芥菜型油菜黄化突变体叶片叶绿素合成代谢变化

以芥菜型油菜黄化突变体(L638-y)及其野生型(L638-g)为材料,测定了叶片叶绿素(Chl)、类胡萝卜素(Caro)、Chl合成代谢中间产物含量变化,及叶绿素分解代谢的关键酶叶绿素酶(Chlase)和叶绿素合成代谢相关的氨基乙酰脱氢酶(ALAD)、胆色素原脱氨酶(PBGD)、粪卟啉原Ⅲ氧化酶(CPOX)和原卟啉原Ⅸ氧化酶(PPOX)的活性变化,以揭示芥菜型油菜黄化突变体L638-y叶片缺绿的生化机制.结果显示:(1)从油菜苗期到现蕾期,L638-y叶片总Chl和Caro含量均显著低于L638-g,且Chl b含量降低程度大于Chl a,总Chl含量降低程度大于Caro;(2)L638-y叶片Chlase活性在3～5叶期显著低于L638-g, 其它各生长时期二者Chlase活性均无显著差异;(3)L638-y叶片叶绿素合成代谢中间产物氨基乙酰丙酸(ALA)、胆色素原(BPG)、尿卟啉原Ⅲ(Urogen Ⅲ)和粪卟啉原Ⅲ(Coprogen Ⅲ)含量均显著大于L638-g,而原卟啉Ⅸ(Proto Ⅸ)、镁原卟啉(Mg-Proto)、原脱植基叶绿素(Pchlide)的含量却显著低于L638-g;与L638-g相比,L638-y叶片ALAD活性无显著差异,BPGD活性显著增加,而COPX和PPOX活性却显著降低.研究表明,L638-y叶片叶绿素缺乏的主要原因是叶绿素合成代谢受阻,而并非其叶绿素降解所致,受阻位点在由Coprogen Ⅲ-Proto Ⅸ的反应.这是一种不同于前人报道的新型缺绿突变体.     

叶面喷施亚精胺对盐碱胁迫下番茄幼苗生长及其叶绿素合成前体含量的影响

以盐碱敏感型番茄品种‘中杂9号’幼苗为试验材料,研究叶面喷施0.25mmol·L-1亚精胺(Spd)对75mmol·L-1盐碱溶液胁迫下番茄幼苗生长、净光合速率及叶绿素合成前体物质含量的影响,探讨Spd在缓解番茄盐碱胁迫伤害的生理机制。结果显示:(1)盐碱胁迫下番茄叶片叶绿素合成前体物质原卟啉Ⅸ(ProtoⅨ)、镁-原卟啉Ⅸ(Mg-protoⅨ)、原叶绿素酸(Pchl)含量均显著降低,而δ-氨基酮戊酸(ALA)、胆色素原(PBG)、尿卟啉原Ⅲ(UroⅢ)显著积累,UroⅢ到ProtoⅨ的转化受阻,引起叶片叶绿素a(Chl a)、叶绿素b(Chl b)和总叶绿素(Chl)含量显著降低,以及幼苗叶片净光合速率(Pn)、叶面积、叶片相对含水量、地上和地下部干鲜重的生长指标均显著降低。(2)盐碱胁迫下,叶面喷施Spd可显著促进番茄幼苗的生长,抑制叶片内ALA、PBG、UroⅢ的积累,并提高ProtoⅨ、Mg-protoⅨ、Pchl、Chl a、Chl b、Chl含量和相应Pn值。研究表明,盐碱胁迫显著抑制番茄幼苗的生长,叶面喷施Spd可通过缓解UroⅢ到ProtoⅨ的转化受阻程度,促进盐碱胁迫下番茄叶片的叶绿素合成,提高叶绿素含量和净光合速率,减轻盐碱胁迫对幼苗生长的伤害。     

