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Summary: Correct chloroplast development and function require co‐ordinated expression of chloroplast and nuclear genes. This is achieved through chloroplast signals that modulate nuclear gene expression in accordance with the chloroplast's needs. Genetic evidence indicates that GUN1, a chloroplast‐localized pentatricopeptide repeat (PPR) protein with a C‐terminal Small MutS‐Related (SMR) domain, is involved in integrating multiple developmental and stress‐related signals in both young seedlings and adult leaves. Recently, GUN1 was found to interact physically with factors involved in chloroplast protein homeostasis, and with enzymes of tetrapyrrole biosynthesis in adult leaves that function in various retrograde signalling pathways. Here we show that following perturbation of chloroplast protein homeostasis: (i) by growth in lincomycin‐containing medium; or (ii) in mutants defective in either the FtsH protease complex (ftsh), plastid ribosome activity (prps21‐1 and prpl11‐1) or plastid protein import and folding (cphsc70‐1), GUN1 influences NEP‐dependent transcript accumulation during cotyledon greening and also intervenes in chloroplast protein import. Significance Statement: Correct chloroplast development and function require co‐ordinated regulation of chloroplast and nuclear gene expression. Here, we propose that GUN1, a main player of chloroplast‐to‐nucleus retrograde communication, influences NEP‐dependent transcript accumulation during cotyledon greening and also intervenes in chloroplast protein import. [ABSTRACT FROM AUTHOR]

Pinaceae are the predominant photosynthetic species in boreal forests, but so far no detailed description of the protein components of the photosynthetic apparatus of these gymnosperms has been available. In this study we report a detailed characterization of the thylakoid photosynthetic machinery of Norway spruce (Picea abies (L.) Karst). We first customized a spruce thylakoid protein database from translated transcript sequences combined with existing protein sequences derived from gene models, which enabled reliable tandem mass spectrometry identification of P. abies thylakoid proteins from two-dimensional large pore blue-native/SDS-PAGE. This allowed a direct comparison of the two-dimensional protein map of thylakoid protein complexes from P. abies with the model angiosperm Arabidopsis thaliana. Although the subunit composition of P. abies core PSI and PSII complexes is largely similar to that of Arabidopsis, there was a high abundance of a smaller PSI subcomplex, closely resembling the assembly intermediate PSI* complex. In addition, the evolutionary distribution of light-harvesting complex (LHC) family members of Pinaceae was compared in silico with other land plants, revealing that P. abies and other Pinaceae (also Gnetaceae and Welwitschiaceae) have lost LHCB4, but retained LHCB8 (formerly called LHCB4.3). The findings reported here show the composition of the photosynthetic apparatus of P. abies and other Pinaceae members to be unique among land plants. [ABSTRACT FROM AUTHOR]

Trans‐methylation reactions are vital in basic metabolism, epigenetic regulation, RNA metabolism, and posttranslational control of protein function and therefore fundamental in determining the physiological processes in all living organisms. The plant kingdom is additionally characterized by the production of secondary metabolites that undergo specific hydroxylation, oxidation and methylation reactions to obtain a wide array of different chemical structures. Increasing research efforts have started to reveal the enzymatic pathways underlying the biosynthesis of complex metabolites in plants. Further engineering of these enzymatic machineries offers significant possibilities in the development of bio‐based technologies, but necessitates deep understanding of their potential metabolic and regulatory interactions. Trans‐methylation reactions are tightly coupled with the so‐called activated methyl cycle (AMC), an essential metabolic circuit that maintains the trans‐methylation capacity in all living cells. Tight regulation of the AMC is crucial in ensuring accurate trans‐methylation reactions in different subcellular compartments, cell types, developmental stages and environmental conditions. This review addresses the organization and posttranslational regulation of the AMC and elaborates its critical role in determining metabolic regulation through modulation of methyl utilization in stress‐exposed plants. [ABSTRACT FROM AUTHOR]

Glucosinolates ( GSL) of cruciferous plants comprise a major group of structurally diverse secondary compounds which act as deterrents against aphids and microbial pathogens and have large commercial and ecological impacts. While the transcriptional regulation governing the biosynthesis and modification of GSL is now relatively well understood, post-translational regulatory components that specifically determine the structural variation of indole glucosinolates have not been reported. We show that the cytoplasmic protein phosphatase 2A regulatory subunit B′γ ( PP2A-B′γ) physically interacts with indole glucosinolate methyltransferases and controls the methoxylation of indole glucosinolates and the formation of 4-methoxy-indol-3-yl-methyl glucosinolate in Arabidopsis leaves. By taking advantage of proteomic approaches and metabolic analysis we further demonstrate that PP2A-B′γ is required to control the abundance of oligomeric protein complexes functionally linked with the activated methyl cycle and the trans-methylation capacity of leaf cells. These findings highlight the key regulatory role of PP2A-B′γ in methionine metabolism and provide a previously unrecognized perspective for metabolic engineering of glucosinolate metabolism in cruciferous plants. [ABSTRACT FROM AUTHOR]

