Purine biosynthetic enzyme ATase2 is involved in the regulation of early chloroplast development and chloroplast gene expression in Arabidopsis

Phosphoribosyltransferases and Their Roles in Plant Development and Abiotic Stress Response

Glycosylation is a widespread glycosyl modification that regulates gene expression and metabolite bioactivity in all life processes of plants. Phosphoribosylation is a special glycosyl modification catalyzed by phosphoribosyltransferase (PRTase), which functions as a key step in the biosynthesis pathway of purine and pyrimidine nucleotides, histidine, tryptophan, and coenzyme NAD(P)⁺ to control the production of these essential metabolites. Studies in the past decades have reported that PRTases are indispensable for plant survival and thriving, whereas the complicated physiological role of PRTases in plant life and their crosstalk is not well understood. Here, we comprehensively overview and critically discuss the recent findings on PRTases, including their classification, as well as the function and crosstalk in regulating plant development, abiotic stress response, and the balance of growth and stress responses. This review aims to increase the understanding of the role of plant PRTase and also contribute to future research on the trade-off between plant growth and stress response.

SvSTL1 in the large subunit family of ribonucleotide reductases plays a major role in chloroplast development of Setaria viridis

Ribonucleotide reductases (RNRs) are essential enzymes in DNA synthesis. However, little is known about the RNRs in plants. Here, we identified a svstl1 mutant from the self-created ethyl methanesulfonate (EMS) mutant library of Setaria viridis. The mutant leaves exhibited a bleaching phenotype at the heading stage. Paraffin section analysis showed the destruction of the C₄ Kranz anatomy. Transmission electron microscopy results further demonstrated the severely disturbed development of some chloroplasts. MutMap analysis revealed that the SvSTL1 gene is the primary candidate, encoding a large subunit of RNRs. Complementation experiments confirmed that SvSTL1 is responsible for the phenotype of svstl1. There are two additional RNR large subunit homologs in S. viridis, SvSTL2 and SvSTL3. To further understand the functions of these three RNR large subunit genes, a series of mutants were generated via CRISPR/Cas9 technology. In striking contrast to the finding that all three SvSTLs interact with the RNR small subunit, the phenotype varied along with the copies of chloroplast genome among different svstl single mutants: the svstl1 mutant exhibited pronounced chloroplast development and significantly fewer copies of the chloroplast genome than the svstl2 or svstl3 single mutants. These results suggested that SvSTL1 plays a major role in the optimal function of RNRs and is essential for chloroplast development. Furthermore, through the analysis of double and triple mutants, the study provides new insights into the finely tuned coordination among SvSTLs to maintain normal chloroplast development in the emerging C₄ model plant S. viridis.

Identification of a delayed leaf greening gene from a mutation of pummelo

Delayed greening of young leaves is an unusual phenomenon of plants in nature. Citrus are mostly evergreen tree species. Here, a natural mutant of "Guanxi" pummelo (Citrus maxima), which shows yellow leaves at the young stage, was characterized to identify the genes underlying the trait of delayed leaf greening in plants. A segregating population with this mutant as the seed parent and a normal genotype as the pollen parent was generated. Two DNA pools respectively from the leaves of segregating seedlings with extreme phenotypes of normal leaf greening and delayed leaf greening were collected for sequencing. Bulked segregant analysis (BSA) and InDel marker analysis demonstrated that the delayed leaf greening trait is governed by a 0.3 Mb candidate region on chromosome 6. Gene expression analysis further identified a key candidate gene (Citrus Delayed Greening gene 1, CDG1) in the 0.3 Mb region, which showed significantly differential expression between the genotypes with delayed and normal leaf greening phenotypes. There was a 67 bp InDel region difference in the CDG1 promoter and the InDel region contains a TATA-box element. Confocal laser-scanning microscopy revealed that the CDG1-GFP fusion protein signals were co-localized with the chloroplast signals in the protoplasts. Overexpression of CDG1 in tobacco and Arabidopsis led to the phenotype of delayed leaf greening. These results suggest that the CDG1 gene is involved in controlling the delayed leaf greening phenotype with important functions in chloroplast development.

