Molecular Aspects of Specificity in Interaction of Arabidopsis and Host-Specific Toxin Produced by Alternaria brassicicola

十字花科黑叶斑病是十字花科栽培植物的主要病害。最近，我们发现病原菌产生宿主特异性毒素（host-specific toxin， HST），称为AB-toxin。只有将孢子接种到寄主叶片上，才会从萌发的孢子中释放出毒素，说明病原菌产生ab毒素是受寄主叶片中的物质诱导的。有趣的是，ab毒素的活性被分子量为5000的膜保留了下来。此外，该酶的热稳定性较差，用蛋白酶处理后活性降低。因此，与其他HSTs不同，ab -毒素具有较高的分子量，可能是一种蛋白质。作为一种蛋白质，ab毒素可能具有基因直接产物的优势。拟南芥（Arabidopsis thariana）属于十字花科，被认为是分子生物学的模式植物。因此，研究了对拟南芥病原菌和ab -毒素的反应。将病原菌孢子接种在7个生态型的叶片上，除1个生态型RLD外，6个生态型均出现黑斑病。测定ab毒素对拟南芥的生物活性。毒素对6个生态型均有毒性，但对RLD无害。这些结果表明存在对病原菌和抗体毒素敏感和抗性的生态型。为了研究拟南芥应答的遗传背景，将敏感生态型和抗性生态型进行杂交。杂交植株的抗性反应表明，抗性是由一个显性基因决定的。另一方面，ab毒素在处理后1小时内诱导敏感生态型叶片的电解质渗漏，表明ab毒素对宿主细胞的质膜起主要作用。

Black leaf spot of Cruciferae caused by Alternaria brassicicola is a major disease in cultivated Brassica plants. Recently, we found that the pathogen produces host-specific toxin (HST), named AB-toxin. The toxin was released from the germinating spores only when spores were inoculated on host leaves, indicating that the production of AB-toxin by the pathogen is induced by substances in host leaves. Interestingly, the activity of AB-toxin was retained by membranes with a molecular weight cutoff of 5,000. In addition, the activity was heat unstable, and was lost by treatment with proteinase. Thus, unlike other HSTs, AB-toxin has a high molecular weight and is probably a protein (s). As a protein, AB-toxin may offer the advantage of being a direct gene product.Arabidopsis thariana which is known as a model plant for molecular biology, belongs to Cruciferae. Therefore, responses to the pathogen and AB-toxin of A.thariana were investigated. When the spores of the pathogen were inoculated on leaves of 7 ecotypes of A.thariana, black spot lesions appeared on 6 ecotypes except an ecotype RLD.AB-toxin was examined for biological activity on A.thariana. The toxin was toxic to 6 ecotypes, but was harmless to RLD.These results indicate that there are ecotypes susceptible and resistant to the pathogen and AB-toxin. To investigate hereditary background of the responses of A.thariana, susceptible and resistant ecotypes were crossed. The responses of the crossed plants indicated that the resistance is determined by a single dominant gene. On the other hand, AB-toxin induced electrolyte leakage from susceptible ecotype leaves within 1 hr after treatment, indicating that AB-toxin has a primary effect on the plasma membranes of host cells.