Responses of ferns to red light are mediated by an unconventional photoreceptor

Efficient photosynthesis is essential for plant survival. To optimize photosynthesis, plants have developed several photoresponses. Stems bend towards a light source (phototropism), chloroplasts move to a place of appropriate light intensity (chloroplast photorelocation) and stomata open to absorb carbon dioxide. These responses are mediated by the blue-light receptors phototropin 1 (phot1) and phototropin 2 (phot2) in Arabidopsis (refs 1-5). In some ferns, phototropism and chloroplast photorelocation are controlled by red light as well as blue light. However, until now, the photoreceptor mediating these red-light responses has not been identified. The fern Adiantum capillus-veneris has an unconventional photoreceptor, phytochrome 3 (phy3), which is a chimaera of the red/far-red light receptor phytochrome and phototropin. We identify here a function of phy3 for red-light-induced phototropism and for red-light-induced chloroplast photorelocation, by using mutational analysis and complementation. Because phy3 greatly enhances the sensitivity to white light in orienting leaves and chloroplasts, and PHY3 homologues exist among various fern species, this chimaeric photoreceptor may have had a central role in the divergence and proliferation of fern species under low-light canopy conditions.     

Chloroplast avoidance movement reduces photodamage in plants

When plants are exposed to light levels higher than those required for photosynthesis, reactive oxygen species are generated in the chloroplasts and cause photodamage. This can occur even under natural growth conditions. To mitigate photodamage, plants have developed several protective mechanisms. One is chloroplast avoidance movement, in which chloroplasts move from the cell surface to the side walls of cells under high light conditions, although experimental support is still awaited. Here, using different classes of mutant defective in chloroplast avoidance movement, we show that these mutants are more susceptible to damage in high light than wild-type plants. Damage of the photosynthetic apparatus and subsequent bleaching of leaf colour and necrosis occur faster under high light conditions in the mutants than in wild-type plants. We conclude that chloroplast avoidance movement actually decreases the amount of light absorption by chloroplasts, and might therefore be important to the survival of plants under natural growth conditions.     

Chloroplast to nucleus communication triggered by accumulation of Mg-protoporphyrinIX

Plant cells coordinately regulate the expression of nuclear and plastid genes that encode components of the photosynthetic apparatus. Nuclear genes that regulate chloroplast development and chloroplast gene expression provide part of this coordinate control. There is evidence that information also flows in the opposite direction, from chloroplasts to the nucleus. Until now, at least three different signalling pathways have been identified that originate in the plastid and control nuclear gene expression but the molecular nature of these signals has remained unknown. Here we show that the tetrapyrrole intermediate Mg-protoporphyrin (Mg-ProtoIX) acts as a signalling molecule in one of the signalling pathways between the chloroplast and nucleus. Accumulation of Mg-ProtoIX is both necessary and sufficient to regulate the expression of many nuclear genes encoding chloroplastic proteins associated with photosynthesis.     

Phot1 and phot2 mediate blue light regulation of stomatal opening.

The stomatal pores of higher plants allow for gaseous exchange into and out of leaves. Situated in the epidermis, they are surrounded by a pair of guard cells which control their opening in response to many environmental stimuli, including blue light. Opening of the pores is mediated by K(+) accumulation in guard cells through a K(+) channel and driven by an inside-negative electrical potential. Blue light causes phosphorylation and activation of the plasma membrane H(+)-ATPase that creates this potential. Thus far, no blue light receptor mediating stomatal opening has been identified, although the carotenoid, zeaxanthin, has been proposed. Arabidopsis mutants deficient in specific blue-light-mediated responses have identified four blue light receptors, cryptochrome 1 (cry1), cryptochrome 2 (cry2), phot1 and phot2. Here we show that in a double mutant of phot1 and phot2 stomata do not respond to blue light although single mutants are phenotypically normal. These results demonstrate that phot1 and phot2 act redundantly as blue light receptors mediating stomatal opening.     