Quantification of guanosine triphosphate and tetraphosphate in plants and algae using stable isotope-labelled internal standards

Guanosine tetraphosphate (G4P) and guanosine pentaphosphate (G5P) are signalling nucleotides found in bacteria and photosynthetic eukaryotes that are implicated in a wide-range of processes including stress acclimation, developmental transitions and growth control. Measurements of G4P/G5P levels are essential for studying the diverse roles of these nucleotides. However, G4P/G5P quantification is particularly challenging in plants and algae due to lower cellular concentrations, compartmentalization and high metabolic complexity. Despite recent advances the speed and accuracy of G4P quantification in plants and algae can still be improved. Here, we report a new approach for rapid and accurate G4P quantification which relies on the use of synthesized stable isotope-labelled as internal standards. We anticipate that this approach will accelerate research into the function of G4P signaling in plants, algae and other organisms.

Crystal Structure of the Chloroplastic Glutamine Phosphoribosylpyrophosphate Amidotransferase GPRAT2 From Arabidopsis thaliana

Chloroplastic glutamine phosphoribosylpyrophosphate amidotransferase (GPRATase) catalyzes the first committed step of de novo purine biosynthesis in Arabidopsis thaliana, and DAS734 is a direct and specific inhibitor of AtGPRAT, with phytotoxic effects similar to the leaf beaching phenotypes of known AtGPRAT genetic mutants, especially cia1 and atd2. However, the structure of AtGPRAT and the inhibition mode of DAS734 still remain poorly understood. In this study, we solved the structure of AtGPRAT2, which revealed structural differences between AtGPRAT2 and bacterial enzymes. Kinetics assay demonstrated that DAS734 behaves as a competitive inhibitor for the substrate phosphoribosyl pyrophosphate (PRPP) of AtGPRAT2. Docking studies showed that DAS734 forms electrostatic interactions with R264 and hydrophobic interactions with several residues, which was verified by binding assays. Collectively, our study provides important insights into the inhibition mechanism of DAS734 to AtGPRAT2 and sheds light on future studies into further development of more potent herbicides targeting Arabidopsis GPRATases.

UMP Kinase Regulates Chloroplast Development and Cold Response in Rice

Pyrimidine nucleotides are important metabolites that are building blocks of nucleic acids, which participate in various aspects of plant development. Only a few genes involved in pyrimidine metabolism have been identified in rice and the majority of their functions remain unclear. In this study, we used a map-based cloning strategy to isolate a UMPK gene in rice, encoding the UMP kinase that phosphorylates UMP to form UDP, from a recessive mutant with pale-green leaves. In the mutant, UDP content always decreased, while UTP content fluctuated with the development of leaves. Mutation of UMPK reduced chlorophyll contents and decreased photosynthetic capacity. In the mutant, transcription of plastid-encoded RNA polymerase-dependent genes, including psaA, psbB, psbC and petB, was significantly reduced, whereas transcription of nuclear-encoded RNA polymerase-dependent genes, including rpoA, rpoB, rpoC1, and rpl23, was elevated. The expression of UMPK was significantly induced by various stresses, including cold, heat, and drought. Increased sensitivity to cold stress was observed in the mutant, based on the survival rate and malondialdehyde content. High accumulation of hydrogen peroxide was found in the mutant, which was enhanced by cold treatment. Our results indicate that the UMP kinase gene plays important roles in regulating chloroplast development and stress response in rice.

Purine nucleotide biosynthetic gene GARS controls early chloroplast development in rice (Oryza sativa L.)

GARS encodes an enzyme catalyzing the second step of purine nucleotide biosynthesis and plays an important role to maintain the development of chloroplasts in juvenile plants by affecting the expression of plastid-encoded genes. A series of rice white striped mutants were previously described. In this research, we characterized a novel gars mutant with white striped leaves at the seedling stage. By positional cloning, we identified the mutated gene, which encodes a glycinamide ribonucleotide synthetase (GARS) that catalyzes the second step of purine nucleotide biosynthesis. Thylakoid membranes were less abundant in the albinic sectors of mutant seedling leaves compared to the wild type. The expression levels of genes involved in chlorophyll synthesis and photosynthesis were changed. Contents of ATP, ADP, AMP, GTP and GDP, which are crucial for plant growth and development, were decreased in the mutant seedlings. Complementation and CrispR tests confirmed the role of the GARS allele, which was expressed in all rice tissues, especially in the leaves. GARS protein displayed a typical chloroplast location pattern in rice protoplasts. Our results indicated that GARS was involved in chloroplast development at early leaf development by affecting the expression of plastid-encoded genes.

pTAC10, an S1-domain-containing component of the transcriptionally active chromosome complex, is essential for plastid gene expression in Arabidopsis thaliana and is phosphorylated by chloroplast-targeted casein kinase II

