Strategies of Cruciferous Pathogenic Fungi:  Detoxification of the Phytoalexin Cyclobrassinin by Mimicry

Abstract

The remarkable metabolism of the cruciferous phytoalexin cyclobrassinin by the phytopathogenic root rot (Rhizoctonia solani Kuhn) and blackleg [Phoma lingam (Tode ex Fr.) Desm., asexual stage of Leptosphaeria maculans (Desm.) Ces. et de Not.] fungi is reported. It was established that R. solani metabolized and detoxified cyclobrassinin via the phytoalexin brassicanal A, which was further transformed into nontoxic products. Detoxification of cyclobrassinin in P. lingam avirulent isolate Unity occurred via the phytoalexin brassilexin, whereas the detoxification in P. lingam virulent isolate BJ 125 occurred via the phytoalexin dioxibrassinin. The chemistry involved in the structure determination of the intermediates of these three apparently different pathways and their antifungal activities are described. In addition, efficient syntheses of both phytoalexins brassicanal A and brassilexin by mimicry of the fungal biotransformation route are reported. Implications of these unprecedented transformations are discussed.

Camalexin induces detoxification of the phytoalexin brassinin in the plant pathogen Leptosphaeria maculans

“…Stock solutions of each compound in DMSO (0.05 M) were used to prepare assay solutions in PDA media (concentrations shown in Table 1); control solutions contained 1% DMSO in PDA (Pedras and Okanga, 1999). Sterile tissue culture plates (6-well, 35 mm diameter) containing test solutions and control solutions (2.5 ml/well) were inoculated with mycelial plugs placed upside down on the center of each plate (6 mm cut from 7-day-old PDA plates) and incubated under constant light for 5 days.…”

Cloning, purification and characterisation of brassinin glucosyltransferase, a phytoalexin-detoxifying enzyme from the plant pathogen Sclerotinia sclerotiorum

“…Crucifer phytoalexins can be detoxified in vitro by Leptosphaeria maculans and Leptosphaeria biglobosa (Pedras and Ahiahonu, 2005;Pedras et al, , 2007aPedras et al, , 2007b, Rhizoctonia solani, which causes root rot (Pedras and Okanga, 1999;Pedras and Khan, 2000), and S. sclerotorium (Pedras and Ahiahonu, 2002;Pedras andHossain, 2006, 2007). Moreover, similar detoxification reactions occur in planta; for example, brassinin detoxification by L. maculans in infected leaves of Brassica napus .…”

Leptosphaeria maculans, the Causal Agent of Blackleg Disease of Brassicas

“…Cyclobrassinin is the biosynthetic precursor of brassilexin in B. juncea and of dioxybrassinin in B. rapa (Monde et al, 1994). Thus, these results suggest that different fungal species can metabolize the phytoalexin cyclobrassinin "mimicking" pathways that also operate in the plant (Pedras and Okanga, 1999). Considering that fungal pathogens have been coevolving with plants for many generations, the detoxification of phytoalexins by "mimicry" is plausible.…”

“…The metabolism of another Brassica phytoalexin, cyclobrassinin, occurs also via different pathways that include other phytoalexins as intermediates by different pathotype groups of L. maculans (Pedras and Okanga, 1999;Pedras et al, 2000). An A pathotype group isolate, BJ-125 (IBCN57), detoxifies cyclobrassinin via the phytoalexin dioxybrassinin, whereas isolate Unity (IBCN81) detoxifies it via the phytoalexin brassilexin (Fig.…”