LOW TEMPERATURE–INDUCIBLE PROTEINS. The low temperature–inducible protein litera-
ture is replete with various acronyms depending on the reporting laboratory’s designation. For example,
inArabidopsis, several gene families have been described, the members of which are variously referred
to as cold-responsive (COR), cold-inducible (KIN), responsive to desiccation (RD), low temperature in-
ducible (LTI), early-dehydration inducible (ERD), and cold acclimation protein (CAP).
Low temperature–inducible proteins and mRNAs fall into several categories, including various sig-
naling molecules or transcription factors (described in the following), metabolic enzymes, heat shock pro-
teins (see earlier), and many hydrophobic or hydrophilic gene products. The proteins in the latter cate-
gories generally resemble or belong to the LEA or dehydrin protein classes (see earlier). Interestingly,
several of these proteins show sequence homology and activities resembling those of the antifreeze (ther-
mal hysteresis) proteins of certain cold-water fish [149–152] (see also Ref. 153). Many of these proteins
have shown the ability to inhibit ice propagation or recrystallization either in vitro or in vivo.
The metabolic enzymes constitute a small portion of the inducible proteins, and those characterized
are primarily associated with glycolytic or fermentative pathways. The fermentative enzymes could serve
to compensate for a reduction in oxidative ATP production from mitochondrial membrane disruption.
Glycolytic enzymes could function by providing sugars to stabilize membranes or as compatible solutes
(see preceding sections). Synthesis of compatible solutes in the form of quaternary amines is also possi-
ble. Kishitani et al. [154] reported the accumulation of glycine betaine in barley plants during low-tem-
perature acclimation and freezing tolerance. It is likely that many of the glycine betaine biosynthetic en-
zymes are induced, as Kishitani et al. [154] also noted the induction of betaine dehydrogenase (BADH).
Numerous studies [155] have demonstrated that alterations in membrane lipid composition occur
during cold acclimation. Several cold-inducible genes or gene products involved in this process have been
characterized. An -3 desaturase, whose activity could increase membrane fluidity, has been described
[156]. In addition, at least one nonspecific lipid transfer protein from barley is low temperature inducible
[116,157]. As noted previously, LTPs could also act to increase membrane fluidity.
- Low Water Status Signal Transduction
The emerging view of signal transduction is one of a network of pathways with considerable cross talk.
Figure 2 has been compiled to provide context for the reader in understanding how the various stresses
sharing diminished water status interact. The initial perception step(s) for these stresses is still a matter of
conjecture. Some researchers have posited mechanosensors or stretch-activated channels, followed by
Ca^2 fluxes coupled with various cation transporters and cation ATPases [158].
Regardless of the nature of initial perception, abscisic acid (ABA) has been shown to be a key inter-
mediary in the expression of many, but not all, genes induced by decreased water status. Shinozaki and
Yamaguchi-Shinozaki [159] proposed a model consisting of two ABA-dependent and two ABA-inde-
pendent signaling pathways (Figure 2). Pathway I results in the synthesis of a variety of transcription fac-
tors, which in turn bind to appropriate promotor elements, leading to gene expression. Pathway II appears
to be specific for a family of bZIP transcription factors that bind to abscisic acid response elements
(ABREs) in the promotors of specific genes. Wu et al. [160] uncovered portions of the ABA signaling
pathway, particularly the use of cADPR (cyclic ADP-ribose), two specific phosphatases, possibly IP 3 (in-
ositol 1,4,5-triphosphate), as well as Ca^2 fluxes. The Ca^2 fluxes lead to activation of various protein
kinases and phosphatases, which directly or indirectly result in the induction or activation of the pathway
I and II transcription factors. It should be noted that low temperature alone also stimulates ABA but con-
siderably less than water deficit or salinity stress [161]. This may account for the relatively weak induc-
tion of some otherwise highly ABA-inducible proteins by low temperature [157].
The ABA-independent pathway IV is primarily a function of low temperature [112,157,162]. Mu-
rata and Los [163] postulated that the initial perception may be via a change in membrane fluidity, possi-
bly coupled with a change in the conformation of a membrane-bound protein. The low-temperature sig-
nal is then probably transduced via Ca^2 fluxes, specific protein kinases, possibly an MAP kinase
cascade, and inactivation of specific phosphatases [153]. Gilmour et al. [164] proposed a model in which
these early events activate a protein termed ICE (inducer of CBF expression), where ICE and CBF are
proposed to be transcription factors. CBFs 1, 2, and 3 have been characterized and are also known as
DREB 1B, 1C, and 1A, respectively. These CBFs/DREBs bind to a promotor element described as the
CRT/DRE (C-repeat/ drought responsive element), hence CBF stands for CRT/DRE binding factor, and
DREB is an acronym for DRE binding [164]. Many low temperature–inducible genes must contain the
670 ARTLIP AND WISNIEWSKI