四吡咯代谢中间产物在植物质体向细胞核信号转导中的作用

在植物细胞内,除了顺向的信号转导通路,即核基因控制着质体基因的转录和翻译之外,还存在着逆向的信号转导通路,即质体的代谢状况作为一种信号去调控核基因的表达。过去对这条逆向的信号转导通路,亦称质体因子,研究得非常少。近几年来,随着对基因组解偶联突变体的深入研究,人们对这条通路的认识大大加深了。现着重介绍质体中的四吡咯代谢中间产物参与信号的产生,以及质体向细胞质搬运这些中间产物启动了对编码质体蛋白的核基因的表达调控。     

Plastid signalling under multiple conditions is accompanied by a common defect in RNA editing in plastids

Retrograde signalling from the plastid to the nucleus, also known as plastid signalling, plays a key role in coordinating nuclear gene expression with the functional state of plastids. Inhibitors that cause plastid dysfunction have been suggested to generate specific plastid signals related to their modes of action. However, the molecules involved in plastid signalling remain to be identified. Genetic studies indicate that the plastid-localized pentatricopeptide repeat protein GUN1 mediates signalling under several plastid signalling-related conditions. To elucidate further the nature of plastid signals, investigations were carried out to determine whether different plastid signal-inducing treatments had similar effects on plastids and on nuclear gene expression. It is demonstrated that norflurazon and lincomycin treatments and the plastid protein import2-2 (ppi2-2) mutation, which causes a defect in plastid protein import, all resulted in similar changes at the gene expression level. Furthermore, it was observed that these three treatments resulted in defective RNA editing in plastids. This defect in RNA editing was not a secondary effect of down-regulation of pentatricopeptide repeat protein gene expression in the nucleus. The results indicate that these three treatments, which are known to induce plastid signals, affect RNA editing in plastids, suggesting an unprecedented link between plastid signalling and RNA editing.     

Plastid-to-nucleus signalling

The function of the eukaryotic cell depends on the reciprocal interaction between its different compartments. Plastids emit signals that regulate nuclear gene expression to ensure the stoichiometric assembly of plastid protein complexes and to initiate macromolecular reorganisation in response to environmental cues. It is now clear that several different plastid processes produce signals that influence the expression of photosynthetic genes in the nucleus. The genome uncoupled (gun) mutants recently revealed one of the plastid signals, the chlorophyll intermediate Mg-protoporphyrinIX.     

A new gun mutant of oilseed rape with a reduced porphyrin flux through Mg-chelatase

Plastid-to-nucleus retrograde signaling coordinates the expression of nuclear photosynthetic genes with the developmental and functional state of the plastid. These signals are essential not only for coordinating the expression of photosynthetic genes both in the plastome and nuclear genome, but also for plants to respond optimally to environmental stress. In the present study, we found that the expression of the nuclear genes that encode plastid and non-plastid photosynthesis-related proteins was still maintained or slightly higher in cr3529, a chlorophyll deficient mutant of oilseed rape that possesses an arrested development of chloroplasts, suggesting that the expression of photosynthesis-related nuclear genes was uncoupled from the normal dependence on the developmental state of the chloroplast in cr3529. When the development of the plastid in cr3529 and the wild type was completely inhibited by lincomycin, much higher expression of photosynthesis-related nuclear genes was observed in cr3529, suggesting that the genomes uncoupled (gun) phenotype of cr3529 is even more apparent than under normal growth conditions. Lincomycin treatment also derepressed the expression of plastid genes in cr3529. The determination of porphyrin flux through Mg-chelatase showed that the content of protoporphyrin IX and Mg-protoporphyrin decreased in cr3529. The obvious gun phenotype of cr3529 under normal growth conditions and the pattern of tetrapyrrole metabolism in cr3529 suggest that it is a new gun mutant that could be used to study the regulation of the expression of nuclear and plastid genes by plastid-to-nucleus retrograde signaling under more physiological conditions and the mechanism of plant stress responses mediated by plastid signals.     

Retrograde signaling and plant stress: plastid signals initiate cellular stress responses

Retrograde signaling coordinates the expression of nuclear genes encoding organellar proteins with the metabolic and developmental state of the organelle. These plastid signals are essential not only for coordinating photosynthetic gene expression in both the nucleus and in the chloroplasts but also for mediating plant stress responses. The chloroplasts therefore act as sensors of environmental changes and complex networks of plastid signals coordinate cellular activities and assist the cell during plant stress responses. Recent work suggests that information from both cytosolic-signaling and plastid-signaling networks must be integrated for the plant cell to respond optimally to environmental stress.     

