Mechanisms of Gravitropism and Phototropism

In the late 1800's Charles Darwin outlined many of the critical questions in phototropism and gravitropism. Since that time, very little has been learned concerning the mechanism of these sensory processes. We wish to achieve an understanding of reception and sensory processing from the behavioral to the molecular levels. Our approach is to define the physiological processes, to isolate mutants with alterations in the sensory responses, to characterize these mutants physiologically, biophysically and genetically, to use particularly interesting mutants as material from which we can clone the genes of interest, and to use the genes to identify the corresponding proteins. We have chosen to work with Arabidopsis thaliana and have isolated a large number of mutants with which we can dissect the phototropism and gravitropism pathways.

By analyzing gravitropism and phototropism of a number of mutants we have shown that we can alter phototropism, leaving gravitropism normal; we can alter gravitropism, leaving phototropism normal; or we can simultaneously alter phototropism and gravitropism. Thus, it seems clear that phototropism and gravitropism share common elements. One possible pathway to account for these data shown converging pathways for phototropism and gravitropism. This clear evidence for the relationship between gravitropism and phototropism could have been obtained only through a genetic approach. This approach can also be used to determine the approximate location of an alteration in one of the pathways. For example, it is clear that the photoreceptor pigment itself should be specific to the phototropism pathway. Thus, a photoreceptor pigment mutant should be altered in phototropism but not in gravitropism. One mutant that appears to be altered in the photoreceptor pigment also exhibits an alteration in the blue light-induced phosphorylation of a plasma membrane protein. Thus, this mutant may permit us to identify a biochemical step close to the photoreceptor pigment itself.

Under the conditions found in nature, plants are exposed to constant gravitational forces and long times of irradiation. During these long stimulus times, the plant adapts to the stimulus. Thus, a dark-grown plant responds differently at the end of a long irradiation than at the beginning of that irradiation. We are emphasizing the dissection of the adaptation process because an understanding of adaptation is required for an understanding and eventual manipulation of the sensory process in "field conditions."