Extrinsic Ice Nucleation in Plants

Abstract

In the early 1980's a considerable amount of research focused on the role of extrinsic ice nucleation and its= role in inducing plants to freeze at warm sub-zero temperatures. The working hypothesis was that by controlling extrinsic nucleation events, plants could supercool well below 0 °C and thus avoid freezing (Lindow, 1995). It was felt that such a strategy could provide a significant level of frost protection to frost sensitive plants or plant parts. While the majority of reports dealt with the role of ice-nucleating-active (INA) bacteria (e.g. Pseudomonas syringae), related research focused on the role of other extrinsic nucleating agents and whether or not plants could actually supercool to temperatures several degrees below 0 °C due to the presence of intrinsic nucleating agents which induced the plants to freeze at warm temperatures (Ashworth and Kieft, 1995). The identification of a wide range of both extrinsic and intrinsic ice nucleating agents made the practical application of blocking extrinsic ice nucleation complex. Since that time, research emphasis has switched to identifying genes that impart cold tolerance and the transcriptional activators that regulate cold hardiness genes (Thomashow, 1998; Jaglo, et al., 2001). The hypothesis here is that by the overexpression of these types of genes, a non-acclimated or freeze-sensitive plant could be made freezing tolerant.While great progress has been made in understanding the genetic basis of cold hardiness, manipulation of this trait by molecular biology has also demonstrated itself to be complicated due to the “additional” effects of the overexpression of several cold hardiness genes on the physiology and development of the target plant. Therefore, blocking extrinsic ice nucleation, although complicated, may still be a valuable approach to providing protection to frost sensitive plants.