Handbook of Plant and Crop Physiology

(Steven Felgate) #1

stress or, more commonly, “heat shock” [81]. For many temperate climate crops, temperatures in excess
of 32 to 33°C constitute a heat shock [81]. Many of the protein synthetic responses to elevated tempera-
ture are found throughout the eukaryotic kingdom as well as in prokaryotes.



  1. Decreased Translation


Quantitatively, a decline in overall protein synthesis occurs as a result of the translational repression of
most mRNAs during severe heat shock [82]. Many explanations have been suggested, including general
instability of polysomes at high temperature [83], loosely bound translational factors, changes in the cy-
toskeleton, and inhibition at the initiation or elongation step [82]. Some of the translationally repressed
mRNAs are sequestered during the heat shock and are expressed after the stress is relieved [82].



  1. Heat Shock Proteins/Molecular Chaperones


Qualitatively, several classes of proteins with different molecular masses are rapidly (20 min to 3 hr) and
preferentially translated [82,84]. These proteins have been termed heat shock proteins (HSPs) and
grouped by their molecular masses: low molecular weight (LMW; 15 to 30 kDa), which are seen only in
plants, and the HSP60, HSP70, HSP90, and HSP110 classes. The appearance of these proteins has been
correlated with enhanced thermotolerance as well as some measure of cross protection to other environ-
mental stresses [82,85]. The HSPs are typically seen regardless of whether the temperature rise is rapid
or slow [86] and have been shown to occur under field conditions [86,87]. In plants, 27- and 70-kDa pro-
teins produced in response to elevated temperature also appear in response to variety of environmental
stresses [88,89]. The 70-kDa HSPs are also seen to have seasonal expression patterns in some woody
plant species, being more prevalent in autumn and winter months than in summer months in the species
examined [90]. It has been suggested that these commonly produced proteins constitute a form of gen-
eral-purpose stress tolerance.
The intensive study of HSPs and their constitutively synthesized heat shock cognates (HSCs) has led
to the more general biological description of HSPs as molecular chaperones. Molecular chaperones com-
prise several classes (Ref. 91 and references therein) and act by assisting the self-assembly of nascent
polypeptides into their correctly folded tertiary structures. HSPs are generally considered to be molecu-
lar chaperones, although in some cases the function of a particular HSP class remains in question. HSPs
are thought to act by preventing the aggregation of nonfunctional proteins resulting from heat denatura-
tion [92]. Homologues of the high MW HSPs have been reported in the cytoplasm, mitochondria, and
chloroplasts. The HSP60 class appears to be restricted to mitochondria and chloroplasts, despite its nu-
clear origin. The function of the LMW HSPs is poorly understood even though they show a distribution
similar to that of the higher MW HSPs.
Regulation of HSP synthesis occurs at the level of both transcription and translation. HSP70 provides
a paradigm for such regulation [93,94]. A transcription factor, termed HSF, exists in a latent, monomeric
form prior to heat shock. Upon heat shock, the monomer trimerizes and then binds to the appropriate pro-
moter element (HSE) along with other transcription factors, resulting in transcription of HSP70 mRNA.
Attenuation of transcription is apparently due to HSF being bound by HSP70 and possibly other factors.
Phosphorylation may play a role in both the trimerization and the attenuation processes. In concert with
this, HSP70 mRNA is stabilized at elevated temperatures and is efficiently translated, unlike most other
mRNAs that are translationally repressed. Multiple HSFs have been reported from several plant species,
suggesting functional differences and activities [94]. This is not surprising, considering that HSP70 is
synthesized in response to multiple environmental stresses as well as normal growth and development
[95].



  1. Other Inducible Proteins


Other proteins or mRNAs also increase in abundance during elevated temperature but are not consid-
ered HSPs. They include several glycolytic enzymes [96], protein kinases [97], and ubiquitin
[96,98,99]. Veirling [81] suggested that glycolytic enzymes and protein kinases are involved in
metabolic readjustment. The activation or deactivation of regulatory proteins and enzymes by phos-
phorylation could be especially important. Ubiquitin is involved in protein degradation, and its en-
hanced expression is probably required to remove aberrant proteins resulting from damage to transla-
tional machinery or thermally denatured proteins.


666 ARTLIP AND WISNIEWSKI

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