The levels of two distinct species of phytochrome are regulated differently during germination in Avena sativa L.

The abundance and molecular mass of phytochrome in germinating embryos of A. sativa (oat) grown in light or darkness have been monitored using immunoblot and spectrophotometric assays. Immunoblot analysis shows that imbibed but quiescent embryos have two immunochemically distinct species of phytochrome with monomeric molecular masses of 124 and 118 kDa (kdalton). The 118-kDa species has the properties of the 118-kDa phytochrome extracted from fully green oat tissue (J.G. Tokuhisa, S.M. Daniels, P.H. Quail, 1985, Planta 164, 321-332), whereas the 124-kDa polypeptide appears similar to the well-characterized photoreceptor of etiolated tissue. The capacity of antibodies directed against etiolated-oat phytochrome to immunoprecipitate the 124-kDa species but not the 118-kDa species has been exploited to quantitate the levels of each separately over a 72-h time course of germination and seedling development. The abundance of the 124-kDa molecule increases at least 200-fold in etiolated seedlings over 72 h whereas in light-grown seedlings the level of this molecule is relatively constant. In contrast, the amount of the 118-kDa species increases only twofold in both dark- and light-grown seedlings over the same period of time. These data indicate that whereas the abundance of 124-kDa phytochrome is regulated at the protein level by the well-documented, differential stability of the red- and far-red-absorbing forms in vivo, the 118-kDa molecule is present at a low constitutive level, presumably reflecting no such difference in the stability of the two spectral forms.