植物叶片衰老过程中叶绿素降解代谢研究进展

本文对近年来植物叶片衰老过程中叶绿素降解代谢研究进展作一介绍,包括叶绿素降解产物分离、检测和命名;叶绿素降解途径及降解酶系。此外,对叶绿素降解意义及今后研究趋势进行了评述。     

植物叶片衰老过程中叶绿素降解代谢研究进展

 本文对近年植物叶片衰老过程中叶绿素降解代谢研究进展作一介绍,包括叶绿素降解产物分离、检测和命名;叶绿素降解途径及降解酶系。此外,对叶绿素降解意义及今后研究趋势进行了评述。     

光呼吸途径及其功能

光呼吸是C3植物体内重要的代谢过程,是光合作用研究的热点之一。本文阐述了光呼吸的正常代谢途径及乙醛酸代谢的交替途径,交替途径的功能,及途径中关键酶的生物学特性。就光呼吸在减轻逆境伤害、减缓叶绿素的降解、驱动卡尔文循环、参与三羧酸循环、氮素代谢、蛋白质积累以及PSI和PSⅡ之间的状态转换等生物学功能进行了综述。     

光呼吸途径及其功能

光呼吸是C3植物体内重要的代谢过程,是光合作用研究的热点之一。本文阐述了光呼吸的正常代谢途径及乙醛酸代谢的交替途径,交替途径的功能,及途径中关键酶的生物 学特性。就光呼吸在减轻逆境伤害、减缓叶绿素的降解、驱动卡尔文循环、参与三羧酸循环、氮素代谢、蛋白质积累以及PSⅠ和PSⅡ之间的状态转换等生物学功能进行了综述。     

氮代谢参与植物逆境抵抗的作用机理研究进展

近年来,植物所受到的诸如干旱、盐、高温、低氧、重金属胁迫和营养元素缺乏等环境胁迫越来越多,严重影响了植物的生长发育及作物的质量和产量。氮素是植物生长发育所需的必需营养元素,同时也是核酸、蛋白质和叶绿素的重要组成成分,其代谢过程与植物抵抗逆境的能力息息相关。氮代谢是指植物对氮素的吸收、同化和利用的全过程,是植物体内基础代谢途径之一。氮代谢主要从氮素吸收、同化及氨基酸代谢等方面参与植物的抗逆性,并通过调节离子吸收和转运、稳定细胞形态和蛋白质结构、维持激素平衡和细胞代谢水平、减少体内活性氧(reactive oxygen species,ROS)生成以及促进叶绿素合成等生理机制来影响植物抵抗非生物胁迫的能力。因此,提高植物在逆境下的氮代谢水平是减轻外界胁迫对其损伤的一种潜在途径。该文从氮素同化的基本途径出发,分别阐述了氮代谢在干旱胁迫、盐胁迫和高温胁迫等多个方面的逆境抵抗过程中的作用机理,为氮代谢参与植物抗逆性研究提供了有利参考。     

植物叶绿素缺失突变体分子机制研究进展

植物叶绿素缺失突变体在自然界中广泛存在,是研究叶绿素形成和叶绿体发育等代谢途径的良好材料.该文主要从分子层面上阐述了叶绿素缺失突变体产生的原因,如叶绿素合成受阻、叶绿体光合蛋白合成或输入受阻、叶绿体RNA转录物未被编辑、过量光损伤和卟啉循环各物质之间的相互抑制,并归纳了近年来鉴定出来的一些叶绿素缺失突变基因,简要介绍了叶绿素和叶绿体之间的关系以及叶绿素缺失突变体的应用.     

植物叶绿素降解机制研究进展

叶绿素降解与作物产量密切相关,叶绿素降解延迟,能延长作物后期的光合能力,并提高作物产量。近年随着结构生物学、基因组测序和生物信息学的发展,人们已经在植物叶绿素降解机制的研究上取得了一系列进展,特别是对叶绿素降解的主要生化途径——脱镁叶绿酸氧化酶(pheide a oxygenase,Pa O)途径已有深入的了解。主要对近年来叶绿素降解代谢、调控机理、滞绿突变体等三方面的研究进展进行综述,并对未来研究方向进行了展望,旨为作物育种和光高效利用提供理论依据。     

叶绿素酶的研究进展

叶绿素的降解代谢被公认是一个难解的生物学之谜.叶绿素酶是迄今为止了解最多的叶绿素酶促降解途径的重要组成酶之一.已有多位学者通过不同方法对该酶进行了分离纯化鉴定.研究发现,叶绿素酶主要定位于叶绿体膜上,与底物叶绿素分子存在着空间隔离,其催化反应的最适pH值是7.8～8.5,Km值是3.1～278μmol/L ;催化反应的最适温度因反应微环境的不同而各异,叶绿素酶至少存在两种同工酶;并对叶绿素酶基因学及酶活调控等进行了探讨.     

