Toward understanding the ecological functions of tropisms: interactions among and effects of light on tropisms.

Tropisms of higher plants have been investigated for well over a century. Only recently, however, we have begun to establish their mechanisms firmly, mainly thanks to the availability of mutants and genome sequence information. For example, the starch-statolith hypothesis is now best supported as the main mechanism by which plants perceive gravity direction. Phototropins have been identified as the photoreceptors for the major blue-light-sensitive phototropism. Investigations have been extended to elucidate the relationships among tropisms and the controlling roles played by environmental factors, such as light. We are now finding examples in which phototropic and hydrotropic responses are modified through the environmental control of counteracting gravitropism. We are also finding that seedlings generally become phototropically competent only after phytochrome is activated. Such results are providing insights into how plants use tropisms to achieve adaptive growth movements.