外源亚精胺对盐胁迫下菠菜叶绿素合成前体含量的影响

采用营养液水培的方法,以耐盐性较弱的菠菜(Spinacia oleracea L.)品种‘全能菠菜’为试材,研究外源亚精胺(Spd)对盐胁迫下菠菜生长及叶绿素前体含量的影响。结果显示:(1)菠菜植株在250mmol·L-1 NaCl胁迫下生长受到显著抑制,单株总鲜重、总干重、地上部鲜重、地上部干重、地下部鲜重和地下部干重分别较对照降低57.23%、53.08%、63.14%、55.05%、42.22%和42.86%,叶面喷施1.0mmol·L-1Spd使其分别比盐胁迫处理提高62.83%、71.19%、60.57%、71.74%、70.31%和69.23%;(2)250 mmol·L-1盐胁迫使菠菜叶片叶绿素a(Chl a)、叶绿素b(Chl b)及总叶绿素Chl(a+b)含量分别较对照显著降低42.31%、54.55%和44.53%,叶面喷施1.0mmol·L-1 Spd使其分别较单纯盐胁迫处理显著提高46.24%、51.85%和47.11%;(3)盐胁迫使菠菜叶片Chl a、Chl b、叶绿素酸(Pchl)、镁原卟啉Ⅸ(Mg-protoⅨ)、原卟啉Ⅸ(ProtoⅨ)和尿卟啉Ⅲ(UroⅢ)含量均显著降低,而使胆色素原(PBG)和δ-氨基酮戊酸(ALA)含量升高,而叶片Chl a、Chl b、Pchl、Mg-protoⅨ、ProtoⅨ和UroⅢ含量在施用外源Spd后均显著提高,而其PBG和ALA含量有所降低。可见,盐胁迫条件下,菠菜叶片叶绿素合成受阻,受阻位点在PBG向UroⅢ的转化过程,外源Spd能够缓解盐胁迫下菠菜叶片叶绿素合成受阻的程度,促进叶绿素合成,从而提高叶绿素含量。     

高等植物叶绿素生物合成的联乙烯还原酶及编码基因研究进展

联乙烯还原酶(DVR)将各种叶绿素中间物质的8-乙烯基转化为乙基,是叶绿素生物合成必不可少的一个关键酶。迄今已在高等植物中检测到5种DVR活性。水稻和玉米的重组DVR蛋白能将联乙烯叶绿素a、叶绿素酸酯a、原叶绿素酸酯a、镁原卟啉Ⅸ单甲酯和镁原卟啉Ⅸ分别转化为相应的单乙烯物质,从而证实了这5种DVR活性。在高等植物中各种DVR活性是由一个基因编码的具有广谱底物专化性的DVR蛋白所催化,但来源于不同物种的DVR蛋白的催化活性可能具有极显著的差异,并且即使是同一个DVR蛋白,对不同的联乙烯底物也可能具有显著不同的催化活性。在此基础上,提出了"源于一个联乙烯还原酶的叶绿素生物合成多分支路径"假说。该文对近年来国内外有关高等植物叶绿素生物合成途径中联乙烯中间物质与联乙烯还原酶活性、联乙烯还原酶基因的克隆及重组酶活性检测、联乙烯还原酶的数目与叶绿素生物合成的多分支路径等方面的研究进展进行综述,并讨论了有待进一步探讨的若干问题。     

油菜叶绿素b减少突变体Cr3529叶绿素生物合成的研究

利用吸收光谱和荧光光谱法测定了油菜叶绿素b减少突变体Cr3529子叶叶绿素生物合成途径中几种主要前体物质的含量.结果显示:突变体子叶中叶绿素生物合成第一个限速步骤的前体物质δ-氨基乙酰丙酸(ALA)含量与野生型油菜大致相同,饲喂ALA后的突变体及野生型油菜子叶中ALA含量均显著增加,但二者无显著差异;胆色素原含量在突变体中也未降低,而尿卟啉原Ⅲ含量仅为野生型的一半,粪卟啉原Ⅲ、原卟啉Ⅸ、镁原卟啉Ⅸ和原植基叶绿素的含量都明显低于野生型.结果证明,Cr3529突变体中叶绿素生物合成受阻于由胆色素原形成尿卟啉原Ⅲ的步骤,其叶绿素合成缺陷的机制和前体物质的累积与其它叶绿素b减少突变体明显不同.     