绿色器官衰老进程中叶绿素降解代谢及其调控的研究进展

褪绿(degreening)现象是绿色器官衰老的显著标志,其中涉及的叶绿素急速降解是植物衰老的特征性生理生化过程。近十余年来,叶绿素降解的主要生化途径(PAO途径)已基本被阐明。游离的叶绿素及其代谢中间产物具有潜在的光毒性,因此精准调控的叶绿素降解被认为是一个高效有序的解毒过程。目前关于叶绿素降解的全局性调控机制依然知之甚少;多个物种中叶绿素降解关键调控因子SGRs/NYEs的鉴别及其相关作用机理的初步揭示是该领域近年来的最显要进展。上述研究基础预示着近期在叶绿素降解调控的多个方向上可能会取得实质性乃至突破性进展。     

植物3-羟基-3-甲基戊二酰辅酶A还原酶基因研究进展

细胞分裂素、赤霉素、脱落酸、叶绿素、萜类等类异戊二烯物质,是植物中广泛存在的一类代谢产物,在植物生长发育过程中起着非常重要的作用。一些萜类化合物作为药物的合成前体或有效的药用成分在工农业及医药生产上具有重要的经济价值。类异戊二烯物质主要通过甲羟戊酸代谢途径中的一系列酶催化合成,其中,3-羟基-3-甲基戊二酰辅酶A还原酶(3-hydroxy-3-methylglutaryl coenzyme A reductase, HMGR)是该代谢途径中的第一个关键限速酶,能够将3-羟基-3-甲基戊二酰辅酶A转化成中间代谢产物甲羟戊酸。对植物HMGR基因的克隆、酶结构和功能分析、基因组织表达及调控等方面进行了综述,旨在为其在重要农作物的遗传改良、代谢产物工程植物创制以及植物亲缘关系分析中的应用等研究提供理论依据。     

The control of chlorophyll catabolism and the status of yellowing as a biomarker of leaf senescence

The pathway of chlorophyll catabolism during leaf senescence is known in a fair amount of biochemical and cell biological detail. In the last few years, genes encoding a number of the catabolic enzymes have been characterized, including the key ring-opening activities, phaeophorbide a oxygenase (PaO) and red chlorophyll catabolite reductase (RCCR). Recently, a gene that modulates disassembly of chlorophyll–protein complexes and activation of pigment ring-opening has been isolated by comparative mapping in monocot species, positional cloning exploiting rice genomics resources and functional testing in Arabidopsis. The corresponding gene in pea has been identified as Mendel's I locus (green/yellow cotyledons). Mutations in this and other chlorophyll catabolic genes have significant consequences, both for the course of leaf senescence and senescence-like stress responses, notably hypersensitivity to pathogen challenge. Loss of chlorophyll can occur via routes other than the PaO/RCCR pathway, resulting in changes that superficially resemble senescence. Such 'pseudosenescence' responses tend to be pathological rather than physiological and may differ from senescence in fundamental aspects of biochemistry and regulation.     

Update on the biochemistry of chlorophyll breakdown

In land plants, chlorophyll is broken down to colorless linear tetrapyrroles in a highly conserved multi-step pathway. The pathway is termed the ‘PAO pathway’, because the opening of the chlorine macrocycle present in chlorophyll catalyzed by pheophorbide a oxygenase (PAO), the key enzyme of the pathway, provides the characteristic structural basis found in all further downstream chlorophyll breakdown products. To date, most of the biochemical steps of the PAO pathway have been elucidated and genes encoding many of the chlorophyll catabolic enzymes been identified. This review summarizes the current knowledge on the biochemistry of the PAO pathway and provides insight into recent progress made in the field that indicates that the pathway is more complex than thought in the past.     

