The evolution of light stress proteins in photosynthetic organisms

The Elip (early light-inducible protein) family in pro- and eukaryotic photosynthetic organisms consists of more than 100 different stress proteins. These proteins accumulate in photosynthetic membranes in response to light stress and have photoprotective functions. At the amino acid level, members of the Elip family are closely related to light-harvesting chlorophyll a/b-binding (Cab) antenna proteins of photosystem I and II, present in higher plants and some algae. Based on their predicted secondary structure, members of the Elip family are divided into three groups: (a) one-helix Hlips (high light-induced proteins), also called Scps (small Cab-like proteins) or Ohps (one-helix proteins); (b) two-helix Seps (stress-enhanced proteins); and (c) three-helix Elips and related proteins. Despite having different physiological functions it is believed that eukaryotic three-helix Cab proteins evolved from the prokaryotic Hlips through a series of duplications and fusions. In this review we analyse the occurrence of Elip family members in various photosynthetic prokaryotic and eukaryotic organisms and discuss their evolutionary relationship with Cab proteins.     

Suppression of both ELIP1 and ELIP2 in Arabidopsis does not affect tolerance to photoinhibition and photooxidative stress

ELIPs (early light-induced proteins) are thylakoid proteins transiently induced during greening of etiolated seedlings and during exposure to high light stress conditions. This expression pattern suggests that these proteins may be involved in the protection of the photosynthetic apparatus against photooxidative damage. To test this hypothesis, we have generated Arabidopsis (Arabidopsis thaliana) mutant plants null for both elip genes (Elip1 and Elip2) and have analyzed their sensitivity to light during greening of seedlings and to high light and cold in mature plants. In particular, we have evaluated the extent of damage to photosystem II, the level of lipid peroxidation, the presence of uncoupled chlorophyll molecules, and the nonphotochemical quenching of excitation energy. The absence of ELIPs during greening at moderate light intensities slightly reduced the rate of chlorophyll accumulation but did not modify the extent of photoinhibition. In mature plants, the absence of ELIP1 and ELIP2 did not modify the sensitivity to photoinhibition and photooxidation or the ability to recover from light stress. This raises questions about the photoprotective function of these proteins. Moreover, no compensatory accumulation of other ELIP-like proteins (SEPs, OHPs) was found in the elip1/elip2 double mutant during high light stress. elip1/elip2 mutant plants show only a slight reduction in the chlorophyll content in mature leaves and greening seedlings and a lower zeaxanthin accumulation in high light conditions, suggesting that ELIPs could somehow affect the stability or synthesis of these pigments. On the basis of these results, we make a number of suggestions concerning the biological function of ELIPs.     

Plastid development in albescent maize

Plastid development in albescent (al/al) and wild-type (+/al) strains of Zea mays has been studied in the electron microscope. Etiolated seedlings of the mutant are severely deficient in colored carotenoid pigments and accumulate carotenoid precursors tentatively identified as phytoene and phytofluene. The fine structure of proplastids in etiolated wild-type and mutant leaves is similar with 1 notable exception. Osmiophilic bodies found in the wild-type were lacking in all sections of albescent proplastids examined suggesting that these structures may be storage centers for carotenoid pigments. Plastid pigments are destroyed, chlorophyll synthesizing potential is lost, and the ultrastructure of plastids is irreversibly altered when mutant seedlings are placed directly in high intensity light. However, synthesis of plastid pigments and development of the photosynthetic apparatus as seen in the electron microscope is normal, and indistinguishable from that in the wild-type, in seedlings of the albescent mutant preilluminated with low intensity light prior to high intensity illumination. During treatment in low intensity light carotenogenesis is initiated in the mutant and proceeds normally thereafter.     