葡萄糖和δ-氨基酮戊酸促进水稻在黑暗中合成叶绿素

本研究检测单子叶被子植物水稻(Oryza sativa L.)在完全黑暗中是否能合成叶绿素.以5 cm高的水稻黄化幼苗为研究材料,在黑暗中用不同浓度的葡萄糖和δ-氨基酮戊酸(δ-aminolevulinic acid, ALA)处理之.定时采收叶片检测其叶绿素、原卟啉Ⅸ (Proto)、 Mg-原卟啉Ⅸ (Mg-Proto)及原叶绿酸酯(Pchlide)的含量,并计算它们的卟啉的摩尔百分比.在黑暗中12 d,水稻幼苗的叶绿素从2.5 μg/g增加到7.5 μg/g,但叶绿素总量从0.36 μg/g增至3.6 μg/g.在黑暗中未经处理的幼苗Proto、Mg-Proto及Pchlide的摩尔百分比分别为65%、27.5%和7.5%; 而光照下幼苗相应的摩尔百分比分别为42.5%、35.0%和22.5%.在黑暗中用葡萄糖处理水稻黄化幼苗2 d,其卟啉的摩尔百分比即可恢复到正常值(如光照下之相同比例).在黑暗中以3%和6%的葡萄糖处理水稻黄化幼苗2 d,其叶绿素含量分别增加2.5和4.0倍;若同时辅以1 mmol/L δ-氨基酮戊酸,其叶绿素含量分别增加22和24倍.因此,被子植物在黑暗中可以合成叶绿素;葡萄糖或δ-氨基酮戊酸可以促进被子植物在黑暗中合成叶绿素;葡萄糖和δ-氨基酮戊酸并用有加成作用.葡萄糖或δ-氨基酮戊酸促进水稻在黑暗中合成叶绿素的生理机制有待研究.     

葡萄糖和δ—氨基酮戊酸促进水稻在黑暗中合成叶绿素

本研究检测单子叶被子植物水稻（Oryza sativaL.)在完全黑暗中是否能合成叶绿素。以5cm高的水稻黄化幼苗为研究材料,在黑暗中用不同浓度的葡萄糖和δ-氨基酮戊酸（δ-aminolevulinic acid,ALA)处理之,定时采收叶片检测其叶绿素,原卟啉Ⅸ（Proto),Mg-原卟啉Ⅸg-Proto)及原叶绿酸酯（Pchlide)的含量,并计算它们的卟啉的摩尔百分比。在黑暗中12d,水稻幼苗的叶绿素从2.5μg/g增加到7.5μg/g,但叶绿素总量从0.36μg/g增至3.6μg/g。在黑暗中未经处理的幼苗Proto,Mg-Proto及Pchlide的摩尔百分比分别为65%,27.5%和7.5%;而光照下幼苗相应的摩尔百分比分别为42.5%,35.0%和22.5%。在黑暗中用葡萄糖处理水稻黄化幼苗2d。其卟啉的摩尔百分比即可恢复到正常值（如光照下之相同比例）。在黑暗中以3%和6%的葡萄糖处理水稻黄化幼苗2d,其叶绿素含量分别增加2.5和4.0倍;若同时辅以1mmool/Lδ-氨基酮戊酸,其叶绿素含量分别增加22和24倍,因此,被子植物在黑暗中可以合成叶绿素;葡萄糖或δ-氨基酮戊酸可以促进被子植物在黯部合成叶绿素;葡萄糖和δ-氨基酮戊酸并用有加成作用。葡萄糖或δ-氨基酮戊酸促进水稻在黑暗中合成叶绿素在生理机制有待研究。     

