A fallacy in phytochrome pelletability due to in vitro instability of the phytochrome-particle association

Two fractionation procedures were used to study the phenomenonof phytochrome pelletability or binding to a particulate fractionof maize coleoptiles. Using a revised procedure, we were unableto show an increase of phytochrome pelletability during darkincubation of red irradiated plant tissue, reported by Manabeand Furuya for pea seedlings (6), and the P_fr-enhanced affinityfor P_r in R/FR irradiated tissue as reported by Quail et al.(11). However, we were able to match these reported observationsusing a procedure which we have regarded as standard. In thestandard procedure, the irradiated tissue is homogenized andthe brei permitted to incubate in the dark at 0?C before fractionationby differential centrifugation prior to measurements of phytochromepelletability. In the revised procedure this incubation is eliminatedand fractionation and measurement follow directly on tissuehomogenization. A progressive decrease of particulate phytochromewas observed during dark incubation at 0?C of the brei fromred irradiated tissue, but no substantial decrease was observedduring dark incubation of the red irradiated tissue at 0?C.The decrease was not dependent on in vitro P_frP_r reversion.In the case of R/FR irradiated tissues, phytochrome pelletabilitywas found to decrease during dark incubation of both the irradiatedtissue and its brei at 0?C. With these results and a recognitionof the tendency of phytochrome to dissociate from the particulatefraction in vitro, we have thus rationalized the results ofQuail et al. (11) and Manabe and Furuya (6). (Received August 12, 1976; )