The protein phosphatase 7 regulates phytochrome signaling in Arabidopsis

Protein phosphatases and their targets: Comprehending the interactions in plant signaling pathways

Protein phosphorylation is a vital reversible post-translational modification. This process is established by two classes of enzymes: protein kinases and protein phosphatases. Protein kinases phosphorylate proteins while protein phosphatases dephosphorylate phosphorylated proteins, thus, functioning as 'critical regulators' in signaling pathways. The eukaryotic protein phosphatases are classified as phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine (Ser)/threonine (Thr) specific phosphatases (STPs) that dephosphorylate Ser and Thr residues. The PTP family dephosphorylates Tyr residues while dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. The composition of these enzymes as well as their substrate specificity are important determinants of their functional significance in a number of cellular processes and stress responses. Their role in animal systems is well-understood and characterized. The functional characterization of protein phosphatases has been extensively covered in plants, although the comprehension of their mechanistic basis is an ongoing pursuit. The nature of their interactions with other key players in the signaling process is vital to our understanding. The substrates or targets determine their potential as well as magnitude of the impact they have on signaling pathways. In this article, we exclusively overview the various substrates of protein phosphatases in plant signaling pathways, which are a critical determinant of the outcome of various developmental and stress stimuli.

Chromosome-level genome assembly of the diploid blueberry Vaccinium darrowii provides insights into its subtropical adaptation and cuticle synthesis

Vaccinium darrowii is a subtropical wild blueberry species that has been used to breed economically important southern highbush cultivars. The adaptive traits of V. darrowii to subtropical climates can provide valuable information for breeding blueberry and perhaps other plants, especially against the background of global warming. Here, we assembled the V. darrowii genome into 12 pseudochromosomes using Oxford Nanopore long reads complemented with Hi-C scaffolding technologies, and we predicted 41 815 genes using RNA-sequencing evidence. Syntenic analysis across three Vaccinium species revealed a highly conserved genome structure, with the highest collinearity between V. darrowii and Vaccinium corymbosum. This conserved genome structure may explain the high fertility observed during crossbreeding of V. darrowii with other blueberry cultivars. Analysis of gene expansion and tandem duplication indicated possible roles for defense- and flowering-associated genes in the adaptation of V. darrowii to the subtropics. Putative SOC1 genes in V. darrowii were identified based on phylogeny and expression analysis. Blueberries are covered in a thick cuticle layer and contain anthocyanins, which confer their powdery blue color. Using RNA sequencing, we delineated the cuticle biosynthesis pathways of Vaccinium species in V. darrowii. This result can serve as a reference for breeding berries whose colors are appealing to customers. The V. darrowii reference genome, together with the unique traits of this species, including its diploid genome, short vegetative phase, and high compatibility in hybridization with other blueberries, make V. darrowii a potential research model for blueberry species.

A ménage à trois: salicylic acid, growth inhibition, and immunity

Salicylic acid (SA) is a plant hormone almost exclusively associated with the promotion of immunity. It is also known that SA has a negative impact on plant growth, yet only limited efforts have been dedicated to explain this facet of SA action. In this review, we focus on SA-related reduced growth and discuss whether it is a regulated process and if the role of SA in immunity imperatively comes with growth suppression. We highlight molecular targets of SA that interfere with growth and describe scenarios where SA can improve plant immunity without a growth penalty.

Conformational Change of Tetratricopeptide Repeats Region Triggers Activation of Phytochrome-Associated Protein Phosphatase 5

Phytochrome activity is not only controlled by light but also by post-translational modifications, e. g. phosphorylation. One of the phosphatases responsible for plant phytochrome dephosphorylation and thereby increased activity is the phytochrome-associated protein phosphatase 5 (PAPP5). We show that PAPP5 recognizes phospho-site mimicking mutants of phytochrome B, when being activated by arachidonic acid (AA). Addition of AA to PAPP5 decreases the α-helical content as tracked by CD-spectroscopy. These changes correspond to conformational changes of the regulatory tetratricopeptide repeats (TPR) region as shown by mapping data from hydrogen deuterium exchange mass spectrometry onto a 3.0 Å crystal structure of PAPP5. Surprisingly, parts of the linker between the TPR and PP2A domains and of the so-called C-terminal inhibitory motif exhibit reduced deuterium uptake upon AA-binding. Molecular dynamics analyses of PAPP5 complexed to a phyB phosphopeptide show that this C-terminal motif remains associated with the TPR region in the substrate bound state, suggesting that this motif merely serves for restricting the orientations of the TPR region relative to the catalytic PP2A domain. Given the high similarity to mammalian PP5 these data from a plant ortholog show that the activation mode of these PPP-type protein phosphatases is highly conserved.

