W.C. Mueller

A system of defence in depth provided by vascular parenchyma cells of tomato in response to vascular infection with Fusarium oxysporum f. sp. lycopersici, race 1

Abstract The penetration and colonization of xylem parenchyma cells surrounding xylem vessels of tomato infected with Fusarium oxysporum f. sp. lycopersici were examined. The primary xylem tissues of both resistant and susceptible near-isolines were found to be highly resistant to infection with only 5% and 3%, respectively, of the first tier of cells and none of the second layer of cells being invaded. The secondary xylem tissues were considerably more susceptible to infection, and here the difference between resistant and susceptible cultivars was pronounced. Secondary tissues were selected in which at least three layers of xylem parenchyma cells abutted the infected vessels. The cells in these layers were designated contact cells (those in immediate contact with the vessels) and first and second adjacent cells. The progress of infection and apparent defence responses were recorded within these three layers of cells at 3 and 6 days after inoculation. These data were later combined when it became apparent that by 3 days after inoculation a successful defence had already been established within the three cell layers, or the infection front had moved beyond them. The percentages of cells infected within these three cell layers were 52, 15 and 3, respectively, in the resistant cultivar, with a mean of 23%. In the susceptible cultivar these percentages were 90, 83, and 70, respectively, with a mean of 81%. Although similar responses, callose deposition and an apparent hypersensitive reaction were observed in both cultivars, the responses, especially that of callose deposition, appeared to be much stronger and to provide a more effective defence in the resistant cultivar than in the susceptible cultivar.

Ultrastructural responses of vessel contact cells in cotton plants resistant or susceptible to infection by Fusarium oxysporum f. sp. vasinfectum

When vascular elements of cotton plants became infected with Fusarium oxysporum f. sp. vasinfectum , contact parenchyma cells surrounding the initially infected vessels responded with changes in their ultrastructural characteristics that could be categorized into three groups. In one group, contact cells were invaded by the fungus and rapidly disintegrated within 24 h after inoculation. In the second group, the cytoplasm in contact cells degenerated within 48 h after inoculation but without apparent hyphal invasion. The evidence suggests that this cytoplasmic degeneration may represent a type of defence response. In the third group, which comprised the majority of contact cells, the cytoplasm remained uninvaded and appeared healthy after inoculation; however, it showed ultrastructural evidence of cytoplasmic reorganization and increased metabolic activity that resulted in the deposition of a complex wall apposition, as well as the development of osmiophilic droplets in the cytoplasm and their apparent secretion through the plasmalemma into the apposition layers and vessel lumens. Lateral colonization by the fungus into the vascular parenchyma cells in both cvs Seabrook Sea Island SBSI and Rowden seemed to be prevented by these cellular responses. The responses, however, were produced faster and were more pronounced in the contact cells of the resistant cultivar than in the susceptible cultivar. Thus is appears that quantitative differences in these reactions may contribute to the degree of resistance to Fusarium .

Ultrastructure and histochemistry of lipoidal droplets in vessel contact cells and adjacent parenchyma cells in cotton plants infected by Fusarium oxysporum f.sp. vasinfectum

Abstract Osmiophilic droplets developed in contact cells and adjacent vascular parenchyma cells of cotton accompanied by changes in cytoplasmic organization after vessel infection by Fusarium oxysporum f.sp. vasinfectum. In the early stages of the cellular response to vessel infection, the droplets were observed in dilations of ER cisternac, the membrane envelopes of mitochondria, and occasionally the membrane envelopes of plastids and nuclei. As the cellular response to vessel infection proceeded and cytoplasmic reorganization became apparent, the droplets were associated with vesicles or remained as individual droplets within the cytoplasm. The developmental sequence suggests that the osmiophilic substances in these droplets may be secreted through the plasmalemma, infused into apposition layers, and exuded into the infected vessels through pits. Sudan Black B staining and organic solvent extraction indicate that the osmiophilic droplets are lipoidal in nature. It is suggested that the lipoidal substances are terpenoids that could function as phytoalexins.

Enlargement and vacuolization of the cytoplasm in contact cells of resistant and susceptible tomato plants following inoculation with Fusarium oxysporum f. sp. Iycopersici, race 1*

This study deals with rapid early cytoplasmic responses that occur in contact parenchyma cells that lie immediately adjacent to vascular infections. Earlier studies have shown that such contact cells in tomato and cotton respond to vascular infections by the deposition of apposition wall layers and the synthesis of stress metabolites several hours to several days after inoculation. These responses appear to be part of a complex system of defence. The studies reported here demonstrated that pronounced cytoplasmic changes, which seemingly enable subsequent morphological changes, could be detected 60–90 min after inoculation. The changes included, first, an increased cytoplasmic width (plasmalemma to vacuolar membrane) on the side of the cell proximal to the infection, and later, increased cytoplasmic width on the distal side. No changes in the numbers and/or distribution of mitochondria were detected during this early period. The cytoplasmic response was found to be more rapid and greater in a resistant than in a near-isogenic, susceptible cultivar of tomato. Vacuolization of the cytoplasm was a conspicuous feature associated with this apparent cytoplasm distension.