In higher plant chloroplasts, the plastid-encoded RNA polymerase (PEP) consists of four catalytic subunits and numerous nuclear-encoded accessory proteins, including pTAC10, an S1-domain-containing protein. In this study, pTAC10 knockout lines were characterized. Two ptac10 mutants had an albino phenotype and severely impaired chloroplast development. The pTAC10 genomic sequence fused to a four-tandem MYC tag driven by its own promoter functionally complemented the ptac10-1 mutant phenotype. pTAC10 was present in both the chloroplast stroma and thylakoids. Two-dimensional blue native polyacrylamide gel electrophoresis (BN-PAGE), and immunoblotting assays showed that pTAC10:MYC co-migrates with one of the PEP core subunits, RpoB. A comprehensive investigation of the plastid gene expression profiles by quantitative RT-PCR revealed that, compared with wild-type plants, the abundance of PEP-dependent plastid transcripts is severely decreased in the ptac10-1 mutant, while the amount of plastid transcripts exclusively transcribed by NEP either barely changes or even increases. RNA blot analysis confirmed that PEP-dependent chloroplast transcripts, including psaB, psbA and rbcL, substantially decrease in the ptac10-1 mutant. Immunoblotting showed reduced accumulation of most chloroplast proteins in the ptac10 mutants. These data indicate the essential role of pTAC10 in plastid gene expression and plastid development. pTAC10 interacts with chloroplast-targeted casein kinase 2 (cpCK2) in vitro and in vivo and can be phosphorylated by Arabidopsis cpCK2 in vitro at sites Ser95, Ser396 and Ser434. RNA-EMSA assays showed that pTAC10 is able to bind to the psbA, atpE and accD transcripts, suggesting a non-specific RNA-binding activity of pTAC10. The RNA affinity of pTAC10 was enhanced by phosphorylation and decreased by the amino acid substitution Ser434-Ala of pTAC10. These data show that pTAC10 is essential for plastid gene expression in Arabidopsis and that it can be phosphorylated by cpCK2.

Multiwall carbon nanotubes modulate paraquat toxicity in Arabidopsis thaliana

Carbon nanotubes can be either toxic or beneficial to plant growth and can also modulate toxicity of organic contaminants through surface sorption. The complex interacting toxic effects of carbon nanotubes and organic contaminants in plants have received little attention in the literature to date. In this study, the toxicity of multiwall carbon nanotubes (MWCNT, 50 mg/L) and paraquat (MV, 0.82 mg/L), separately or in combination, were evaluated at the physiological and the proteomic level in Arabidopsis thaliana for 7-14 days. The results revealed that the exposure to MWCNT had no inhibitory effect on the growth of shoots and leaves. Rather, MWCNT stimulated the relative electron transport rate and the effective photochemical quantum yield of PSII value as compared to the control by around 12% and lateral root production up to nearly 4-fold as compared to the control. The protective effect of MWCNT on MV toxicity on the root surface area could be quantitatively explained by the extent of MV adsorption on MWCNT and was related to stimulation of photosynthesis, antioxidant protection and number and area of lateral roots which in turn helped nutrient assimilation. The influence of MWCNT and MV on photosynthesis and oxidative stress at the physiological level was consistent with the proteomics analysis, with various over-expressed photosynthesis-related proteins (by more than 2 folds) and various under-expressed oxidative stress related proteins (by about 2-3 folds). This study brings new insights into the interactive effects of two xenobiotics (MWCNT and MV) on the physiology of a model plant.

A novel tetratricopeptide repeat protein, WHITE TO GREEN1, is required for early chloroplast development and affects RNA editing in chloroplasts

The chloroplast is essential for plant photosynthesis and production, but the regulatory mechanism of chloroplast development is still elusive. Here, a novel gene, WHITE TO GREEN1 (WTG1), was identified to have a function in chloroplast development and plastid gene expression by screening Arabidopsis leaf coloration mutants. WTG1 encodes a chloroplast-localized tetratricopeptide repeat protein that is expressed widely in Arabidopsis cells. Disruption of WTG1 suppresses plant growth, retards leaf greening and chloroplast development, and represses photosynthetic gene expression, but complemented expression of WTG1 restored a normal phenotype. Moreover, WTG1 protein is associated with the organelle RNA editing factors MORF8 and MORF9, and RNA editing of the plastid petL-5 and ndhG-50 transcripts was affected in wtg1 mutants. These results indicate that WTG1 affects both transcriptional and posttranscriptional regulation of plastid gene expression, and provide evidence for the involvement of a tetratricopeptide repeat protein in chloroplast RNA editing in Arabidopsis.