BrPP5.2 Overexpression Confers Heat Shock Tolerance in Transgenic Brassica rapa through Inherent Chaperone Activity, Induced Glucosinolate Biosynthesis, and Differential Regulation of Abiotic Stress Response Genes

Plant phosphoprotein phosphatases are ubiquitous and multifarious enzymes that respond to developmental requirements and stress signals through reversible dephosphorylation of target proteins. In this study, we investigated the hitherto unknown functions of Brassica rapa protein phosphatase 5.2 (BrPP5.2) by transgenic overexpression of B. rapa lines. The overexpression of BrPP5.2 in transgenic lines conferred heat shock tolerance in 65–89% of the young transgenic seedlings exposed to 46 °C for 25 min. The examination of purified recombinant BrPP5.2 at different molar ratios efficiently prevented the thermal aggregation of malate dehydrogenase at 42 °C, thus suggesting that BrPP5.2 has inherent chaperone activities. The transcriptomic dynamics of transgenic lines, as determined using RNA-seq, revealed that 997 and 1206 (FDR < 0.05, logFC ≥ 2) genes were up- and down-regulated, as compared to non-transgenic controls. Statistical enrichment analyses revealed abiotic stress response genes, including heat stress response (HSR), showed reduced expression in transgenic lines under optimal growth conditions. However, most of the HSR DEGs were upregulated under high temperature stress (37 °C/1 h) conditions. In addition, the glucosinolate biosynthesis gene expression and total glucosinolate content increased in the transgenic lines. These findings provide a new avenue related to BrPP5.2 downstream genes and their crucial metabolic and heat stress responses in plants.

Plant protein phosphatases: What do we know about their mechanism of action?

Protein phosphorylation is a major reversible post-translational modification. Protein phosphatases function as 'critical regulators' in signaling networks through dephosphorylation of proteins, which have been phosphorylated by protein kinases. A large understanding of their working has been sourced from animal systems rather than the plant or the prokaryotic systems. The eukaryotic protein phosphatases include phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine(Ser)/threonine(Thr)-specific phosphatases (STPs), while PTP family is Tyr specific. Dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. PTPs lack sequence homology with STPs, indicating a difference in catalytic mechanisms, while the PPP and PPM families share a similar structural fold indicating a common catalytic mechanism. The catalytic cysteine (Cys) residue in the conserved HCX₅ R active site motif of the PTPs acts as a nucleophile during hydrolysis. The PPP members require metal ions, which coordinate the phosphate group of the substrate, followed by a nucleophilic attack by a water molecule and hydrolysis. The variable holoenzyme assembly of protein phosphatase(s) and the overlap with other post-translational modifications like acetylation and ubiquitination add to their complexity. Though their functional characterization is extensively reported in plants, the mechanistic nature of their action is still being explored by researchers. In this review, we exclusively overview the plant protein phosphatases with an emphasis on their mechanistic action as well as structural characteristics.

PP7L is essential for MAIL1-mediated transposable element silencing and primary root growth

The two paralogous Arabidopsis genes MAINTENANCE OF MERISTEMS (MAIN) and MAINTENANCE OF MERISTEMS LIKE1 (MAIL1) encode a conserved retrotransposon-related plant mobile domain and are known to be required for silencing of transposable elements (TE) and for primary root development. Loss of function of either MAIN or MAIL1 leads to release of heterochromatic TEs, reduced condensation of pericentromeric heterochromatin, cell death of meristem cells and growth arrest of the primary root soon after germination. Here, we show that they act in one protein complex that also contains the inactive isoform of PROTEIN PHOSPHATASE 7 (PP7), which is named PROTEIN PHOSPHATASE 7-LIKE (PP7L). PP7L was previously shown to be important for chloroplast biogenesis and efficient chloroplast protein synthesis. We show that loss of PP7L function leads to the same root growth phenotype as loss of MAIL1 or MAIN. In addition, pp7l mutants show similar silencing defects. Double mutant analyses confirmed that the three proteins act in the same molecular pathway. The primary root growth arrest, which is associated with cell death of stem cells and their daughter cells, is a consequence of genome instability. Our data demonstrate so far unrecognized functions of an inactive phosphatase isoform in a protein complex that is essential for silencing of heterochromatic elements and for maintenance of genome stability in dividing cells.

