Geoscience Reference
In-Depth Information
thermostable conformations of the membrane-connected thyla-
koid protein subunits and the adjustment of membrane lipid flu-
id it y.
In vivo
interaction of xanthophyll-cycle pigments with the
membrane lipid matrix is supported by a series of experimental
facts (reviewed by Sarry et al. 1994). It has been reported that
zeaxanthin synthesis modulates the fluidity (Gruszecki and
Strzalka 1991) and the lipid peroxidation status (Sarry et al.
1994) of thylakoid membranes.
High temperature negatively affects both metabolic (Mahan
and Mauget 2005) and reproductive (Snider et al. 2010) efficien-
cies. Heat stress effects are notable at various levels, including
plasma membrane and biochemical pathways operative in the
cytosol or cytoplasmic organelles (Sung et al. 2003). Initial
effects of heat stress, however, are on plasma lemma, which
shows more fluidity of lipid bilayer under heat stress. This leads
to the induction of Ca
2+
influx and cytoskeletal reorganisa-
tion, resulting in the upregulation of mitogen-activated protein
kinases and calcium-dependent protein kinase. Signalling of
these cascades at the nuclear level leads to the production of
antioxidants and compatible osmolytes for cell water balance
and osmotic adjustment. Production of ROS in the organelles
(e.g. chloroplast and mitochondria) is of great significance for
signalling as well as production of antioxidants. The antioxi-
dant defence mechanism is a part of heat-stress adaptation, and
its strength is correlated with acquisition of thermotolerance
(Figure 14.4). Accordingly, in a set of wheat genotypes the
capacity to acquire thermotolerance was correlated with activi-
ties of catalase (CAT) and SOD (Superoxide dis mutase), higher
ascorbic acid content and less oxidative damage. One of the most
closely studied mechanisms of thermotolerance is the induction
of HSPs. Each major HSP family has a unique mechanism of
action with chaperonic activity. The protective effects of HSPs
can be attributed to the network of the chaperone machinery,
in which many chaperones act in concert. An increasing num-
ber of studies suggest that the HSPs/chaperones interact with
other stress-response mechanisms. The HSPs/chaperones can
play a role in stress signal transduction and gene activation as
well as in regulating cellular redox state. They also interact
with other stress-response mechanisms such as production of
osmolytes and antioxidants. HSPs are generally classified into
five evolutionarily conserved groups: HSP100, HSP90, HSP70,
HSP60 and small HSPs. Most, but not all, HSPs are molecu-
lar chaperones, which bind and stabilise proteins at intermedi-
ate stages of folding, assembly, degradation and translocation
across membranes. Membrane lipid saturation is considered
