Active defense in plants is associated with marked metabolic alterations, but little is known about the exact role of the reported changes in specific activity of several enzymes in infected plant tissues. ,j-Fructosidase (invertase), the enzyme that converts sucrose into glucose and fructose, increases upon infection by fungi and bacteria. To understand the relationship between fungal growth and #l-fructosidase accumulation, we used an antiserum raised against a purified deglycosylated carrot cell wall j-fructosidase to study by immunogold labeling the spatial and temporal distribution of the enzyme in susceptible and resistant tomato (Lycopersicon esculentum) root tissues infected with the necrotrophic fungus, Fusarium oxysporum f. sp. racidis-Iycopersici. In susceptible plants, the enzyme started to accumulate in host cell walls about 72 hours after inoculation. Accumulation occurred only in colonized cells and was mainly restricted to areas where the walls of both partners contacted each other. In resistant plants, accumulation of #l-fructosidase was noticeable as soon as 48 hours after inoculation and appeared to reach an optimum by 72 hours after inoculation. Increase in wall-bound #-fructosidase was not restricted to infected cells but occurred also, to a large extent, in tissues that remained uncolonized during the infection process. The enzyme also accumulated in wall appositions (papillae) and intercellular spaces. This pattern of enzyme distribution suggests that induction of s-fructosidase upon fungal infection is part of the plant's defense response. The possible physiological role(s) of this enzyme in infected tomato plants is discussed in relation to the high demand in energy and carbon sources during pathogenesis.An attack by potential pathogens elicits a complex plant defense response in which numerous genes are activated, resulting in the synthesis and accumulation of secondary metabolites (phytoalexins and phenolics), structural macromolecules (callose, lignin, and hydroxyproline-rich glycoproteins), and a variety ofproteins (pathogenesis-related proteins, enzyme inhibitors, hydrolytic enzymes, and enzymes of secondary metabolism) (2,8,12,18,20). Thus, infection of a plant by a pathogen is accompanied by a considerable metabolic change (15). One physiology that has not received much attention concerns the reported increase in the metabolic rate ofinfected plant tissues (24, 34, 35). Little is known about the enzymatic processes underlying this marked increase, which allows the plant cells to sustain their defense response. A growing body of evidence from several reports supports the concept that fungal infection causes a significant increase in fl-fructosidase, the enzyme that hydrolyzes sucrose into glucose and fructose (21,24, 28, 31). In accord with these observations, Sturm and Chrispeels (37) found that infection of carrot storage roots by Erwinia carotovora was associated with a rapid and marked increase in the level ofmRNA encoding extracellular ,B-fructosidase. Another line o...