Photoreceptor Interaction in Plant Photomorphogenesis: The Limits of Experimental Techniques and Their Interpretations

Abstract

Problems concerning the interpretation of interactions of higher plant photomorphogenetic receptors are discussed. The theory that action of a blue light photoreceptor serves only to maintain responsiveness to phytochrome (Pesponsiveness Theory) is demonstrated to be unable to be properly tested with present techniques. This theory is also unable to explain experimental results any better than an alternative theory that a blue light photoreceptor may require the presence of the active form of phytochrome to express its activity (Presence Theory). This latter theory is also incapable of being fully tested. There does not appear to be an adequate current theory to explain photoreceptor interactions. Other issues discussed include the use of displacement transducers in growth studies, the induction of phytochrome-type responses by blue light, and the relative importance of the photoreceptors. New data are introduced on the effect of blue light in the end-of-day growth response to phytochrome of the lightgrown Cucumis sativus L. hypocotyl, and on the light equivalence principle in the same species.

Cryptochrome, Phytochrome, and Anthocyanin Production

“…Anthocyanin production and other plant photomorphogenic responses to prolonged R and FR irradiations are mediated by phytochrome (10,11,16 acting or acting independently of one another (2,7,(19)(20)(21)24).…”

“…The unknown nature of the UV-B-photoreceptor and cryptochrome and the fact that UV and BL excite not only these two photoreceptors, but also phytochrome, complicate studies on photoreceptor involvement and interaction in the mediation of responses to UV and BL (7,14). The selection of light treatments that can be used to discriminate the action of different photoreceptors is based on criteria ( 18,24) that take into account the differences between the known (phytochrome) and the inferred (cryptochrome and UV-B-photoreceptor) spectral properties of the photoreceptors.…”

“…The extent of the response is a function of light quality, fluence rate, and exposure duration; action peaks have been found in the UV, BL,3 R, and FR spectral regions. The relative efficiencies of these regions vary significantly, depending on the species and experimental conditions (10,11).Anthocyanin production and other plant photomorphogenic responses to prolonged R and FR irradiations are mediated by phytochrome (10,11,16 acting or acting independently of one another (2,7,(19)(20)(21)24).The unknown nature of the UV-B-photoreceptor and cryptochrome and the fact that UV and BL excite not only these two photoreceptors, but also phytochrome, complicate studies on photoreceptor involvement and interaction in the mediation of responses to UV and BL (7,14). The selection of light treatments that can be used to discriminate the action of different photoreceptors is based on criteria ( 18,24) that take into account the differences between the known (phytochrome) and the inferred (cryptochrome and UV-B-photoreceptor) spectral properties of the photoreceptors.…”

“…Anthocyanin production and other plant photomorphogenic responses to prolonged R and FR irradiations are mediated by phytochrome (10,11,16 'Abbreviations: BL, blue; R, red; FR, far red; RF, mixture of R and FR; sP, Pfr/P ratio at photoequilibrium (Pfr/P, ratio between Pfr and total phytochrome; k, rate constant for phytochrome photoconversion (k = k, + k2; k1 and k2, rate constants for Pr to Pfr and Pfr to Pr photoconversions, respectively); H, light-dependent rate of cycling between Pr and Pfr at photoequilibrium [H = (1 -P)k, = 'Pk2 = ('P -'p2)k]; BL + R, simultaneous irradiation with BL and R; BL + RF, simultaneous irradiation with BL and RF; BL + FR, simultaneous irradiation with BL and FR; N, photon fluence rate; cyBL + coR, simultaneous irradiation with cyclic BL and continuous R; AR, extent of the R-FR reversible response. acting or acting independently of one another (2,7,(19)(20)(21)24).…”

Photophysiology and Phytochrome Content of Long-Hypocotyl Mutant and Wild-Type Cucumber Seedlings

“…There is interaction between these photoreceptor systems in time and space during differentiation (13,14). The interpretation of physiological experiments is therefore often difficult, if not impossible, particularly in experiments utilizing light which is absorbed by more than one photoreceptor, as is the case for phytochrome and cryptochrome in the BL1 region of the spectrum (8,13). To understand the roles played by the different photoreceptor systems we have taken a genetic approach in an attempt to resolve the inevitable confusion that has arisen (1,(10)(11)(12).…”