A blue-light-activated adenylyl cyclase mediates photoavoidance in Euglena gracilis

Blue light regulates processes such as the development of plants and fungi and the behaviour of microbes. Two types of blue-light receptor flavoprotein have been identified: cryptochromes, which have partial similarity to photolyases, and phototropins, which are photoregulated protein kinases. The former have also been found in animals with evidence of essential roles in circadian rhythms. Euglena gracilis, a unicellular flagellate, abruptly changes its swimming direction after a sudden increase or decrease in incident blue light intensity, that is, step-up or step-down photophobic responses, resulting in photoavoidance or photoaccumulation, respectively. Although these photobehaviours of Euglena have been studied for a century, the photoreceptor molecules mediating them have remained unknown. Here we report the discovery and biochemical characterization of a new type of blue-light receptor flavoprotein, photoactivated adenylyl cyclase, in the photoreceptor organelle of Euglena gracilis, with molecular genetic evidence that it mediates the step-up photophobic response.     

Homologous plant and bacterial proteins chaperone oligomeric protein assembly

An abundant chloroplast protein is implicated in the assembly of the oligomeric enzyme ribulose bisphosphate carboxylase-oxygenase, which catalyses photosynthetic CO2-fixation in higher plants. The product of the Escherichia coli groEL gene is essential for cell viability and is required for the assembly of bacteriophage capsids. Sequencing of the groEL gene and the complementary cDNA encoding the chloroplast protein has revealed that these proteins are evolutionary homologues which we term 'chaperonins'. Chaperonins comprise a class of molecular chaperones that are found in chloroplasts, mitochondria and prokaryotes. Assisted post-translational assembly of oligomeric protein structures is emerging as a general cellular phenomenon.     

Single gene circles in dinoflagellate chloroplast genomes.

Photosynthetic dinoflagellates are important aquatic primary producers and notorious causes of toxic 'red tides'. Typical dinoflagellate chloroplasts differ from all other plastids in having a combination of three envelope membranes and peridinin-chlorophyll a/c light-harvesting pigments. Despite evidence of a dinoflagellete satellite DNA containing chloroplast genes, previous attempts to obtain chloroplast gene sequences have been uniformly unsuccessful. Here we show that the dinoflagellate chloroplast DNA genome structure is unique. Complete sequences of chloroplast ribosomal RNA genes and seven chloroplast protein genes from the dinoflagellate Heterocapsa triquetra reveal that each is located alone on a separate minicircular chromosome: 'one gene-one circle'. The genes are the most divergent known from chloroplast genomes. Each circle has an unusual tripartite non-coding region (putative replicon origin), which is highly conserved among the nine circles through extensive gene conversion, but is very divergent between species. Several other dinoflagellate species have minicircular chloroplast genes, indicating that this type of genomic organization may have evolved in ancestral peridinean dinoflagellates. Phylogenetic analysis indicates that dinoflagellate chloroplasts are related to chromistan and red algal chloroplasts and supports their origin by secondary symbiogenesis.     

psbA genes indicate common ancestry of prochlorophytes and chloroplasts

It has long been suspected that chloroplasts evolved after an endosymbiotic event involving a photosynthetic prokaryote, presumably a cyanobacterium, and a eukaryotic organism. Recent studies have provided strong evidence about the cyanobacterial nature of chloroplasts. Since the discovery of prochlorophytes, oxygen-evolving photosynthetic prokaryotes containing chlorophyll a and chlorophyll b and lacking phycobiliproteins, there has been speculation that these represent evolutionary intermediates between cyanobacteria and chloroplasts. Prochloron sp., the first described prochlorophyte, proved difficult to work with because it is an obligate symbiont of marine ascidians. Prochlorothrix hollandica, a recently isolated, freshwater filamentous prochlorophyte, is easily maintained in the laboratory. Overall pigment composition and thylakoid membrane structure of P. hollandica suggest it has intermediate characteristics between cyanobacteria and the chloroplasts of higher plants. The P. hollandica psbA genes, which encode the photosystem II thylakoid protein D1, were cloned and sequenced and the sequences compared to those reported for cyanobacteria, a green alga, a liverwort, and several higher plants. The two psbA genes present in P. hollandica encode an identical amino-acid sequence. As in all chloroplast psbA genes, there is a seven amino-acid gap near the C terminus of the derived protein relative to the protein predicted by cyanobacterial genes, suggesting that P. hollandica is part of the lineage that led to chloroplasts after a divergence from cyanobacteria. This hypothesis is also supported by phylogenetic analysis of derived D1 amino-acid sequences from psbA genes of thirteen taxa on the basis of parsimony.     

