Given the previously mentioned low specific activity of known
plant AFPs, perhaps better described as ice binding proteins (IBPs),
it seems unlikely that expression of these proteins in frost-sensitive
crop plants would result in increased freeze avoidance, and there-
fore frost protection. Also, because freeze tolerance is a complex
process involving a host of biochemical changes and adaptations,
it is improbable that expression of IBPs in frost-sensitive plants
would render them freeze tolerant. It is possible, however, that
plant IBPs could exhibit ice nucleation inhibition activity. If so,
this could be of value in situations where inhibition of intrinsic
ice nucleators is critically important. Most plant IBPs appear to be
present in the apoplast, where ice forms in freeze tolerant plants,
and consequently these IBPs apparently do not inhibit ice nucleation. Wisniewski et al. (1999), however, reported a dehydrin with
thermal hysteresis activity, in the cytoplasm of acclimated peach
bark and xylem cells, including xylem ray parenchyma cells, which
are known for their ability to supercool to very low temperatures.
A leucine rich carrot AFP (IBP) gene, structurally similar to the
polygalacturonase inhibitor protein (Worral et al., 1998) was used
to produce transformed Arabidopsis that accumulated the IBP at
low temperatures (Meyer et al. (1999). AFP activity was defined as
the ability of extracts from transformed plants to affect ice crystal
shape. These transgenic plants would not be expected to exhibit
a lower freezing temperature (i.e., increased freezing avoidance)
since the specific thermal hysteresis activity of plant IBPs (including
the one from carrot) are low. The plants were not tested for cold
tolerance.