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

High Temperature Stress
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reduced after heat stress in the light in ndh mutants, suggesting alternate electron
routes. On the other hand, Bukhov et al. (2001) found that the heat-stimulated cyclic
electron transport was sensitive to rotenone and so concluded that NADPH is the
primary stromal reductant involved. Thus, increased cyclic electron transport may lead
to a decline in the NADPH levels inside the stroma as more electrons are diverted back
into the electron transport chain.
Several authors have found a decline in NADP-MDH activity, which is dependent
on the stromal redox status and the NADPH/NADP + ratio, in heat treated bean and
cotton leaves as low as 35°C (Pastenes and Horton, 1996b; Schrader et al., 2004). However,
Weis (1981b) found no decline in NADP-MDH activity at 38°C in isolated chloroplasts,
and Sharkey et al. (2001a) found no decline in NADP-MDH activity in WT
tobacco leaves and an increase in activity in rubisco-activase-deficient transgenic tobacco
leaves at 40°C.
During a 39°C heat pulse, we demonstrated a stimulation of PSI with little
concurrent effect on PSII (Schrader et al., 2004) thus confirming the work of others that
cyclic electron transport is dramatically accelerated during heat stress. The stimulation
of PSI happens at lower temperature than the inhibition of PSII usually associated with
heat stress (Terzaghi et al., 1989; Thompson et al., 1989; Gombos et al., 1994; Cajánek et
al., 1998). The finding that lowered redox status can lead to significant stimulation of
PSI-mediated cyclic electron transport (Joët et al., 2002) raises a question.
One of the mechanisms involved in stimulating cyclic electron flow is phosphorylation
of light harvesting chlorophyll complex of PSII (LHCII). Phosphorylated
LHCII moves from the appressed thylakoid regions, where PSII is located, to the
unappressed regions, where PSI is located (Chow et al., 1991). The phosphorylated
LHCII becomes energetically disconnected from PSII core complex (slowing its turnover
rate) and energetically coupled to a PSI core (increasing its turnover rate) and this
process requires a specific polypeptide within PSI (Lunde et al., 2000). This is the well
known state transition (Allen, 1992) and is known to accompany an increase in cyclic
electron flow around PSI. However, heat (which also stimulates PSI-mediated cyclic
flow) has been reported to stimulate dephosphorylation of a number of PSII core proteins
namely D1, D2 and CP43 (Rokka et al., 2000; Vener et al., 2001). Therefore, the
regulation of phosphorylation of LHCII may well be quite different from the regulation
of phosphorylation of the PSII core proteins (Harrison and Allen, 1991; Pursiheimo et
al., 2003). The regulation of phosphorylation of thylakoid proteins interacts with redox
status (Vener et al., 1995). Likewise, dephosphorylation of LHCII appears to be catalyzed
by a different phosphatase than dephosphorylation of other thylakoid-associated
proteins (Hammer et al., 1997; Vener et al., 1999). There are several thylakoid
associated kinases (TAKs) (Snyders and Kohorn, 1999, 2001) but also another kinase
that appears unrelated to TAKs that is necessary for state transitions and phosphorylation
of LHCII (Depège et al., 2003).
Thus, there may be two different regulatory systems that control thylakoid
protein phosphorylation/dephosphorylation, one that controls phosphorylation /de-