Process of tomato root colonization by a pathogenic strain of Fusarium oxysporum f. sp. lycopersici in comparison with a non‐pathogenic strain

Abstract

A pathogenic strain of Fusarium oxysporum f. sp. lycopersici transformed with the glucuronidase (GUS) reporter gene was used to study the colonization process of tomato (Lycopersicon esculentum) roots in hydroponic culture. The plants were treated exactly as those used in a different study with a non‐pathogenic strain of F. oxysporum in order to compare the two types of colonization process. The pathogenic strain rapidly colonized the root surface, forming a dense network of hyphae – shown by the GUS staining assay – as early as 48 h after inoculation. The first images of the pathogen penetrating into the epidermis of the root were observed 24 h after inoculation. The plant showed defence reactions mainly in the hypodermis, but also in the cortex. Generally these barriers failed to prevent the centripetal growth of the pathogen towards the stele. The GUS activity moved with the actively growing hyphae into the tissues; the stele appeared intensely stained 7 d after inoculation, whereas by this time the hyphae at the root surface did not show any more staining. The hyphae formed a dense network at the apex of the root, but direct contact between hyphae and living cells was prevented by several layers of sloughed cap cells. Although rarely observed, penetration of the pathogen into the apex was possible and led to a rapid destruction of apical cells. The only differences between this pattern of root colonization and that observed for a non‐pathogenic strain of F. oxysporum appear to concern the frequency of dead apices and the intensity of fungal colonization in the cortex. When the pathogen passed around the barrier formed in the hypodermis it always reached the xylem, although this barrier and other defence reactions induced at different levels in the cortex always prevented the non‐pathogenic strain from reaching the stele. These observations suggest that the main differences between the two types of interaction – between the plant and either the pathogenic or the non‐pathogenic strain – are quantitative rather than qualitative.

Fusarium oxysporum (F221-B) as biocontrol agent against plant pathogenic fungi in vitro and in hydroponics

“…Interestingly, F221-B could survive pretty well in such a system (in nutrient solution and plant root). This was in line with two findings when one demonstrated the ability of non-pathogenic F. oxysporum (V2w2 and III4w1) to penetrate the intact host tissue and recolonise the host internally upon inoculation in nutrient solution (Paparu et al 2006) and the other revealed the capability of non-pathogenic F. oxysporum in colonising the lower parts of plantlets such as roots (Olivain & Alabouvette 1999). The mechanism of action associated with nonpathogenic F. oxysporum can be divided into two broad categories: direct antagonism of non-pathogenic strains to the pathogen and indirect antagonism mediated through the host plant such as induced resistance (Fuchs et al 1997).…”

Reaction of the native Greek tomato varieties ‘Chondrokatsari Messinias’ and ‘Katsari Santorinis’ to Fusarium oxysporum f. sp. lycopersici and Rhizoctonia solani infection

“…lycopersici was able to perform a vascular infection of tomato root tissue producing lesions on the roots. However, these lesions had limited expansion probably due to intense defense reactions occurring in the superficial cell layers (Olivain and Alabouvette, 1999). Brown discoloration of the vascular system of the plants was also observed in the present study indicating colonization of xylem vessels by the pathogen ( Figure 1B).…”

“…In the present study, symptoms were also observed on the surface of the roots as dark brown to black, necrotic, circular or irregular lesions ( Figure 1A). According to Olivain and Alabouvette (1999), F. oxysporum f. sp. lycopersici was able to perform a vascular infection of tomato root tissue producing lesions on the roots.…”

“…This is considered a typical symptom of infection of tomato plants by F. oxysporum f. sp. lycopersici following root tissue penetration and colonization of the vascular system by the pathogen (Bishop and Cooper, 1983;Agrios, 1997;Olivain and Alabouvette, 1999).…”

Expression of effector gene SIX1 of Fusarium oxysporum requires living plant cells

“…Even though all isolates of F. oxysporum appear to have a very similar lifestyle, notably feeding on plant debris and colonizing the rhizosphere of living plants, some are capable of entering and infecting living plant tissue, while others are not (Gordon and Martyn, 1997;Olivain and Alabouvette, 1999). This suggests the existence of isolate specific genes and the capacity to switch from a saprophytic to an infectious lifestyle, probably involving the reprogramming of gene expression.…”