The chloroplast division mutant caa33 of Arabidopsis thaliana reveals the crucial impact of chloroplast homeostasis on stress acclimation and retrograde plastid-to-nucleus signaling

Retrograde plastid-to-nucleus signaling tightly controls and coordinates the nuclear and plastid gene expression that is required for plastid biogenesis and chloroplast activity. As chloroplasts act as sensors of environmental changes, plastid-derived signaling also modulates stress responses of plants by transferring stress-related signals and altering nuclear gene expression. Various mutant screens have been undertaken to identify constituents of plastid signaling pathways. Almost all mutations identified in these screens target plastid-specific but not extraplastidic functions. They have been suggested to define either genuine constituents of retrograde signaling pathways or components required for the synthesis of plastid signals. Here we report the characterization of the constitutive activator of AAA-ATPase (caa33) mutant, which reveals another way of how mutations that affect plastid functions may modulate retrograde plastid signaling. caa33 disturbs a plastid-specific function by impeding plastid division, and thereby perturbing plastid homeostasis. This results in preconditioning plants by activating the expression of stress genes, enhancing pathogen resistance and attenuating the capacity of the plant to respond to plastid signals. Our study reveals an intimate link between chloroplast activity and the susceptibility of the plant to stress, and emphasizes the need to consider the possible impact of preconditioning on retrograde plastid-to-nucleus signaling.     

'happy on norflurazon' (hon) mutations implicate perturbance of plastid homeostasis with activating stress acclimatization and changing nuclear gene expression in norflurazon-treated seedlings

Various mutant screens have been undertaken to identify constituents involved in the transmission of signals from the plastid to the nucleus. Many of these screens have been performed using carotenoid-deficient plants grown in the presence of norflurazon (NF), an inhibitor of phytoene desaturase. NF-treated plants are bleached and suppress the expression of nuclear genes encoding chloroplast proteins. Several genomes uncoupled (gun) mutants have been isolated that de-repress the expression of these nuclear genes. In the present study, a genetic screen has been established that circumvents severe photo-oxidative stress in NF-treated plants. Under these modified screening conditions, happy on norflurazon (hon) mutants have been identified that, like gun mutants, de-repress expression of the Lhcb gene, encoding a light-harvesting chlorophyll protein, but, in contrast to wild-type and gun mutants, are green in the presence of NF. hon mutations disturb plastid protein homeostasis, thereby activating plastid signaling and inducing stress acclimatization. Rather than defining constituents of a retrograde signaling pathway specifically associated with the NF-induced suppression of nuclear gene expression, as proposed for gun, hon mutations affect Lhcb expression more indirectly prior to initiation of plastid signaling in NF-treated seedlings. They pre-condition seedlings by inducing stress acclimatization, thereby attenuating the impact of a subsequent NF treatment.     

Photosynthetic redox control of nuclear gene expression

Chloroplasts contain 3000-4000 different proteins but only a small subset of them is encoded in the plastid genome while the majority is encoded in the nucleus. Expression of these genes therefore requires a high degree of co-ordination between nucleus and chloroplast. This is achieved by a bilateral information exchange between both compartments including nucleus-to-plastid (anterograde) and plastid-to-nucleus (retrograde) signals. The latter represent a functional feedback control which couples the expression of nuclear encoded plastid proteins to the actual functional state of the organelle. The efficiency of photosynthesis is a very important parameter in this context since it is influenced by many environmental conditions and therefore represents a sensor for the residing environment. Components of the photosynthetic electron transport chain exhibit significant changes in their reduction/oxidation (redox) state depending on the photosynthetic electron flow and therefore serve as signalling parameters which report environmental influences on photosynthesis. Such redox signals control chloroplast and nuclear gene expression events and play an important role in the co-ordination of both genetic compartments. It is discussed here which photosynthetic parameters are known to control nuclear gene expression, how these signals are transduced toward the nucleus, and how they interact with other plastid retrograde signals and cytosolic light perception systems.     