The plant mobile domain proteins MAIN and MAIL1 interact with the phosphatase PP7L to regulate gene expression and silence transposable elements in Arabidopsis thaliana

Transposable elements (TEs) are DNA repeats that must remain silenced to ensure cell integrity. Several epigenetic pathways including DNA methylation and histone modifications are involved in the silencing of TEs, and in the regulation of gene expression. In Arabidopsis thaliana, the TE-derived plant mobile domain (PMD) proteins have been involved in TE silencing, genome stability, and control of developmental processes. Using a forward genetic screen, we found that the PMD protein MAINTENANCE OF MERISTEMS (MAIN) acts synergistically and redundantly with DNA methylation to silence TEs. We found that MAIN and its close homolog MAIN-LIKE 1 (MAIL1) interact together, as well as with the phosphoprotein phosphatase (PPP) PP7-like (PP7L). Remarkably, main, mail1, pp7l single and mail1 pp7l double mutants display similar developmental phenotypes, and share common subsets of upregulated TEs and misregulated genes. Finally, phylogenetic analyses of PMD and PP7-type PPP domains among the Eudicot lineage suggest neo-association processes between the two protein domains to potentially generate new protein function. We propose that, through this interaction, the PMD and PPP domains may constitute a functional protein module required for the proper expression of a common set of genes, and for silencing of TEs.

The plant mobile domain (PMD) is a protein domain of unknown function that is widely spread in the angiosperm plants. Although most PMDs are associated with repeated DNA sequences called transposable elements (TEs), plants have domesticated the PMD to produce genic versions that play important roles within the cell. In Arabidopsis thaliana, MAINTENANCE OF MERISTEMS (MAIN) and MAIN-LIKE 1 (MAIL1) are genic PMDs that are involved in genome stability, developmental processes, and silencing of TEs. The mechanisms involving MAIN and MAIL1 in these cellular processes remain elusive. Here, we show that MAIN, MAIL1 and the phosphoprotein phosphatase (PPP) named PP7-like (PP7L) interact to form a protein complex that is required for the proper expression of genes, and the silencing of TEs. Phylogenetic analyses revealed that PMD and PP7-type PPP domains are evolutionary connected, and several plant species express proteins carrying both PMD and PPP domains. We propose that interaction of PMD and PPP domains would create a functional protein module involved in mechanisms regulating gene expression and repressing TEs.

Extrachloroplastic PP7L Functions in Chloroplast Development and Abiotic Stress Tolerance

Chloroplast biogenesis is indispensable for proper plant development and environmental acclimation. In a screen for mutants affected in photosynthesis, we identified the protein phosphatase7-like (pp7l) mutant, which displayed delayed chloroplast development in cotyledons and young leaves. PP7L, PP7, and PP7-long constitute a subfamily of phosphoprotein phosphatases. PP7 is thought to transduce a blue-light signal perceived by crys and phy a that induces expression of SIGMA FACTOR5 (SIG5). We observed that, like PP7, PP7L was predominantly localized to the nucleus in Arabidopsis (Arabidopsis thaliana), and the pp7l phenotype was similar to that of the sig6 mutant. However, SIG6 expression was unaltered in pp7l mutants. Instead, loss of PP7L compromised translation and ribosomal RNA (rRNA) maturation in chloroplasts, pointing to a distinct mechanism influencing chloroplast development. Promoters of genes deregulated in pp7l-1 were enriched in PHYTOCHROME-INTERACTING FACTOR (PIF)-binding motifs and the transcriptome of pp7l-1 resembled those of pif and CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1) signalosome complex (csn) mutants. However, pif and csn mutants, as well as cop1, cryptochromes (cry)1 cry2, and phytochromes (phy)A phyB mutants, do not share the pp7l photosynthesis phenotype. PhyB protein levels were elevated in pp7l mutants, but phyB overexpression plants did not resemble pp7l These results indicate that PP7L operates through a different pathway and that the control of greening and photosystem biogenesis can be separated. The lack of PP7L increased susceptibility to salt and high-light stress, whereas PP7L overexpression conferred resistance to high-light stress. Strikingly, PP7L was specifically recruited to Brassicales for the regulation of chloroplast development. This study adds another player involved in chloroplast biogenesis.

