Rapid Changes in Plant Genomes

Beginning with Mendel, plant research has been the source of revolutionary discoveries that have changed our views of heredity. Therefore, baffling new observations in plant genetics that challenge current concepts continue a long tradition. One of these challenges comes from the study of heritable phenotypic and genetic alterations that are consistently produced after a flax cultivar has undergone an environmental change. Another challenge comes from the startling discovery that mutations in the Arabidopsis HOTHEAD (HTH) gene can revert at an extraordinarily high frequency with genome-wide effects. This study has received attention in the popular press because it is thought to reveal the existence of extragenomic inheritance in the form of an RNA cache of correction templates. The observations in flax have received relatively little attention, even though they are remarkable by any standard, in that environmental induction consistently leads to the sudden appearance of a large element in a genome that previously lacked it. Both studies reveal the appearance of sequence information from an unidentified source in plants, and it is attractive to think that they occur by related mechanisms. Here, I compare the two phenomena and attempt to weigh the relative merits of extragenomic versus intragenomic explanations.