The effect of light on the biosynthesis of the light-harvesting chlorophyll a/b protein

The effect of light on the biosynthesis of the light-harvesting chlorophyll a/b protein (LHCP) is investigated in wild-type barley (Hordeum vulgare L.) and in the chlorophyll b-less mutant chlorina f2. In dark-grown plants a short red light pulse triggers the appearance of mRNA activity for the LHCP. While the accumulation of this mRNA is controlled by phytochrome (Apel (1979) Eur. J. Biochem. 97, 183–188), the red light treatment is not sufficient to induce the appearance of the LHCP within the membrane. Thus, at least one of the subsequent steps in the biosynthetic pathway leading to the assembly of the LHCP is controlled by light. The red light-induced mRNA is taken up into the polysomes during the subsequent dark period and is translated in vitro in a cell-free protein synthesizing system. However, an accumulation of the freshly synthesized polypeptide within the plant is not observed. The apparent instability of the polypeptide might be explained by the deficiency of chlorophyll in the red light-treated plants. In the chlorophyll b-less barley mutant chlorina f2 an accumulation of the freshly synthesized apoprotein of the LHCP can be observed in the light. Thus, chlorophyll a formation seems to be a light-dependent step which is required for the stabilization of the LHCP.Abbreviations mRNA messenger RNA - EDTA ethylenediaminetetraacetic acid - SDS sodium dodecylsulfate - LHCP light-harvesting chlorophyll a/b protein     

Ultrastructure of the plastids of 3 types of Chlamydomonas reinhardi mutants phenotypically yellow in the light or in darkness]

A study was made of the ultrastructure of plastids of three mutant types of Chlamydomonas reinhardi which are phenotypically revealed either in the light or in the darkness as yellow mutants. Characteristics of pigments for each mutant have been given. Mutant Y-4 unable to synthesize chlorophyll either in the light or in the darkness shows a complete reduction of photosynthesizing membranes. Mutant Y-1 capable of synthesizing chlorophyll develops a normal system of photosynthesizing membranes. The dark synthesis of chlorophyll in this mutant is broken, the mutant accumulates only carotenoids, the membrane system of its plastid being reduced. On the contrary, mutant Y-3 has in the darkness a complete set of pigments and a well developed membrane system. In the light this mutant yellows due to chlorophyll photodestruction that is followed by destruction of the membrane system of chloroplasts.     

Chloroplast photooxidation inhibits the expression of a set of nuclear genes

Summary Mutations or herbicides which inhibit the accumulation of carotenoid pigments in higher plants also result in the arrest of chloroplast development at a very early stage. The cause is extensive photooxidative damage within the chloroplast in the absence of protective carotenoids. Because the extent of photooxidation is dependent upon light intensity, normal chloroplast development can occur when carotenoid-deficient seedlings are grown in very dim light. Normal accumulation of chloroplastic and cytosolic mRNAs encoding chloroplast proteins proceeds only under permissive dim light conditions. Illumination with higher intensity light causes rapid chlorophyll photooxidation and the loss of two cytosolic mRNAs coding for proteins destined for the chloroplast, but does not affect another light-regulated cytosolic mRNA encoding a cytosolic protein. This experimental system may have uncovered a mechanism which coordinates the expression of genes in different cellular compartments.Abbreviations LHCP light-harvesting chlorophyll a/b protein - SSu small subunit - RuBP fibulose 1,5-bisphoshate - PEP phosphoenolpyruvate     

Inactivation of the chloroplast ATP synthase gamma subunit results in high non-photochemical fluorescence quenching and altered nuclear gene expression in Arabidopsis thaliana

