Proline content does not influence pest and disease susceptibility of barley

Abstract

The genetic engineering of crop plants for specific desired characters is of potential importance in agriculture¹. Serious consideration must therefore be given to undesirable side effects of particular modifications. The soluble nitrogen content of leaves is important in the relationship between plants and their insect predators and increases during periods of environmental stress^2,3. Soluble proline increases markedly during water stress in a wide range of plant species including barley^4,5 and it has been suggested that plants selected for higher proline content may be more tolerant of drought stress⁶. However, proline is phagostimulatory to locusts when presented on an inert matrix⁷ and this work has been extended by Haglund⁸ to suggest that proline (or valine) may be a cue detected by grasshoppers to lead them to drought-stressed nitrogen-enriched plants. Plants selected for a higher content of free proline in an attempt to increase their tolerance of drought stress might therefore prove more attractive to insect predators. To test this hypothesis we have now compared the responses of locusts, slugs, aphids and a mildew fungus to normal barley and a barley mutant⁹ which has up to six times the normal content of free proline in the leaves. We find no evidence that the mutant is more susceptible than its parent to damage by any of these organisms.