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Title: CYTOKININS AS BIOREGULATORS PROMOTE INSECT RESISTANCE IN PLANTS TRANSFORMED WITH THE IPT GENE

Author
item Smigocki, Anna
item HEU, SUNGGI - CHONGUI KOREA
item BUTA, GEORGE - ARS-RETIRED

Submitted to: Plant Growth Regulation Society of America Quarterly
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2002
Publication Date: N/A
Citation: N/A

Interpretive Summary: The possible mechanism of insect resistance associated with the expression of a gene involved in the synthesis of a major plant growth regulating hormone (cytokinin) was investigated. The gene involved in the synthesis of cytokinin was reconstructed for expression in plants upon insect attack or mechanical wounding. Genetically engineered plants were screened for resistance to tobacco hornworms and green peach aphids. All transformed tobacco showed resistance to these insects. Leaf extracts from transgenic plants were chemically analyzed for insect deterrence compounds. The resistance compounds found in the leaf extracts were lethal to the hornworms at very low concentrations (0.05 to l% suspensions). The activity was stable for about 2 days at ambient temperatures below 42C and for more than 1 year when refrigerated. The activity is light sensitive and within 24 hours of continuous exposure to light, it is completely inactivated suggesting along with further analyses that the active compounds could be of the diterpene-type. This fits in well with many examples of natural products that have been shown to have insecticidal properties. We speculate that the production or secretion of these compounds is related to the production of cytokinins in plants. This information will be used by scientists to increase our knowledge of the mechanisms controlling plant responses to insect-induced wounding and lead to insect resistant plants thus reducing the usage of environmentally damaging pesticides.

Technical Abstract: The expression of the bacterial cytokinin biosynthesis gene (PI-II-ipt ) in Nicotiana plumbaginifolia plants has been associated with antifeedent and toxic effects on Manduca sexta and Myzus persicae larvae. To characterize the components responsible for the resistance, leaf extracts prepared from transgenic plants were bioassayed for insecticidal activity. Surface extracts killed l00% of M. sexta second instars at concentrations of 0.05% (w/v). Partial purification by HPLC yielded a fraction that had three major peaks of molecular mass of 354, 374 and 472 daltons as determined by chemical ionization mass spectroscopy (CIMS). This fraction reduced the hatch rate of M. sexta eggs by 30% and killed first, second and third instars within 24, 48 and 72 hours of exposure, respectively. The extract was stable for up to 2 days at ambient temperatures and for longer than 1 year at 4C when stored in the dark. In transgenic N. tabacum plants, similar antifeedent effects were observed but surface extracts were at least 20 times less active than those from transgenic N. plumbaginifolia plants. Partial characterization of the compounds indicates that they are oxygen-containing aliphatic molecules in the molecular weight range of diterpenes.