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Voigt, Christian (2010): Characterization of the gun phenotype under photo-bleaching conditions. Dissertation, LMU München: Faculty of Biology



Protein complexes involved in biochemical processes of organelles are composed of subunits encoded in the organelles and the nucleus. To guarantee energysaving assembly and efficient functioning of such protein complexes, a proper regulatory network is required. The anterograde control of the nucleus over the organelles is extensive and the principal parameters are known. It is also accepted that organelles send information about their developmental and metabolic state to the nucleus (‘retrograde signaling’) in order to adapt nuclear gene expression. But, the nature of the molecules that relay information to the nucleus is still unclear. In a mutant screen, designed to find factors involved in retrograde signaling in A. thaliana, the genomes uncoupled (gun) mutants were identified more than 15 years ago. Under photo-bleaching conditions induced by norflurazon (NF), an inhibitor of carotenoid biosynthesis, the expression of the nuclear localized gene encoding photosystem II chlorophyll a/b-binding protein (LHCB1.2) is suppressed in wild-type plants. In the gun mutants, this suppression is less pronounced. Since four out of the five known gun mutants are affected in the tetrapyrrole biosynthesis pathway, it was suggested that tetrapyrroles are involved in retrograde signaling. However, recent studies have cast doubt on that theory. In this thesis the performance of photo-bleached gun mutants was characterized in more detail. A before unknown phenotype of NF-treated gun mutants is described which is not due to NF resistance. In comparison to NF-treated wild-type seedlings the gun2-5 mutants affected in tetrapyrrole biosynthesis showed an enhanced growth capability, carotenoid enrichment, less anthocyanin accumulation and they retained plastome-encoded proteins. Replacement of NF by other inhibitors of carotenoid biosynthesis (such as amitrole) revealed that the growth and pigmentation phenotype is not coupled to the LHCB1.2 mRNA accumulation phenotype. Furthermore, it is shown that no simple correlation between any single metabolite, pigment or reactive oxygen species and LHCB1.2 expression exist. The observed heme accumulation caused by NF treatment is also not related to the LHCB1.2 de-repression phenotype. Application of abscisic acid (ABA) to NF-treated wild-type plants was sufficient to increase LHCB1.2 mRNA levels, but ABA is not involved in GUNdependent signaling associated with tetrapyrrole biosynthesis. It is discussed that more natural conditions are necessary to uncover the regulatory network of GUN signaling.