Modes of Action of Plant Protection Chemistries – Part 3

Natural Plant Defence Mechanisms Salicylic acid pathway – Induces SAR (systemic acquired resistance), a natural biological defence response to pathogen attack. Jasmonic Acid Pathway - Induces the production of disease and insect defence compounds. Salicylic Acid Pathway Production of active oxygen (hydrogen peroxide, peroxidase) o Peroxidases have been associated with fungal cell wall degradation and pathogen defence signaling • Thickening plant cell wall - Increasing lignification - Production of phenolic esters that strengthen cross linking Chitinases such as ‘Softguard®’ The simple version of it's function is that chitosan (poly-D-glucosamine) is found in the shells of almost all insects and crustaceans, as well as in most fungi, algae and yeasts. Chitin is the second most abundant polymer in nature after cellulose, and it is a structural component of some fungi, insects, various crustaceans, and nematode eggs. Chitin is insoluble in water, organic or inorganic acids. Chitin is the major component of all fungal cell walls except for the Oomycetes Chitinases break down fungal cell walls Chitinases can break down insect exo-skeletons Activity is greatly enhanced by Glucanase. Chitosan is deacetylated chitin, and referred to as poly â-(1-4)-D glucosamine. Chitosan occurs naturally as a component of the chitin matrix. Chitinases are a class of enzymes capable of degrading chitin, the N-acetyl glucosamine polymer that makes up fungal cell walls. Chitinases are expressed at low levels in healthy plants but increased when under pathogen attack being located primarily in the cell wall and vacuole. Extracts containing chitinases will directly inhibit the growth of fungi by degrading their cell walls and also indirectly promote the release of oligosaccharides that act as elicitors of defence reactions

When acetylated chitosan (SoftGard®) is sprayed onto the leaves of plants the chitosan content triggers the plants natural defence mechanisms into thinking the plant is under attack by fungi, and the plant defends itself. If treatment is carried out regularly during hot, wet or humid weather, and BEFORE the outbreak of fungal activity, it can prevent many forms of fungal outbreaks. Addition of chitin to soil stimulates growth of bacteria actinomycetes, and a limited number of fungal species with chitinolytic properties. A beneficial function of these microorganisms is that they may attack and reduce parasitic nematode colonies, whereby more soil macronutrients and micronutrients become available to the plant through its healthy root system. Jasmonic Acid Pathway Insect predation also causes the synthesis of jasmonic acid and its ester methyl jasmonate. Jasmonate is highly volatile and can be detected by surrounding plants to warn them of potential insect predation by the inducing the production of systemin and the synthesis of proteinase inhibitors. Farmer and Ryan (1990) discovered that jasmonic acid volatilized from sagebrush could trigger defense gene expression in adjacent tomatoes Jasmonic acid volatiles act as attractants for beneficial insects Jasmonic acid induces the production of disease and insect defense compounds. - Defence Proteins - Phytochemicals Phytochemicals • Different from phytoalexins in that phytochemicals are induced by wounding. - Phenolics such as furanocoumarins, Coumarins, Tannins, Lignin, other phenolics - Terpenoids - Alkaloids Mode of action-plant activators such as Alexin

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