Ultrastructure of the Penetration and Infection of Pansy Roots by Thielaviopsis basicola

Abstract

Transmission electron microscopy was used to study the penetration and infection of pansy roots by Thielaviopsis basicola. Events observed in 7- to 10-day-old roots produced on moist filter paper differed slightly from those in roots from 4-week-old plants washed free of potting media prior to inoculation. By 3 h postinoculation (PI), epidermal cells of roots produced on filter paper exhibited aggregated cytoplasm and papilla formation in response to germ tube tips. The presence of callose in papillae was demonstrated using immunogold labeling. Papilla formation was not effective in preventing host cell penetration. A slender infection hypha emerged from a germ tube tip and grew through a papilla. Its tip then expanded to form a globose infection vesicle. By 6 h PI, infection hyphae emerged from infection vesicles, and invaded host cells showed signs of necrosis. By 8 h PI, infection hyphae had grown into cortical cells in spite of papilla formation in these cells. By 24 h PI, distinctive intracellular hyphae were present in necrotic cortical cells. In washed roots, most epidermal cells failed to respond to invasion. Hyphae simply grew through these cells and contacted cortical cells that exhibited aggregated cytoplasm and papillae formation. Infection structures similar to those produced in epidermal cells from roots grown on filter paper then formed in cortical cells of washed roots. The fact that T. basicola formed infection structures only in cells that responded to invasion suggests that T. basicola has a more complex relationship with its host than would be expected in a nectrotrophic pathogen. We believe that T. basicola is best described as a necrotrophic hemibiotroph.

Colonization of plant roots by egg‐parasitic and nematode‐trapping fungi

“…Fungal growth rate, although decreased by such plant cell wall modifications, was not arrested. This was also observed for several fungal root pathogens (Rodríguez‐Gálvez & Mendgen, 1995; Mims et al ., 2000). However, other factors might be controlling fungus spread in tomato, since no long lignitubers were found in A. oligospora ‐colonized roots.…”

Interaction of the Arabidopsis GTPase RabA4c with Its Effector PMR4 Results in Complete Penetration Resistance to Powdery Mildew

“…Regarding defense responses to pathogen attack, particular attention has been focused on the formation of cell wall thickenings. So-called papillae contain callose as the most common chemical constituent, but also proteins (e.g., peroxidases and thionins), phenolics, and other antimicrobial and antifungal constituents (Aist and Williams, 1971;Sargent et al, 1973;Mercer et al, 1974;Sherwood and Vance, 1976;Mims et al, 2000). Even though callose deposition appears to be an ubiquitous plant defense response to invading pathogens and has been studied for over 150 years (deBary, 1863), the specific function of callose in this defense reaction has remained unclear.…”

Secreted Fungal Effector Lipase Releases Free Fatty Acids to Inhibit Innate Immunity-Related Callose Formation during Wheat Head Infection

“…Since then, examinations have identified callose as the most abundant chemical constituent in papillae, which may also include proteins (e.g. peroxidases and antimicrobial thionins), phenolics, and other constituents (Aist and Williams, 1971;Sherwood and Vance, 1976;Mims et al, 2000). Papillae have been regarded as an early defense reaction that may not completely stop the pathogen; rather, they have been considered to act as a physical barrier to slow pathogen invasion (Stone and Clarke, 1992;Voigt and Somerville, 2009) and to contribute to the plant's innate immunity (Jones and Dangl, 2006;Schwessinger and Ronald, 2012).…”