More than 200 years ago, the poet, philosopher, and natural scientist Johann Wolfgang von Goethe made the remarkable suggestion that flower organs represent modified leaves. By analyzing mutations in four different but related genes, researchers have found that these genes are critical for the conversion of leaves into flower organs.
Although people have wondered about the mysteries of flower formation for ages, scientists have carried out molecular studies on flowering only during the last two decades. Numerous genes controlling flower development have been identified during this time. Many of the studies have focused on the small weed Arabidopsis thaliana, which possesses the typical set of flower organs--sepals (typically outer-most in the flower whorl), petals, stamens, and carpels--and have relied on a traditional genetic approach, where mutations in key regulatory genes lead to alterations in flowering. Although exceedingly powerful, such an approach nevertheless often fails to reveal the essential nature of closely related genes that are functionally redundant. Functional redundancy refers to the situation where two or more genes play very similar roles and must be simultaneously inactivated if the abnormal characteristic is to be observed. For example, recent studies had shown that a so-called triple mutant, lacking the activities of three closely related genes, produces flowers that consist only of sepals.
If flower organs represent modified leaves, and if this recently discovered triple mutant has only sepals, is it possible that mutations in another gene might convert these sepals into leaves? New work, by Gary Ditta and coworkers in the laboratory of Martin Yanofsky at the University of California at San Diego, has identified such a gene. The gene, SEP4, is closely related to the three genes corresponding to the triple mutant previously characterized by Yanofsky's group. When the researchers constructed the quadruple mutant, they found that the flowers consisted only of leaf-like organs. Together, these studies show that four closely related genes, all of the "MADS-box" class, are necessary for the formation of sepals, petals, stamens, and carpels and that the loss of activity of all four genes results in the conversion of flower organs toward leaves.
Gary Ditta, Anusak Pinyopich, Pedro Robles, Soraya Pelaz, and Martin F. Yanofsky: "The SEP4 Gene of Arabidopsis thaliana Functions in Floral Organ and Meristem Identity"
Publishing in Current Biology, Volume 14, Number 21, November 9, 2004, pages 1935-1940.