Inhibition of Plastocyanin to P700  +Electron Transfer inChlamydomonas reinhardtiiby Hyperosmotic Stress

Abstract

Oxygen electrode and fluorescence studies demonstrate that linear electron transport in the freshwater alga Chlamydomonas reinhardtii can be completely abolished by abrupt hyperosmotic shock. We show that the most likely primary site of inhibition of electron transfer by hyperosmotic shock is a blockage of electron transfer between plastocyanin (PC) or cytochrome c 6 and P 700 . The effects on this reaction were reversible upon dilution of the osmolytes and the stability of plastocyanin or photosystem (PS) I was unaffected. Electron micrographs of osmotically shocked cells showed a significant decrease in the thylakoid lumen volume. Comparison of estimated lumenal width with the x-ray structures of plastocyanin and PS I suggest that lumenal space contracts during HOS so as to hinder the movement of docking to PS I of plastocyanin or cytochrome c 6 .The effects of high osmotic potentials on primary photosynthetic processes are of great interest because they are thought to influence the ability of plants to survive desiccation and salt stress. It has been known for some time that photosynthesis can be inhibited by hyperosmotic shock (HOS), and several studies have been made on the effects of such conditions on the primary processes of photosynthesis (for review, see Kirst, 1990). However, the nature of the primary lesions to photosynthesis caused by HOS has remained elusive. To date, the most intensive studies on the responses of green algae to HOS have been performed on marine species, in particular Duneliella salina (e.g. Wiltens et al., 1978; Satoh et al., 1983;Gilmour et al., 1984). Because such species are found in waters of variable salinity, they are expected to have robust osmoregulatory systems. On the other hand, freshwater algae are likely to respond differently to salt or osmotic stresses. Because of the detailed genetic information and its ability to be transformed, the fresh water Chlorophyte, Chlamydomonas reinhardtii, has become an important laboratory species, particularly in studies of photosynthesis; thus, understanding its ecophysiology has become critical. Considerable work has been done on the effects of HOS on cyanobacteria species, such as Anacystis nidulans and Synechococcus sp. PCC 7942 (Grodzinski and Colman, 1973;Fulda et al., 1999; Allakhverdiev et al., 2000a), which pointed to direct effects of HOS on the photosynthetic reaction centers, particularly photosystem (PS) II (Allakhverdiev et al., 2000b). However, direct comparisons of the effects with those in freshwater green algae may be difficult considering their evolutionary and physiological differences.Previous research has shown that overall photosynthetic capacity of C. reinhardtii is severely inhibited by HOS (Reynoso and Gamboa, 1982;Berkowitz et al., 1983;Gamboa et al., 1985; Neale and Melis, 1989; Kirst, 1990;Endo et al., 1995; Leó n and Galván, 1995). Furthermore, Neale and Melis (1989) have shown that the photosynthetic apparatus of C. reinhardtii cells is significantly more susceptible to photoinhibition or photodamage...

New direct dynamic models of protein interactions coupled to photosynthetic electron transport reactions

“…The distance decrease below the optimal value also leads to the rate constant decrease because Pc diffusion in the luminal space becomes difficult and even impossible at the small lumen width. This conclusion corresponds to the known effect of hyperosmotic stress, changing the lumen volume and inhibiting the Pc-cyt reaction, which is observed experimentally (Cruz et al 2001).…”

Recent Discoveries on the Roles of Polyphosphates in Plants

“…Likewise, the acidocalcisome can migrate toward the chloroplast under stressful conditions. Osmotic stress induces changes in volume in other cellular structures such as the thylakoids of C. reinhardtii, which swell when exposed to hyperosmotic stress (Cruz et al 2001;Teun et al 2000). Thus, it is reasonable to hypothesize that polyP hydrolysis and/or synthesis and parallel changes in acidocalcisomal osmotic potential create a favorable osmotic gradient that may allow water flux into the acidocalcisome to regulate the volume of cellular organelles.…”

Modulation of the multilamellar membrane organization and of the chiral macrodomains in the diatom Phaeodactylum tricornutum revealed by small-angle neutron scattering and circular dichroism spectroscopy

“…Although, we have no evidence for the causal relationship between the reorganizations in the lamellar order (SANS data) as well as the chiral macro-organization of the complexes (CD data) and the capability of cells for NPQ, our observations show that this possibility warrants a more detailed, systematic investigation, during which the possibility of osmotic influence on the photosynthetic electron flow should also be considered (cf. Cruz et al 2001). Taking into account the technical possibilities, these types of experiments are now feasible, as SANS, fluorescence (NPQ) and electron transport experiments are possible to carry out on the same sample under essentially the same conditions.…”