Peroxidative Responses of Leaves in Two Soybean Genotypes Injured by Twospotted Spider Mites (Acari:Tetranychidae) D. F. HILDEBRAND,' J. G. RODRIGUEZ,2 G. C. BROWN,2 K. T. LUU,2.3 AND C. S. VOLDEN2 University of Kentucky, Lexington, Kentucky 40546-0091 J. Econ. Entomol. 79: 1459-1465 (1986) ABSTRACT Levels of resistance in soybean genotypes to the twospotted spider mite, Te- tranychus urticae Koch, and cumulative lipid peroxidation of the tissues were strongly related. Both were also correlated with loss of carotenoids and chlorophyll. Activity of the prooxidant enzyme Iipoxygenase (LOX) and of peroxidase increased in parallel with T. urticae population levels and lipid peroxidation. Antioxidant enzymes catalase and super- oxide dismutase did not change with increasing T. urticae population levels and lipid peroxidation. The mite-resistant soybean cultivar showed greater loss of carotenoids and chlorophyll and greater lipid peroxidation with initial T. urticae infestation than the mite- susceptible soybean cultivar. Increased resistance could be induced in remote tissues of the mite-resistant soybean cultivar by brief exposure to T. urticae feeding. Possible direct and indirect roles of lipid oxidation and LOX in pest resistance are discussed. KEY WORDS soybeans, Tetranychus urticae, lipid oxidation, pest resistance, Iipoxygen- ase, peroxidase, catalase THE LOSS OF LEAF chlorophyll and formation of necrotic or senescent tissue is one of the most com- mon events associated with mite damage in plants (Tanigoshi & Davis 1978, Mothes & Seitz 1982). The loss of carotenoids and chlorophyll is the result of oxidation induced by increased peroxidizing conditions (Rustin et al. 1983). Enhanced produc- tion of free radicals is one of many events associ- ated with senescence. Free radicals have been im- plicated in membrane deterioration and loss of important physiological functions in both senes- cing and stressed plant tissues (Leshem et al. 1981, Lynch & Thompson 1984). One of the primary characteristics in the host-parasite interaction is the accumulation of peroxides and free radicals (Hal- liweIl1979). The production of free radicals such as superoxide anion 0- 2 as well as lipid peroxides (LHPOX) during wounding and senescence could be a consequence of lipoxygenase (LOX) (lino- leate: oxygen oxidoreductase, EC 1.13.11.12) ac- tion on polyunsaturated fatty acids released by li- pases (Rustin et al. 1983, Lynch & Thompson 1984) (Fig. 1). Apparently, LOX can have both a direct and indirect effect on promotion of tissue senescence and pest resistance. Free radicals, hydroperoxides, and carbonyl compounds, which can result from LOX action as well as other processes, are known to cause damage to proteins, membrane structures, I Dep. of Agronomy. , Dep. of Entomology. 'Current address: Dep. of Agronomy, Univ. of Tennessee, Knoxville, TN 37919. and cellular organization, and can damage DNA (Brawn & Fridovich 1983). The toxic effects of peroxides and free radicals on viruses, microor- ganisms, and mammalian cells is well established (Brawn & Fridovich 1983). Shukle & Murdock (1983) have recently presented the first evidence that LOX could directly function as plant defenses against insect attack. One possible pest-resistance mechanism is the increased production of peroxy radicals in tissues of pest-resistant plants that render those tissues a less favorable substrate for growth of that pest (Shukle & Murdock 1983). Another possible pest- resistance mechanism might involve formation of peroxy radicals in tissues of resistant and suscep- tible genotypes upon pest-induced wounding; with greater ability to detoxify the radicals and perox- ides, the resistant genotype sustains less tissue damage (Fig. 1). The enzymes involved in the pro- tection of cells against the cytotoxicity of peroxides and free radicals are superoxide dismutase (SOD) (EC 1.15.1.1), catalase (CAT) (EC 1.11.1.6), and peroxidase (POX) (EC 1.11.1.7) (Mavelli et al. 1982). SOD catalyzes the disproportionation of su- peroxide (resulting from the reaction of peroxy radicals with oxygen) to hydrogen peroxide and molecular oxygen (16) (Fig. 1). CAT and POX can catalyze the reduction of toxic intermediates of O 2 metabolism (Faccioli 1979, Dhindsa et al. 1981, Mavelli et al. 1982, Jablonski & Anderson 1984). In soybean leaves, LOX is always present but is separated from its substrate (e.g., linoleic, linole- nic, and arachidonic acids) until wounding or gen- eralized tissue damage, which can bring LOX and 1459 Downloaded from https://academic.oup.com/jee/article/79/6/1459/842372 by guest on 12 June 2022