Higher eukaryotes are characterized by the allocation of distinct functions to numerous types of differentiated cells. Whereas in animals the well-defined, protected cells of the germ line separate early, germ cells in plants differentiate from somatic cells only after many cycles of mitotic division. Therefore somatic mutations in plants can be transmitted via the germ cells to the progeny. There is thus a clear need for somatic tissues to maintain their genetic integrity in the face of environmental challenges, and two types of interactions have been shown to play important roles in the...