Ultrastructural and Cytochemical Investigation of Asymptomatic Infection by Pythium spp.

ABSTRACT The influence exerted by Pythium group F (a minor pathogen ubiquitous in soilless cultures) and P. uncinulatum (a nonpathogenic species) colonization on tomato roots was investigated. In both interactions, infected roots did not exhibit obvious symptoms; however, major physiological changes occurred within the host tissues colonized by Pythium group F compared to P. uncinulatum. According to our cytological observations, Pythium group F colonization involved a series of events: first, development and growth of the fungus in the epidermis and outer cortex tissues, which was associated with marked host cell disorganization and even breakdown. In colonized roots, symptoms were not easily discernible because alterations were restricted to the epidermis and outer cortex tissues. Second, pathogen ingress in the inner cortex and stele tissues was associated with massive induction of host defense reactions and alteration of invading hyphae. In a complex interaction that involved major metabolic changes in root cells, an array of defense-related reactions was produced, as exemplified by the formation of wall appositions and plugging of host cells with osmiophilic, electron-dense, granular, or fibrillar materials. P. uncinulatum growth was restricted to the epidermis and outer cortex tissues and associated with relatively minor damage to host cells. P. uncinulatum colonization of root tissues did not result in defense events similar to those observed in Pythium group F-infected inner cortex and stele tissues. Moreover, most of the invading P. uncinulatum hyphae were moribund. The results of the current study indicated that symptomless infections can be insignificant in terms of pathology for the plant (i.e., P. uncinulatum) or can be potentially dangerous (i.e., Pythium group F). Our results clearly showed a striking difference between nonpathogenic and minor pathogenic Pythium spp. attacks. The description of the Pythium group F infection process suggests that this interaction is unique, because the fungus behaves as a necrotrophic fungus in the epidermis and outer cortex tissues, whereas it is a potential inducer of plant defense reactions in the inner cortex and stele tissues. To our knowledge, cytological events similar to those observed with Pythium group F have not been described until now, even though descriptions of these events could help elucidate several aspects of the relationships between plants and minor pathogens.