Episode 4: Proteinase inhibitors
Some of the proteins induced upon infection are very small, below 10 kDa. They may also be called PR-peptides. Family 6 gathered such peptides showing proteinase inhibitor activity: they have the property to bind proteinases and control proteinase activity. Several types of inhibitors have been characterized, targeted to different types of proteinases: serine proteinase (e.g. trypsin, Chymotrypsin), cysteine proteinase (e.g. papain, caspase) or aspartate/metallo proteinases. Reduction of the digestibility of the pathogen reduces its pathogenicity but also indirectly its growth.
Although mainly induced by incompatible pathogens, defense mechanisms depicted in the cartoons (see previous episodes, too) can also be activated in response to abiotic stress, such as ozone or UV radiation.
The HR-dying cells are restricting the pathogen at its site of penetration but some signal molecules known as stress phytohormones (salicylic acid, jasmonic acid and ethylene) are diffusing out from the HR developing cells and lead to the establishing of resistance both locally (local acquired resistance, LAR) and systemically (systemic acquired resistance, SAR).
Interestingly, although the recognition step is highly specific, the HR response is not. Especially SAR that can be long-lasting often confers broad-based resistance to a variety of different pathogens. For this reason, HR has been the focus of plant protection strategies.
Plant immunization using elicitors is a powerful and promising strategy for sustainable crop production.
A maybe older strategy was the manipulation of the defense mechanisms in transgenic plants. Increased production of PR-proteins has been of particular interest, or the overexpression of enzyme involved in the production of phytoalexines. A major drawback of those strategies was the problem of numerous synthesizing steps for phytoalexines or the limited action of only one overexpressed PR-proteins (e.g. hydrolases).
A much more powerful and promising strategy for sustainable crop production is plant immunization using elicitors: by mimicking an incompatible pathogen attack, it is possible to induce the defense mechanisms, and more particularly the SAR in the whole plant for protection against a wide range of future attacks. Particularly, pathogen-associated molecular patterns (PAMPs), a generic name for pathogen-derived elicitors, have been used in agriculture. The other option is the exogenous application of phytohormones involved in SAR establishment.
Limited number of studies has focused on forestry, but a few reported successful use of methyl jasmonate to induce defense responses in conifers. Research is needed in this field to minimize the use of pesticides in plant protection and integrated pest management.
Any food safety issues?
Plants are producing a large set of secondary metabolites in response to pathogen attacks. Many have beneficial health promoting effects in humans (see episode 2), but others, as they are expected to be, have direct toxic effects: they act as toxins and also affect growth, development, and digestibility (e.g. proteinase inhibitors). Those effects are not limited to pathogens and may also apply to animals and humans eating the plants. Solanine in potato or nicotine in tobacco are two good examples.
Already 20 years ago, Dr. Fritig was warning us about the possible problems due to the accumulation of those toxic compounds in challenged plants: “Not all phenolic compounds are good!” – they are not all phenolics, though.
Organic farming limits the use of synthetic pesticides and lets the “nature” do its job. Several studies report a higher content of antioxidants in organic food compared to that of conventionally produced food. However, although this certainly does not underrate the benefits of organic farming, there are grounds to wonder about higher production of natural defense-related toxins in organically-grown food and the possible detrimental effect on organic food quality. This is certainly species-dependent and clearly requires further research.