Downstream integrators of red, far-red, and blue light signaling for photomorphogenesis

Light energy capture by leaves, water, and mineral absorption by roots are crucial for plant survival. To guide their growth plant organs sense a variety of environmental cues, among which the direction of gravity and the direction of light are the most important. Other environmental factors dictating tropic responses like moisture and touch play minor roles in land plants and are discussed in a recent review (Blancaflor and Masson 2003). Gravitropism and phototropism refer to the directional curvature of an organ in response to lateral differences in gravity or light, respectively. Plants respond to changes in gravity and light direction by modulating the rate of cellular elongation on opposite flanks of the stimulated organ. Such asymmetric growth leads to a curvature and subsequent realignment with the right orientation. The machinery that sustains tropism in roots, hypocotyls, and shoots of higher plants are at least partially distinct (Blancaflor and Masson 2003). This review focuses on the mechanisms underlying light interaction with gravitropism and phototropism, particularly in hypocotyls of higher plants. We will not discuss events occurring in root gravitropism or other phototropin-mediated responses. Recent reviews cover these fields more specifically (Boonsirichai et al 2002, Briggs and Christie 2002, Liscum 2003, Kiss et al 2003).