Effects of Filipin and Steroids on Phytochrome Pelletability

Abstract

ABSTRACrRed light given to dark-grown etiolated leaves of Hordeum vulgare L. in vio or to cnrde homogenates increases the phytochrome content of the 20,000g pelet on centrifugation. The steroids cholesterol and stgmasterol ihibit this red light-induced phytochrome pelletability. Filipin (a polyene antibiotic, which is known to combine with steroids) inhibits red lghtinduced phytochrome pelletabiElty. Fdipin and steroids at the appropriate concentration appie together prevent the inhibition caused by either when aplied aloe. These results suggest that phytochrome may bind to a steroid component of mranes. The phospholid pbophatidyl choline dipalmitoyl has no effect on red lght-induced phytochrome pelletability.Preliminary evidence demonstrates a direct association of soluble phytochrome in its active form and steroids. The physiological significance of red light-induced pelletability and the primary anism of phytochrome action are discussed in terms of a hypothetical steroid-binding site.

Phytochrome Pelletability in Barley

“…In conclusion we have demonstrated Pfr binding in barley after in vivo and in vitro irradiation that is independent of Mg^ •" concentration. Elsewhere (Roth-Bejerano and Kendrick 1979) we have provided evidence for the association of Pfr with suspensions of steroids and we have postulated that Pfr binds to plant membranes at a steroid binding site. Such Pfr binding to a steroid site is concluded from competition experiments using filipin (a polyene antibiotic which binds specifically with steroids) and steroids.…”

“…This in vivo association occurs either in vivo or immediately upon homogenization of red irradiated tissue (Quail 1978). Evidence exists which indicates that in vivo association takes place upon red irradiation in vivo (Roth-Bejerano and Kendrick 1979). Moreover, it was suggested that both in vivo and in vitro association takes place to a steroid component of the membranes.…”

“…Mg^+ or Ca^"*" were found to be essential for the fed light enhancement of phytochrome pelletability (Marme et al 1973, Pratt andMarme 1976). There is an increasing body of evidence which demonstrates that an increase of phytochrome pelletability can occur on exposure to red light in the absence or presence of low Mg^*^ concentrations (Georgevich et al 1977, Smith et al 1978, Roth-Bejerano and Kendrick 1979. The efTect of pH on the in vivo enhanced phytochrome pelletability is small (Pratt and Marme 1977), whereas in vitro high pH eliminates enhancement in mustard (Pratt and Marme 1976) and zuchini squash (Marme el al.…”

Binding of phytochrome to plasma membranes in vitro

“…The phytochrome binding to liposomes {Kim and Song 1981) and to isolated organelles has been regarded as a hydrophobie binding (Roth-Bejerano and Kendrick 1979). The influence of NaCl and Triton X-100 on the binding of phytochrome to plasma membrane also points to a hydrophobic binding (Tab.…”

“…The significance of these results has been discussed and experimentally tested (Pratt 1976, Quail and Briggs 1980. However, it is still not known whether the association is to a specific protein receptor or to a steroid binding site (Roth-Bejerano and Kendrick 1979). Another possibility is that the binding is artificial since Pfr is more hydrophobic than Pr (Hahn andSong 1981, Tokutomi et al 1981) and thus could be more disposed to associate unspecifically with any membrane.…”