Highly Diverse Endophytic and Soil Fusarium oxysporum Populations Associated with Field-Grown Tomato Plants

Abstract

The diversity and genetic differentiation of populations of Fusarium oxysporum associated with tomato fields, both endophytes obtained from tomato plants and isolates obtained from soil surrounding the sampled plants, were investigated. A total of 609 isolates of F. oxysporum were obtained, 295 isolates from a total of 32 asymptomatic tomato plants in two fields and 314 isolates from eight soil cores sampled from the area surrounding the plants. Included in this total were 112 isolates from the stems of all 32 plants, a niche that has not been previously included in F. oxysporum population genetics studies. Isolates were characterized using the DNA sequence of the translation elongation factor 1␣ gene. A diverse population of 26 sequence types was found, although two sequence types represented nearly two-thirds of the isolates studied. The sequence types were placed in different phylogenetic clades within F. oxysporum, and endophytic isolates were not monophyletic. Multiple sequence types were found in all plants, with an average of 4.2 per plant. The population compositions differed between the two fields but not between soil samples within each field. A certain degree of differentiation was observed between populations associated with different tomato cultivars, suggesting that the host genotype may affect the composition of plant-associated F. oxysporum populations. No clear patterns of genetic differentiation were observed between endophyte populations and soil populations, suggesting a lack of specialization of endophytic isolates. Fusarium oxysporum is a well-known phytopathogenic fungus that affects hundreds of crops worldwide. Most studies of the fungus focus on the ability of F. oxysporum to cause vascular wilt on a particular host. However, environmental surveys of a wide range of habitats frequently find F. oxysporum in the absence of plant disease (1). These F. oxysporum populations, often referred to as nonpathogenic, although lack of pathogenicity is usually not confirmed, are a cosmopolitan component of soil communities. F. oxysporum can also infect and colonize plants asymptomatically as an endophyte. This endophytic lifestyle has not been well researched, despite reports of endophytic colonization by F. oxysporum on nearly 100 plant species (2).The interaction of endophytic F. oxysporum and plants may give insight into how pathogen recognition and plant resistance operate. In susceptible plants, pathogenic F. oxysporum penetrates the root, colonizes the root cortex, and then spreads through the xylem to the rest of the plant, causing occlusion of the xylem and ultimately wilting and death (3, 4). In host cultivars that are resistant to the pathogen, the spread of the fungus is blocked, and only limited colonization of host tissue occurs (5, 6, 7). Nonpathogenic F. oxysporum inoculated onto plants can colonize the root cortex similarly to pathogens, but host responses, such as thickening of cell walls and protuberances into intercellular spaces, seem to limit the growth of endophytes, similar to ...

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