Expression of a Bacterial Ice Nucleation Gene in Plants

We have introduced an ice nucleation gene (inaZ) from Pseudomonas syringae pv. syringae into Nicotiana tabacum, a freezing-sensitive species, and Solanum commersonii, a freezing-tolerant species. Transformants of both species showed increased ice nucleation activity over untransformed controls. The concentration of ice nuclei detected at -10.5 degrees C in 15 different primary transformants of S. commersonii varied by over 1000-fold, and the most active transformant contained over 100 ice nuclei/mg of tissue. The temperature of the warmest freezing event in plant samples of small mass was increased from approximately -12 degrees C in the untransformed controls to -4 degrees C in inaZ-expressing transformants. The threshold nucleation temperature of samples from transformed plants did not increase appreciably with the mass of the sample. The most abundant protein detected in transgenic plants using immunological probes specific to the inaZ protein exhibited a higher mobility on sodium dodecyl sulfate polyacrylamide gels than the inaZ protein from bacterial sources. However, some protein with a similar mobility to the inaZ protein could be detected. Although the warmest ice nucleation temperature detected in transgenic plants is lower than that conferred by this gene in P. syringae (-2 degrees C), our results demonstrate that the ice nucleation gene of P. syringae can be expressed in plant cells to produce functional ice nuclei.