PsaI represents one of three low molecular weight peptides of PSI. Targeted inactivation of the plastid PsaI
gene in Nicotiana tabacum has no measurable effect on photosynthetic
electron transport around PSI or on accumulation of proteins involved in
photosynthesis. Instead, the lack of PsaI
destabilizes the association of PsaL and PsaH to PSI, both forming the
light-harvesting complex (LHC)II docking site of PSI. These alterations
at the LHCII binding site surprisingly did not prevent state transition
but led to an increased incidence of PSI-LHCII complexes, coinciding
with an elevated phosphorylation level of the LHCII under normal growth
light conditions. Remarkably, LHCII was rapidly phosphorylated in ΔpsaI
in darkness even after illumination with far-red light. We found that
this dark phosphorylation also occurs in previously described mutants
impaired in PSI function or state transition. A prompt shift of the
plastoquinone (PQ) pool into a more reduced redox state in the dark
caused an enhanced LHCII phosphorylation in ΔpsaI
Since the redox status of the PQ pool is functionally connected to a
series of physiological, biochemical, and gene expression reactions, we
propose that the shift of mutant plants into state 2 in darkness
represents a compensatory and/or protective metabolic mechanism. This
involves an increased reduction and/or reduced oxidation of the PQ pool,
presumably to sustain a balanced excitation of both photosystems upon
the onset of light.