Evolutionary transfer of the chloroplast tufA gene to the nucleus

Evolutionary gene transfer is a basic corollary of the now widely accepted endosymbiotic theory, which proposes that mitochondria and chloroplasts originated from once free-living eubacteria. The small organellar chromosomes are remnants of larger bacterial genomes, with most endosymbiont genes having been either transferred to the nucleus soon after endosymbiosis or lost entirely, with some being functionally replaced by pre-existing nuclear genes. Several lines of evidence indicate that relocation of some organelle genes could have been more recent. These include the abundance of non-functional organelle sequences of recent origin in nuclear DNA, successful artificial transfer of functional organelle genes to the nucleus, and several examples of recently lost organelle genes, although none of these is known to have been replaced by a nuclear homologue that is clearly of organellar ancestry. We present gene sequence and molecular phylogenetic evidence for the transfer of the chloroplast tufA gene to the nucleus in the green algal ancestor of land plants.     

Cloning and functional analysis of chloroplast division gene NtFtsZ2-1 in Nicotiana tabacum

FtsZ protein plays an important role in the division of chloroplasts. With the finding and functional analysis of higher plant FtsZ proteins, people have deepened the understanding in the molecular mechanism of chloroplast division. Multiple ftsZ genes are diversified into two families in higher plants, ftsZ1 and ftsZ2. On the basis of the research on ftsZ1 family, we analyzed the function of NtFtsZ2-1 gene in Nicotiana tabacum. Microscopic analysis of the sense and antisense NtFtsZ2-1 transgenic tobacco plants revealed that the chloroplasts were abnormal in size and also in number when compared with wild-type tobacco chloroplasts. Our investigations confirmed that the NtFtsZ2-1 gene is involved in plant chloroplast division.     

盐度对盐生杜氏藻叶绿体超微结构的影响

研究不同盐度对盐生杜氏藻（ Ｄｕｎａｌｉｅｌｌａｓａｌｉｎａ（ Ｄｕｎａｌ．） Ｔｅｏｄ ．） 叶绿体超微结构的影响．结果显示,在ＮａＣｌ 浓度为２ ．７ ｍｏｌ／Ｌ 的培养液中,其叶绿体为典型的杯状结构．在ＮａＣｌ 浓度从２ ．７ ｍｏｌ／Ｌ 增加到４ ．３ ｍｏｌ／Ｌ时,其杯状叶绿体逐渐离解为多个相互连结或分离的部分;叶绿体内的类囊体片层系统解体并出现大量的嗜锇质体小球;在分离的叶绿体间隙中形成多个线粒体．在培养液的ＮａＣｌ 浓度逐步从４ ．３ ｍｏｌ／Ｌ 降低到２ ．７ ｍｏｌ／Ｌ后,盐藻的叶绿体又逐步恢复为典型的杯状结构．由此可知,不同盐度下盐藻叶绿体的结构变化是一个可逆的过程,这种可逆变化是盐藻得以在高盐度,强光照,易蒸发的自然环境中生存的一种适应机制．     

Cloning and functional analysis of chloroplast division gene NtFtsZ2-l in Nicotiana tabacum

FtsZ protein plays an important role in the division of chloroplasts. With the finding and functional analysis of higher plant FtsZ proteins, people have deepened the understanding in the molecular mechanism of chloroplast division. Multiple ftsZ genes are diversified into two families in higher plants, ftsZ1 and ftsZ2 . On the basis of the research on ftsZl family, we analyzed the function of NtFtsZ2-l gene in Nicotiana tabacum . Microscopic analysis of the sense and antisense NtFtsZ2-l transgenic tobacco plants revealed that the chloroplasts were abnormal in size and also in number when compared with wild-type tobacco chloroplasts. Our investigations confirmed that the NtFtsZ2-l gene is involved in plant chloroplast division.     