Localization of chlorophyllase in the chloroplast envelope

Chlorophyllase catalyzes the first step in the catabolic pathway of chlorophyll. It is a constitutive enzyme located in chloroplast membranes. In isolated plastids the hydrolysis of the endogenous chlorophyll does not take place unless the membranes are solubilized in the presence of detergent. The structural latency of chlorophyllase activity appears to be due to the differential locations of substrate and enzyme within the plastids. Envelope membranes prepared from both chloroplasts and gerontoplasts contain chlorophyllase activity. The isolation of envelopes is associated with a marked increase in chlorophyllase activity per unit of protein. Yields of chlorophyllase and of specific envelope markers in the final preparations are similar, suggesting that the enzyme may be located in the envelope. It is hypothesized that the breakdown of chlorophyll during leaf senescence requires a mechanism that mediates the transfer of chlorophyll from the thylakoidal pigment-protein complexes to the sites of catabolic reactions in the envelope.Abbreviations ACT acyl CoA thioesterase - Chl chlorophyll - Chlide chlorophyllide - PC phosphatidylcholine     

Pheophytin Pheophorbide Hydrolase (Pheophytinase) Is Involved in Chlorophyll Breakdown during Leaf Senescence in Arabidopsis

During leaf senescence, chlorophyll is removed from thylakoid membranes and converted in a multistep pathway to colorless breakdown products that are stored in vacuoles. Dephytylation, an early step of this pathway, increases water solubility of the breakdown products. It is widely accepted that chlorophyll is converted into pheophorbide via chlorophyllide. However, chlorophyllase, which converts chlorophyll to chlorophyllide, was found not to be essential for dephytylation in Arabidopsis thaliana. Here, we identify pheophytinase (PPH), a chloroplast-located and senescence-induced hydrolase widely distributed in algae and land plants. In vitro, Arabidopsis PPH specifically dephytylates the Mg-free chlorophyll pigment, pheophytin (phein), yielding pheophorbide. An Arabidopsis mutant deficient in PPH (pph-1) is unable to degrade chlorophyll during senescence and therefore exhibits a stay-green phenotype. Furthermore, pph-1 accumulates phein during senescence. Therefore, PPH is an important component of the chlorophyll breakdown machinery of senescent leaves, and we propose that the sequence of early chlorophyll catabolic reactions be revised. Removal of Mg most likely precedes dephytylation, resulting in the following order of early breakdown intermediates: chlorophyll → pheophytin → pheophorbide. Chlorophyllide, the last precursor of chlorophyll biosynthesis, is most likely not an intermediate of breakdown. Thus, chlorophyll anabolic and catabolic reactions are metabolically separated.     

Chlorophyllase is a rate-limiting enzyme in chlorophyll catabolism and is posttranslationally regulated

Chlorophyll is a central player in harvesting light energy for photosynthesis, yet the rate-limiting steps of chlorophyll catabolism and the regulation of the catabolic enzymes remain unresolved. To study the role and regulation of chlorophyllase (Chlase), the first enzyme of the chlorophyll catabolic pathway, we expressed precursor and mature versions of citrus (Citrus sinensis) Chlase in two heterologous plant systems: (1) squash (Cucurbita pepo) plants using a viral vector expression system; and (2) transiently transformed tobacco (Nicotiana tabacum) protoplasts. Expression of full-length citrus Chlase resulted in limited chlorophyll breakdown in protoplasts and no visible leaf phenotype in whole plants, whereas expression of a Chlase version lacking the N-terminal 21 amino acids (ChlaseDeltaN), which corresponds to the mature protein, led to extensive chlorophyll breakdown in both tobacco protoplasts and squash leaves. ChlaseDeltaN-expressing squash leaves displayed a dramatic chlorotic phenotype in plants grown under low-intensity light, whereas under natural light a lesion-mimic phenotype occurred, which was demonstrated to follow the accumulation of chlorophyllide, a photodynamic chlorophyll breakdown product. Full-length and mature citrus Chlase versions were localized to the chloroplast membrane fraction in expressing tobacco protoplasts, where processing of the N-terminal 21 amino acids appears to occur. Results obtained in both plant systems suggest that Chlase functions as a rate-limiting enzyme in chlorophyll catabolism controlled via posttranslational regulation.     

Chlorophyll breakdown in higher plants

Chlorophyll breakdown is an important catabolic process of leaf senescence and fruit ripening. Structure elucidation of colorless linear tetrapyrroles as (final) breakdown products of chlorophyll was crucial for the recent delineation of a chlorophyll breakdown pathway which is highly conserved in land plants. Pheophorbide a oxygenase is the key enzyme responsible for opening of the chlorin macrocycle of pheophorbide a characteristic to all further breakdown products. Degradation of chlorophyll was rationalized by the need of a senescing cell to detoxify the potentially phototoxic pigment, yet recent investigations in leaves and fruits indicate that chlorophyll catabolites could have physiological roles. This review updates structural information of chlorophyll catabolites and the biochemical reactions involved in their formation, and discusses the significance of chlorophyll breakdown. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.     

