Episode 3: Hydrolases
After strengthening their cell walls and synthesizing antimicrobial chemicals, the HR-dying plant cells are synthesizing antimicrobial proteins called pathogenesis-related proteins (PR-proteins). They are not present or at very low basal level in healthy tissues and are rapidly induced in response to fungal, bacterial, viral attacks, as well as wounding and application of plant hormones ethylene, jasmonate and salicylic acid.
PR-proteins are typically small proteins (5–75 kDa), currently categorized into 17 families according to their properties and functions.
The most characterized and studied PR-proteins are glycoside hydrolases with β-glucanase and chitinase activities. Both enzymes are important hydrolytic enzymes that are accumulating in many plant species after pathogen infection. They appear to be coordinately expressed after fungal infection and have a direct effect on pathogen by degrading chitin and β-1,3-glucan, which are major structural components of the cell walls of many pathogenic fungi.
Glucanases are classified as PR2 family. They cleave the β-1,3-glucosidic bonds in β-1,3-glucan. Multiple isoforms exist and these enzymes may have other physiological functions than in plant defense, such as plasmodesmata regulation (see also episode 1), cell division and elongation, fruit ripening, pollen germination and tube growth, fertilization, seed germination and flower formation.
Chitinases are classified into four families of PRs (3, 4, 8 and 11) based on the sequence homology and the presence or absence of chitin binding domain. They cleave the bond between two consecutive N-acetyl-D-glucosamine monomers of chitin. Some chitinases have dual lysozyme and chitinase functions and show higher activity on bacterial cell wall peptidoglycan.
In addition to the direct effect on pathogen, chitin and β-1,3-glucan released by extracellular glucanases and chitinases act as elicitors to stimulate host defense response and induce acquired resistance to further infection.
Many PR families exist in addition to the above mentioned glucanases and chitinases. In addition to its antifungal in vitro activity, PR5 (thaumatin-like) also showed β-glucanase activity. Several other PR-proteins (defensins, thionins, and lipid transfer proteins) act directly on the pathogen by membrane permeabilization. The most abundant PR-proteins, the PR-1 family, have demonstrated antimicrobial activity but their exact mechanism of action is unclear, like it is the case with many other PR-proteins.
Not all PR-proteins are induced in response to pathogen attack, which highlights the importance of the different signaling pathways involved in induction of defense mechanisms.
4 – Protease inhibitors and concluding remarks