Synthesis and import of GDP-l-fucose into the Golgi affect plant-water relations.

Land plants evolved multiple adaptations to restrict transpiration. However, the underlying molecular mechanisms are not sufficiently understood. We used an ozone-sensitivity forward genetics approach to identify Arabidopsis thaliana mutants impaired in gas exchange regulation. High water loss from detached leaves and impaired decrease of leaf conductance in response to multiple stomata-closing stimuli were identified in a mutant of MURUS1 (MUR1), an enzyme required for GDP-l-fucose biosynthesis. High water loss observed in mur1 was independent from stomatal movements and instead could be linked to metabolic defects. Plants defective in import of GDP-l-Fuc into the Golgi apparatus phenocopied the high water loss of mur1 mutants, linking this phenotype to Golgi-localized fucosylation events. However, impaired fucosylation of xyloglucan, N-linked glycans, and arabinogalactan proteins did not explain the aberrant water loss of mur1 mutants. Partial reversion of mur1 water loss phenotype by borate supplementation and high water loss observed in boron uptake mutants link mur1 gas exchange phenotypes to pleiotropic consequences of l-fucose and boron deficiency, which in turn affect mechanical and morphological properties of stomatal complexes and whole-plant physiology. Our work emphasizes the impact of fucose metabolism and boron uptake on plant-water relations.

Multi-modal meta-analysis of cancer cell line omics profiles identifies ECHDC1 as a novel breast tumor suppressor.

Molecular and functional profiling of cancer cell lines is subject to laboratory-specific experimental practices and data analysis protocols. The current challenge therefore is how to make an integrated use of the omics profiles of cancer cell lines for reliable biological discoveries. Here, we carried out a systematic analysis of nine types of data modalities using meta-analysis of 53 omics studies across 12 research laboratories for 2,018 cell lines. To account for a relatively low consistency observed for certain data modalities, we developed a robust data integration approach that identifies reproducible signals shared among multiple data modalities and studies. We demonstrated the power of the integrative analyses by identifying a novel driver gene, ECHDC1, with tumor suppressive role validated both in breast cancer cells and patient tumors. The multi-modal meta-analysis approach also identified synthetic lethal partners of cancer drivers, including a co-dependency of PTEN deficient endometrial cancer cells on RNA helicases.

Red and blue light treatments of ripening bilberry fruits reveal differences in signalling through abscisic acid-regulated anthocyanin biosynthesis.

The biosynthesis of anthocyanins has been shown to be influenced by light quality. However, the molecular mechanisms underlying the light-mediated regulation of fruit anthocyanin biosynthesis are not well understood. In this study, we analysed the effects of supplemental red and blue light on the anthocyanin biosynthesis in non-climacteric bilberry (Vaccinium myrtillus L.). After 6 days of continuous irradiation during ripening, both red and blue light elevated concentration of anthocyanins, up to 12- and 4-folds, respectively, compared to the control. Transcriptomic analysis of ripening berries showed that both light treatments up-regulated all the major anthocyanin structural genes, the key regulatory MYB transcription factors and abscisic acid (ABA) biosynthetic genes. However, higher induction of specific genes of anthocyanin and delphinidin biosynthesis alongside ABA signal perception and metabolism were found in red light. The difference in red and blue light signalling was found in 9-cis-epoxycarotenoid dioxygenase (NCED), ABA receptor pyrabactin resistance-like (PYL) and catabolic ABA-8'hydroxylase gene expression. Red light also up-regulated expression of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) domain transporters, which may indicate involvement of these proteins in vesicular trafficking of anthocyanins during fruit ripening. Our results suggest differential signal transduction and transport mechanisms between red and blue light in ABA-regulated anthocyanin and delphinidin biosynthesis during bilberry fruit ripening.

Broad-range RNA modification analysis of complex biological samples using rapid C18-UPLC-MS.

