Resistance Of Tea Plant To Insect Pests Induced By Exogenous Methyl Jasmonate And Its Mechanism

Using the tritrophic relationships of the tea plant Camellia sinensis, the geometrid Ectropis obliqua and the parasitoid Apanteles sp. as a model system, we studied the application of exogenous methyl jasmonate (MJA) to the plant on the lipoxygenase (LOX), polyphenol oxidase (PPO) and proteinase inhibitor (PI) activity in the leaves of tea plants, the growth and midgut proteinase activity of geometrid larvae, parasitism of the caterpillars by the wasp, releases of volatiles from the tea shoots, and host-selection behavior of the geometrid and the parasitoid. Our objectives were to investigate the exogenous MJA-inducible direct and indirect resistance of tea plants to insects as well as the physiological and biochemical mechanisms of the induced resistance. (1) Effect of exogenous MJA treatment of tea plants on the growth of geometrid larvaeEffect of exogenous MJA treatment of tea plants on the body weight of and leaf area consumed by the geometrid larvae were studied. When 1st instar geometrid larvae fed for five days on leaves of tea plants treated by either MJA-spraying or MJA-exposure at a dosage of 200 μl/plant, body weight and leaf area consumption were decreased by 21.59%, 21.69% and 17.21%, 20.05% respectively, than those of the larvae feeding on control tea plants. When 1st instar geometrid larvae fed for four days on leaves of tea plants treated by either MJA-spraying or MJA-exposure at a dosage of 50, 100, 200 or 400 ul/plant, increase in body weight of geometrid larvae in each of the four dosages was lower than that in the control, but no deaths of larvae were observed. Moreover, the difference in body weight of larvae between the treatments and control increased as the dosage of MJA increased, but there was no significant difference between the treatments of 200 and 400 ul/plant. These results indicate that when geometrid larvae fed on leaves of tea plants after MJA treatment, their growth was reduced, and the level of reduction increased as the concentrations of MJA increased up to a plateau, where no further decrease could be observed.(2) Effect of exogenous MJA treatment of tea plants on the parasitism of Apanteles sp.Effect of exogenous MJA treatment of tea plants on the percentage of the geometrid larvae parasitized by Apanteles sp. was studied under field condition. The results showed that, when the tea plants were treated with MJA for two days, the percentage of geometrid caterpillars parasitized under the canopy of the tea plant in the two treatments, i.e., MJA-spraying and MJA-exposure, was increased by 13.97% and 13.38% respectively in 2002, and by 11.21% and 10.28% respectively in 2003, than those of the control tea plants. The differences between MJA-treated tea plants and controls were highly significant; however, the differences between the two methods of MJA treatments were not significant. Volatiles of tea plants following exogenous MJA treatments exhibited a significant increase of attractiveness to the female adults of Apanteles sp.(3) Effect of exogenous MJA treatment of tea plants on enzyme activity in the tea leavesEffect of exogenous MJA treatment of tea plants on LOX, PPO and PI activity in the tea leaves was studied. MJA significantly induced LOX, PPO and PI activity in the leaves of tea plants. Two, five and seven days after MJA treatment of the tea plants, either by MJA-spraying and MJA-exposure, LOX activities in the leaves of tea plants were increased by 32.5-63.5% and 44.3-61.7% respectively, than those of the control tea plants. LOX activities at 2 days after MJA-spraying treatment were 11.8% lower than mat after MJA-exposure treatment; but the differences of LOX activities at 5 and 7 days between MJA-spraying and MJA-exposure treatments were not significant. Two, five and seven days after MJA-treatment of tea plants, either by MJA-spraying or MJA-exposure, PPO activities in the leaves were increased by 20.28-23.39% and 18.00-18.59% respectively, than those of the control tea plants; but the differences between the two treatment methods were not significant. Two, five and seven days after MJA-treatment of tea plants, either by MJA-spraying or MJA-exposure, PI activities in the leaves were increased by 19.13-24.78 % and 37.64-42.38 % respectively, than those of the control tea plants; and PI activities after MJA-spraying treatment were higher than those after MJA-exposure treatment(4) Effect of exogenous MJA treatment of tea plants on midgut proteinase activity of thegeometrid larvaeEffect of exogenous MJAtreatment of tea plants on midgut proteinase activity of the geometrid larvae was studied. When geometrid larvae fed on leaves of tea plants at 5 days aftereither MJA-spraying or MJA-exposure treatment, the activities of the high alkaline trypsin-like enzyme and chymotrypsin-like enzyme in their midgut were 15.6 %, 24.1 % and 40.3%, 44.6% lower, than