STN7 kinase catalyzes the phosphorylation of the globally most common membrane proteins, the light-harvesting complex II ( LHCII) in plant chloroplasts. STN7 itself possesses one serine (Ser) and two threonine (Thr) phosphosites. We show that phosphorylation of the Thr residues protects STN7 against degradation in darkness, low light and red light, whereas increasing light intensity and far red illumination decrease phosphorylation and induce STN7 degradation. Ser phosphorylation, in turn, occurs under red and low intensity white light, coinciding with the client protein ( LHCII) phosphorylation. Through analysis of the counteracting LHCII phosphatase mutant tap38/ pph1, we show that Ser phosphorylation and activation of the STN7 kinase for subsequent LHCII phosphorylation are heavily affected by pre-illumination conditions. Transitions between the three activity states of the STN7 kinase (deactivated in darkness and far red light, activated in low and red light, inhibited in high light) are shown to modulate the phosphorylation of the STN7 Ser and Thr residues independently of each other. Such dynamic regulation of STN7 kinase phosphorylation is crucial for plant growth and environmental acclimation. [ABSTRACT FROM AUTHOR]

Plants survive periods of unfavourable conditions with the help of sensory mechanisms that respond to reactive oxygen species (ROS) as signalling molecules in different cellular compartments. We have previously demonstrated that protein phosphatase 2A (PP2A) impacts on organellar cross-talk and associated pathogenesis responses in A rabidopsis thaliana. This was evidenced by drastically enhanced pathogenesis responses and cell death in cat2 pp2a-b′γ double mutants, deficient in the main peroxisomal antioxidant enzyme CATALASE 2 and PP2A regulatory subunit B′ γ (PP2A-B′ γ). In the present paper, we explored the impacts of PP2A-B′ γ and a highly similar regulatory subunit PP2A-B′ ζ in growth regulation and light stress tolerance in Arabidopsis. PP2A - B ′γ and PP2A - B ′ζ display high promoter activities in rapidly growing tissues and are required for optimal growth under favourable conditions. Upon acclimation to a combination of high light, elevated temperature and reduced availability of water, however, pp2a-b′γζ double mutants grow similarly to the wild type and show enhanced tolerance against photo-oxidative stress. We conclude that by controlling ROS homeostasis and signalling, PP2A-B′ γ and PP2A-B′ ζ may direct acclimation strategies upon environmental perturbations, hence acting as important determinants of defence responses and light acclimation in plants. [ABSTRACT FROM AUTHOR]

Organellar reactive oxygen species ( ROS) signalling is a key mechanism that promotes the onset of defensive measures in stress-exposed plants. The underlying molecular mechanisms and feedback regulation loops, however, still remain poorly understood. Our previous work has shown that a specific regulatory B′γ subunit of protein phosphatase 2A ( PP2A) is required to control organellar ROS signalling and associated metabolic adjustments in Arabidopsis thaliana. Here, we addressed the mechanisms through which PP2A-B′γ impacts on organellar metabolic crosstalk and ROS homeostasis in leaves., Genetic, biochemical and pharmacological approaches, together with a combination of data-dependent acquisition ( DDA) and selected reaction monitoring ( SRM) MS techniques, were utilized to assess PP2A-B′γ-dependent adjustments in Arabidopsis thaliana., We show that PP2A-B'γ physically interacts with the cytoplasmic form of aconitase, a central metabolic enzyme functionally connected with mitochondrial respiration, oxidative stress responses and regulation of cell death in plants. Furthermore, PP2A-B'γ impacts ROS homeostasis by controlling the abundance of specific alternative oxidase isoforms, AOX1A and AOX1D, in leaf mitochondria., We conclude that PP2A-B'γ-dependent regulatory actions modulate the functional status of metabolic enzymes that essentially contribute to intracellular ROS signalling and metabolic homeostasis in plants. [ABSTRACT FROM AUTHOR]

Oxidative stress responses are influenced by growth day length, but little is known about how this occurs. A combined reverse genetics, metabolomics and proteomics approach was used to address this question in Arabidopsis thaliana., A catalase-deficient mutant ( cat2), in which intracellular oxidative stress drives pathogenesis-related responses in a day length-dependent manner, was crossed with a knockdown mutant for a specific type 2A protein phosphatase subunit ( pp2a-b′γ). In long days ( LD), the pp2a-b′γ mutation reinforced cat2-triggered pathogenesis responses., In short days ( SD), conditions in which pathogenesis-related responses were not activated in cat2, the additional presence of the pp2a-b′γ mutation allowed lesion formation, PATHOGENESIS-RELATED GENE1 ( PR1) induction, salicylic acid ( SA) and phytoalexin accumulation and the establishment of metabolite profiles that were otherwise observed in cat2 only in LD. Lesion formation in cat2 pp2a-b′γ in SD was genetically dependent on SA synthesis, and was associated with decreased PHYTOCHROME A transcripts. Phosphoproteomic analyses revealed that several potential protein targets accumulated in the double mutant, including recognized players in pathogenesis and key enzymes of primary metabolism., We conclude that the cat2 and pp2a-b′γ mutations interact synergistically, and that PP2A-B′γ is an important player in controlling day length-dependent responses to intracellular oxidative stress, possibly through phytochrome-linked pathways. [ABSTRACT FROM AUTHOR]