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.