Characterization of the gun phenotype under photo-bleaching conditions

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.