Defensive compounds against pathogens

Flavonoids are important secondary metabolites involved in pigmentation of flowers, disease resistance, and so on. Among the flavonoids, kaempferol (67), quercetin (68), and naringenin (69) are most often cited as allelochemicals. (zh)-Catechin was identified in the root exudates from spotted knapweed Centaurea maculosa) as the compound responsible for its invasive behavior.45 The plant uses (-)-catechin (70) as an allelochemical facilitating its invasion and (+)-catechin (71) as a defense compound against pathogens. 

Immunization of Plants by Attenuated Forms of Pathogens. As has been argued by Kuc and Caruso (47), plants can be immunized to achieve higher levels of resistance to pathogens. Similar mechanisms may conceivably provide a line of defense against phytophagous insects without the challenge-independent accumulation of defensive compounds. 

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