DNA-binding protein phosphatase AtDBP1 acts as a promoter of flowering in Arabidopsis

We provide evidence that AtDBP1 promotes flowering by regulating the transcript levels of several important integrators and floral meristem identity genes, including FLC, CO, SOC1, LFY, FT and FD. DNA-binding protein phosphatases (DBP) which exhibit both sequence specific DNA-binding and protein phosphatase 2C activities are important regulators that are involved in both the transcriptional and post-translational regulations. DBP factors are known to mediate susceptibility to potyviruses; however, whether they are involved in other processes is still unclear. In this study, under both long day (LD) and short day conditions, AtDBP1 overexpressing plants displayed early flowering, while the knock out mutants, atdbp1, exhibited a delay in flowering relative to the wild-type plants; both the overexpressing lines and atdbp1 mutants remained photoperiodic sensitive, indicating that AtDBP1 was involved in the autonomous pathway. AtDBP1 does not respond to vernalization at transcript level, and both AtDBP1 overexpressing plants and atdbp1 mutants remain responsive to vernalization, indicating that AtDBP1 may not be directly involved in vernalization. Real-time PCR analysis showed that AtDBP1 can suppress FLOWERING LOCUC C (FLC) expression, a key integrator of the autonomous and vernalization pathways, and enhance the expression levels of CONSTANS and FLOWERING LOCUC T, key regulators of the LD pathway. Furthermore, expression of floral meristem identity genes including SUPPRESSOR OF OVEREXPRESSION OF CO 1, LEAFY and FD was also promoted in AtDBP1 overexpressing plants. AtDBP1 transcription can be detected in root, leaf, stem, flower and silique. AtDBP1-GFP and YFP-AtDBP1 fusion protein were localized in the cytosol and nucleus. Our results provide the evidence demonstrating the effective role of AtDBP1 for flowering time regulation and report a novel function of DBP factors in planta besides in plant defense.

Role of phytochromes A and B in the regulation of cell death and acclimatory responses to UV stress in Arabidopsis thaliana

Plants coordinate their responses to various biotic and abiotic stresses in order to optimize their developmental and acclimatory programmes. The ultimate response to an excessive amount of stress is local induction of cell death mechanisms. The death of certain cells can help to maintain tissue homeostasis and enable nutrient remobilization, thus increasing the survival chances of the whole organism in unfavourable environmental conditions. UV radiation is one of the environmental factors that negatively affects the photosynthetic process and triggers cell death. The aim of this work was to evaluate a possible role of the red/far-red light photoreceptors phytochrome A (phyA) and phytochrome B (phyB) and their interrelations during acclimatory responses to UV stress. We showed that UV-C treatment caused a disturbance in photosystem II and a deregulation of photosynthetic pigment content and antioxidant enzymes activities, followed by increased cell mortality rate in phyB and phyAB null mutants. We also propose a regulatory role of phyA and phyB in CO2 assimilation, non-photochemical quenching, reactive oxygen species accumulation and salicylic acid content. Taken together, our results suggest a novel role of phytochromes as putative regulators of cell death and acclimatory responses to UV.

Phosphatases in plants

Reversible protein phosphorylation is an essential posttranslational modification mechanism executed by opposing actions of protein phosphatases and protein kinases. About 1,000 predicted kinases in Arabidopsis thaliana kinome predominate the number of protein phosphatases, of which there are only ~150 members in Arabidopsis. Protein phosphatases were often referred to as "housekeeping" enzymes, which act to keep eukaryotic systems in balance by counteracting the activity of protein kinases. However, recent investigations reveal the crucial and specific regulatory functions of phosphatases in cell signaling. Phosphatases operate in a coordinated manner with the protein kinases, to execute their important function in determining the cellular response to a physiological stimulus. Closer examination has established high specificity of phosphatases in substrate recognition and important roles in plant signaling pathways, such as pathogen defense and stress regulation, light and hormonal signaling, cell cycle and differentiation, metabolism, and plant growth. In this minireview we provide a compact overview about Arabidopsis protein phosphatase families, as well as members of phosphoglucan and lipid phosphatases, and highlight the recent discoveries in phosphatase research.

