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.     

Heme synthesis by plastid ferrochelatase I regulates nuclear gene expression in plants

Chloroplast signals regulate hundreds of nuclear genes during development and in response to stress, but little is known of the signals or signal transduction mechanisms of plastid-to-nucleus (retrograde) signaling. In Arabidopsis thaliana, genetic studies using norflurazon (NF), an inhibitor of carotenoid biosynthesis, have identified five GUN (genomes uncoupled) genes, implicating the tetrapyrrole pathway as a source of a retrograde signal. Loss of function of any of these GUN genes leads to increased expression of photosynthesis-associated nuclear genes (PhANGs) when chloroplast development has been blocked by NF. Here we present a new Arabidopsis gain-of-function mutant, gun6-1D, with a similar phenotype. The gun6-1D mutant overexpresses the conserved plastid ferrochelatase 1 (FC1, heme synthase). Genetic and biochemical experiments demonstrate that increased flux through the heme branch of the plastid tetrapyrrole biosynthetic pathway increases PhANG expression. The second conserved plant ferrochelatase, FC2, colocalizes with FC1, but FC2 activity is unable to increase PhANG expression in undeveloped plastids. These data suggest a model in which heme, specifically produced by FC1, may be used as a retrograde signal to coordinate PhANG expression with chloroplast development.     

Effect of chlorophyll reduction in Arabidopsis thaliana by methyl jasmonate or norflurazon on antioxidant systems.

Methyl jasmonate (MeJA) and norflurazon (NF) treatments resulted in a substantial decrease in photosynthetic activities and chlorophylls (Chls) in Arabidopsis thaliana plants, causing a senescence-like yellowing and a bleaching in MeJA- and NF-treated plants, respectively. Non-radiative energy dissipation through q(N) and non-photochemical quenching increased greatly in NF-treated plants in concomitance with an increase in photoprotectants antheraxanthin and zeaxanthin from interconversion of violaxanthin, although they were not changed in MeJA-treated plants. A significant accumulation of anthocyanin was observed only in MeJA-treated plants, not in NF-treated plants. Total activities of catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7), superoxide dismutase (EC 1.15.1.1) and glutathione reductase (EC 1.6.4.2) increased greatly in response to MeJA, particularly a 100-fold increase in POD activity 7 days after MeJA treatment. NF application to plants exhibited less increase in antioxidant enzymes than MeJA-treated plants. NF-treated young leaves had a greater decline in Chls and CAT activity, and less zeaxanthin accumulation compared to NF-treated mature leaves, indicating that NF-treated young leaves are more susceptible to excess light exposure and a possible photooxidative stress. Both MeJA- and NF-treated Arabidopsis plants suffered destruction of Chls, however, they developed differential antioxidant responses during the stress, in large part by an increased anthocyanin level in the epidermis and enzymatic antioxidants in MeJA-treated plants and by accumulation of antheraxanthin and zeaxanthin, and enhanced energy dissipation in NF-treated plants.     

Carotenoid-deficient young wheat etioplasts are able to bind precursor proteins on the plastid surface but are impaired in their translocation ability

Young carotenoid-deficient etioplasts, isolated from Norflurazon (NF)-treated wheat seedlings, were used to study the role of coloured carotenoids in the binding and import reactions of different nuclear-encoded plastid proteins. Plastids from control seedlings exhibited significantly higher import efficiencies than did plastids from NF-treated plants. Etioplasts containing normal levels of carotenoids imported approximately 2000 and 800 molecules per plastid of the precursors of the small Rubisco subunit (pSS) and the Rieske FeS protein (pFeS), respectively. Plastids from NF-treated plants imported approximately 100 and 70 pSS and pFeS molecules per plastid, respectively. In addition, a maximum binding capacity of NF-treated plastids of 1200 protein molecules per plastid was observed for both pSS and pFeS when assayed at 25°C: and a maximum binding capacity of approximately 1300 molecules per plastid was noted at 4°C. For control plastids, a similar amount of binding, or approximately 1400 protein molecules per plastid, could only be observed if import was inhibited by low ATP concentrations at 4°C. When these plastids were washed and transferred to conditions promoting import at 25°C and 10 mM Mg-ATP, close to 60% of the envelope-associated precursor protein molecules were imported. These results indicate that control and NF-treated young etioplasts contain similar amounts of binding sites for precursor proteins. However, only in the case of control plastids the binding was productive and lead to import and processing in the stroma upon transfer to conditions promoting import. Plastids isolated from wheat seedlings grown in weak red light and containing different amounts of carotenoids, were assayed for their ability to bind and import a protein with unusual import characteristics, the Chlamydomonas reinhardtii PsaF precursor of PSI (pPsaF) and transit peptide deletion constructs. The PsaF protein was imported in a transit peptide-dependent manner into control etioplasts, whereas import of pPsaF into young wheat etioplasts isolated from NF-treated plants was inhibited at low levels of plastid carotenoids.     

Tetrapyrrole involvement in expression regulation of a nuclear gene of low-molecular-weight plastid protein ELIP

An inhibitor analysis was used for studying the tetrapyrrole role in the regulation of the expression of the nuclear gene encoding a low-molecular-weight protein, a stress plastid light-inducible protein ELIP. 2,2'-Dipyridyl and norflurazon were used as inhibitors. Experiments with dipyridyl demonstrated that tetrapyrroles were involved in the regulation of Elip gene expression, inhibiting it by approximately 50%. Similar results were obtained when there was photodestruction of the chloroplasts, caused by a plant treatment with norflurazon. The results confirm the involvement of the chloroplasts in the regulation of the nuclear gene expression coding for plastid proteins. Tetrapyrroles are important contributors to this process.     

Tetrapyrrole involvement in expression regulation of a nuclear gene of low-molecular-weight plastid protein ELIP

An inhibitor analysis was used for studying the tetrapyrrole role in the regulation of the expression of the nuclear gene encoding a low-molecular-weight protein, a stress plastid light-inducible protein ELIP. 2,2′-Dipyridyl and norflurazon were used as inhibitors. Experiments with dipyridyl demonstrated that tetrapyrroles were involved in the regulation of Elip gene expression, inhibiting it by ～50%. Similar results were obtained when there was photodestruction of the chloroplasts, caused by a plant treatment with norflurazon. The results confirm the involvement of the chloroplasts in the regulation of the nuclear gene expression coding for plastid proteins. Tetrapyrroles are important contributors to this process.     

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。