ACTIN MICROFILAMENTS ARE REQUIRED FOR THE EXPRESSION OF NONHOST RESISTANCE IN HIGHER-PLANTS

We investigated the role of actin microfilaments in nonhost resistance of higher plants, Here we present several lines of evidence to indica te that microfilaments are indeed involved in blocking fungal penetrat ion of nonhost plants, Erysiphe pisi, a pathogen of pea, normally fail s to penetrate into nonhost plants such as barley, wheat, cucumber and tobacco. When tissues of these nonhost plants were treated with cytoc halasins, specific inhibitors of actin polymerization, this fungus bec ame able to penetrate and formed haustoria in epidermal cells of these plants, Moreover, treatment of these plants with various kinds and co ncentrations of cytochalasins allowed several other nonpathogenic fung i, E. graminis hordei, E,graminis tritici, Sphaerotheca fuliginea, Col letotrichum graminicola, Mycosphaella pinodes, C, lagenarium, Alternar ia kikuchiana and Corynespora melonis, to also penetrate the cells of these plants, The degree of microfilament depolymerization varied depe nding on the kinds and concentrations of cytochalasins applied and we show that this is significantly correlated with the penetration effici ency of C. graminicola, This indicates that the polymerized, filamento us state of actin is necessary for plants to block fungal penetration. These results strongly suggest that actin microfilaments may play imp ortant roles in the expression of nonhost resistance of higher plants.