still an important role in the resistance of the moderately resistant cv. Petrus to FHB.