Production of novel semiochemicals through synthetic biology for crop protection

With an ever-increasing world population and pressure to reduce pollution from toxic chemicals the challenge to secure a sufficient food supply for the future has never been higher. Development of an environmentally benign method for protecting crop plants from herbivorous insects and the extension of our knowledge of terpene biosynthesis are the overall goals of this project.

For both defense against damage from insects and attraction of beneficial pollinators plants generate volatile signaling molecules (semiochemicals). These can be toxic, repellent or attractive to the insects depending upon the nature of the plant-insect interaction. Exploiting such molecules for crop defense is an attractive goal. They have low environmental impact since they are very likely to be insect-species specific and of low general toxicity thereby alleviating the problems associated with the current use of broad-spectrum, highly toxic insecticides. Such compounds kill general insect populations leading to collateral damage such as catastrophic loss of beneficial insects and are also toxic to other species including humans. Furthermore since they cause high mortality this leads to a selective pressure for insect populations to develop resistance. Hence we wish to develop synthetic volatile semiochemicals that will protect crops through either repelling damaging insects from valuable crops or through attracting them into traps. The naturally produced semiochemicals however often have undesirable properties such as low-persistence in the environment and/or insufficient potency and so generation of similar chemicals that retain the desired bioactivity but with improved efficacy is necessary.

In previous work within our group we prepared analogues of the volatile sesquiterpene semiochemical, germacrene D, through preparation of analogues of the precursor compound, E,E-farnesyl diphosphate (EE-FDP) and using the enzyme germacrene D synthase to catalyse the conversion of these to germacrene D analogues. This led to several promising analogues of this semiochemical that showed repellent activity and, in one case, attractive properties towards grain aphids, a serious pest to farmers of cereal crops. Here we will extend this methodology towards a new target, tomatoes, to demonstrate the generality of our approach to protecting commercially important crops and also to investigate the mechanism of action of a new class of sesquiterpene synthases.

Epizingiberene synthase (EZS) is a sesquiterpene synthase that generates 7-epizingibere, a repellent and toxin of whitefly, insects that cause multi-million pound damage to tomato crops each year. EZS is a member of a recently discovered class of sesquiterpene synthases that do not employ the more usual, EE-FDP substrate, but instead a close analogue, ZZ-FDP as substrate. Hence we have the opportunity to develop novel crop protection agents whilst simultaneously extending our knowledge of terpene biosynthesis through study of this new class of enzyme. A series of modified ZZ-FDPs will be prepared and converted using EZS (and genetically engineered EZS mutants) into 7-epizingiberene analogues that will be tested for activity against whitefly in collaboration with Rothamsted Research. The results will reveal both molecular level detail of the catalytic action of EZS and useful compounds for protection of tomato crops.