Effects of a triterpenoid saponin on spectral properties of undegraded pea phytochrome.

Soyasaponin I, a triterpenoid saponin isolated from etiolated pea (Pisum sativum cv. Alaska) shoots and identified as Pfr killer, was examined for its effects on spectral properties of undegraded pea phytochrome. When soyasaponin I in concentrations of 100 micromolar or lower was added to Pr in the dark, the spectrum of Pr was not significantly affected, whereas in the presence of 120 micromolar or higher concentrations the absorption maximum of Pr shifted from 666 to 658 nanometer with slight decrease of absorbance. After a brief exposure of the mixture to red light, the increase in absorbance at 666 nanometers that occurs in the dark was inhibited at 26 micromolar and higher soyasaponin I concentrations; the maximum effect being reached at about 180 micromolar. The decrease in absorbance at 724 nanometers in the dark after red light irradiation was somewhat inhibited by 60 micromolar and totally prevented by 410 micromolar soyasaponin I. When P(658) was irradiated with red light in the presence of 220 micromolar or higher soyasaponin I concentrations, a bleached form (P(bl)) was produced instead of Pfr. P(bl) showed no dark spectral changes, and the phototransformation of P(bl) to P(658) required a significantly high irradiance of far-red light. When the saponin was added to Pfr in the dark, none of the above-described spectral changes occurred, although the same effects were observed after the mixture was exposed briefly to far-red light followed by red light.