Involvement of the phytoalexin medicarpin in the differential response of callus lines of lucerne (Medicago sativa) to infection by Verticillium albo-atrum

Callus cultures derived from seeds of lucerne (Medicago saliva L.) were challenged with a lucerne isolate of Verticillium albo-atrum using agar blocks or spore suspensions as inoculum. The rate of colonization of different callus lines by the fungus varied from complete overgrowth of tissues in susceptible lines, to a rapid browning reaction of host cells accompanied by a restriction of fungal growth in resistant lines. The phytoalexin medicarpin accumulated in both resistant and susceptible reaction types, but resistant callus lines accumulated higher levels of this compound upon challenge with spore suspensions of V. albo-atrum. Pretreatment of resistant calli with subherbicidal levels of the inhibitor glyphosate lowered the production of medicarpin, reduced the browning response of callus cells and enhanced colonization by the fungus. Possible links between these in vitro responses and those of intact lucerne plants to V. albo-atrum are discussed.