Kinetics of Phytochrome Phototransformation

Abstract

Hillman (5) has comparatively recently reviewed the accumulating evidence suggesting that phytochrome in plant cells may be physiologically compartmentalized into "active" and "inactive" populations of molecules, possibly showing quite different phototransformation, dark reversion, and dark destruction properties. We recently published results of some experiments on phytochrome phototransformation (7) designed to determine whether or not one could detect multiple species of phytochrome in vivo or in vitro on the basis of phototransformation kinetics. The published data were interpreted as revealing a minimum of two molecular species both in plant tissues and in extracts. One species was four or five times more readily transformed than the other.The purpose of the present paper is to present evidence that the transformation kinetics previously presented (7) were in fact partially if not entirely a result of instrument artifact, and that phototransformation of phytochrome in both directions shows the log linearity to be expected of single first order photochemical reactions. In addition to our own paper, phytochrome phototransformation curves have been published on at least three other occasions. Butler (1), using quite different instrumentation, observed strict log linearity in both directions for phototransformation in crude phytochrome extracts. Butler et al. (2) later published curves again suggesting log linearity in both directions, for extracted phytochrome, though as we mentioned earlier (7), the small number and scatter of points precluded being completely certain of strict first order kinetics. Pratt and Briggs (6) published transformation curves for intact corn coleoptile tips which, though considered log-linear, could possibly be resolved into two components. In addition, Fox and Hillman (4) reported confirmation of the Purves and Briggs two-component transformation curves though they did not present the data graphically.On reconsideration, it became clear that kinetically distinguishable components could be clearly and repeatedly observed only when the measurements were made with a Ratiospect R-2 difference spectrophotometer (Agricultural Specialties Co., Beltsville, Md.), the instrument used by Pratt and Briggs (6), Purves and Briggs (7), and Fox and Hillman (4). We therefore studied phototransformation of phytochrome in vivo and in vitro using two other spectrophotometers for comparison with the

The Dark Reactions of Rye Phytochrome in Vivo and in Vitro

“…All grass and most dicot phytochrome examined to date, in vivo and in vitro, has shown strict log-linearity in phototransformation kinetics (5,13,25). This implies that there is only one population of phytochrome molecules in terms of phototransformation.…”

PHYTOCHROME PHOTOCONVERSIONin vivo. EFFECT OF THE INITIAL Pfr/Ptot RATIO

“…The data available at present on the comparative spectral properties of the different types and the relative abundance of different types in etiolated seedlings (a type I seems to be predominant) are not sufficient to justify suggesting a cause-effect relationship or even a correlation between deviations from log-linear kinetics and the presence of different phytochrome types. Deviations from log-linear photoconversion kinetics have also been attributed to measurement artifacts in the spectrophotometric assay (Everett et al, 1970), a possibility that should always be taken into consideration, especially with in vivo assays (Pratt, 1983). It is not easy to identify the type of measurement artifact that might be responsible for a deviation from log-linear kinetics for only one of six photoconversion curves (Figs.…”