适于双向电泳分析的银杏叶绿体蛋白质提取方法

【目的】获得银杏叶绿体蛋白质的提取方法,在蛋白质水平探讨银杏光合作用对环境的响应机制。【方法】建立一种适合银杏叶绿体蛋白双向电泳分离的实验体系,采用Tris-平衡酚抽提法对银杏叶绿体蛋白质进行提取,并与三氯乙酸(TCA)-丙酮沉降法进行对比分析。【结果】Percoll密度梯度离心法适用于银杏叶绿体的提取,提取的叶绿体平均完整率在85%左右。单向电泳结果显示,与TCA-丙酮沉降法相比,在用Tris-平衡酚抽提法分离的叶绿体蛋白质泳道中,低分子质量蛋白质条带更多、更清晰。进一步的双向电泳结果表明,用Tris-平衡酚抽提法对银杏叶绿体蛋白质进行提取,蛋白质产量更高,图谱清晰,所分离的蛋白点更多,形态更好,条纹影响相对较小。【结论】Tris-平衡酚抽提法可有效地提取高质量的叶绿体蛋白质并进行双向电泳,可用于银杏叶绿体的蛋白质组学分析。     

不同光照条件下少花桂幼苗部分光合指标的变化

研究了生成在不同光照条件下的少花桂幼苗叶绿体结构、净光合日变化、光合与生长和ＡＴＰ含量的生长期变化,结果表明：遮荫条件使叶肉细胞中的叶绿全变大,数量减少,基质片层退化;且遮荫条件明显影响体内ＡＴＰ含量、净光合速率和生物量的积累状况。     

ATPG is required for the accumulation and function of chloroplast ATP synthase in Arabidopsis

The subunit Ⅱ of chloroplast ATP synthase is one of the two peripheral stalks, which associates the catalytic CF1 with mem-brane-spanning CFo . Although the structural and functional roles of chloroplast ATP synthase have been extensively examined, the physiological significance of subunit Ⅱ in vivo is still unclear. In this work, we identified one Arabidopsis T-DNA insertion mutant of atpG gene encoding the subunit Ⅱ of chloroplast ATP synthase. The atpg null mutant displayed an albino lethal pheno-type, as it could not grow photoautotrophically. Transmission electron microscopy analysis showed that chloroplasts of atpg lacked the organized thylakoid membranes. Loss of subunit Ⅱ affected the accumulation of CF1-CFo complex, however, it did not seem to have an effect on the CF1 assembly. The light induced ATP formation of atpg was significantly reduced compared with the wild type. Based on these results, we suggested that ATPG was essential for the accumulation and function of chloroplast ATP synthase.     

A photosynthetic alveolate closely related to apicomplexan parasites

Many parasitic Apicomplexa, such as Plasmodium falciparum, contain an unpigmented chloroplast remnant termed the apicoplast, which is a target for malaria treatment. However, no close relative of apicomplexans with a functional photosynthetic plastid has yet been described. Here we describe a newly cultured organism that has ultrastructural features typical for alveolates, is phylogenetically related to apicomplexans, and contains a photosynthetic plastid. The plastid is surrounded by four membranes, is pigmented by chlorophyll a, and uses the codon UGA to encode tryptophan in the psbA gene. This genetic feature has been found only in coccidian apicoplasts and various mitochondria. The UGA-Trp codon and phylogenies of plastid and nuclear ribosomal RNA genes indicate that the organism is the closest known photosynthetic relative to apicomplexan parasites and that its plastid shares an origin with the apicoplasts. The discovery of this organism provides a powerful model with which to study the evolution of parasitism in Apicomplexa.     

