Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

106
T.D. Sharkey and S.M. Schrader
when Arabidopsis plants were modified to lack essentially all capability to make triply
unsaturated thylakoid lipids (Murakami et al., 2000). Thus, there is clear evidence that
thylakoid membranes can affect plant tolerance to high temperature, and that this can
affect overall plant thermotolerance, but the effect may not be universal. The importance
of membrane fluidity in high temperature stress remains an unsettled question.
2.2. Proteins and Heat Stress
Because of the temperature sensitivity of the forces responsible for protein folding
(Pace et al., 1996), proteins are easily denatured by high temperature. Biological organisms
have a suite of proteins that are made in response to high temperature that appear
to be designed to prevent or reverse the effects of heat on protein denaturation. These
are called heat shock proteins because they are made in abundance when organisms are
subjected to potentially damaging high temperature. Many of these proteins are also
made in response to other stresses, notably osmotic and oxidative stress. It is believed
that these proteins help other proteins fold correctly or refold after damage by heat
stress (Boston et al., 1996) although recent data has established unique roles for each
of the families of heat shock proteins.
Heat shock proteins were discovered as proteins that were not expressed or
not very prominent in assays of plants grown in moderate temperature but which became
prominent when organisms were subjected to a heat shock (Lindquist, 1986). The
hsps are divided into families based on their molecular weight. For example, most
organisms have heat-inducible genes with a molecular weight between 100 and 104;
these are called Hsp100. The different hsp families appear to have different functions.
Some are constitutive, expressed at a somewhat higher level in response to heat stress,
while others are nearly completely inducible, with no protein detectable in unstressed
plants and lots of protein in stressed plants. The following is a quick tour of some of the
important aspects of each class of heat shock proteins.
Hsp 100 - This class of hsp has been shown to be directly related to thermotolerance
(Queitsch et al., 2000). Plants lacking Hsp101 can grow at normal temperatures and
reproduce, but unlike wild type plants, a pretreatment at moderately high temperature
does not induce thermotolerance. Hsp101 is produced by Arabidopsis seedlings
and confers tolerance to heat stress (Hong and Vierling, 2000; Queitsch et al.,
2000) but there is now evidence for an additional three genes that are not hsps that
contribute to inducible thermotolerance as measured in hypocotyl elongation assays
(Hong et al., 2003). Expression of heat shock proteins and heat shock factors
(proteinaceous factors that stimulate expression of heat shock proteins as well as
other heat tolerance genes) results in increased thermotolerance in many plants
(Sun et al., 2002). Hsp101 is highly inducible and plants that have not experienced
high temperature have only very low level, if any, Hsp101. The Hsp100 family has
sequence similarities to caseinolytic protease B (ClpB) and is often referred to as
Hsp100/clpB (Agarwal et al., 2002). These proteins have nucleotide binding mo-