Unsere Forschung

Photosynthesis supplies almost all of the energy necessary for life, and is responsible for maintaining atmospheric O2 levels. During photosynthesis, light energy is converted into chemical energy and stored as sugars and organic acids. However, active photosynthesis results also in the production of reactive oxygen species (ROS) and reactive carbonyl species (RCS). Depending on their subcellular production site and concentrations, these compounds can act as signalling molecules, but also cause oxidative damage. ROS and RCS are often produced in excess as a result of environmental challenges; to avoid harmful effects, plants upregulate their photoprotective capacities and activate detoxification systems.

All organisms carrying out oxygenic photosynthesis – plants, algae, and cyanobacteria – use Rubisco for the fixation of CO2. During this process, the oxygenase activity of Rubisco produces 2-phosphoglycolate, a toxic intermediate that is eliminated through photorespiration. This process serves as a carbon recovery system. Many photosynthetic lineages independently evolved complex processes to reduce photorespiratory carbon losses by concentrating CO2 at the site of Rubisco. In plants, the most prominent such mechanism is C4 photosynthesis resulting in higher yields as well as water and nitrogen-use efficiencies under challenging environments.

The long-term goal of my research is a deep systemic understanding of four central aspects of photosynthesis: (1) photoinduced production of reactive oxygen and carbonyl species;      (2) photoprotective acclimation in dynamic environments; (3) involvement of organic acids in photosynthetic carbon assimilation in C3 and C4 plants; and (4) photorespiration. We strive to provide the basis for the development and implementation of strategies to improve organisms in terms of efficient resource utilization and enhanced yield. To achieve these goals, my group combines extensive expertise in plant physiology, biochemistry, and molecular biology into a vigorous, interdisciplinary research program.

Projects

(1) Photoinduced production of reactive oxygen and carbonyl species
During photosynthesis, the chloroplasts and the peroxisomes are the main sources of reactive oxygen species, especially of hydrogen peroxide (H2O2). H2O2 causes oxidative damage, but also acts in signalling. A major aim of my research is to dissect the genetic networks that control H2O2 signalling and to assess specific and common responses towards H2O2 signalling after its production in different subcellular compartments. Using a non-invasive approach to modulate the metabolic production of H2O2 in A. thaliana chloroplasts system, we showed that a moderate production of H2O2 from chloroplasts or peroxisomes induces two types of responses: Chloroplast-produced H2O2 has crucial sensory and signalling functions, while peroxisome-produced H2O2 most likely induces stress tolerance responses.
Reactive carbonyl species, like methylglyoxal and glyoxal, are highly reactive toward different cellular macromolecules. The primary source of these RCS is the glycolytic pathway. However, as a unique feature of plant cells, the activity of the Calvin Benson cycle results in a second source of reactive carbonyl species. Ongoing work indicates that the individual components of the system might be active at specific plant developmental stages and metabolic conditions induced by cellular and environmental changes. The involvement of different organelles during the detoxification of methylglyoxal implies a synchronization of the corresponding enzymatic and transport activities between the cellular compartments.

(2) Photoprotective acclimation under dynamic environments
Excess light energy, which is absorbed by light-harvesting pigments but not utilized for photochemical reactions, can lead to the production of reactive carbonyl species, resulting in (photo-) oxidative damage in oxygenic photosynthetic organisms. When excess light conditions persist, plants are able to upregulate their photoprotective capacities via acclimation. Acclimation under constant light involves adjustments of leaf architecture, chloroplast structure, composition of the photosynthetic electron transport chain, and regulation of photosynthetic light utilization. Fluctuating light selectively induces photoprotective acclimation in wild-type Arabidopsis and rapid switches in the grana structure. So far it is not known if any H2O2 scavenging pathways are upregulated in chloroplasts during fluctuating light acclimation. We recently found that excess fluctuating light can rescue the patchy pale-green phenotype of plants producing H2O2 from the chloroplasts, probably through upregulation of H2O2 scavenging. We are using these plants as a tool to study the mechanisms of dynamic photoprotective acclimation.

(3) Involvement of organic acids in photosynthetic carbon assimilation in C3 and C4 plants

Organic acids represent intermediates of major carbon metabolism in plant cells. In species that use the C4 photosynthetic metabolism, malate has been co-opted as a key metabolite in the photosynthetic process. Despite the complexity of C4 photosynthesis, this trait constitutes a striking example of convergent evolution: it has evolved independently in more than 60 angiosperm lineages. About 40 C4 lineages contain species using NADP-Malic enzyme (ME) as their primary decarboxylase, while NAD-ME is used by species from about 20 lineages. The occurrence of gene duplications is widely believed to be a prerequisite for the evolution of C4 metabolism, as gene duplicates allow the plant to retain one gene copy for the ancestral C3 function, while its duplicate can acquire alterations advantageous for the evolving C4 function. Importantly, all enzymes required for C4 photosynthesis have orthologs in C3 species, where they perform other functions.  In the evolution of C4 biochemistry, these enzymes required concerted changes in their cell type-specific gene expression as well as adjustment of their structural and kinetic properties. Research in my group center on the mechanisms behind the evolution of C4 decarboxylases and in different aspects of organic acid metabolism in plants using different photosynthetic types.  

(4) Photorespiration

During photorespiration, dephosphorylation of 2-PG in the chloroplasts produces glycolate. Regardless of the mechanisms of 2-PG/glycolate toxicity, even minute traces of them exert cumulative detrimental effects. Enzymatic removal of glycolate is an essential part of photorespiration and is catalyzed by two phylogenetically unrelated enzymes. In cyanobacteria and chlorophyta, glycolate oxidation is catalyzed by glycolate dehydrogenase (GlcDH). In contrast, charophyceae and land plants only have glycolate oxidase (GOX) activity, which produces H2O2 in peroxisomes. We recently elucidated the evolution of GOX and discovered that GOX and GlcDH activities never co-exist in an organism. Currently we are elucidating the composition of the photorespiratory pathway in algae that arose through secondary endosymbiosis.

Arbeitsgruppenleiterin

Vertretungsprofessorin in Botanik (Universität zu Köln)

Heisenberg-Stipendiatin

Priv. Doz. Dr. Veronica G. Maurino

Entwicklungs- und Molekularbiologie der Pflanzen
Heinrich-Heine-Universität
Düsseldorf
Universitätsstraße 1
Gebäude: 26.03
Etage/Raum: U1.23
Tel.: +49 211 81-12368
Verantwortlich für den Inhalt: E-Mail sendenPD. Dr. Veronica G. Maurino