Trends in Plant Science
Abiotic stress seriesAntifreeze proteins in overwintering plants: a tale of two activities
Section snippets
Interaction between antifreeze proteins and ice
Unlike most solutes that are simply pushed ahead of the ice face during freezing, AFPs bind irreversibly to the surface of ice and are incorporated into the ice crystal lattice [5]. Although it seems counterintuitive, the ice-binding domains of fish and insect AFPs are flat and relatively hydrophobic and their adsorption onto ice is a hydrophobic interaction driven by the increase in entropy gained by releasing hydration water from the ice and protein surfaces. Binding to ice is stabilized by a
Antifreeze activity in plants
Antifreeze activity was first reported in plants in 1992 12, 13 and has since been found in many more overwintering vascular plants, including ferns, gymnosperms, and monocotyledonous and dicotyledonous angiosperms (see Supplementary material) 11, 13, 18, 19. Antifreeze activity is present in overwintering plants only after they have been exposed to low temperatures and only in plants that tolerate the presence of ice in their tissues. Antifreeze activity has been observed in different parts of
Plant antifreeze proteins are also pathogenesis-related proteins
AFPs have been isolated from six plants and full-length nucleotide sequences are available for genes encoding five AFPs (Table 1). The surprising result of sequencing plant AFPs, or their corresponding genes, is that most of them are homologous to pathogenesis-related (PR) proteins [20]. Normally, PR proteins are released into the apoplast in response to pathogen infection and act together to degrade fungal cell walls enzymatically and inhibit fungal enzymes. Secreted PR proteins with
Regulation of antifreeze proteins
To date, no plant has been reported to have constitutive antifreeze activity; rather all studies have shown that transcripts and translation products of AFP genes accumulate during cold acclimation 12, 13, 20, 21, 28. The conditions used for cold acclimation mimic autumn when days become shorter and colder. Therefore, low temperature and daylength are important environmental cues for AFP production. For example, winter rye plants grown at low temperatures accumulate more apoplastic protein
Role of antifreeze proteins in freezing in planta
The lower limit of freezing tolerance of a plant population is measured as LT50, the lethal temperature for 50% of the individuals. In single plants, LT50 is often determined as the loss of 50% of the electrolytes from plant tissues after freezing. As plants acclimate to low temperatures in autumn, they acquire freezing tolerance and the LT50 becomes progressively lower. In breeding programs, plants are often selected for increased freezing tolerance based on changes in LT50; however, any
Plant transformation with genes encoding antifreeze proteins
Agricultural production in many areas is limited by freezing temperatures. Higher yields could be achieved either by improving the freezing tolerance of an overwintering crop or by increasing the survival of freezing-sensitive crop plants following light frosts. Moreover, AFPs could increase the shelf life and improve the quality of frozen foods by inhibiting the recrystallization of ice if the AFPs are targeted to accumulate in fruits and vegetables before harvest [41]. Therefore, it is not
Conclusions
Plant AFPs are unusual proteins: they have multiple, hydrophilic ice-binding domains that appear to function as inhibitors of ice recrystallization and ice nucleation. AFPs have little effect on LT50 but could enhance winter survival by slowing freezing processes. Moreover, most AFPs from plants are modified PR proteins that retain high sequence identify and even the antifungal activities of the progenitor PR proteins. Therefore, genes encoding plant AFPs are excellent models for use in
Acknowledgements
We thank Maja Stressmann and André Peters, University of Waterloo, and Alejandra M. Regand and Alejandro G. Marangoni, University of Guelph, for assistance with Figure 2. Our work is supported by the Natural Science and Engineering Research Council of Canada. M.G. is a Killam Research Fellow.
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