Cytokinins, abscisic acid and light affect accumulation of chloroplast proteins in Lupinus luteus cotyledons without notable effect on steady-state mRNA levels

Abstract

Etiolated lupine (Lupinus luteus L.) cotyledons respond in a highly sensitive manner to phytohormones and light. The effects of cytokinin, abscisic acid, gibberellic acid (GA3) and indolylacetic acid (IAA) have been studied at the ultrastructural, steady-state mRNA and protein levels using 15 gene-specific probes for plastid proteins and corresponding antisera. No effect was noted with GA3 and IAA. As in other systems, N6-benzylaminopurine (BAP) and abscisic acid (ABA) operated antagonistically. In both instances, the steady-state mRNA levels remained relatively unaffected for plastid-encoded polypeptides, but not for those nuclear-encoded genes that could be tested. On the other hand, synthesis and accumulation of proteins of nuclear and plastid origin varied significantly. Cytokinin strongly promoted the accumulation of cytochrome b 559 and subunit IV of the cytochrome b/f complex, while little effect was observed for cytochrome b 6, the β subunit of the chloroplast ATP synthase or the large subunit of ribulose-1,5-bisphosphate carboxylase. In etiolated seedlings the level of chlorophyll-binding proteins (the 43-kDa chlorophyll a protein of photosystem II and subunits I a, b of photosystem I) was below the level of detectability. Their accumulation in light was promoted by cytokinin and inhibited by ABA though to different extents. Cytochrome b 559 and the 33-kDa polypeptide of the water-oxidizing complex were not detectable in water-(control) and ABA-treated cotyledons. Cytokinin induced the synthesis of these proteins, even in darkness. These results indicate a protein-specific response to phytohormones, which can differ even for polypeptides belonging to the same membrane complex. They also suggest different modes of interaction between hormones and light, quite different phytohormone action in the two compartments, and demonstrate that phytohormones influence the biogenesis of the thylakoid membrane mainly posttranscriptionally.