菠菜叶绿体基因组定点整合表达载体构建及原核表达

分析菠菜叶绿体基因组全序列,选用了rbcL基因和accD基因的间隔区作为外源基因的定点整合位点,并从菠菜叶绿体基因组中克隆了rbcL基因全长和accD基因的5′端部分,长度分别为1956 bp和1320 bp。以这2个DNA片段作为同源重组片段,以烟草叶绿体基因的启动子Prrn和终止子psbA3′控制外源基因的转录,构建了包含筛选标记基因aadA基因(编码氨基糖苷-3′-腺苷酸转移酶,具有壮观霉素和链霉素抗性)和报告基因GFP(编码绿色荧光蛋白)的菠菜叶绿体基因组定点整合表达载体pRAGA。酶切结果显示构建正确。将该载体转化大肠杆菌,在激光扫描共聚焦显微镜下用488 nm蓝光激发,发现大肠杆菌发出强烈的绿色荧光,而对照菌体没有荧光,表明GFP基因在原核大肠杆菌中已经成功表达。实验结果说明构建的菠菜叶绿体定点整合表达载体pRAGA可以用于菠菜叶绿体转化。     

盐生杜氏藻的完整叶绿体分离

盐生杜氏藻(Dunaliella salina)是世界上最耐盐的一种单细胞真核绿藻,可在含0．1～5．0mol／L NaCl的培养液中正常生长．盐生杜氏藻细胞内的主要调渗物质是甘油,在甘油代谢途径中,3-磷酸甘油脱氢酶是一个关键酶．近年来的研究表明杜氏藻(Dunaliella tertiolecta)细胞质和叶绿体中都存在3-磷酸甘油脱氢酶的同工酶,且叶绿体上的同工酶与渗透调节作用直接相关,为了解D．satina中3-磷酸甘油脱氢酶同工酶的细胞定位和分布,我们通过冰浴超声波破碎和蔗糖密度梯度离心,获得盐生杜氏藻的完整叶绿体,用相差显微镜镜检、酶的活性分析等方法对其完整性作了初步分析。     

转SOD基因烟草的光合特性初探

 以转基因Fe-SOD,Mn-SOD高表达、转基因Mn-SOD反义低表达烟草及非转基因品系为供试材料,比较CO₂光强对烟草光合作用和蒸腾作用的影响及不同叶位叶片的光合差异.在相同条件下,Mn-SOD高表达烟草CO₂补偿点最高(105μmol·mol^-1),Mn-SOD低表达烟草最低(78μmol·mol^-1),CO₂浓度对非转基因烟草蒸腾作用的影响比对其他三个转基因品系的影响要强.Mn-SOD低表达烟草的光补偿点最高(45μmol·m^-2·s^-1),而Mn-SOD高表达烟草最低(25μmol·m^-2·s^-1),并且强光对非转基因烟草的光合作用抑制大,转基因品系对强光具有较强的耐受力.同一叶位叶片,转基因烟草的光合速率、蒸腾速率更强.但转基因SOD高表达烟草在整体上并没有表现出明显的光合作用优势.     

Regulation of Arabidopsis cryptochrome 2 by blue-light-dependent phosphorylation

Cryptochromes are blue/ultraviolet-A light receptors that mediate various light responses in plants and animals. But the initial photochemical reaction of cryptochrome is still unclear. For example, although most photoreceptors are known to undergo light-dependent protein modification such as phosphorylation, no blue-light dependent phosphorylation has been reported for a cryptochrome. Arabidopsis cryptochrome 2 (cry2) mediates light regulation of seedling development and photoperiodic flowering. The physiological activity and cellular level of cry2 protein are light-dependent, and protein protein interactions are important for cry2 function. Here we report that cry2 undergoes a blue-light-dependent phosphorylation, and that cry2 phosphorylation is associated with its function and regulation. Our results suggest that, in the absence of light, cry2 remains unphosphorylated, inactive and stable; absorption of blue light induces the phosphorylation of cry2, triggering photomorphogenic responses and eventually degradation of the photoreceptor.