Phenolic acids are generally not abundant in most plants. There are a few exceptions: gallic acid (1.5) and salicylic acid (SA; 1.8). Gallic acid is a precursor for the ellagitannins and gallotannins, which will be discussed in more detail in section 15 of this chapter. Salicylic acid is an important defense compound because it mediates systemic acquired resistance (SAR), a resistance mechanism whereby SA is used as a signaling molecule to relay information on pathogen attack to other parts of the plant. Upon receiving the SA signal, a general defense response is activated that includes the biosynthesis of pathogenesis-related (PR) proteins. This will be discussed in more detail in Chapter 6, Section 2.4.
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