Agriculture Reference
In-Depth Information
to high temperature stress. Auxin and gibberellic acid content are reduced
in plants in the glasshouse under high temperatures with an increase in
proline content (Muthuvelet al.
,
1999). Auxin production is reduced with a
simultaneous high level of abscissic acid in the plant at high temperatures,
whichfavors premature senescence and abscission of reproductive organs
(El-Ahmadi and Stevens, 1979a; Iwahori, 1967; Levy et al.
,
1978). High
temperature stress also suppresses ethylene production (Inaba et al., 1996;
Johjima, 1995; Lurie et al.
,
1996) causing impaired ripening of fruits.
10.4.5 MOLECULAR REACTIONS
10.4.5.1
HEAT SHOCK PROTEINS
Production and accumulation of special proteins occur when heat stress
is rapid and these proteins are identified as heat shock proteins (HSPs).
Enhanced production of HSPs is found to incur when crops experience
either sudden or slow rise in temperature (Nakamoto and Hiyama, 1999;
Schoffl et al., 1999). HSPs expression is limited to certain steps of growth,
including germination, embryogenesis, growth of pollen and maturation
of fruit (Prasinos et al., 2005). Three sorts of proteins, as detected by mo-
lecular weight, account for most HSPs, viz., HSP90, HSP70 and less mo-
lecular weight proteins of 15-30 kDa. The ratios of these different types
of protein vary between crop species (Feussner et al., 1997). The gene for
a nuclear-encoded HSP, Hsa32, that encode a 32 kDa protein, has been
cloned in tomato (Liu et al., 2006). In tomato crops suffering from heat
stress, HSPs gather into a granular structure in the cytoplasm, probably
preserving the protein bio production machinery (Miroshnichenko et al.,
2005). Presence of HSPs can hinder denaturation of other proteins that
can be influenced by high temperature. The dynamic nature and aggregate
state of small HSPs may be vital for their roles in protection of crop cells
from harmful influences of heat stress (Schöffl et al., 1999; Iba, 2002).
The specific significance of small HSPs in crops is proposed by their ab-
normal diversity and abundance. The capability of small HSPs to gather
into heat shock granules (HSGs) and their decomposition is a prerequisite
for survival of crop cells under constant stress environments at sub-lethal
temperatures (Miroshnichenko et al., 2005). Low molecular weight HSPs
may have structural functions in stability of cell membrane. The presence
