natural-products-in-plant-pest-management

56 J.C. Pretorius and E. van der Wattp-Coumaric acid is the direct precursor of other coumarins such asumbelliferone and scopoletin but coumarin itself is synthesized fromcinnamic acid (Fig. 3.19).The potential of phenyl propanoids to be developed into natural productsfor the agricultural industry is probably underestimated at present. Forexample, coumarin, isolated from Mellilotus alba (White Clover; a legume), isa strong inhibitor of seed germination and has the potential to be developedas a pre-emergence herbicide. However, depending on the concentration, itis also known to stimulate IAA activity and therefore growth. Its potential asbio-stimulant has not been researched to date. Coumarin can also combinewith monosaccharide sugars such as glucose to form coumarin glycosides,e.g. umbelliferone glycoside (Fig. 3.20).These glycosides inhibit the activity of the enzyme 6-phosphogluconatedehydrogenase, the regulatory enzyme of the oxidative pentose phosphate(OPP)-pathway, and therefore possess excellent potential to be developed asnatural herbicides. Umbelliferone also has an inhibiting effect on the growthOHRFig. 3.18. Two well-known phenyl propanoids.COOHp-Coumaric acid (R=H)Caffeic acid (R=OH)OOHCH 3 OHOOOHOOOOOUmbelliferoneScopoletinCoumarinCinnamic acidFig. 3.19. Phenyl propanoids synthesized from the precursor, p-coumaric acid.glycosylOOOFig. 3.20. Umbelliferone glycoside.