Figure 9.

Schematic Model of QR1 and QR2 Action.

(A) The lignin component (i) is oxidatively decarboxylated to DMBQ (ii) through the actions of fungal peroxidases, host peroxidases, or mild abrasion (Caldwell and Steelink, 1969; Keyes et al., 2007).

(B) DMBQ (ii) and peonidin (iii) enter the parasite cells, possibly by nonselective diffusion (Shann and Blum, 1987). Peonidin is drawn in one of several possible conformations (Brouillard and Cheminat, 1982).

(C) The p-benzoquinone motive (iv) is reduced by a single electron to a free radical semiquinone (v) by the action of QR1.

(D) Further nonenzymatic reduction of the semiquinone leads to an equilibrium with the relatively stable hydroquinone (vi) (Testa, 1995).

(E) In the presence of oxygen, the semiquinone promotes the generation of superoxide anion-radicals (O₂•⁻), which are further modified to highly toxic hydroxyl radicals.

(F) The haustorium signaling pathway is initiated either by the radical semiquinone or by the ROS species generated in air.

(G) QR2 encodes a two electron detoxifying reduction that avoids the radical semiquinone.