小麦突变体返白系返白阶段叶绿素代谢的变化

小麦 ( Triticum aestivum)返白系在返白阶段 Chl、胡萝卜素 ( Caro)含量均下降 ,但 Caro/ Chl的比值大于对照 ,表明叶片白化不是因 Caro减少引起的。Chl下降的同时 ,Chla和 Chlb均下降 ,表明该突变体属阶段性缺总 Chl型。返白初期 Chlase活性增高 ,返白中期活性下降 ,表明 Chl降解不是造成叶片失绿的主要原因 ;Chl合成中间物 δ-氨基酮戊酸 ( ALA)、胆色素原 ( PBG)积累 ,尿卟啉 ( Uro )、原卟啉 ( Proto )、镁 -卟啉 ( Mg- Proto )、原叶绿素酸 Pchl减少 ,特别是 Uro 在返白中期含量最低 ,复绿初期却急剧积累 ,表明叶绿素合成受阻于尿卟啉原 ( Urogen )的形成上。     

高等植物叶绿素生物合成的研究进展

叶绿素是植物叶绿体内参与光合作用的重要色素,其功能是捕获光能并驱动电子转移到反应中心.整个叶绿素生物合成过程(L-谷氨酰-tRNA→叶绿素a→叶绿素b)需要15步反应,涉及15种酶,迄今在模式植物拟南芥中已分离到27个编码这些酶的基因,完成了以拟南芥为代表的被子植物叶绿素生物合成全部基因的克隆.本文主要对近年来国内外有关植物叶绿素的生物合成过程及相关酶基因的克隆、生物合成途径中2个关键步骤(σ-氨基酮戊酸(ALA)合成和Mg离子插入原卟啉Ⅸ的调节)、影响叶绿素生物合成的主要因素(光、温度、营养元素等),以及叶绿素生物合成相关酶的其他生物学功能等的研究进展进行综述.     

叶面喷施亚精胺对高温胁迫下番茄叶绿素合成代谢的影响

以设施番茄栽培品种‘金棚朝冠’幼苗为试验材料,研究叶面喷施0.25 mmol·L~(-1)亚精胺(Spd)对高温胁迫下(38℃/28℃,昼/夜)番茄幼苗生长及叶绿素合成过程中前体物质含量、关键酶活性和叶绿素含量的影响,探讨Spd缓解番茄高温胁迫伤害的生理机制。结果表明:(1)高温胁迫显著抑制了番茄幼苗的生长,叶面喷施Spd可有效缓解高温胁迫对番茄幼苗地上部生长的抑制作用,但对于地下部的生长无显著缓解作用。(2)高温胁迫下番茄叶片叶绿素合成前体物质δ-氨基酮戊酸(ALA)和胆色素原(PBG)的含量显著提高,而尿卟啉原Ⅲ(UroⅢ)、原卟啉Ⅸ(ProtoⅨ)、镁-原卟啉Ⅸ(Mg-protoⅨ)和原叶绿素酸(Pchl)的含量显著降低,证明叶绿素前体由PBG向UroⅢ的转化受阻,导致叶绿素a(Chla)、总叶绿素(Chlt)含量和Chla/Chlb显著降低。(3)叶面喷施Spd能增强高温胁迫下胆色素原脱氨酶(PBGD)活性,有效缓解了高温胁迫对PBG到UroⅢ转化的阻碍作用,促进PBG之后叶绿素合成前体物质的合成,Chla含量和Chla/Chlb显著增加。研究发现,高温胁迫显著抑制番茄幼苗的生长,叶面喷施Spd可通过显著增强PBGD活性来缓解由PBG向UroⅢ的受阻程度,促进高温胁迫下番茄叶片的叶绿素前体合成,提高叶绿素含量和番茄植株的生长量,减轻高温胁迫对番茄幼苗生长的伤害。     

The formation of chlorophyll from chlorophyllide in leaves containing proplastids is a four-step process

The time course of the different esters of chlorophyllide (Chlide) during the formation of chlorophyll a (Chl) in embryonic bean leaves containing proplastids was investigated by HPLC. After the reduction of photoactive Pchlide (Pchlide) to Chlide, three intermediates, i.e. Chlide geranylgeraniol, Chlide dihydrogeranylgeraniol and Chlide tetrahydrogeranylgeraniol were detected before the formation of Chlide phytol, i.e. authentic Chl. The transformation of Chlide to Chl was found to be much faster in leaves containing proplastids than in etiolated leaves with etioplasts.     

