Bacterial Pathogen Infection Triggers Magic Spot Nucleotide Signaling in Arabidopsis thaliana Chloroplasts through Specific RelA/SpoT Homologues

How to quantify magic spots - a brief overview of (p)ppGpp detection and quantitation methods

Guanosine tetra- and penta-phosphates, collectively known as (p)ppGpp, are well-known second messengers of cellular stress responses in bacteria and plants. Their intracellular concentration is tightly regulated and can vary widely-from undetectable levels under optimal growth conditions, through intermediate concentrations, to extremely high levels that match or even exceed GTP concentrations when cells are exposed to severe stress. Importantly, the effects exerted by (p)ppGpp are often concentration-dependent, making their quantitative analysis a crucial aspect of studying cellular responses to stress. To gain a deeper understanding of the regulatory mechanisms associated with (p)ppGpp, it is essential to monitor its accumulation in vivo and conduct detailed molecular studies in vitro. Various methods have been developed for detecting and quantifying (p)ppGpp, enabling researchers to track its levels in living cells and analyse its function under controlled laboratory conditions. In this work, we provide an overview of the available techniques for (p)ppGpp detection and quantification. We present their advantages, limitations, and potential applications in research on metabolic regulation and cellular stress responses.

Structural Insights into Broad-Range Polyphosphate Kinase 2-II Enzymes Applicable for Pyrimidine Nucleoside Diphosphate Synthesis

Polyphosphate kinases (PPK) play crucial roles in various biological processes, including energy storage and stress responses, through their interaction with inorganic polyphosphate (polyP) and the intracellular nucleotide pool. Members of the PPK family 2 (PPK2s) catalyse polyP-consuming phosphorylation of nucleotides. In this study, we characterised two PPK2 enzymes from Bacillus cereus (BcPPK2) and Lysinibacillus fusiformis (LfPPK2) to investigate their substrate specificity and potential for selective nucleotide synthesis. Both enzymes exhibited a broad substrate scope, selectively converting over 85 % of pyrimidine nucleoside monophosphates (NMPs) to nucleoside diphosphates (NDPs), while nucleoside triphosphate (NTP) formation was observed only with purine NMPs. Preparative enzymatic synthesis of cytidine diphosphate (CDP) was applied to achieve an yield of 49 %. Finally, structural analysis of five crystal structures of BcPPK2 and LfPPK2 provided insights into their active sites and substrate interactions. This study highlights PPK2-II enzymes as promising biocatalysts for the efficient and selective synthesis of pyrimidine NDPs.

Nucleotides and nucleotide derivatives as signal molecules in plants

In reaction to a stimulus, signaling molecules are made, generate a response, and are then degraded. Nucleotides are classically associated with central metabolism and nucleic acid biosynthesis, but there are a number of nucleotides and nucleotide derivatives in plants to which this simple definition of a signaling molecule applies in whole or at least in part. These include cytokinins and chloroplast guanosine tetraposphate (ppGpp), as well as extracellular canonical nucleotides such as extracellular ATP (eATP) and NAD+ (eNAD+). In addition, there is a whole series of compounds derived from NAD+ such as ADP ribose (ADPR), and ATP-ADPR dinucleotides and their hydrolysis products (e.g. pRib-AMP) together with different variants of cyclic ADPR (cADPR, 2´-cADPR, 3´-cADPR), and also cyclic nucleotides such as 3´,5´-cAMP and 2´,3´-cyclic nucleoside monophosphates. Interestingly, some of these compounds have recently been shown to play a central role in pathogen defense. In this review, we highlight these exciting new developments. We also review nucleotide derivatives that are considered as candidates for signaling molecules, for example purine deoxynucleosides, and discuss more controversial cases.

Exosome-like nanoparticles derived from fruits, vegetables, and herbs: innovative strategies of therapeutic and drug delivery

Over the past ten years, significant advancements have been made in exploring plant-derived exosome-like nanoparticles (PELNs) for disease therapeutics and drug delivery. PELNs, as inherent nanoscale particles comprised of proteins, lipids, nucleic acids, and secondary metabolites, exhibit the capacity for cellular uptake by human cells. This intercellular interaction transcends biological boundaries, effectively influencing biological functions in animals. PELNs have outstanding biocompatibility, low immunogenicity, enhanced safety, and environmentally friendly sustainability. This article summarized the preparation methods and characteristics of PELNs. It provided a systematic review of the varied roles of PELNs derived from fruits, vegetables, and herbs in disease therapeutics and drug delivery. The challenges in their production and application were discussed, and future prospects in this rapidly evolving field were explored.

Complete Loss of RelA and SpoT Homologs in Arabidopsis Reveals the Importance of the Plastidial Stringent Response in the Interplay between Chloroplast Metabolism and Plant Defense Response

The highly phosphorylated nucleotide, guanosine tetraphosphate (ppGpp), functions as a secondary messenger in bacteria and chloroplasts. The accumulation of ppGpp alters plastidial gene expression and metabolism, which are required for proper photosynthetic regulation and robust plant growth. However, because four plastid-localized ppGpp synthases/hydrolases function redundantly, the impact of the loss of ppGpp-dependent stringent response on plant physiology remains unclear. We used CRISPR/Cas9 technology to generate an Arabidopsis thaliana mutant lacking all four ppGpp synthases/hydrolases and characterized its phenotype. The mutant showed over 20-fold less ppGpp levels than the wild type under normal growth conditions and exhibited leaf chlorosis and increased expression of defense-related genes as well as salicylic acid and jasmonate levels upon transition to nitrogen-starvation conditions. These results demonstrate that proper levels of ppGpp in plastids are required for controlling not only plastid metabolism but also phytohormone signaling, which is essential for plant defense.

Effects of Light and Dark Conditions on the Transcriptome of Aging Cultures of Candidatus Puniceispirillum marinum IMCC1322

To elucidate the function of proteorhodopsin in Candidatus Puniceispirillum marinum strain IMCC1322, a cultivated representative of SAR116, we produced RNA-seq data under laboratory conditions. We examined the transcriptomes of six different cultures, including sets of expression changes under constant dark (DD), constant light (LL), and diel-cycled (LD; 14 h light: 10 h dark) conditions at the exponential and stationary/death phases. Prepared mRNA extracted from the six samples was analyzed on the Solexa Genome Analyzer with 36 cycles. Differentially expressed genes on the IMCC1322 genome were distinguished as four clusters by K-mean clustering and each CDS (n = 2546) was annotated based on the KEGG BRITE hierarchy. Cluster 0 (n = 1573) covered most constitutive genes including proteorhodopsin, retinoids, and glycolysis/TCA cycle. Cluster 1 genes (n = 754) were upregulated in stationary/death phase under constant dark conditions and included genes associated with bacterial defense, membrane transporters, nitrogen metabolism, and senescence signaling. Cluster 2 genes (n = 197) demonstrated upregulation in exponential phase cultures and included genes involved in genes for oxidative phosphorylation, translation factors, and transcription machinery. Cluster 3 (n = 22) contained light-stimulated upregulated genes expressed under stationary/phases. Stringent response genes belonged to cluster 2, but affected genes spanned various cellular processes such as amino acids, nucleotides, translation, transcription, glycolysis, fatty acids, and cell wall components. The coordinated expression of antagonistic stringent genes, including mazG, ppx/gppA, and spoT/relA may provide insight into the controlled cultural response observed between constant light and constant dark conditions in IMCC1322 cultures, regardless of cell numbers and biomass.