Geoscience Reference
In-Depth Information
cytoplasm into the apoplast. Antifreeze proteins (AFPs) have
been reported in ferns, gymnosperms, as well as mono- and
dicotyledonous angiosperms. AFPs inhibit ice crystal growth
by binding to the surface of a growing ice crystal via hydro-
gen bonding between specific hydrophilic amino acids pres-
ent in the AFPs and water within the crystal lattice of ice. The
presence of AFPs in cold-tolerant plants is not constitutive but
requires exposure to low temperature and they accumulate in
virtually all plant tissue, including seeds, stems, leaves, flowers
and roots.
The CBFs are the most extensively studied among the stress-
related transcription factors because of their critical role in the
regulation of low-temperature stress response in
Arabidopsis
and other plant species (reviewed by Thomashow et al. 2001).
CBF transcriptional activators, namely Cbf1, Cbf2 and Cbf3
(DREB1a, DREB1b, DREB1c), bind to CRT/DRE elements
found in the regulatory regions of many cold inducible genes
and induce their transcription activating the plant response to
low temperature. The
Arabidopsis
Cbf genes are organised in
a tandem arrangement localised on chromosome 4 and their
amino acid sequences share a common AP2/EREBPDNA-
binding domain (Medina et al. 1999). The expression of the
Cbf-like transcripts is transiently up-regulated by cold after
15 min of low-temperature exposure (Medina et al. 1999). A
sudden cold stress, transferring the plants directly from 20°C
to 4°C, leads to a fast accumulation of Cbf transcripts with a
maximum after 3 h of stress. Then a drop of the mRNA steady-
state level can be detected, and after 9-21 h of cold stress only
a very low amount of Cbf mRNAs can be found. The same
expression profile can be recorded after a gradual temperature
decrease, suggesting that cold shock is not required to induce
Cbf expression; rather, an absolute temperature is being sensed.
The threshold temperature, promoting transcript accumulation,
is approximately 14°C (Zarka et al. 2003).
Acclimation
of the
photosynthetic
apparatus due
to high
temperatures
Plants that can acclimate to high temperatures are called ther-
motolerant plants. Photosynthetic capacity measured as the
maximum light saturated rate of CO
2
assimilation is sensitive
to temperature. In most cases the productivity of a plant is
directly related to the rate of photosynthesis. Photosynthesis
is, like all other physiological processes, temperature depen-
dent. For a specific plant there exists an optimal temperature
at which the net rate of carbon dioxide fixation is maximal.
It was found that among all cell functions, the photosyn-
thetic activity of chloroplasts is one of the most heat sensitive