Plastid development in germinating wheat (Triticum aestivum) is enhanced by gibberellic acid and delayed by gabaculine

Etioplast development and protochlorophyllide (Pchlide) accumulation was studied in wheat seedlings ( Triticum aestivum L. cv. Walde, Weibull) grown in darkness on gibberellic acid (GA₃), gabaculine (3-amino-2,3-dihydrobenzoic acid), or on a combination of the two. The results were compared with the features of seedlings grown on water only. GA₃ enhanced shoot growth and promoted etioplast development. A correlation was observed between the appearance of prolamellar bodies (PLBs) and of phototransformable Pchlide. Gabaculine, a known tetrapyrrole biosynthesis inhibitor, delayed growth, slowed down the rate of PLB formation and caused structural alterations of the etioplasts up to 48 h of germination. Gabaculine also delayed the formation of phototransformable Pchlide as well as overall Pchlide biosynthesis, as determined by low-temperature fluorescence emission in vivo. The spectral blue-shift of newly formed chlorophyllide (Chlide) was delayed in irradiated dark-grown gabaculine-grown seedlings, indicating an inhibited dissociation of Chlide and NADPH-Pchlide oxidoreductase (Pchlide reductase: EC 1.3.1.33). Thus there is a close correlation between accumulation of Pchlide and etioplast development, also under conditions when development is enhanced or delayed.     

High salt stress in wheat leaves causes retardation of chlorophyll accumulation due to a limited rate of protochlorophyllide formation

When exposed to salt stress, leaves from dark-grown wheat seedlings ( Triticum aestivum , cv. Giza 168) showed reduced accumulation of chlorophyll during irradiation. To elucidate the mechanism behind salt-influenced reduction of chlorophyll biosynthesis, we have investigated the effect of salt stress on the spectral forms of Pchlide, the phototransformation of Pchlide to Chlide, the Shibata shift, the regeneration of Pchlide and the accumulation of Pchlide from 5-aminolevulinic acid (ALA). We found that the phototransformation of Pchlide to Chlide was not affected by salt stress. The blue shift (Shibata shift) of newly formed Chlide was delayed both after flash irradiation and in continuous light. The reformation of Pchlide in darkness after a flash irradiation or after a period of 3-h irradiation was retarded in the salt-treated leaves. However, after a 20-h dark period, Pchlide was reformed even in salt-treated leaves but the formation of short-wavelength Pchlide was suppressed. Compared to controls, salt treatment also reduced the amount of Pchlide accumulated in leaves floated on ALA. The increase in the low temperature fluorescence emission spectrum at 735 nm, which occurred gradually during several hours of irradiation with continuous light in control leaves, was completely suppressed in salt-treated leaves. It is concluded that salt stress inhibits chlorophyll accumulation partly by reducing the rate of porphyrin formation but, as discussed, also by a possible reduction in the formation of chlorophyll-binding proteins.     

Spectroscopic analysis of desiccation-induced alterations of the chlorophyllide transformation pathway in etiolated barley leaves

Effects of water deficit on the chlorophyllide (Chlide) transformation pathway were studied in etiolated barley (Hordeum vulgare) leaves by analyzing absorption spectra and 77-K fluorescence spectra deconvoluted in components. Chlide transformations were examined in dehydrated leaves exposed to a 35-ms saturating flash triggering protochlorophyllide (Pchlide) and Chlide transformation processes. During the 90 min following the flash, we found that dehydration induced modifications of Chlide transformations, but no effect on Pchlide phototransformation into Chlide was observed. During this time, content of NADPH-Pchlide oxydoreductase in leaves did not change. Chlide transformation process in dehydrated leaves was characterized by the alteration of the Shibata shift process, by the appearance of a new Chlide species emitting at 692 nm, and by the favored formation of Chl(ide) A(668)F(676). The formation of Chl(ide) A(668)F(676), so-called "free Chlide," was probably induced by disaggregation of highly aggregated Chlide complexes. Here, we offer evidence for the alteration of photoactive Pchlide regeneration process, which may be caused by the desiccation-induced inhibition of Pchlide synthesis.     

