Cytoplasmic Susceptibility in Plant Disease

Biologists now generally accept the existence of extrachromosomal inheritance. In recent years cytoplasmic genes have been discovered and their role in cell heredity studied (38). Several important traits are inherited through the cytoplasm. The United States corn blight epidemic of 1970 dramatically revealed the importance of plant cyto­ plasm in susceptibility to disease (19, 33, 48). While the importance of extrachromosal inheritance (cytoplasmic genes) relative to chromosomal inheritance (nuclear genes) is not completely understood, far greater advances have been made in the knowledge of nuclear gene inheritance and its application to plant and animal improvement than has that of cytoplasm genes. The same can be said for reaction to disease in plants (21). A brief discussion of what is known about the plant cytoplasm, cytoplasmic inheritance, and cytoplasmic male sterility is given as background information to aid the reader in understanding how the cytoplasm may play a role in the inheritance of disease susceptibility. This is followed by a review of selected reports on disease reaction and a discussion of possible mechanisms involved in cytoplasmic suscepti­ bility in plant disease.