Plant immunity cut to size

As a universal process in all multicellular organisms, including humans and plants, damaged cells send out signals to alert the surrounding tissue of the wound. These signals can activate the immune system to prevent infection and promote tissue regeneration, eventually leading to wound healing. In plants, short protein fragments or peptides play an important role in the immune system. These peptides are produced from precursor proteins that are 'cut into shape' by so-called proteolytic enzymes or proteases.

The problem is that there are a lot of proteases, which means that it is essential to identify which ones perform which roles in the plant immune system. By wounding leaves of the thale cress, Arabidopsis thaliana, the teams of Thomas Boller (Prof. emeritus, University of Basel), Frank Van Breusegem (VIB-UGent Center for Plant Systems Biology) and Kris Gevaert (VIB-UGent Center for Medical Biotechnology) found that a class of proteolytic enzymes called metacaspases played an important role in the plant's response which involves the release of calcium and the peptide precursor protein PROPEP1. They checked their findings by producing a plant with a mutation in the gene coding for an important metacaspase. This plant was unable to release the immune signal.

To understand the speed and extent of the immune response in Arabidopsis, Simon Stael, the postdoc who led the efforts, damaged the roots with lasers. The targeted plant cells responded quickly. Simon Stael says: "We were really excited to see those first laser shots followed by calcium waves and PROPEP1 signal dispersion." The newly uncovered process can be summarized as follows: damage elicits high calcium levels in the cell interior that activate metacaspases. These metacaspases go to work on PROPEP1, which regulates the immune response and associated damage limitation efforts.

This opens up new avenues of research since proteases usually cleave more than one protein. So, which other plant processes are influenced by metacaspases and contribute to wound response and immunity? We can now also use the laser ablation technique to look at a variety of other responses in the damaged cells and their surroundings, to learn more about the details of local wound response. Finally, crop breeding strategies mostly select for optimal growth, yield and quality of food or feed in combination with intensive pesticide use, potentially crippling the plant immune system. Metacaspases now emerge as potential targets for improved breeding techniques and better crop immunity.