Post-transcriptional RNA modifications play an important role in cellular metabolism with homoeostatic disturbances manifesting as a wide repertoire of phenotypes, reduced stress tolerance and translational perturbation, developmental defects, and diseases, such as type II diabetes, leukaemia, and carcinomas. Hence, there has been an intense effort to develop various methods for investigating RNA modifications and their roles in various organisms, including sequencing-based approaches and, more frequently, liquid chromatography-mass spectrometry (LC-MS)-based methods. Although LC-MS offers numerous advantages, such as being highly sensitive and quantitative over a broad detection range, some stationary phase chemistries struggle to resolve positional isomers. Furthermore, the demand for detailed analyses of complex biological samples often necessitates long separation times, hampering sample-to-sample turnover and making multisample analyses time consuming. To overcome this limitation, we have developed an ultra-performance LC-MS (UPLC-MS) method that uses an octadecyl carbon chain (C18)-bonded silica matrix for the efficient separation of 50 modified ribonucleosides, including positional isomers, in a single 9-min sample-to-sample run. To validate the performance and versatility of our method, we analysed tRNA modification patterns of representative microorganisms from each domain of life, namely Archaea (Methanosarcina acetivorans), Bacteria (Pseudomonas syringae), and Eukarya (Saccharomyces cerevisiae). Additionally, our method is flexible and readily applicable for detection and relative quantification using stable isotope labelling and targeted approaches like multiple reaction monitoring (MRM). In conclusion, this method represents a fast and robust tool for broad-range exploration and quantification of ribonucleosides, facilitating future homoeostasis studies of RNA modification in complex biological samples.

Effect of wet storage conditions on potato tuber transcriptome, phytohormones and growth.

BACKGROUND: Stored potato (Solanum tuberosum L.) tubers are sensitive to wet conditions that can cause rotting in long-term storage. To study the effect of water on the tuber surface during storage, microarray analysis, RNA-Seq profiling, qRT-PCR and phytohormone measurements were performed to study gene expression and hormone content in wet tubers incubated at two temperatures: 4 °C and 15 °C. The growth of the plants was also observed in a greenhouse after the incubation of tubers in wet conditions. RESULTS: Wet conditions induced a low-oxygen response, suggesting reduced oxygen availability in wet tubers at both temperatures when compared to that in the corresponding dry samples. Wet conditions induced genes coding for heat shock proteins, as well as proteins involved in fermentative energy production and defense against reactive oxygen species (ROS), which are transcripts that have been previously associated with low-oxygen stress in hypoxic or anoxic conditions. Wet treatment also induced senescence-related gene expression and genes involved in cell wall loosening, but downregulated genes encoding protease inhibitors and proteins involved in chloroplast functions and in the biosynthesis of secondary metabolites. Many genes involved in the production of phytohormones and signaling were also affected by wet conditions, suggesting altered regulation of growth by wet conditions. Hormone measurements after incubation showed increased salicylic acid (SA), abscisic acid (ABA) and auxin (IAA) concentrations as well as reduced production of jasmonate 12-oxo-phytodienoic acid (OPDA) in wet tubers. After incubation in wet conditions, the tubers produced fewer stems and more roots compared to controls incubated in dry conditions. CONCLUSIONS: In wet conditions, tubers invest in ROS protection and defense against the abiotic stress caused by reduced oxygen due to excessive water. Changes in ABA, SA and IAA that are antagonistic to jasmonates affect growth and defenses, causing induction of root growth and rendering tubers susceptible to necrotrophic pathogens. Water on the tuber surface may function as a signal for growth, similar to germination of seeds.

Short oligogalacturonides induce pathogen resistance-associated gene expression in Arabidopsis thaliana.

BACKGROUND: Oligogalacturonides (OGs) are important components of damage-associated molecular pattern (DAMP) signaling and influence growth regulation in plants. Recent studies have focused on the impact of long OGs (degree of polymerization (DP) from 10-15), demonstrating the induction of plant defense signaling resulting in enhanced defenses to necrotrophic pathogens. To clarify the role of trimers (trimeric OGs, DP3) in DAMP signaling and their impact on plant growth regulation, we performed a transcriptomic analysis through the RNA sequencing of Arabidopsis thaliana exposed to trimers. RESULTS: The transcriptomic data from trimer-treated Arabidopsis seedlings indicate a clear activation of genes involved in defense signaling, phytohormone signaling and a down-regulation of genes involved in processes related to growth regulation and development. This is further accompanied with improved defenses against necrotrophic pathogens triggered by the trimer treatment, indicating that short OGs have a clear impact on plant responses, similar to those described for long OGs. CONCLUSIONS: Our results demonstrate that trimers are indeed active elicitors of plant defenses. This is clearly indicated by the up-regulation of genes associated with plant defense signaling, accompanied with improved defenses against necrotrophic pathogens. Moreover, trimers simultaneously trigger a clear down-regulation of genes and gene sets associated with growth and development, leading to stunted seedling growth in Arabidopsis.