The nuclear atpC1 gene encoding the gamma subunit of the plastid ATP synthase has been inactivated by T-DNA insertion mutagenesis in Arabidopsis thaliana. In the seedling-lethal dpa1 (deficiency of plastid ATP synthase 1) mutant, the absence of detectable amounts of the gamma subunit destabilizes the entire ATP synthase complex. The expression of a second gene copy, atpC2, is unaltered in dpa1 and is not sufficient to compensate for the lack of atpC1 expression. However, in vivo protein labeling analysis suggests that assembly of the ATP synthase alpha and beta subunits into the thylakoid membrane still occurs in dpa1. As a consequence of the destabilized ATP synthase complex, photophosphorylation is abolished even under reducing conditions. Further effects of the mutation include an increased light sensitivity of the plant and an altered photosystem II activity. At low light intensity, chlorophyll fluorescence induction kinetics is close to those found in wild type, but non-photochemical quenching strongly increases with increasing actinic light intensity resulting in steady state fluorescence levels of about 60% of the minimal dark fluorescence. Most fluorescence quenching relaxed within 3 min after dark incubation. Spectroscopic and biochemical studies have shown that a high proton gradient is responsible for most quenching. Thylakoids of illuminated dpa1 plants were swollen due to an increased proton accumulation in the lumen. Expression profiling of 3292 nuclear genes encoding mainly chloroplast proteins demonstrates that most organelle functions are down-regulated. On the contrary, the mRNA expression of some photosynthesis genes is significantly up-regulated, probably to compensate for the defect in dpa1.     

Discrete character of the development of the photosynthetic apparatus in greening barley leaves

Biogenesis of the photosynthetic apparatus in greening etiolated leaves of barley (Hordeum vulgare L) was investigated by an approach permitting investigation of this process under conditions that minimize differences in plastid development. Distributions of barley leaves greening for 24 h as to chlorophyll content and of chloroplast grana as to number of thylakoids were shown to be of a multimodal character. The shape of time-course curves of chlorophyll accumulation in local sites of greening etiolated leaves was of a stepped or (at the end of greening) undulated character. The stepwise accumulation of chlorophyll was accompanied by wave-like changes in chlorophyll b/a ratio, intensity of low-temperature chlorophyll fluorescence and photosynthetic activity with minima at the time points of transition to accelerated chlorophyll accumulation. It is assumed that (1) development of the photosynthetic apparatus in local sites of greening etiolated leaves occurs stepwise, from one steady level to another, but not as gradually as is generally accepted, and (2) every separate step in development of the photosynthetic apparatus seems to begin with formation of photosystem cores and to end with the synthesis of light-harvesting complexes. This revised version was published online in June 2006 with corrections to the Cover Date.     

The distribution of NADPH-protochlorophyllide oxidoreductase in relation to chlorophyll accumulation along the barley leaf gradient

The possible regulatory role of NADPH-protochlorophyllide oxidoreductase for chlorophyll accumulation has been investigated in barley plants. Within the primary leaf of etiolated plants the different maturation stages of etioplasts are found in a linear series with the youngest in cells near the base and the oldest in cells near the tip. This distribution of different plastid forms is paralleled by drastic differences in the NADPH-protochlorophyllide-oxidoreductase content of the plastids and their capacity to accumulate chlorophyll during illumination. The amount of enzyme and the rate of chlorophyll accumulation are highest in the mature etioplast in the tip of the leaf and both decline rapidly with decreasing age of the leaf tissue, being almost undetectable in the leaf base. The translatable mRNA coding for the enzyme shows a different distribution pattern within the leaf. The highest concentration is found in the middle part of the leaf while in the top part only traces of this mRNA are detectable. It is concluded that during leaf development the enzyme is synthesized rapidly only during a limited time period and that it is stored subsequently in the mature etioplast as a stable protein. The close correlation between the distribution of the enzyme within the barley leaf and that of the potential to accumulate chlorophyll during illumination would favour a control of chlorophyll accumulation by the amount of NADPH-protochlorophyllide oxidoreductase. Dark-grown plants which were exposed to far-red light were used to test this possibility. The far-red-absorbing form of phytochrome (P_fr) has an inverse effect on the kinetics of chlorophyll accumulation and the enzyme concentration. Our results indicate that the rate of chlorophyll accumulation in barley is not determined by the level of NADPH-protochlorophyllide oxidoreductase present in the leaves.