Mechanistic Reappraisal of Early Stage Photochemistry in the Light-Driven Enzyme Protochlorophyllide Oxidoreductase

The light-driven enzyme protochlorophyllide oxidoreductase (POR) catalyzes the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). This reaction is a key step in the biosynthesis of chlorophyll. Ultrafast photochemical processes within the Pchlide molecule are required for catalysis and previous studies have suggested that a short-lived excited-state species, known as I675*, is the first catalytic intermediate in the reaction and is essential for capturing excitation energy to drive subsequent hydride and proton transfers. The chemical nature of the I675* excited state species and its role in catalysis are not known. Here, we report time-resolved pump-probe spectroscopy measurements to study the involvement of the I675* intermediate in POR photochemistry. We show that I675* is not unique to the POR-catalyzed photoreduction of Pchlide as it is also formed in the absence of the POR enzyme. The I675* species is only produced in samples that contain both Pchlide substrate and Chlide product and its formation is dependent on the pump excitation wavelength. The rate of formation and the quantum yield is maximized in 50∶50 mixtures of the two pigments (Pchlide and Chlide) and is caused by direct energy transfer between Pchlide and neighboring Chlide molecules, which is inhibited in the polar solvent methanol. Consequently, we have re-evaluated the mechanism for early stage photochemistry in the light-driven reduction of Pchlide and propose that I675* represents an excited state species formed in Pchlide-Chlide dimers, possibly an excimer. Contrary to previous reports, we conclude that this excited state species has no direct mechanistic relevance to the POR-catalyzed reduction of Pchlide.     

Prolonged salt stress alters the ratios of protochlorophyllide spectral forms in dark-grown wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) and influences chlorophyll <Emphasis Type="Italic">a</Emphasis> accumulation following irradiation

To examine the effects of salt stress on dark-grown wheat (Triticum aestivum), seedlings of the salt-tolerant cultivar Sids 1 and the susceptible cultivar Giza 168 were grown in darkness for 14 days in nutrient solution with and without 200 mM of supplementary salt (100 mM of NaCl and 100 mM of KCl). During this time, we monitored their protochlorophyllide (Pchlide) contents, ratios of photoactive to non-photoactive forms of Pchlide (from 655/633-nm emission ratios in their 77 K fluorescence emission spectra) and (following flash irradiation) ratios of newly formed chlorophyllide (Chlide) to non-photoactive Pchlide. In addition, the accumulation of chlorophyll a in leaf sections was monitored during prolonged (24 h) irradiation. The results depended on the developmental state of the seedlings. However, the salt stress treatment caused marked increases in both Pchlide contents in dark-grown leaves and in Chlide contents following irradiation of leaf sections of both cultivars. The ratio of phototransformable to non-phototransformable Pchlide and the abundance of newly formed Chlide were also increased by the salt stress. Further, leaves of salt-stressed seedlings consistently accumulated more chlorophyll a than leaves of unstressed seedlings when floating on the nutrient solution (with or without supplementary salt) in continuous white light. The findings are consistent with the hypothesis that increased levels of the long-wavelength form of Pchlide contribute to protective mechanisms against salt stress.     

