Effect of Tagetitoxin on the Levels of Ribulose 1,5-Bisphosphate Carboxylase, Ribosomes, and RNA in Plastids of Wheat Leaves

Abstract

Growth of wheat seedlings in the presence of the phytotoxin tagetitoxin produces pigment-deficient leaves of normal size and morphology whose cells contain only rudimentary plastids. We could not detect the accumulation of either the plastid-encoded large subunit or the nuclear-encoded small subunit of the chloroplast stromal enzyme ribulose 1,5-bisphosphate carboxylase (RuBPCase) in western blots of protein extracted from leaves of such seedlings. Sucrose gradient centrifugation profiles showed that plastid ribosomes were essentially absent in toxin-treated leaf tissue while cytoplasmic ribosomes were relatively unaffected. Northern blot analysis of RNA in toxin-treated leaves showed a deficiency of plastid ribosomal RNA (16S and 23S) as well as reduced levels of plastid mRNAs for the large subunit of RuBPCase and for the 32 kilodalton thylakoid Q(B) polypeptide. Northern analysis also showed that the nuclear-encoded rbcS mRNA for the small subunit of RuBPCase is present in only trace amounts in toxin-treated leaves.