Time threshold for second positive phototropism is decreased by a preirradiation with red light.

Abstract

A second positive phototropic response is exhibited by a plant after the time of irradiation has exceeded a time threshold. The time threshold of dark-grown seedlings is about 15 minutes for Arabidopsis thaliana. This threshold is decreased to about 4 minutes by a 669-nanometer preirradiation. Tobacco (Nicotiana tabacum) seedlings show a similar response. The time threshold of darkgrown seedlings is about 60 minutes for tobacco, and is decreased to about 15 minutes after a preirradiation with either 450-or 669-nanometer light. The existence of a time threshold for second positive phototropism and the dependence of this threshold on the irradiation history of the seedling contribute to the complexity of the fluence response relationship for phototropism.Understanding the mechanism for phototropism has been hindered by the complexity of the fluence response relationship for this response by seedlings. Phototropic curvature increases with increasing fluence in an ascending arm above a threshold fluence, reaches a maximum, and then decreases in a descending arm. This relatively symmetrical curve is known as first positive phototropism. At even higher fluences, curvature increases again with increasing fluence in what is known as second positive phototropism. Reciprocity is valid for first positive phototropism but not for second positive phototropism (3,8).The suggestion by Zimmerman and Briggs (12,13) that first and second positive phototropism result from separate systems has been interpreted to mean that the two phototropic responses are controlled by different photoreceptor pigments. However, Janoudi and Poff (5) suggested that the apparent lack of valid reciprocity for second positive phototropism is, at least in part, a consequence of an irradiation time requirement for that response. When that time requirement is met in Arabidopsis thaliana, fluence response relationships for second positive phototropism show a fluence threshold that is the same as the fluence threshold for first positive phototropism. Based on this common fluence threshold for first and second positive phototropism, they noted that it is not necessary to postulate a difference in the

PINOID AGC Kinases Are Necessary for Phytochrome-Mediated Enhancement of Hypocotyl Phototropism in Arabidopsis

“…3C). The results indicated that red light pretreatment, presumably through phytochromes, is necessary for the induction of time-dependent phototropism as reported previously (Janoudi et al, 1992;Liu and Iino, 1996b;Haga and Sakai, 2012). The pid quadruple mutant showed obvious responses of time-dependent phototropism with red light pretreatment and no clear responses without red light pretreatment similar to wild-type response, although the magnitudes of the curvature responses were partially impaired in the mutant (Fig.…”

“…For the first positive pulse-induced phototropism, phytochromes are involved in the enhancement of phototropic curvatures in addition to desensitization (Liu and Iino, 1996a;Janoudi et al, 1997a;Whippo and Hangarter, 2004;Haga and Sakai, 2012). For the second positive phototropism, phytochromes participate in the establishment of a phototropic system, which is necessary to respond to a prolonged irradiation of blue light, the promotion of phototropic curvature rates, and the reduction of the time required to induce curvature responses (Janoudi et al, 1992;Liu and Iino, 1996b;Hangarter, 1997;Haga and Sakai, 2012). All such regulation is controlled by multiple phytochromes (Parks et al, 1996;Hangarter, 1997;Janoudi et al, 1997aJanoudi et al, , 1997bWhippo and Hangarter, 2004).…”

Phototropism: Some History, Some Puzzles, and a Look Ahead

“…Janoudi and Poff (1991) first showed that red light treatment enhanced phototropic curvature in the first positive range, subsequently confirmed and shown to be mediated by phyA (Hennig, 1996;Parks et al, 1996). Janoudi et al (1992) showed that red light pretreatment actually shortened the lag period between the start of blue light irradiation and the onset of curvature, again an enhancement. Whippo and Hangarter (2004) demonstrated that the role of phytochromes was more complex and that phyB and phyD could also mediate the sensitization of Arabidopsis phototropism to higher blue light fluences (for further details, see Han et al, 2008).…”

Phytochrome modulation of blue‐light‐induced phototropism

“…The red light enhancement of phototropism, which decreases the latency of the response, is mediated by the phytochrome family of red/far‐red photoreceptors (Janoudi et al . 1992, 1997; Hangarter 1997).…”