Contribution of a Cyanide-insensitive Alternate Respiratory System to Increases in Formamide Hydro-lyase Activity and to Growth in Stemphylium loti in Vitro

Abstract

Stemphylium lotn, a pathogen of a cyanogenc plant, possesses a cyanide-isensitive alternate respiratory pathway. In the absence of cytochrome inhibitors, the alternate system had only a minor role in resption. When S. loi was grown in medium amended with antimycin to block the cytochrome chain, the alternate system accounted for the total oxygen consumption ocited with respiration. Hydrogen cyanide is released from cyanogenic glucosides in birdsfoot trefoil (Lotus corniculatus L.) upon infection by Stemphylium loti (16). This fungus grows in the presence of relatively high concentrations of cyanide in vitro (3) and is more tolerant of cyanide than are other fungi (16). reported that cyanide tolerance in S. loti is induced upon exposure to cyanide and is due to the production of the enzyme formamide hydro-lyase which converts cyanide to nontoxic formamide.We Cyanide-insensitive respiration is generally associated with an unidentified alternate terminal oxidase (ATO) (8) which reportedly connects with the normal electron transport chain at ubiquinone (24). This alternate respiratory pathway coexists with the normal respiratory chain in mitochondria (8). Studies of the normal and alternate respiratory systems have shown the following: (a) antimycin, cyanide, azide, and carbon monoxide, through inhibition of certain Cyt in the normal electron transport chain, inhibit the normal respiratory system but not the alternate pathway (9, 14, 25); (b) salicylhydroxamic acid specifically inhibits the alternate pathway but not the normal pathway (20); and (c) the addition of Cyt inhibitors to a growth medium promotes the appearance of the alternate respiratory pathway in microorganisms such that 100% of the respiration is insensitive to cyanide but is sensitive to SHAM (6,10,12).Our objectives were to determine if S. loti possesses a cyanideinsensitive alternate respiratory pathway and to assess the contribution of such an alternate pathway to growth and to increases in FHL activity. A preliminary report of this work has been published (17). MATERIALS AND METHODS

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

“…Evidence for this is that the ao gene from Arabidopsis thaliana can complement the growth of Escherichia coli mutants lacking a terminal oxidase (Kumar and Soll, 1992). Involvement of this HCN-insensitive oxidase in pathogenesis by fungal pathogens of cyanogenic plants was proposed by Rissler and Millar (1977) for the fungus S. loti because it could still synthesize CHT when only the AO respiratory pathway was functioning. However, if G. sorghi contains an AO or another HCN metabolizing enzyme that is activated by exposure to HCN, it does not confer much tolerance to HCN as the CHT mutant is highly sensitive to HCN (Fig.…”

An ABC Transporter and a Cytochrome P450 of Nectria haematococca MPVI Are Virulence Factors on Pea and Are the Major Tolerance Mechanisms to the Phytoalexin Pisatin

“…Stemphylium loti, a phytopathogenic fungus of a cyanogenic plant (Lotus corniculatus L.), also appears to have a sequence of tolerance mechanisms that allows the fungus to overcome the toxic effects of cyanide released by its host (Rissler and Miller 1977). Upon exposure to hydrogen cyanide, the inhibition of the fungal cytochrome-based respiration activates a cyanide-insensitive respiratory system.…”

Cyanide degradation by immobilised fungi

“…The reaction catalysed is: H20 + HCN + HCONH 2 It is carried out by an enzyme, cyanide hydratase (formamide hydrolyase), that can be induced in spores and mycelia that have been exposed to cyanide (FqJ & Millar, 1972;Rissler & Millar, 1977). Other fungi and possibly bacteria are known to degrade cyanide to CO 2 , presumably via formamide (Knowles, 1976;Bunch & Knowles, 1980).…”