A reverse genetics approach identifies novel mutants in light responses and anthocyanin metabolism in petunia

Flower color and plant architecture are important commercially valuable features for ornamental petunias (Petunia x hybrida Vilm.). Photoperception and light signaling are the major environmental factors controlling anthocyanin and chlorophyll biosynthesis and shade-avoidance responses in higher plants. The genetic regulators of these processes were investigated in petunia by in silico analyses and the sequence information was used to devise a reverse genetics approach to probe mutant populations. Petunia orthologs of photoreceptor, light-signaling components and anthocyanin metabolism genes were identified and investigated for functional conservation by phylogenetic and protein motif analyses. The expression profiles of photoreceptor gene families and of transcription factors regulating anthocyanin biosynthesis were obtained by bioinformatic tools. Two mutant populations, generated by an alkalyting agent and by gamma irradiation, were screened using a phenotype-independent, sequence-based method by high-throughput PCR-based assay. The strategy allowed the identification of novel mutant alleles for anthocyanin biosynthesis (CHALCONE SYNTHASE) and regulation (PH4), and for light signaling (CONSTANS) genes.

Arabidopsis PPP family of serine/threonine protein phosphatases: many targets but few engines

The major plant serine/threonine protein phosphatases belong to the phosphoprotein phosphatase (PPP) family. Over the past few years the complement of Arabidopsis thaliana PPP family of catalytic subunits has been cataloged and many regulatory subunits identified. Specific roles for PPPs have been characterized, including roles in auxin and brassinosteroid signaling, in phototropism, in regulating the target of rapamycin pathway, and in cell stress responses. In this review, we provide a framework for understanding the PPP family by exploring the fundamental role of the phosphatase regulatory subunits that drive catalytic engine specificity. Although there are fewer plant protein phosphatases compared with their protein kinase partners, their function is now recognized to be as dynamic and as regulated as that of protein kinases.

Hypersensitive to red and blue 1 and its modification by protein phosphatase 7 are implicated in the control of Arabidopsis stomatal aperture

The stomatal pores are located on the plant leaf epidermis and regulate CO(2) uptake for photosynthesis and the loss of water by transpiration. Their stomatal aperture therefore affects photosynthesis, water use efficiency, and agricultural crop yields. Blue light, one of the environmental signals that regulates the plant stomatal aperture, is perceived by the blue/UV-A light-absorbing cryptochromes and phototropins. The signal transduction cascades that link the perception of light to the stomatal opening response are still largely unknown. Here, we report two new players, Hypersensitive to Red and Blue 1 (HRB1) and Protein Phosphatase 7 (PP7), and their genetic and biochemical interactions in the control of stomatal aperture. Mutations in either HRB1 or PP7 lead to the misregulation of the stomatal aperture and reduce water loss under blue light. Both HRB1 and PP7 are expressed in the guard cells in response to a light-to-dark or dark-to-light transition. HRB1 interacts with PP7 through its N-terminal ZZ-type zinc finger motif and requires a functional PP7 for its stomatal opening response. HRB1 is phosphorylated in vivo, and PP7 can dephosphorylate HRB1. HRB1 is mostly dephosphorylated in a protein complex of 193 kDa in the dark, and blue light increases complex size to 285 kDa. In the pp7 mutant, this size shift is impaired, and HRB1 is predominately phosphorylated. We propose that a modification of HRB1 by PP7 under blue light is essential to acquire a proper conformation or to bring in new components for the assembly of a functional HRB1 protein complex. Guard cells control stomatal opening in response to multiple environmental or biotic stimuli. This study may furnish strategies that allow plants to enjoy the advantages of both constitutive and ABA-induced protection under water-limiting conditions.

