The Role of Sugar Accumulation in Leaf Frost Hardiness — Investigations with Transgenic Tobacco Expressing a Bacterial Pyrophosphatase or a Yeast Invertase Gene

Abstract

Summary In order to assess the contribution of increased leaf osmolality to plant frost hardiness, we have used transgenic tobacco ( Nicotiana tabacum ) plants that accumulate soluble carbohydrates. The leaves from plants of the clone U-ppa-1-10 expressing a bacterial pyrophosphatase gene displayed an increase in frost hardiness of 1.2 °C when compared with wild type control plants. Most strikingly, these plants showed a higher capacity to increase their hardiness during exposure to 4 °C growth temperature for 10 to 14 days: frost hardiness increased by 1.1 °C in transgenic plants as compared with 0.2 °C in wild type controls. Of the other three independent clones transformed with the pyrophosphatase gene, none showed a statistically significant increase in hardiness compared with wild type plants, or increased hardiness after cold acclimation. There was no correlation between leaf osmolality and hardiness when leaves from cold acclimated and from non-acclimated wild type and all clones of transformed tobacco were compared. Tobacco plants expressing an apoplastic yeast invertase gene were more susceptible to freeze-thaw stress than wild type controls, in spite of increased leaf osmolality due to sugar accumulation in the leaf cells. Cold acclimation of such plants resulted in increased frost hardiness, which, however, did not exceed the hardiness of untransformed controls. When the expressed invertase gene contained a signal sequence for targeting the protein to the vacuole only moderate increases in leaf osmolality were obtained. None of the three independent clones that we investigated showed improved frost hardiness compared with the wild type.