Plants have evolved a variety of mechanisms including physical and chemical barriers (repellents, toxins) to protect themselves from herbivory. In Arabidopsis and other crucifers, the glucosinolate-myrosinase system acts as a chemical defense against herbivore attack. Glucosinolates, a class of thioglucosides, and the enzyme myrosinase (b-thioglucoside glucohydrolase, TGG) are compartmentalized in different plant cells. Tissue disruption, e.g. wounding caused by insect herbivores, allows myrosinase to cleave glucosinolates and results in the release of toxic products such as isothiocyanates, nitriles and thiocyanates. The goal of this work is to understand the role of the Arabidopsis myrosinase enzymes in glucosinolate turnover and insect defense. T-DNA insertion lines with defects in TGG1 and TGG2, the two known functional myrosinase genes in Arabidopsis, are being studied. Complete knockout mutations of the respective TGG genes were verified by RT-PCR. The mutants do not differ morphologically from the wild-type plants. However, at different developmental stages and in various tissues, myrosinase activity, determined spectrophotometrically as degradation of the glucosinolate sinigrin, is approximately 5% of wild-type activity in tgg1 mutants, but is not significantly different from wild type in tgg2 mutants. The mutant lines were investigated to determine whether altered myrosinase activity is involved in defense against the generalist herbivore Myzus persicae (green peach aphid) and the specialist herbivore Brevicoryne brassicae (cabbage aphid). Reproduction of M. persicae was not significantly different on wild type and mutant plants. In contrast, the specialist B. brassicae reproduced two to three times better on the tgg1 mutants than on the wild type and the tgg2 mutant plants.