Arabidopsis thaliana Plants Lacking the PSI-D Subunit of Photosystem I Suffer Severe Photoinhibition, Have Unstable Photosystem I Complexes, and Altered Redox Homeostasis in the Chloroplast Stroma

Abstract

The PSI-D subunit of photosystem I is a hydrophilic subunit of about 18 kDa, which is exposed to the stroma and has an important function in the docking of ferredoxin to photosystem I. We have used an antisense approach to obtain Arabidopsis thaliana plants with only 5-60% of PSI-D. No plants were recovered completely lacking PSI-D, suggesting that PSI-D is essential for a functional PSI in plants. Plants with reduced amounts of PSI-D showed a similar decrease in all other subunits of PSI including the light harvesting complex, suggesting that in the absence of PSI-D, PSI cannot be properly assembled and becomes degraded. Plants with reduced amounts of PSI-D became light-stressed even in low light although they exhibited high non-photochemical quenching (NPQ). The high NPQ was generated by upregulating the level of violaxanthin de-epoxidase and PsbS, which are both essential components of NPQ. Interestingly, the lack of PSI-D affected the redox state of thioredoxin. During the normal light cycle thioredoxin became increasingly oxidized, which was observed as decreasing malate dehydrogenase activity over a 4-h light period. This result shows that photosynthesis was close to normal the first 15 min, but after 2-4 h photoinhibition dominated as the stroma progressively became less reduced. The change in the thiol disulfide redox state might be fatal for the PSI-D-less plants, because reduction of thioredoxin is one of the main switches for the initiation of CO 2 assimilation and photoprotection upon light exposure.

A Cytoplasmically Inherited Barley Mutant Is Defective in Photosystem I Assembly Due to a Temperature-Sensitive Defect in ycf3 Splicing

“…In general, PSI is known to be much more stable, also at high temperatures, than PSII (Berry and Bjö rkman, 1980;Takeuchi and Thornber, 1994;Al-Khatib and Paulsen, 1999), but in the CL3 mutant PSI was more affected than PSII. The biochemical, spectroscopic, and physiological phenotype resembles that described for several other PSI mutants, where the amount of PSI is decreased and a secondary damage to PSII and other thylakoid proteins is observed especially at higher light intensity and with light that predominantly excites PSII (Haldrup et al, 2000(Haldrup et al, , 2003.…”

GLK Transcription Factors Coordinate Expression of the Photosynthetic Apparatus in Arabidopsis

“…Likewise, changes in core complex subunits of PSI were relatively weak with PsaD and PsaK upregulated two-to threefold. Beyond sustaining PSI function, the role of PsaD is unclear (Haldrup et al, 2003); however, PsaK appears to interact with LHCI and may stabilize the association of LHCI with the PSI core complex (Jensen et al, 2000). The lumenal electron carrier plastocyanin was also stimulated by GLK1 expression, perhaps reflecting the increased electron flow associated with enhanced light interception by LHC antennae.…”

In-Depth Analysis of the Thylakoid Membrane Proteome ofArabidopsis thalianaChloroplasts: New Proteins, New Functions, and a Plastid Proteome Database[W]

“…Six of the PSI core proteins (PsaD, PsaE, and PsaH) are found in pairs of closely related paralogues. Functional analysis has been performed for several of them (Varotto et al, 2000(Varotto et al, , 2002Scheller et al, 2001;Haldrup et al, 2003). Despite the very high level of sequence identity between psaE1 and psaE2 and between psaH1 and psaH2 (89 and 99%, respectively), we found unique sequence tags (by ESI-MS/MS) for each and a unique (diagnostic) tag for psaD-2 (98% sequence identity with psaD-1) in the pepsin digest of fraction C/M 1:1 RP-HPLC.…”