A post-translational protein modification that is poorly characterized in plants is ADP-ribosylation, i.e. the covalent attachment of one or more ADP-ribose units onto proteins. This protein modification is carried out by enzymes of the poly(ADP-ribose) polymerase (PARP) family, that are localized in the cell nucleus. Another family of enzymes, the poly(ADP-ribose) glycohydrolases (PARGs), can remove single ADP-ribose units and thus shorten or entirely remove the protein modification. Under stress conditions, ADP-ribose chains are dynamically built up and shortened on stress-responsive proteins. One function of protein ADP-ribosylation is the regulation of plant responses to abiotic and biotic stress. This includes infections by plant pathogens as well as drought, heat or salt stress. However, it is largely unknown which proteins become ADP-ribosylated under stress conditions and how this strengthens plant resistance to environmental stress and diseases.

Many plant pathogens shuttle proteins into host cells that ADP-ribosylate plant proteins to inhibit defence mechanisms [1]. Precise information about which plant proteins are modified provides a better understanding of pathogen infection strategies. In addition, this knowledge contributes to breeding plants with increased disease resistance. In collaboration with the group Proteome Analytics of the BPI department, we have developed a protein mass spectrometry method that can detect proteins that are ADP-ribosylated by plant pathogens [2]. Further development of this and similar methods will help us to understand how ADP-ribosylation influences the biological function of plant proteins and how this enables plants to better defend themselves against adverse environmental conditions and pathogen infection.

Although SRO proteins are related to the above-mentioned PARPs, only some SRO proteins can transfer ADP-ribose units onto target proteins. Other SRO proteins have lost this ability but nevertheless contribute to plant stress tolerance [3]. To understand why some SRO proteins are inactive, we solved the 3D structures of the PARP-related regions of SRO proteins from wheat and the model plant Arabidopsis thaliana (thale cress) [4 and 5]. These insights on the molecular level form the basis for a better understanding of the differences between active and inactive SRO proteins. Our future goal is to make reliable predictions about the activity of SRO proteins from crop plants based solely on the protein sequence and AI-based protein structure modelling. In times of climate change, a better understanding of the molecular functions of SRO proteins will help to identify particularly beneficial SRO variants that can increase the stress tolerance of crops.