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

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T.D. Sharkey and S.M. Schrader
when fed exogenous isoprene (Sharkey et al., 2001b). Fosmidomycin inhibits the production
of isoprene without interfering with photosynthesis. However, Logan and
Monson (1999) did not find an enhanced thermal tolerance from exogenous isoprene on
leaf discs. This led to the hypothesis that isoprene protected against heatflecks or
short, high temperature events that leaves frequently encounter (Singsaas and Sharkey,
1998; Sharkey et al., 2001b). The exact mechanism of thermal tolerance due to isoprene
is unknown, but it has been proposed that isoprene embeds in the thylakoid membrane
and increases hydrophobic interactions (Sharkey and Singsaas, 1995; Sharkey and
Yeh, 2001). The increase in hydrophobic interactions may have one or both of two
effects. The first would be to stabilize the thylakoid membrane and decrease ion leakage
or inhibit non-bilayer formation. The second may be to increase lipid protein
interactions and thus stabilize proteins associated with the electron transport chain. If
the increase in hydrophobic interactions in the thylakoid membrane is the reason for
the increased thermal tolerance of photosynthesis, then other mechanisms that increase
these hydrophobic interactions should also increase the thermal tolerance of
photosynthesis, and this has been confirmed. Alkenes similar to isoprene in structure
increased the photosynthetic thermal tolerance although alkanes did not (Sharkey et
al., 2001b).
Plants grown at high temperature have a reduced degree of unsaturation in
their membrane lipids (Pearcy, 1978). Mutant Arabidopsis plants, fad7fad8 (Hugly et
al., 1989; Kunst et al., 1989), and transgenic tobacco plants, with a silenced FAD7 gene,
have increased lipid saturation show increased growth and photosynthetic thermal
tolerance (Murakami et al., 2000). Thermal tolerance was also increased in chloroplasts
and thylakoids when their lipids were chemically hydrogenated (Quinn et al., 1989).
Sugars and other solutes that do not interact with the lipid phase of thylakoid
membranes have also been shown to provide increased thermal tolerance to thylakoid
membranes and PSII (Feldman, 1962; Molotkovsky and Zhestkova, 1965; Molotkovsky,
1968; Santarius, 1973; Williams et al., 1992). Glycinebetaine deficient maize and transgenic
Arabidopsis and rice showed decreased tolerance to heat stress (Yang et al., 1996; Alia
et al., 1998; Kishitani et al., 2000). Glycinebetaine stabilizes PSII against thermal inactivation
possibly by stabilizing the water splitting complex (Gorham, 1995; Allakhverdiev
et al., 1996, 2003; Klimov et al., 2003). Digalactosyl diacylglycerol (Yang et al., 2002a,
2002b) and SQDG have also been shown to enhance thermal tolerance. Plants grown
under enriched CO 2
atmospheres have shown a greater thermal tolerance in PSII chlorophyll
fluorescence (Faria et al., 1996; Huxman et al., 1998; Taub et al., 2000; Hamerlynck
et al., 2000). Bicarbonate has also been shown to interact with PSII and enhance thermal
tolerance (Allakhverdiev et al., 1997; Klimov et al., 1997a, 1997b; Hulsebosch et al.,
1998; Yruela et al., 1998; van Rensen et al., 1999; Klimov and Baranov, 2001; Klimov et
al., 2003).