Plastid signals remodel light signaling networks and are essential for efficient chloroplast biogenesis in Arabidopsis

Plastid signals are among the most potent regulators of genes that encode proteins active in photosynthesis. Plastid signals help coordinate the expression of the nuclear and chloroplast genomes and the expression of genes with the functional state of the chloroplast. Here, we report the isolation of new cryptochrome1 (cry1) alleles from a screen for Arabidopsis thaliana genomes uncoupled mutants, which have defects in plastid-to-nucleus signaling. We also report genetic experiments showing that a previously unidentified plastid signal converts multiple light signaling pathways that perceive distinct qualities of light from positive to negative regulators of some but not all photosynthesis-associated nuclear genes (PhANGs) and change the fluence rate response of PhANGs. At least part of this remodeling of light signaling networks involves converting HY5, a positive regulator of PhANGs, into a negative regulator of PhANGs. We also observed that mutants with defects in both plastid-to-nucleus and cry1 signaling exhibited severe chlorophyll deficiencies. These data show that the remodeling of light signaling networks by plastid signals is a mechanism that plants use to integrate signals describing the functional and developmental state of plastids with signals describing particular light environments when regulating PhANG expression and performing chloroplast biogenesis.     

Evidence that the plastid signal and light operate via the samecis-acting elements in the promoters of nuclear genes for plastid proteins

Nuclear-encoded genes for proteins of the photosynthetic maschinery represent a particular subset of genes. Their expression is cooperatively stimulated by discrete factors including the developmental stage of plastids and light. We have analyzed in transgenic tobacco the plastid- and light-dependent expression of a series of 5′ promoter deletions of various nuclear genes from spinach, of fusions of defined promoter segments with the 90-bp 35S RNA CaMV minimal promoter, as well as with mutations in sequences with homologies to characterizedcis-elements, to address the question of whether the plastid signal and light operate via the same or differentcis-acting elements. In none of the 160 different transgenic lines (representing 32 promoter constructs from seven genes) analyzed, could significant differences be identified in the responses to the two regulatory pathways. The data are compatible with the idea that both signals control the expression of nuclear genes for plastid proteins via the samecis-acting elements.     

The discovery of plastid-to-nucleus retrograde signaling—a personal perspective

DNA and machinery for gene expression have been discovered in chloroplasts during the 1960s. It was soon evident that the chloroplast genome is relatively small, that most genes for chloroplast-localized proteins reside in the nucleus and that chloroplast membranes, ribosomes, and protein complexes are composed of proteins encoded in both the chloroplast and the nuclear genome. This situation has made the existence of mechanisms highly probable that coordinate the gene expression in plastids and nucleus. In the 1970s, the first evidence for plastid signals controlling nuclear gene expression was provided by studies on plastid ribosome deficient mutants with reduced amounts and/or activities of nuclear-encoded chloroplast proteins including the small subunit of Rubisco, ferredoxin NADP+ reductase, and enzymes of the Calvin cycle. This review describes first models of plastid-to-nucleus signaling and their discovery. Today, many plastid signals are known. They do not only balance gene expression in chloroplasts and nucleus during developmental processes but are also generated in response to environmental changes sensed by the organelles.     

In vivo visualization of Mg-protoporphyrin IX, a coordinator of photosynthetic gene expression in the nucleus and the chloroplast

The photosynthetic apparatus is composed of proteins encoded by genes from both the nucleus and the chloroplast. To ensure that the photosynthetic complexes are assembled stoichiometrically and to enable their rapid reorganization in response to a changing environment, the plastids emit signals that regulate nuclear gene expression to match the status of the plastids. One of the plastid signals, the chlorophyll intermediate Mg-ProtoporphyrinIX (Mg-ProtoIX) accumulates under stress conditions and acts as a negative regulator of photosynthetic gene expression. By taking advantage of the photoreactive property of tetrapyrroles, Mg-ProtoIX could be visualized in the cells using confocal laser scanning spectroscopy. Our results demonstrate that Mg-ProtoIX accumulated both in the chloroplast and in the cytosol during stress conditions. Thus, the signaling metabolite is exported from the chloroplast, transmitting the plastid signal to the cytosol. Our results from the Mg-ProtoIX over- and underaccumulating mutants copper response defect and genome uncoupled5, respectively, demonstrate that the expression of both nuclear- and plastid-encoded photosynthesis genes is regulated by the accumulation of Mg-ProtoIX. Thus, stress-induced accumulation of the signaling metabolite Mg-ProtoIX coordinates nuclear and plastidic photosynthetic gene expression.