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

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T.D. Sharkey and S.M. Schrader
1996a; Gounaris et al., 1984; Sundby et al., 1986; Joshi et al., 1995). Changes in the
structure of the thylakoid membrane can be seen in electron micrographs (Armond et
al., 1980; Gounaris et al., 1984) of plants treated for short periods of time at moderately
high temperature.
Finally, a pronounced stimulation of PSI activity by heat stress has been
reported a number of times (Pastenes and Horton, 1996a; Bukhov et al., 1999; Schrader
et al., 2004). The stimulation of PSI activity comes at the expense of the redox status of
the chloroplast (Bukhov et al., 2001; Egorova and Bukhov, 2002; Schrader et al., 2004).
PSI-mediated cyclic electron flow (which would generate the transthylakoid energy
gradient needed to regulate PSII (Demmig-Adams and Adams, III, 1992) could be a
mechanism for limiting PSII activity and so limit the production of AOS (Heber and
Walker, 1992; Heber, 2002).
3.3.4. Cyclic Electron Flow around Photosystem I
Contrary to the detrimental effects of heat on PSII and thylakoid membrane permeability,
it was noted early that heat stimulated PSI activity (Stidham et al., 1982; Monson et
al., 1982), and that this stimulation coincided with the inhibition of PSII activity (Thomas
et al., 1986a). Thomas et al. (1986b) demonstrated in isolated chloroplasts that PSI
stimulation was not due solely to uncoupling of photophosphorylation and that granal
stacking had no influence on heat stimulation of PSI, but that PSI reaction centers
turned over more rapidly under heat. They also suggested that a new electron acceptor
site was exposed on cytochrome b 6
/f, indicating that cyclic electron transport may be
important in the heat stimulation of PSI.
Heating appears to specifically engage cyclic electron flow around PSI (Havaux,
1996; Bukhov et al., 1999, 2000). The dark rereduction of PSI was found to undergo
“spectacular acceleration” with the half-life of P700 + falling from over 500 ms to less
than 50 ms between 34°C and 40°C (Havaux, 1996). Havaux noted that the rise in PSI
activity with heat stress was catalyzed by electron flow from stromal reductants through
plastoquinone, which was further confirmed by several other studies (Bukhov et al.,
1999, 2000, 2001). A flow of electrons from the stroma to the plastoquinone pool in the
dark at 36°C was reported by Yamane et al. (2000). In these studies a specific protein
was hypothesized that might catalyze electron donation to plastoquinone and that
would only be active above 35°C. Yamane et al. (2000) found little effect of antimycin A,
the inhibitor of ferredoxin plastoquinone reductase-type cyclic electron flow. They
also saw only minor inhibition of the high temperature electron flow to plastoquinone
by feeding inhibitors of plastidial NAD(P)H dehydrogenase complex (Ndh1), the protein
which catalyzes the other known cyclic electron transport path. The Ndh-dependent
cyclic electron transport pathway was found to be a high capacity pathway in low
oxygen or when mitochondrial and chlororespiratory metabolism was poisoned (Joët et
al., 2002). Sazanov et al. (1998) found that the plastoquinone pool was still being