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phytoalexin An antimicrobial substance that is produced by a plant in response to infection by fungi or bacteria and helps to defend the plant by inhibiting the growth of invading microbes. Phytoalexins vary in their chemical nature; for example, in legumes they are mainly isoflavonoids, whereas in the potato family (Solanaceae) they are predominantly terpenes. Substances, such as small polysaccharides and proteins, that are produced by the invading pathogen act as elicitors, triggering the manufacture of phytoalexins in the tissues surrounding the infection site. The production of phytoalexins is one component of a broader defensive reaction, known as hypersensitivity.