Ultrastructural and cytochemical study of colonization of xylem vessel elements of susceptible and resistant Dianthus caryophyllus by Fusarium oxysporum f.sp. dianthi

Abstract

The colonization processes of the xylem in the susceptible carnation cv. Early Sam and the resistant cv. Novada were studied ultrastructurally following inoculation with Fusarium oxysporum f.sp. dianthi. Samples from 1 to 3 cm above the incision were collected over 5 weeks and processed following conventional procedures as well as with probes for cellulose, N-acetyl-glucosamine, and pectin. The fungus grew profusely in the vessel lumina of the susceptible cultivar. Some of the colonized vessels were lined with coating material connected to the fungal cell wall and extending into the host cell wall through microfilamentous-like structures. Coatings did not label for pectin or cellulose. The pathogen crossed from one vessel element to another (and at times to parenchyma cells) usually directly through pit membranes; often the invading structures of the fungus appeared to be either only membrane-bound or formed solely of microfilamentous-like entities. The fungus subsequently invaded extensively, generally by means of microhyphae, the vessel intercalary walls from the pit membranes and vessel wall junctures. Microhyphae had thin or imperceptible walls and contained only some of the normal cytoplasmic components. Initially, the invading hyphae dislocated the host cell walls, apparently mechanically more than by lysis; however, more pronounced lysis occurred following general tissue invasion. Host parenchyma cells seemed relatively unaffected, even after the surrounding walls had undergone severe degradation. Colonization of resistant plants was restricted. Degradation of tissues did not occur and microhyphae were not observed. Inoculated vessel elements in the 'Novada' plants contained numerous fungal cells and little occluding material, whereas the surrounding vessels were almost completely occluded. The initially invaded xylem became tangentially compartmentalized by parenchyma cell wall thickenings and by hyperplastic parenchyma. Occasionally, hyperplastic tissues were slightly re-invaded, forming secondary invasion pockets. Vessel-occluding material varied in structure and opacity, not only from vessel to vessel but also within the same vessel, and contained microfilamentous-like structures and other types of fine fibrillar material. Some vessel elements in or near the secondary invasion pockets contained only the finer fibrils that reacted strongly with an antibody specific for pectin. Vessel elements rarely contained tyloses.Key words: cellulose, chitin, Dianthus caryophyllus, Fusarium wilt, gels and gums, host wall degradation, microhyphae, pectin, tyloses.

Histopathology of Fusarium wilt of staghorn sumac (Rhus typhina) caused by Fusarium oxysporum f. sp. callistephi race 3. II. Characteristics of opaque matter associated with extensive host cell and cell wall alterations

“…The occurrence of fungal cell wall modifications, including desquamation, in other wilt-causing fungi, has been described and reported previously (Nicole et al 1994;Ouellette and Baayen 2000;Ouellette et al 1999aOuellette et al , 1999bOuellette et al , 2002. In this study, fungal cell wall desquamation was also observed similar to that reported for O. novo-ulmi (Ouellette et al 1999a), but in some cases, unevenness in the walls might also have been due to irregular wall formation.…”

“…Similar structures often pervaded fungal walls and extracellular matter, and then seemed to stretch freely over void spaces or into the host cell walls. A similar association was equally observed in other fungal wilt pathogens (Nicole et al 1994;Ouellette et al 1995Ouellette et al , 1999bOuellette et al , 2002.…”

Tissue invasion and alteration in eggplant infected with Verticillium dahliae: A light and transmission electron microscopy study

“…These were characterized by three demarcated layers: a shredded one, unlabelled for cellulose, a more compact and opaque cellulose-labelled layer, and the unlabelled VWLM layer. This type of alteration mirrored that of pit membranes of half-bordered pits in the metaxylem in DED affected elms and Fusariuminfected carnations (Ouellette 1978a(Ouellette , 1978bOuellette et al 1999Ouellette et al , 2004bOuellette et al , 2004cOuellette and Rioux 1993) and often of intertracheary pit membranes in staghorn sumac (Ouellette et al 2005a). In all instances, the term "swelling of pit membranes" should not apply, in our opinion, when it is primarily attributed to covering layers of extraneous matter, likely of a pathogen origin in most cases.…”

“…We have already described particular modes of pathogen development, of tissue alterations, of host cell reactions and related aspects concerning Dutch elm disease and Fusarium wilts of carnation, tomato and staghorn sumac (Charest et al 2004;Ouellette 1978b;Ouellette et al 1995Ouellette et al , 1999Ouellette et al , 2004aOuellette et al , 2004bOuellette et al , 2004cOuellette et al , 2004dOuellette et al , 2004e, 2005aOuellette et al , 2005b. These works present peculiar characteristics of pathogen structures, of host cell wall and content alterations related to extensive tissue invasion by opaque matter, not ascribable to a host origin, of various host cell reactions and so on, and more than circumstantial evidence was presented to show that most of the material lining vessel walls did not always correspond to the notions of coating as supported by others.…”

Extracellular sheath formation by Sphaeropsis hypodermia and association with its infection in elm trees

“…Van Dyke and Mims 1991). Opaque material associated with host wall altérations was likewise observed in plants affected with wilt diseases : elm trees with Dutch elm disease (Nicole et al 1994;Ouellette 1978;Ouellette andRioux 1992, 1993); susceptible Ft/sar/um-infected carnation plants (Ouellette and Baayen 2000;Ouellette et al 1999a);…”