The interaction between plant hosts and pathogens are complex systems that consist of a vast multitude of components. These components that contribute to these systems are from host as well as pathogenic origin and compete with one another in an attempt to be more successful than the other party.
There are many different pathogens that infect plants from all five kingdoms of life and each has a different method of infecting the host. There are several traits that are universal requirements for infection such as gaining entry to the host, overcoming the host’s immune response, replicating effectively within the host and spreading to naïve hosts.
Plant cell walls consist of a network of various peptides and polysaccharides. Three categories of polysaccharides exist, namely cellulose, hemicellulose and pectin. Pectin and hemicellulose form the network that the cellulose microfibrils are embedded. The pectin network consists of galacturonic acid (PGA) subunits that are bound by α-1,4-linked glycosidic bonds. This network acts as a primary defence barrier to pathogens that attempt to cross the cell wall to infect the host as well as providing significant structural support.
The biosynthesis of pectin is localised to the Golgi vesicles. It has been estimated that 67 different enzymes are required in the biosynthetic pathway of pectin. 384 The reason for the evolution of pectin is mostly unknown and genome comparisons between different plants would provide a clearer insight into its evolution. What is known about the evolution is that the latest development in angiosperms is the presence of the QUA1 ortholog.
Plant endoPGs are important for the developmental and remodelling processes during their lifecycles.
One of the most prolific enzymes that enable a fungus or bacteria to infect woody plants is the polygalacturonase (PG) enzyme. This enzyme is produced by hosts as well as by the bacteria, fungi and yeasts that infect it. In plants it plays a crucial role in fruit ripening whereas in bacteria and fungi it plays a vital role in the rotting process that occurs upon infection. The group of PG enzymes consist of endoPG (EC 22.214.171.124) and exoPG (EC 126.96.36.199) isozymes. These isozymes both hydrolyse the polygalacturanic acid pectin network, but at different positions. EctoPG cleaves galacturon monomers from the terminals of the polygalacturanic chains whereas endoPG hydrolyses the internal glycosidic bonds within the polygalacturanic acid chain.
EndoPG have been found to be the enzymes that cause tissue maceration due to their ability to degrade the pectin network. This makes them an integral component to the initial rotting process. Once the tissue has been macerated, the plant no longer has the physical barrier to protect its extracellular and intracellular matrices from pathogen intrusion.
When a plant is infected with a pathogen an immune response occurs. One of the compounds that induce an immune response is PGA. PGA induces a conversion of ·OH to...