Light-regulated pigment interconversion in pheophytin/chlorophyll-containing complexes formed during plant leaves greening

Upon illumination of etiolated maize leaves the photoconversion of protochlorophyllide Pchlide 655/650 into chlorophyllide Chlide 684/676 was observed. It was shown that chlorophyllide Chlide 684/676 in the dark is transformed into pheophytin Pheo 679/675 and chlorophyll Chl 671/668 by means of two parallel reactions, occurring at room temperature: Chlide 684/676. The formed pheophytin Pheo 679/675 was unstable and in the dark was transformed into chlorophyll Chl 671/668 in a few seconds: Pheo 679/675 Chl 671/668. The last reaction is reversed by the light: Chl/668 Pheo 679/675. Thus, on the whole in the greening etiolated leaves this process occurs according to the following scheme:The observed light-regulated interconversion of Mg-containing and Mg-free chlorophyll analogs is activated by ATP and inhibited by AMP.Abbreviations Chl- chlorophyll - Chlide- chlorophyllide - Pchlide- protochlorophyllide - Pheo- pheophytin - PS II RC- Photosystem II reaction centres. Abbreviations for native pigment forms: the first number after the pigment symbol corresponds to the maximum position of the low-temperature fluorescence band (nm), the second number to the maximum position of the longwave absorption band     

POR - import and membrane association of a key element in chloroplast development

The development of proplastids or etioplasts to chloroplast is visualized by the accumulation of chlorophyll in leaves of higher plants. The biosynthesis of chlorophyll includes a light-dependent reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). This light-dependent step is catalysed by the nucleus-encoded NADPH:Pchlide oxidoreductase (POR, EC 1.6.99.1). POR is active within plastids and therefore has to be translocated over the plastid envelope membranes. The import of chloroplast proteins seems to follow a general import pathway using translocons at the outer and inner envelope membrane. POR cross-linking to Toc75, one of the major translocon components at the outer envelope membrane, indicates its use of the general import pathway. However, since variations exist within the so-called general import pathway one has to consider previous data suggesting a novel totally Pchlide-dependent import pathway of one POR isoform, PORA. The suggested Pchlide dependency of POR import is discussed since recent observations contradict this idea. In the stroma the POR transit peptide is cleaved off and the mature POR protein is targeted to the plastid inner membranes. The correct and stable association of POR to the membrane requires the cofactor NADPH. Functional activity of POR calls for formation of an NADPH–Pchlide–POR complex, a formation that probably takes place after the membrane association and is dependent on a phosphorylation reaction.     

Identification of NADPH:protochlorophyllide oxidoreductases A and B: a branched pathway for light-dependent chlorophyll biosynthesis in Arabidopsis thaliana.

Illumination releases the arrest in chlorophyll (Chl) biosynthesis in etiolated angiosperm seedlings through the enzymatic photoreduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), the first light-dependent step in chloroplast biogenesis. NADPH: Pchlide oxidoreductase (POR, EC 1.3.1.33), a nuclear-encoded plastid-localized enzyme, mediates this unique photoreduction. Paradoxically, light also triggers a drastic decrease in the amounts of POR activity and protein before the Chl accumulation rate reaches its maximum during greening. While investigating this seeming contradiction, we identified two distinct Arabidopsis thaliana genes encoding POR, in contrast to previous reports of only one gene in angiosperms. The genes, designated PorA and PorB, by analogy to the principal members of the phytochrome photoreceptor gene family, display dramatically different patterns of light and developmental regulation. PorA mRNA disappears within the first 4 h of greening, whereas PorB mRNA persists even after 16 h of illumination, mirroring the behavior of two distinct POR protein species. Experiments designed to help define the functions of POR A and POR B demonstrate exclusive expression of PorA in young seedlings and of PorB both in seedlings and in adult plants. Accordingly, we propose the existence of a branched light-dependent Chl biosynthesis pathway in which POR A performs a specialized function restricted to the initial stages of greening and POR B maintains Chl levels throughout angiosperm development.     

Protochlorophyllide reduction - what is new in 2005?

Because the transformation of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) is an irradiation-dependent process, it is at the heart of the photosynthetic membrane biogenesis, turnover, and adaptation to changes of the environment. I review here the new data published during the year 2004 on Pchlide reduction to Chlide.