Greenhouse-grown conditionally lethal tobacco plants obtained by expression of plastidic glutamine synthetase antisense RNA may contribute to biological safety.

Abstract A cDNA corresponding to plastidic glutamine synthetase (GS-2), an enzyme involved in photorespiration, was expressed in antisense orientation under the control of a leaf-specific soybean ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit gene promotor in transgenic tobacco to yield conditionally lethal plants. Three transgenic tobacco lines with decreased (at most 64%) foliar GS-2 activity were obtained. These plants grew normally when maintained in an atmosphere with a CO 2 partial pressure sufficiently high (300 Pa CO 2 ) to suppress photorespiration. However, when photorespiration was initiated by the transfer of the plants to air (35 Pa CO 2 ), ammonium accumulated in the leaves. With time, the transgenic plants exhibited severe chlorotic lesions and, eventually, the plants died. A stable atmosphere containing at least 300 Pa CO 2 can be established easily in the greenhouse but is unlikely to occur in a natural environment. Therefore, the transgenic tobacco plants with decreased leaf GS-2 activity may contribute to biological safety for production of desired proteins.