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 indicate that microfilaments are indeed involved in blocking fungal penetration of nonhost plants, Erysiphe pisi, a pathogen of pea, normally fails to penetrate into nonhost plants such as barley, wheat, cucumber and tobacco. When tissues of these nonhost plants were treated with cytochalasins, specific inhibitors of actin polymerization, this fungus became able to penetrate and formed haustoria in epidermal cells of these plants, Moreover, treatment of these plants with various kinds and concentrations of cytochalasins allowed several other nonpathogenic fungi, E. graminis hordei, E,graminis tritici, Sphaerotheca fuliginea, Colletotrichum graminicola, Mycosphaella pinodes, C, lagenarium, Alternaria kikuchiana and Corynespora melonis, to also penetrate the cells of these plants, The degree of microfilament depolymerization varied depending on the kinds and concentrations of cytochalasins applied and we show that this is significantly correlated with the penetration efficiency of C. graminicola, This indicates that the polymerized, filamentous state of actin is necessary for plants to block fungal penetration. These results strongly suggest that actin microfilaments may play important roles in the expression of nonhost resistance of higher plants.