Phototropism: Mechanisms and Outcomes

Plants respond to changes in their light environmentthrough a wide array of responses, including changes indevelopmental fate and reversible alterations in morphol-ogy. Phototropism, or the directional curvature of organsin response to lateral differences in light intensity and/orquality, represents one of the most rapid and visuallyobvious of these responses (Darwin, 1880; Sachs, 1887;Iino, 1990). Although a number of plant organs appear tobe responsive to phototropic stimuli (Iino, 1990; Koller,1990), a vast majority of the experimental data related tophototropism deal with the responses observed inseedling stems and primary roots. In particular, stemshave been shown to exhibit positive phototropism, orcurvature towards the light (Figure 1), while roots shownegative phototropism, or curvature away from the light.Stem phototropism it is thought to provide plants with aneffective means for increasing foraging potential (maxi-mizing photosynthetic light capture) and is therefore like-ly to have appreciable adaptive significance (Iino, 1990;Liscum and Stowe-Evans, 2000). Less is known aboutthe phototropic response of roots, yet it is clear thatcooperative interaction between a negative phototropicresponse and a positive gravitropic response could helpto ensure proper growth of the root into the soil wherewater and nutrients are most abundant and available forabsorption. The adaptive advantage provided by stemand root phototropic responses may be particularlyimportant during the early stages of growth and estab-lishment of seedlings (Iino, 1990) and during gap fillingsituations in dense canopy conditions (Ballare, 1999). Phototropic responses are distinguished from othertypes of light-modulated directional growth responses,such as nastic (Satter and Galston, 1981) and circadian-regulated (McClung, 2001) leaf movements, by two crite-ria. First, the direction of phototropic curvatures is deter-mined by the direction of the light stimulus, while the direc-tion of nastic and circadian-regulated movements are not(Salisbury and Ross, 1992). Second, many leaf movementresponses occur as a result of reversible swelling/shrinkingof specialized motor, or pulvinar, cells (Hart, 1988; Koller,1990), whereas all stem and root phototropic responsesare driven by changes in cell elongation rates across thebending organ. With respect to phototropic responses,unidirectional irradiation of seedlings with UV-A/blue lightresults in enhanced growth in the stem on the flank awayfrom the light (“shaded side”) and generally repressesgrowth on the flank facing the incident light (“lit side”)(Baskin et al., 1985; Briggs and Baskin, 1988; Orbovic andPoff, 1993), causing it to bend towards the light (Iino,1990). An essentially opposite response is observed inroots, where growth is enhanced on the “lit side” andrepressed on the “shaded side”, causing the root to bendaway from the light (Okada and Shimura, 1994). The dif-ferential growth rates driving the development of pho-totropic curvatures appear to be established as a result ofdifferential responsiveness to the plant hormone auxin(Went and Thimann, 1937; Iino, 1990; Liscum and Stowe-Evans, 2000).One feature shared between stem/root phototropicresponses and nastic leaf movement responses is co-localization of photoperception and growth response,namely that the cells exhibiting the “growth response”,however mechanistically different, are the same onesperceiving the light signals (Briggs, 1963; Koller, 1990).This is not necessarily the case with either circadian-dependent or true phototropic movements of leaves,where cells in the leaf blade perceive the signal, and pul-vinar cells at the base of the leaf or petiole exhibit thegrowth response (Schwartz and Koller, 1978; Engelmannandd Johnsson, 1988).