of avrRpt2 by RPS2 occurs via the guardee protein RIN4, which is cleaved. Recognition of a pathogen effector leads to a dramatic immune response known as the hypersensitive response, in which the infected plant cells undergo cell death to prevent the spread of the pathogen. Systemic acquired resistance (SAR) is another example of resistance that is better understood in plants because of research done in A. thaliana. Benzothiadiazol (BTH), a salicylic acid (SA) analog, has been used historically as an antifungal compound in crop plants. BTH, as well as SA, has been shown to induce SAR in plants. The initiation of the SAR pathway was first demonstrated in A. thaliana in which increased SA levels are recognized by nonexpresser of PR genes 1 (NPR1) due to redox change in the cytosol, resulting in the reduction of NPR1. NPR1, which usually exists in a multiplex (oligomeric) state, becomes monomeric (a single unit) upon reduction. When NPR1 becomes monomeric, it translocates to the nucleus, where it interacts with many TGA transcription factors, and is able to induce pathogen-related genes such as PR1. Another example of SAR would be the research done with transgenic tobacco plants, which express bacterial salicylate hydroxylase, nahG gene, requires the accumulation of SA for its expression Although not directly immunological, intracellular transport affects susceptibility by incorporating - or being tricked into incorporating - pathogen particles. For example, the Dynamin-related protein 2b/drp2b gene helps to move invaginated material into cells, with some mutants increasing PstDC3000 virulence even further. Evolutionary aspect of plant-pathogen resistance Plants are affected by multiple pathogens throughout their lifetimes. In response to the presence of pathogens, plants have evolved receptors on their cell surfaces to detect and respond to pathogens. Arabidopsis thaliana is a model organism used to determine specific defense mechanisms of plant-pathogen resistance. These plants have special receptors on their cell surfaces that allow for detection of pathogens and initiate mechanisms to inhibit pathogen growth. They contain two receptors, FLS2 (bacterial flagellin receptor) and EF-Tu (bacterial EF-Tu protein), which use signal transduction pathways to initiate the disease response pathway. The pathway leads to the recognition of the pathogen causing the infected cells to undergo cell death to stop the spread of the pathogen. Plants with FLS2 and EF-Tu receptors have shown to have increased fitness in the population. This has led to the belief that plant-pathogen resistance is an evolutionary mechanism that has built