Line Defense Mechanisms

Like in animals, the first line of defense of plants is the skin - the epidermis and the periderm.

If the first line of defense is penetrated, the second line of defense is the triggering of two different immune responses, which involves extensive genetic reprogramming.

The first being a chemical attack that would isolate the pathogen and prevent spread of infection. This immune response is a hypersensitive response called Pathogen-Associated Molecular Pattern (PAMP) triggered immunity. A hypersensitive response is a local cell and tissue death that would occur at and near the site of infection. It can restrict the spread of the pathogen, but also be a consequence of the overall defense response. It involves transcriptional activation of over 10% of the plant's genes that can encode enzymes that would hydrolyze components of cell walls of pathogens. Effector triggered immunity also stimulates the formation of lignin and cross linking of molecules within plant cell walls.

The second response is only triggered if PAMP is not sufficient. The second response is known as a systemic acquired resistance. This arises from plant-wide expression or defensive genes. Methylsalicylic acid is produced around the infection site and carried by the phloem throughout the plant, where it is converted to salicyclic acid in areas remote from sites of infection. The salicyclic acid activates signal transduction pathways that poises the defense system to respond rapidly to another infection. This response generally lasts for a while in order to ensure that infection is completely protected from.