Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition

Abstract

The effect of strong irradiance (Q000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chiamydomonas reinhardtii was investigated. Low light (LL,15 micromole photons per square meter per second) grown C. reinhardtii are photoinhibited upon exposure to strong irradiance, and the loss of photosynthetic functioning is due to damage to PSII. Under physiological growth conditions, PSII is distributed into two pools. The large antenna size (PSIIa) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (OB-reducing centers). The smaller antenna (PSII) account for the remainder of PSII and exist in astate not yet able to photoreduce plastoquinone (0B-nonreducing centers). The exposure of C. reinhardtii cells to 60 minutes of strong irradiance disabled about half of the primary charge separation between P680 and pheophytin. The PSII,content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSIIa decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSlI,6 from a 0.-nonreducing to a QO-reducing state. Parallel measurements of the rate of oxygen evolution revealed that photosynthetic electron transport was maintained at high rates, despite the loss of activity by a majority of PSIIa. The results suggest that PSlI, in C. reinhardtii may serve as a reserve pool of PSII that augments photosynthetic electron-transport rates during exposure to strong irradiance and partially compensates for the adverse effect of photoinhibition on PSII,.Light energy drives photosynthesis but excess light is potentially damaging to the photosynthetic apparatus. The latter phenomenon is called photoinhibition and is manifested as a loss in chloroplast electron-transport capacity (for recent reviews see 38, 39). Though plants differ greatly in their sensitivity to photoinhibition, current evidence indicates widespread occurrence of photoinhibition in both terrestrial and aquatic environments (2, 33). In most cases, the loss of PSII function is responsible for decreased electron-transport activity, though damage to the PSI complex has also been reported (39). When isolated spinach thylakoids were illuminated with strong-irradiance (2500 Amol.m-2 s-') at 0C, both the QA2 (320 nm absorbance change) and Pheo (685 nm absorbance change) signal were lowered in parallel (8,9,12), suggesting inhibition of primary photochemistry. The PSII primary charge separation was also inhibited when intact cells of the green alga, Chiamydomonas reinhardtii, were exposed to strong irradiance (12). The rapid loss of the pheophytin photoreduction during photoinhibition has also been detected using EPR difference spectroscopy (46). These results indicated that photoinhibitory damage affected the ability of PSII to form the P680+ Pheo-charge separation.An alternative proposal (22, 37) is that photoinhibition result...

Recovery of photosynthesis and photosystem II fluorescence in Chlamydomonas reinhardtii after exposure to three levels of high light

“…This was supported by the finding that as much as 15% of the PS II centers were of PS II, form after exposure to a PPFD of 1400 ftmol m"' s"\ while only 7% of the Qg-reducing centers remained intact (Figs 4b and 5b). The results do not contradict the findings of Neale and Melis (1990) and that PS II^ centers are more sensitive to photoinhibition than PS Up centers, since the amount of PS 11^ centers decrease by 48% while 77 K FV/FM decreased by only 30%. However, these data do not support their suggestion that PS lip Qg-nonreducing centers can act as a reserve pool for PS lip Qg-reducing centers during the phase of photoinhibition as the amount of Qg-non-reducing centers increased from 34 to 55% while the Qg-reducing centers decreased more than the amount of PS II^.…”

PHOTOPHYSIOLOGICAL RESPONSES OF FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) TO NITROGEN DEPLETION AT TWO TEMPERATURES1

“…Consequently, maximum fluorescence could only be achieved when Q B was artificially blocked by DCMU. The advantages to forming Q B nonreducing PSII centers are resistance to photoinhibition and the capability to support excess excitation energy dissipation (Neale and Melis 1990, Ö quist et al 1992, Rodriguez-Roman and Iglesias-Prieto 2005, Hill and Ralph 2006. Furthermore, the maintenance of a reduced photosystem provides the appropriate trigger for xanthophyll pigment production (Mock and Kroon 2002) and prevention of diatoxanthin epoxidase (Goss et al 2006).…”

In Vitro Synthesis and Assembly of Photosystem II Core Proteins

“…The replacement of the D1 protein into PSII has been addressed experimentally in vivo using green algae (9,10,18,19) or using Spirodela leaves (20). In most of these studies, a cycling of the damaged PSII reaction center from grana to stroma lamellae was proposed to occur, but a "D1-less PSII" reaction center in the stroma lamellae has never been detected.…”

“…In most of these studies, a cycling of the damaged PSII reaction center from grana to stroma lamellae was proposed to occur, but a "D1-less PSII" reaction center in the stroma lamellae has never been detected. Melis and co-workers (18,19) proposed a repair cycle of PSII involving several putative assembly intermediates consisting of a PSII core with different sized antennae complexes and so-called QB-nonreducing PSII centers. In other studies it was proposed that the D1 protein associates in the stroma lamellae with a PSII subcomplex consisting of D2-cytochrome b 559 -CP47 (9) or with D2-cytochrome b 559 (20).…”

“…Gels were dried prior to exposure to film. The fraction numbers (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) and bands (I-VI) are indicated.…”