Chlorogenic acid (6.4) is an example of a preformed compound that has a relatively low level of toxicity to most microorganisms. It is effective against microorganisms considered as weak pathogens of potato. In potato tubers, chlorogenic acid is present in the periderm (Kojima et al., 1985) and is toxic to the organism that causes potato scab, Streptomyces scabies. In general, there is more of the compound in tubers of cultivars that are resistant to the pathogen than in tubers of cultivars that are susceptible. Furthermore, more of the compound is present during the time of tuber expansion. Chlorogenic acid also affects the growth of the vascular pathogen Verticillium albo-atrum and is present in the vascular tissue of the potato (Dao and Friedman, 1994). The mechanism by which ortho-dihydroxy phenolic compounds such as chlorogenic acid provide defense against insect pests was studied by Felton et al. (1989). They investigated the fate of chlorogenic acid in tomato (Lycopersicon esculentum) on the feeding behavior of the tomato fruit worm (Heliothis zea) and beet army worm (Spodoptera exigua). Upon feeding by the insects, polyphenol oxidases that are compartmentally separated from chlorogenic acid in the plant, come in contact with their substrate and convert chlorogenic acid to the toxic chlorogenoquinone (6.5; Figure 6-1). This quinone is a highly reactive electrophile and will react with nucleophilic -SH and -NH2 moieties in proteins (as indicated by structure 6.6; Matheis and Whitaker, 1984). This results in the cross-linking of proteins with chlorogenic acid,, which reduces the availability of free amino acids and proteins to the insect.
Figure 6-1. Oxidation of chlorogenic acid (6.4) by polyphenoloxidase (PPO), resulting in chlorogenoquinone (6.5), which can react with nucleophilic groups in proteins (6.6) to give the cross-linked compound 6.7, which can react with another protein molecule to yield 6.8. The quinate residue in structures 6.5, 6.7 and 6.8 is represented by R, whereas R1 and R2 indicate different amino acid residues.
Was this article helpful?