Abstract
Plants produce a wide variety of defensive metabolites to protect themselves against herbivores and pathogens. Non-protein amino acids, which are present in many plant species, can have a defensive function through their mis-incorporation during protein synthesis and/or inhibition of biosynthetic pathways in primary metabolism. 5-Hydroxynorvaline was identified in a targeted search for previously unknown non-protein amino acids in the leaves of maize (Zea mays) inbred line B73. Accumulation of this compound increases during herbivory by aphids (Rhopalosiphum maidis, corn leaf aphid) and caterpillars (Spodoptera exigua, beet armyworm), as well as in response to treatment with the plant signalling molecules methyl jasmonate, salicylic acid and abscisic acid. In contrast, ethylene signalling reduced 5-hydroxynorvaline abundance. Drought stress induced 5-hydroxynorvaline accumulation to a higher level than insect feeding or treatment with defence signalling molecules. In field-grown plants, the 5-hydroxynorvaline concentration was highest in above-ground vegetative tissue, but it was also detectable in roots and dry seeds. When 5-hydroxynorvaline was added to aphid artificial diet at concentrations similar to those found in maize leaves and stems, R. maidis reproduction was reduced, indicating that this maize metabolite may have a defensive function. Among 27 tested maize inbred lines there was a greater than 10-fold range in the accumulation of foliar 5-hydroxynorvaline. Genetic mapping populations derived from a subset of these inbred lines were used to map quantitative trait loci for 5-hydroxynorvaline accumulation to maize chromosomes 5 and 7.
Highlights
Plants in their natural environment are subject to attack by is resistant to feeding by most species of insect herbivores. numerous herbivorous insects
In a targeted search for non-protein amino acids that are induced by methyl jasmonate treatment, the free amino acid content in leaves of maize inbred line B73 was analysed by High-performance liquid chromatography (HPLC) fluorescence detection after derivatization using the Waters AccQ-Fluor reagent kit
The results presented here show that accumulation of 5-hydroxynorvaline in maize leaves is induced in response to insect herbivory
Summary
Plants in their natural environment are subject to attack by is resistant to feeding by most species of insect herbivores. numerous herbivorous insects. Plants in their natural environment are subject to attack by is resistant to feeding by most species of insect herbivores. (Fraenkel, 1959), either through direct toxicity (antibiosis) or by making plant tissue so unpalatable that the herbivores are encouraged to feed elsewhere (antixenosis). A few hundred of these are known constituents of primary metabolism, such as sugars, amino acids and lipids, which are found in most or all plants. In many well-studied plant species, structures of the more abundant secondary metabolites, which provide defence against herbivores and pathogens, have been confirmed. In any given plant species the vast majority of the constituent metabolites remain unidentified. There are undoubtedly unknown intermediates in primary metabolism, it is likely that many of the as-yetunidentified plant metabolites contribute to defence against herbivores and pathogens
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