Towards protease inhibitor mediated resistance to western flower thrips in chrysanthemum

Dendranthema grandiflora (chrysanthemum) is a cutflower grown across the world, with an acreage similar to roses. Frankliniella occidentalis (Western flower thrips, WFT) is a major pest against a large number of crops, including chrysanthemum, in both field and glasshouse cultivation. Aiming at WFT resistance in chrysanthemum by a transgene-mediated approach, the prerequisite of an efficient regeneration system was first established. Subsequently, promoters for high level transgene expression were identified. The predominant group of proteases active in WFT was identified as the class of cysteine proteases, and the recombinant cysteine protease inhibitors potato cystatin (PC) and equistatin (EI) were shown in vitro to achieve over 90% inhibition of WFT protease activity. PC and EI ingested by adult WFT on a pollen diet for a period of five days resulted in 50% reduction in the oviposition rate. Effects on adult mortality were not observed during this brief period. An eight domain cysteine protease inhibitor potato multicystatin (PMC) was expressed in chrysanthemum in an attempt to endow it with WFT resistance. Low expression levels ranging at maximum 0.1-0.13% of total protein were generated. Hence, no correlation with thrips resistance could be established with these lines. However, the results in this thesis provide evidence of potentially significant effects of cysteine protease inhibitors on WFT oviposition rate. This would have a strong effect on restricting WFT population build up. The concept of enhancing cysteine protease inhibitor activity by various approaches in the different parts of chrysanthemum to combat WFT is discussed in relation to the existing alternatives.