Red Light-Induced Accumulation of Ubiquitin-Phytochrome Conjugates in Both Monocots and Dicots

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Abstract
Phytochrome is rapidly degraded in vivo after photoconversion from the stable red-absorbing (Pr) form to the far red-absorbing (Pfr) form. Previously, we have shown in etiolated oat seedlings that ubiquitin-phytochrome conjugates (Ub-P) appear after Pfr formation suggesting that oat phytochrome is rapidly degraded by a ubiquitin-dependent proteolytic pathway. Here, we extend this observation to etiolated tissue from other monocotyledonous (corn [Zea mays. (L.)] and rye [Secale cereale (L.)] and dicotyledonous species (pea [Pisum sativum (L,)] and zucchini squash [Cucurbita pepo (L.)]). Following Pfr formation by red light, all four species synthesized a heterogeneous series of Ub-P that appeared and disappeared concomitant with the degradation of the chromoprotein. When Pfr was photoconverted back to Pr by a far-red light pulse, degradation of phytochrome ceased and the levels of Ub-P concomitantly dropped. In pea and zucchini squash, loss of Ub-P after photoconversion of Pfr back to Pr was rapid, occurring with a half-life of approximately 5 to 10 minutes. These data indicate that the accumulation of Ub-P after Pfr formation is a general phenomenon in etiolated seedlings of higher plants and further support the hypothesis that plants degrade Pfr via Ub-P intermediates.