Role of Lon1 protease in post-germinative growth and maintenance of mitochondrial function in Arabidopsis thaliana

Molecular basis of and interference into degenerative processes in fungi: potential relevance for improving biotechnological performance of microorganisms

Biological systems, from simple microorganisms to humans, are characterized by time-dependent degenerative processes which lead to reduced fitness, disabilities, severe diseases, and, finally, death. These processes are under genetic control but also influenced by environmental conditions and by stochastic processes. Studying the mechanistic basis of degenerative processes in the filamentous ascomycete Podospora anserina and in other systems demonstrated that mitochondria play a key role in the expression of degenerative phenotypes and unraveled a number of underlying molecular pathways. Reactive oxygen species (ROS) which are mainly, but not exclusively, formed at the mitochondrial respiratory chain are crucial players in this network. While being essential for signaling processes and development, ROS are, at the same time, a potential danger because they lead to molecular damage and degeneration. Fortunately, a number of interacting pathways including ROS scavenging, DNA and protein repair, protein degradation, and mitochondrial fission and fusion are involved in keeping cellular damage low. If these pathways are overwhelmed by extensive damage, programmed cell death is induced. The current knowledge of this hierarchical system of mitochondrial quality control, although still incomplete, appears now to be ready for the development of strategies effective in interventions into those pathways leading to degeneration and loss of performance also in microorganisms used in biotechnology. Very promising interdisciplinary interactions and collaborations involving academic and industrial research teams can be envisioned to arise which bear a great potential, in particular, when system biology approaches are used to understand relevant networks of pathways in a holistic way.

The lack of mitochondrial AtFtsH4 protease alters Arabidopsis leaf morphology at the late stage of rosette development under short-day photoperiod

AtFtsH4 is one of four inner membrane-bound mitochondrial ATP-dependent metalloproteases in Arabidopsis thaliana, called AAA proteases, whose catalytic site is exposed to the intermembrane space. In the present study, we used a reverse-genetic approach to investigate the physiological role of the AtFtsH4 protease. We found that loss of AtFtsH4 did not significantly affect Arabidopsis growth under optimal conditions (long days); however, severe morphological and developmental abnormalities in late rosette development occurred under short-day conditions. The asymmetric shape and irregular serration of expanding leaf blades were the most striking features of the ftsh4 mutant phenotype. The severe abnormal morphology of the leaf blades was accompanied by ultrastructural changes in mitochondria and chloroplasts. These abnormalities correlated with elevated levels of reactive oxygen species and carbonylated mitochondrial proteins. We found that two classes of molecular chaperones, Hsp70 and prohibitins, were over-expressed in ftsh4 mutants during late vegetative growth under both short- and long-day conditions. Taken together, our data indicate that lack of AtFtsH4 results in impairment of organelle development and Arabidopsis leaf morphology under short-day conditions.

Mitochondria biogenesis via Lon1 selective proteolysis: who dares to live for ever?

Quality control of proteins in eukaryotic organelles is predominantly maintained by members of the ATP-dependent proteases. Even though numerous biological analyses have shed light on the functional implications of such proteases, their involvement in developmental processes of multicellular organisms has not been determined. We recently identified two lon1 mutant alleles, both missing the carboxy terminal proteolytic domain, that show post-embryonic growth retardation resulting in delayed seedling establishment. In this addendum, we enlighten the role of Lon1 selective proteolysis in plant mitochondria biogenesis, a prerequisite for post-embryonic development and growth. In contrast to the weak lon1-2 allele, the polypeptide encoded by the strong lon1-1 allele carries the sensor- and substrate-discrimination domain allowing substrate recognition and binding. This type of molecular recognition hinders further degradation by the complementary Lon-independent proteolytic machineries resulting in an extra deleterious accumulation of protein aggregates into lon1-1 mitochondria. The most challenging and informative task will be to identify the recognition motifs on the Lon protein substrates and elucidate the molecular events that control plant mitochondrial differentiation.