Histology of the pathogenesis of Mycosphaerella graminicola in wheat

Cellular aspects of the pathogenesis of Mycosphaerella graminicola in a susceptible and resistant wheat cultivar were studied by light microscopy and scanning and transmission electron microscopy. Experiments were designed as time-sequence studies in two replications with sampling dates at 12-, 24-, and 48-h postinoculation (hpi), and 4-, 8-, 10-, 12-, 14-, and 16-days postinoculation (dpi). A separate experiment was performed to quantify the mycelial biomass in cultivars Shafir and Kavkaz/K4500 1.6.a.4 at the aforementioned intervals using a double antibody sandwich enzyme-linked immunosorbent assay (ELISA) for M. graminicola. The germination frequency of M. graminicola conidia was high in both compatible and incompatible interactions, but the infection frequency was low. Infection was strictly stomatal, but appeared to be a random process since many germ tubes crossed stomata without penetrating them. Some germ tubes formed branched structures close to or on top of stomata. These structures were small compared with the size of stomata, were formed irregularly, and were not significantly correlated with successful penetrations of the host. Multiple penetrations of stomata occurred regularly. Hyphae of M. graminicola were already observed in the substomatal cavities at 12 hpi and, at 48 hpi, hyphae had reached the nearest mesophyll cells. In the compatible response, colonization was fairly limited until 8 dpi. Hyphae grew intercellularly and in close contact with the mesophyll cells. During the 10- to 12-dpi interval, extensive host cell death occurred, which induced further colonization and, eventually, pycnidium formation in substomatal cavities. Initial and further colonization had marked effects on the number and size of the chloroplasts in the compatible interaction. Nevertheless, leaves remained green until approximately 10 dpi. The resistance response was primarily characterized by very limited colonization, mostly in the vicinity of the substomatal cavity. Quantification of the mycelial mass with ELISA revealed similar mycelial quantities in cultivars Shafir and Kavkaz/K4500 1.6.a.4 until 8 dpi. After 8 days, the mycelial quantity developed exponentially in 'Shafir,' but did not significantly increase in 'Kavkaz/K4500 1.6.a.4.'