| The relationships among cruciferous vegetables, diamondback moth (Plutella xylostella), and Cotesia plutella were investigated on the signal communication among the tritrophic levels which includes the allurement of vegetable volatiles to diamondback moth, the allurement of the volatiles from the vegetable and the larvae of diamondback moth to Cotesia plutella, the ultrastructure and the roles of the antennae of diamondback moth and Cotesia plutella in their host searching, and the identification of the vegetable volatiles. In addition, the physiological responses of the vegetable to the feeding of diamondback moth were tested, and the responses were further investigated at the molecular level. 1. The orientation of diamondback moth and Cotesia plutella to the vegetable volatiles was investigated using Y-tube olfactory in the lab. The results demonstrated that the diamondback moth was attracted by the vegetable volatiles, female adults were attracted more than male adults, and there existed difference of the attraction from different vegetable species. Cotesia plutella was attracted most by the vegetable plants infested by the larvae of diamondback moth, secondly by the artificial damaged plant, thirdly by feces of the larvae, and lastly by the larvae. The experiment of the diamondback moth with the antennae excised in Y-tube olfactory verified that the antennae played an important role in search of the host plant by detecting the odors from vegetable. 2. The orientation of Cotesia plutella was tested in a Petri dish in the laboratory. The results indicated that Brassica oleracea L. var. capitata L. and B. Oleracea L. var. botrytis DC. could equally lure Cotesia plutella. Further bioassay was tested using the leaves of Brassica oleracea L. var. capitata L. as experimental material. Leaves infested by larvae of the diamondback moth, enticed Cotesia plutella more than undamaged and mechanically damaged leaves. Females of Cotesia plutella were more capable of locating the leaves infested by the diamondback moth than the males. Extraction of larval feces of the diamondback moth using different solvent showed different allurements to Cotesia plutella. Methanol extraction had the most allurement to the parasitoid, ethanol extraction also had certain allurement to the parasitoid, and the other solvent extractions showed almost no allurement, including hexane, tetrachloride carbon, ether and water. Learning of Cotesia plutella played an important role in search of the host and its habitat. The parasitoid could increase the capacity of orientation after touching leaves damaged by larvae of the diamondback moth in the previous day, and the times of touching leaves had positive effect on the orientation of the parasitoid. The role of the antennae of Cotesia plutella to receive chemical cues was testified by the behavior of Cotesia plutella with the antennae excised. 3. There were 9 types of sense organs in the antennae of diamondback moth observed by electronic scanning microscope, of which the amount of Trichoid Sensillum was the most. There were some differences in the antennae between females and males. Styloconic Sensilla only existed in the antennae of male adult. There were more Squamiform Sensilla in the antennae of males than that in females, and the reverse to Coeloconic Sensilla. 4. There were 10 types of sense organs observed in the antennae of Cotesia plutella by using electronic scanning microscopy, of which Trichoid Sensillum and Placoid Sensillum were the main receptors with a large quantity. Differences existed in the antennae between females and males. There were more sense organs in the antennae of females with the unique sense organs of Campanile Sensillum and Ampullaceous Sensillum. The quantity of sensillum in the females was more than that in the males. 5. The enzymatic activities of superoxide dismutase (SOD), hydrogen peroxidae (CAT), peroxidase (POD), polyphenol oxidase (PPO), and the content of phenol in the leaves of Chinese cabbage, Brassica campestris L. ssp. chinensis (L.) Makino, changed rapidly after the cabbage leaves were infested by the larvae of diamondback moth (DBM), Plutella xylostella. The activities of SOD, CAT, POD and PPO and the phenol content of damaged leaves declined generally within a day. However, the activities of SOD, CAT and POD were increased at 15:00 and PPO activity was increased at 11:00, which might result from the excessive stress. The content of phenol and the activity of PPO were negatively correlated in the normal leaves, but the behavior was not the same as that in the damaged leaves. It was thus evident that the physiological disorder was caused by feeding activity of DBM in the cabbage plants, and the physiological stress response of the Chinese cabbage was systematic with the feeding of DBM. The defensive system of the Chinese cabbage could be destroyed by the infestation of DBM larvae in general, and there might be other defensive mechanisms in the cabbage resistant to the damage of DBM. 6. There were 34 peaks detected by GC-MS in the volatiles from Brassica oleracea var. capitata L., of which alkyl (including silane and siloxane, same in the following) was the most, alkene (including benzene, naphthalene, phenanthrene and their derivations, same in the following) the second, ketone the third, and a small amount of aldehyde, alcohol, ester and oxime. The undamaged vegetable plants released more volatiles than the plants infested by the larvae of diamondback moth, but the ratio of volatile component were similar with a good amount of alkyl. There were 58 peaks detected by GC-MS in the volatiles from Brassica oleracea L. var. acephala DC., of which the most were alkene and ketone, others were in small amount such as aldehyde, alcohol, ester and acid. The vegetables damaged by the larvae of diamondback moth released twice of volatiles and more components in the volatiles than the undamaged plants. Thevolatiles with low molecular increased in species and quantity in the damaged plants. There were 50 peaks detected by GC-MS in the volatiles from Brassica oleracea L. var. botrytis DC., which were alkyl, alkene, ketone, alcohol, acid, ester, aldehyde and heteroaromatic compound. The amount of volatiles from the damaged plants is about 1.7 times of that from normal plants. The volatiles from normal plants were evenly composed, with high content in alkyl and alkene. The volatiles from damaged plants were mostly composed of ethyl-boronic acid. In the other way, the volatiles of low molecular decreased and the volatiles of high molecular increased. There were 123 peaks detected by GC-MS in the volatiles from Brassica campestris L. ssp. chinensis (L.) Makino, of which alkyl was the most, alkene the second, and aldehyde, alcohol, ketone, alkyne, acid, ester, nitrile, cyanide and heteroaromatic compound were only a small amount. The volatiles from the plants damaged by the larvae of diamondback moth were of higher molecular mass and were increased by 3 times in the amount compared to that from normal plants. 7. To investigate the defense responses of oriental cabbage, Brassica campestris L. ssp. chinensis (L.) Makino, against the feeding of diamondback moth larvae, Plutella xylostella, under the molecular level, several defense genes in plants were selected for the study, which are lipoxygenase, allene oxide synthase, hydroperoxide lyase, cysteine proteinase inhibitor, and trypsin proteinase inhibitor. Based on the nucleotide sequences of these genes studied in plant species, primers were designed for oriental cabbage. After total RNA samples were extracted individually from the normal vegetable leaves and damaged leaves, RT-PCRs were carried out using the designed primers. The results demonstrated that primers for lipoxygenase, allene oxide synathase, cysteine proteinase inhibitor, and trypsin proteinase inhibitor had amplified several gene fragments specifically in the total RNA of damaged cabbage leaves, but the primers for hydroperoxide lyase did not. Therefore, we could conclude that these four inducible genes in oriental cabbage might play important roles against the attack of diamondback moth. In addition, the specifically amplified gene fragment was sequenced, and the sequencing result demonstrated that the fragment consisted of 528 nucleotides. |