Photosynthesis, photorespiration, and light signalling in defence responses

Visible light is the basic energetic driver of plant biomass production through photosynthesis. The constantly fluctuating availability of light and other environmental factors means that the photosynthetic apparatus must be able to operate in a dynamic fashion appropriate to the prevailing conditions. Dynamic regulation is achieved through an array of homeostatic control mechanisms that both respond to and influence cellular energy and reductant status. In addition, light availability and quality are continuously monitored by plants through photoreceptors. Outside the laboratory growth room, it is within the context of complex changes in energy and signalling status that plants must regulate pathways to deal with biotic challenges, and this can be influenced by changes in the highly energetic photosynthetic pathways and in the turnover of the photosynthetic machinery. Because of this, defence responses are neither simple nor easily predictable, but rather conditioned by the nutritional and signalling status of the plant cell. This review discusses recent data and emerging concepts of how recognized defence pathways interact with and are influenced by light-dependent processes. Particular emphasis is placed on the potential roles of the chloroplast, photorespiration, and photoreceptor-associated pathways in regulating the outcome of interactions between plants and pathogenic organisms.

Heat shock modulates phosphorylation status and activity of nucleoside diphosphate kinase in cultured sugarcane cells

Nucleoside diphosphate kinase (NDPK) is involved in the regeneration of nucleoside triphosphates (NTPs) through its phosphotransferase activity via an autophosphorylating histidine residue. Additionally, autophosphorylation of serine and/or threonine residues is documented for NDPKs from various organisms. However, the metabolic significance of serine/threonine phosphorylation has not been well characterized. In this study we report the cloning and characterization of NDPKI from cultured sugarcane (Saccharum officinarum L. line H50-7209) cells, and modulation of serine autophosphorylation of NDPK1 in response to heat-shock (HS). Heat-shock treatment at 40°C for 2 h resulted in a 40% reduction in labeled phosphoserine in NDPK1. This dephosphorylation was accompanied by an increase in NDPK enzyme activity. In contrast, NDPK1 in cultured tobacco (cv. W-38) cells did not show changes in autophosphorylation or increased enzyme activity in response to HS. The mRNA or protein level of NDPK1 did not increase in response to HS. Sugarcane cells sustain the constitutive protein synthesis in addition to heat-shock protein synthesis during HS, while constitutive protein synthesis is significantly reduced in tobacco cells during HS. Thus, HS modulation of NDPK1 activity and serine dephosphorylation in sugarcane cells may represent an important physiological role in maintaining cellular metabolic functions during heat stress.

SHORT HYPOCOTYL UNDER BLUE1 truncations and mutations alter its association with a signaling protein complex in Arabidopsis

Higher plants monitor their ambient light signals through red/far-red absorbing phytochromes and blue/UV-A light absorbing cryptochromes. Subsequent signaling cascades alter gene expression and initiate morphogenic responses. We previously isolated SHORT HYPOCOTYL UNDER BLUE1 (SHB1), a putative transcriptional coactivator in light signaling. SHB1 is homologous to the SYG1 protein family and contains an N-terminal SPX domain and a C-terminal EXS domain. Overaccumulation of the SPX domain caused a long hypocotyl phenotype similar to that of shb1-D under red, far-red, or blue light. By contrast, overaccumulation of the C-terminal EXS domain led to a short hypocotyl phenotype similar to that of shb1 under blue light. The N-terminal SPX domain was associated with a smaller protein complex than the native protein complex associated with endogenous SHB1. By contrast, the EXS domain was associated with a slightly smaller protein complex than the native protein complex, but it largely displaced endogenous SHB1 from its native protein complex. In addition, all six missense mutations that we identified from a suppressor screen were clustered within or close to the SPX domain, and these mutations impaired the assembly of the SHB1-containing protein complex. We propose that both SPX and EXS domains likely anchor SHB1 to a protein complex, and the SPX domain is critical for SHB1 signaling.

PPEF/PP7 protein Ser/Thr phosphatases

PPEF/PP7 represents one of the five subfamilies of the PPP protein Ser/Thr phosphatases. Studies published in recent years point to a role of plant PP7 at a crossroad of different pathways of light and stress signalling. In animals, PPEFs are highly expressed in sensory neurons, and Drosophila PPEF phosphatase, rdgC, is essential for dephosphorylation of rhodopsin. Expression profiling suggests that mammalian PPEF may play a role in stress-protective responses, cell survival, growth, proliferation, and oncogenesis. Despite structural similarities of the catalytic domains and the fact that some of these phosphatases are involved in light perception both in animals and in plants, the plant and non-plant representatives of this group have distinct domain architecture and appear not to be orthologues.