Effects of phytic acid and xanthotoxin on growth and detoxification in caterpillars

Phytic acid is abundant in the fruits and seeds of many plants and is found in foliage to a lesser extent. Among its several properties, phytic acid is a potent chelator of essential minerals and proteins; thus, the possibility exists that heme-based enzymes such as cytochrome P450 monooxygenases in herbivores are detrimentally affected by phytic acid via chelation of dietary iron. Mortality, growth performance, and P450-mediated metabolism of xanthotoxin, a plant allelochemical, were examined in the presence of phytic acid in three lepidopteran species: a polyphagous seed-feeding species (Heliothis virescens), a polyphagous foliage-feeding species (Trichoplusia ni), and a species oligophagous on immature reproductive structures of two genera of Apiaceae (Depressaria pastinacella). While first instar H. virescens experienced no increase in mortality after 120 hours on a diet containing 1% phytic acid compared to a control diet, both T. ni and D. pastinacella experienced virtually complete mortality over the same time period. Ultimate instars of all three species experienced reductions in relative growth rates (RGR) and relative consumption rates (RCR) in the presence of phytic acid, although the only species to experience reduced digestive efficiency (ECI) was H. virescens. Cytochrome P450-mediated metabolism of xanthotoxin was reduced 60% in the presence of phytic acid in D. pastinacella, although metabolism remained unaffected in the two noctuids. These studies suggest a defensive function of phytic acid in addition to its primary functions of phosphorus storage, energy storage, and cell wall precursor source.