Evidence that P_fr is not the active form of phytochrome in light-grown maize

Abstract

The biological activity of the plant photochromic photoreceptor, phytochrome, is generally thought to reside in P_fr the far-red-absorbing form, whilst the red-absorbing P_r is inactive. This view is based solely on incomplete correlations between spectrophotometrically detectable P_fr and responses in etiolated tissues given brief light treatments^1,2. Studies of light-grown seedlings have recently revealed an inverse, linear relationship between phytochrome-regulated extension growth and phytochrome photoequilibrium (defined as P_fr/(P_r+P_fr) and estimated from the spectral photon distribution of the actinic radiation)^3–10. Such a relationship could only fit the ‘P_fr as active form’ theory if P_r+P_fr (that is, P_total) remained constant and independent of the light sources used. Here I report data from herbicide-bleached, light-grown maize seedlings which show that P_total is strongly dependent on the wavelength distribution of the radiation and that the amount of P_fr does not correlate with growth response.