An attempt to localize the threshold reaction in phytochrome-mediated control of lipoxygenase synthesis in the mustard seedling.

Abstract

Abstract—Synthesis* of the enzyme lipoxygenase (LOG)† in the cotyledons of the mustard seedling (Sinapis alba L.) is controlled by phytochrome (Pfr) through a threshold (all‐or‐none) mechanism. The data of the present paper confirm the previous assumption (Oelze‐Karow and Mohr, 1973) that the primary reaction of Pfr (Pfr+ X → PfrX ⇄ PfrX‘) is the site of the highly cooperative threshold reaction. Suppression of LOG synthesis depends on the presence of PfrX’. However, PfrX‘ is only stable above the threshold level of Pfr. If the level of Pfr is below the threshold, PfrX is stable, and no suppression of LOG synthesis occurs. As long as the level of Pfr remains below the threshold, no destruction of Pfr takes place. Destruction of Pfr occurs only as long as [Pfr]†is above the threshold level. Thus the simplest formulation of the actual threshold reaction in the LOG response is PfrX⇄frX’ state at [Pfr] below threshold no Pfr destruction LOG synthesis suppressed state at [Pfr] above threshold Pfr destruction(1kd LOG synthesis unimpaired The reversible threshold reaction is thus an integral part of the “primary reaction” of Pfr occurring at the “matrix” specific for the LOG response. The data and conclusions on the LOG response are consistent with an “open phytochrome‐receptor model” recently advanced by E. Schäfer (1975). The data are not consistent with the concept that a rapid dark reversion (Pfr→Pr) exists in dicotyledonous seedlings and that the degree of Pfr dark reversion strongly depends on the initial photostationary state, φ†, established by a saturating light pulse.