What stay-green mutants tell us about nitrogen remobilization in leaf senescence

Leaf senescence has an important role in the plant's nitrogen economy. Chlorophyll catabolism is a visible symptom of protein mobilization. Genetic and environmental factors that interfere with yellowing tend to modify protein degradation as well. The chlorophyll-protein relationship is much closer for membrane proteins than it is for soluble or total leaf proteins. In stay-greens, genotypes with a specific defect in the chlorophyll catabolism pathway, soluble protein degradation during senescence may be close to normal, but light-harvesting and reaction centre thylakoid membrane proteins are much more stable. Genes for the chlorophyll catabolism pathway and its control are important in the regulation of protein mobilization. Genes for three steps in the pathway are reported to have been isolated. The gene responsible for the stay-green phenotype in grasses and legumes has not yet been cloned but a fair amount is known about it. Pigment metabolism in senescing leaves of the Festuca-Lolium stay-green mutant is clearly disturbed and is consistent with a blockage at the ring-opening (PaO) step in chlorophyll breakdown. PaO is de novo synthesized in senescence and thought to be the key enzyme in the chlorophyll a catabolic pathway. The stay-green mutation is likely to be located in the PaO gene, or a specific regulator of it. These genes may well be in the various senescence-enhanced cDNA collections that have been generated, but functional handles on them are currently lacking. When the stay-green locus from Festuca pratensis was introgressed into Lolium temulentum, a gene encoding F. pratensis UDPG-pyrophosphorylase was shown to have been transferred on the same chromosome segment. A strategy is described for cloning the stay-green gene, based on subtractive PCR-based analyses of intergeneric introgressions and map-based cloning.     

Involvement of chlorophyllase in chlorophyll metabolism in olive varieties with high and low chlorophyll content

Olive fruits of the Arbequina variety are differentiated from those of Hojiblanca and Picual by the differing presence of 13²-OH-chlorophyll a and of dephytylated chlorophyll derivatives during the life cycle of the fruit. During the fruit growth stage, which coincides with chlorophyll synthesis, chlorophyllase (EC: 3.1.1.14) is present in the three varieties but only yields chlorophyllides in Arbequina. The presence of oxidized catabolites of chlorophyll a in fruits of the Arbequina variety during this same period confirms the activity of oxidative enzyme systems. The low synthesis of chlorophylls in the fruits of the Arbequina variety is associated with the fact that, during the natural biosynthetic turnover, the catabolic pathway is more potentiated than the anabolic one. In the ripening phase, in the Hojiblanca and Picual fruits, chlorophyllase activity was measured but the absence of chlorophyllides showed that this enzyme remains latent and that oxidative enzymes are the ones taking part in the chlorophyll disappearance. In the Arbequina variety, both chlorophyllase and oxidative enzymes are responsible for the chlorophyll degradation.     

Evolution of Chlorophyll Degradation: The Significance of RCC Reductase

Abstract: In angiosperms the key process of chlorophyll breakdown in senescing leaves is catalyzed by pheophorbide a oxygenase and RCC reductase which, in a metabolically channeled reaction, cleave the porphyrin macrocycle and produce a colourless primary catabolite, pFCC. RCC reductase is responsible for the reduction of the C20/C1 double bond of the intermediary catabolite, RCC. Depending on plant species, RCC reductase produces one of the two C1 stereoisomers, pFCC-1 or pFCC-2. Screening of a large number of taxa for the type of RCCR revealed that the isomer produced is uniform within families. It also revealed that type RCCR-2 is predominant; RCCR-1 seems to represent a recent derivation which in unrelated lineages has evolved independently from RCCR-2. A third type of pFCC was produced by RCCR from basal pteridophytes and some gymnosperms; its structure is unknown. Collectively, the data suggest that the pathway of chlorophyll breakdown is very conserved in vascular plants. RCCR appears to represent a decisive addition to the catabolic pathway: it allows terrestrial plants to metabolize the porphyrin part of the chlorophyll molecule to photodynamically inactive final products that are stored in the vacuoles of senescing mesophyll cells.