those of the control tea plants respectively. The differences of two enzymes activities between MJA-spraying and MJA-exposure treatments were not significant. However, the activities of the low alkaline trypsin-like enzyme in their midgut were unaffected, leading to imbalance between different types of enzyme activity in the midgut of the larvae. Consequently, the metabolism velocity for proteins in the leaves of tea plants fed by the larvae was reduced and in turn, larval growth was retarded. (5) Collection and identification of volatiles emitted from tea shootsThe volatiles emitted from intact tea shoots, larvae-infested tea shoots, tea shoots treated by MJA-exposure and tea shoots treated by MJA-spraying were collected by Tenax TA and identified by GC-MS. Results showed the followings:The volatiles emitted from intact tea shoots were composed of 21 compounds. The relative abundance of the compounds from higher to lower was as follows: The following six compounds showed the highest relative abundance (relative abundance >1), namely Acetic acid, 2-ethylhexyl ester, 2-ethyl-l-Hexanol, (Z)-3-Hexen-l-ol,acetate, Phenol, Butanioc acid, pentyl ester, Decanoic acid,ethy ester. The following six compounds showed lower relative abundance C 1> relative abundance >0.1), namely Benzoic acid,2-methypropyl ester, Nonanal, formate, (Z)-3-Hexen-l-ol, Naphthalene, P -Ionone, Acetophenone. And the following nine compounds showed only trace relative abundance (relative abundance ^0.1), namely (E)-3,7-dimethyl-l,3,6 -Octatriene, Styrene,(E)-2-Hexenal, (+/-)-3,7-dimethyl-l,6-Octadien-3-ol, (E)-3,7-dimethyl-2, 6-Octadien-l-ol, Formic acid,ethyl ester, 3-Penten-2-ol, 2-ethylhexyl, 2-ethylhexanoate, 1,4-Methanoazulene,decahydro-4,8,8-trimethyl.The volatiles emitted from tea shoots treated by MJA-exposure were composed of 24 compounds. Among these, nine compounds showed higher relative abundance, ten compounds showed lower relative abundance and five compounds showed only trace relative abundance. The following three compounds, which were lacking the intact tea shoots, were detected in tea shoots treated by MJA-exposure: Benzaldeyde, 2-methyl-3-Heptanone, 5-methyl-3-Hepten-2-one, Styrene, which was lacked in tea shoots treated by MJA-spraying, and 2-Methyl-3-Heptanone, which was lacked in larvae infested tea shoots, were also detectedin tea shoots treated by MJA-exposure. The following ten compounds showed higher relative abundance in tea shoots treated by MJA-exposure tea shoots than in intact tea shoots: (E)-3,7-dimethyl-l,3,6-Octatriene, (Z>3-Hexen-l-ol,acetate, (Z)-3-Hexen-l-ol, formate, 2-ethyl -1-Hexanol, Nonanal, 3-Penten-2-ol, Benzoic acid, 2-methypropyl ester, 1,4-Methanoazulene, decahydro-4,8,8-trimethyl, 2-ethylhexyl,2-ethylhexanoate, Naphthalene. The relative abundance of Acetic acid, 2-ethylhexyl ester was lower in tea shoots treated by MJA-exposure than in intact tea shoots. The total relative abundance in tea shoots trated by MJA-exposure was higher than that in intact tea shoots.The volatiles emitted from tea shoots treated by MJA-spraying were composed of 23 compounds. Among these, there were eight compounds with higher relative abundance, eight compounds with lower relative abundance and seven compounds with only trace relative abundance. The following three compounds, which were lacking in intact tea shoots, were detected in tea shoots treated by MJA-spraying: Benzaldeyde, 2-methyl-3-Heptanone, 5-methyl-3-Hepten-2-one, Styrene was lacking in tea shoots treated by MJA-spraying, and 2-Methyl-3-Heptanone was lacking in larvae infested tea shoots. The following ten compounds showed high relative abundance in tea shoots treated by MJA-spraying than in intact tea shoots: (E)-3,7-dimethyl-l,3,6-Octatriene, (Z)-3-Hexen-l-ol,acetate, (Z>3-Hexen-l-ol, formate, 2-ethyl-1-Hexanol, Nonanal, 3-Penten-2-ol, Acetic acid, 2-ethylhexyl ester, 1,4-Methanoazulene, decahydro-4,8,8-trimethyl, 2-ethylhexyl,2-ethylhexanoate, Naphthalene. The relative abundance of benzoic acid, 2-methypropyl ester was lower in tea shoots treated by MJA-spraying than in intact tea shoots. The total relative abundance of volatiles emitted from tea shoots treated by MJA-spraying was higher than that from intact tea shoots.The volatiles emitted from larvae infested tea shoots were composed of 23 compounds. Among them, there were seven compounds with higher relative abundance, ten compounds with lower relative abundance and six compounds with only trace relative abundance. The following two compounds, which were lacking in intact tea shoots, were detected from tea shoots treated by MJA-spraying: Benzaldeyde, 5-methyl-3-Hepten-2-one The following nine compounds showed higher relative abundance in larvae infested tea shoots than in intact tea shoots: (E)-3,7-dimethyl-l,3,6-Octatriene, (Z)-3-Hexen-l-ol,acetate, (Z>3-Hexen-l-ol, formate, 2-ethyl-1-Hexanol, 3-Penten-2-ol, l,4-Methanoazulene,decahydro-4,8,8-trimethyl, Nonanal, 2-...