Cyanogenesis Inhibits Active Defense Reactions in Plants

Abstract

In the course of fungal attack on the cyanogenic rubber tree (Hevea brasiliensis Muell.-Arg.) HCN is liberated from infected tissue. The HCN interferes with plant host and fungal pathogen. It becomes inhibitory to active defense responses which are dependent on biosynthetic processes as far as a threshold concentration is transgressed.Many plants liberate HCN after mechanical or chemical injury to their cells (2) or in the course of fungal infection (20). The role of this cyanogenic process in plants has been widely discussed but still remains unclear. There is considerable evidence that HCN liberated during tissue destruction by grazing animals acts as a feeding deterrent (10), but the data on cyanogenesis are quite controversial in the case of fungal diseases. Fry's working group (4) provided excellent evidence that fungal pathogens of cyanogenic plants generally are highly tolerant to hydrocyanic acid. From these findings it was deduced that plant pathogenic fungi which lack a mechanism to cope with HCN are incapable ofinfecting cyanogenic plants. Therefore, cyanogenic compounds were generally regarded to be preformed resistance factors (8).In contrast to this conclusion, Ludtke and Hahn (18) reported that highly cyanogenic flax varieties are susceptible to Colletotrichum lini, the causal agent of flax anthracnose, whereas moderately cyanogenic varieties are resistant to the same isolates ofthe fungus. In addition, Lieberei (15) reported a distinct correlation between a high capacity for HCN liberation and the high susceptibility of rubber tree leaves to Microcyclus ulei, the causal agent of rubber tree leaf blight. M. ulei is tolerant to HCN, it even grows better in HCNcontaining atmosphere (17). Obviously in flax and the rubber plant, HCN is not a simple resistance factor which can be overcome by pathogens adapted to it (16,17); instead the HCN is impairing the defense response ofthese plants. Screening studies revealed that all strongly cyanogenic Hevea varieties were susceptible to all existing isolates ofM. ulei, whereas low cyanogenic plants respond in typical race-cultivar specific reaction patterns (11,15).

Disruption of the Cyanide Hydratase Gene in Gloeocercospora sorghi Increases Its Sensitivity to the Phytoanticipin Cyanide but Does Not Affect Its Pathogenicity on the Cyanogenic Plant Sorghum

“…In addition, M. ulei produced smaller lesions on leaves of highly cyanogenic lines when the inoculated leaves were incubated in an atmosphere where the HCN generated by infection was removed (Lieberei, 1986). Additional studies suggested that the HCN released under normal infection conditions appears to interfere with the plant's ability to express resistance (Lieberei et al, 1989). An inverse or lack of correlation with the amount of cyanogenic compounds and resistance has also been found in other plant-pathogen interactions (Pourmohseni and Ibenthal, 1991;Trione, 1960).…”

Cassava Plants with a Depleted Cyanogenic Glucoside Content in Leaves and Tubers. Distribution of Cyanogenic Glucosides, Their Site of Synthesis and Transport, and Blockage of the Biosynthesis by RNA Interference Technology

“…The specialist hornworm Erinnyis ello and green mite Mononychellus tanajoa have coevolved with cassava and therefore have no preference for high-or low-cyanide-containing varieties (Bellotti and Riis, 1994). In the rubber tree H. braziliensis, the presence of high amounts of cyanogenic glucosides has been demonstrated to increase the sensitivity of the plant to attack by the fungus Microcyclis ulei, most likely because the cyanide released inhibits simultaneous synthesis of protective phytoalexins (Lieberei, 1986;Lieberei et al, 1989). White clover (Trifolium repens) is polymorphic with respect to cyanogenesis (Corkill, 1942;Hughes, 1991).…”

Plant Cyanogenesis of Phaseolus lunatus and its Relevance for Herbivore–Plant Interaction: The Importance of Quantitative Data

“…Hydrogen cyanide is toxic because of its affinity to the terminal cytochrome oxidase in the mitochondrial respiratory pathway and a number of other reactions (Solomonson, 1981;Brattsten et al, 1983). Despite the obvious toxicity of the products of plant cyanogenesis, its role in the scope of plantY herbivore interaction is very complex (Lieberei, 1988(Lieberei, , 1989. A range of aspects determines its effectiveness as a mechanism of plant defense.…”