Host‐selective toxins as agents of cell death in plant–fungus interactions

Abstract

Summary Host‐selective toxins are known determinants of compatibility in plant–fungus interactions and provide a powerful model for understanding the specificity of these associations. The identification of genes required for toxin biosynthesis has shown that the genes are unique to the toxin producing species and are clustered in complex loci. These loci may have been acquired by horizontal gene transfer. Many, if not all, host‐selective toxins act by disrupting biochemical processes and in several cases the resulting cell death has the characteristics of programmed cell death. This ability to make dead tissue from living has enabled these facultative saprophytic fungi to become plant pathogens.

Horizontal Chromosome Transfer, a Mechanism for the Evolution and Differentiation of a Plant-Pathogenic Fungus

“…Although these transfer mechanisms are generally unknown in eukaryotes such as fungi, interspecific transfer of a virulence gene encoding the production of a critical toxin has been reported in Pyrenophora tritici-repentis (14). There is also clear evidence of recent lateral gene transfer of the ToxA gene from Stagonospora nodorum to P. tritici-repentis (14,30). In Alternaria alternata plant pathogens (37), we have shown that all strains of the A. alternata pathotypes harbor small extra chromosomes of less than 1.7 Mb, whereas nonpathogenic isolates do not have these small chromosomes (5).…”

“…Some plant-pathogenic fungi, especially necrotrophic pathogens that kill plant cells during invasion, produce phytotoxic metabolites to impair host tissue functions (20,30,42,47). Phytotoxins produced by fungal plant pathogens are generally low-molecular-weight secondary metabolites that exert toxic effects on host plants.…”

“…Phytotoxins produced by fungal plant pathogens are generally low-molecular-weight secondary metabolites that exert toxic effects on host plants. Among these phytotoxins, host-specific toxins (HSTs) are critical determinants of pathogenicity or virulence in several plant-pathogen interactions (13,30,33,40,42,47,49).…”

IP3 production in the hypersensitive response of lemon seedlings against Alternaria alternata involves active protein tyrosine kinases but not a G-protein

“…The presence of multiple signaling pathways for the development of the HR could be useful to provide plants with different systems to counteract the attack by microorganisms that use different strategies to penetrate plant tissues. Markham and Hille (2001) described the infection of plants by microorganisms that use toxins to kill plant tissue directly or to inhibit the secretion of inhibitory compounds that prevent fungal development. If any toxin from A. alternata or other phytopathogens (Rotem, 1994;Markham and Hille, 2001) could activate a G-protein, the G-protein dependent signaling pathway could account for the defensive response against this kind of microorganisms.…”

“…Markham and Hille (2001) described the infection of plants by microorganisms that use toxins to kill plant tissue directly or to inhibit the secretion of inhibitory compounds that prevent fungal development. If any toxin from A. alternata or other phytopathogens (Rotem, 1994;Markham and Hille, 2001) could activate a G-protein, the G-protein dependent signaling pathway could account for the defensive response against this kind of microorganisms. The effect of fungal toxins on G-protein activation in plants has not been described and cannot be ruled out.…”