Induced susceptibility and enhanced resistance at the cellular level in barley coleoptiles. I. The significance of timing of fungal invasion

Growth of Erysiphe graminis hordei and Erysiphe pisi on the same cells of barley coleoptiles was observed in detail by light microscopy to determine significant factors conditioning host cells toward susceptibility. When E. pisi attempted penetration more than 60 min earlier than E. graminis on the same coleoptile cell, E. pisi never succeeded in penetration (0% penetration efficiency) and the penetration efficiency of E. graminis was lowered from 75·0 to 28·6%, suggesting that the resistance to E. graminis invasion might be enhanced under this condition. When both fungi attempted penetration of the coleoptile cell almost simultaneously (within 30 min of each other), the penetration efficiency of E. pisi increased to 11·8%. Moreover, the penetration efficiency of E. graminis was 55·8%. When E. graminis attempted penetration 60 min or more earlier than E. pisi, the mean penetration efficiency of E. pisi was 29·2% and that of E. graminis to 75·0%. These observations suggest that the coleoptile cells are conditioned toward susceptibility by prior-penetration of E. graminis or even by its post-penetration if it penetrates after E. pisi E. pisi developed haustoria only in coleoptile cells where E. graminis formed haustoria. The induced susceptibility and enhanced resistance states in coleoptile cells had not transferred to the adjacent cells within 3 h.