Transcriptomics and functional genomics of ROS-induced cell death regulation by RADICAL-INDUCED CELL DEATH1.

Plant responses to changes in environmental conditions are mediated by a network of signaling events leading to downstream responses, including changes in gene expression and activation of cell death programs. Arabidopsis thaliana RADICAL-INDUCED CELL DEATH1 (RCD1) has been proposed to regulate plant stress responses by protein-protein interactions with transcription factors. Furthermore, the rcd1 mutant has defective control of cell death in response to apoplastic reactive oxygen species (ROS). Combining transcriptomic and functional genomics approaches we first used microarray analysis in a time series to study changes in gene expression after apoplastic ROS treatment in rcd1. To identify a core set of cell death regulated genes, RCD1-regulated genes were clustered together with other array experiments from plants undergoing cell death or treated with various pathogens, plant hormones or other chemicals. Subsequently, selected rcd1 double mutants were constructed to further define the genetic requirements for the execution of apoplastic ROS induced cell death. Through the genetic analysis we identified WRKY70 and SGT1b as cell death regulators functioning downstream of RCD1 and show that quantitative rather than qualitative differences in gene expression related to cell death appeared to better explain the outcome. Allocation of plant energy to defenses diverts resources from growth. Recently, a plant response termed stress-induced morphogenic response (SIMR) was proposed to regulate the balance between defense and growth. Using a rcd1 double mutant collection we show that SIMR is mostly independent of the classical plant defense signaling pathways and that the redox balance is involved in development of SIMR.

The F-box protein MAX2 contributes to resistance to bacterial phytopathogens in Arabidopsis thaliana.

BACKGROUND: The Arabidopsis thaliana F-box protein MORE AXILLARY GROWTH2 (MAX2) has previously been characterized for its role in plant development. MAX2 appears essential for the perception of the newly characterized phytohormone strigolactone, a negative regulator of polar auxin transport in Arabidopsis. RESULTS: A reverse genetic screen for F-box protein mutants altered in their stress responses identified MAX2 as a component of plant defense. Here we show that MAX2 contributes to plant resistance against pathogenic bacteria. Interestingly, max2 mutant plants showed increased susceptibility to the bacterial necrotroph Pectobacterium carotovorum as well as to the hemi-biotroph Pseudomonas syringae but not to the fungal necrotroph Botrytis cinerea. max2 mutant phenotype was associated with constitutively increased stomatal conductance and decreased tolerance to apoplastic ROS but also with alterations in hormonal balance. CONCLUSIONS: Our results suggest that MAX2 previously characterized for its role in regulation of polar auxin transport in Arabidopsis, and thus plant development also significantly influences plant disease resistance. We conclude that the increased susceptibility to P. syringae and P. carotovorum is due to increased stomatal conductance in max2 mutants promoting pathogen entry into the plant apoplast. Additional factors contributing to pathogen susceptibility in max2 plants include decreased tolerance to pathogen-triggered apoplastic ROS and alterations in hormonal signaling.

Are solar UV-B- and UV-A-dependent gene expression and metabolite accumulation in Arabidopsis mediated by the stress response regulator RADICAL-INDUCED CELL DEATH1?

Wavelengths in the ultraviolet (UV) region of the solar spectrum, UV-B (280-315 nm) and UV-A (315-400 nm), are key environmental signals modifying several aspects of plant physiology. Despite significant advances in the understanding of plant responses to UV-B and the identification of signalling components involved, there is limited information on the molecular mechanisms that control UV-B signalling in plants under natural sunlight. Here, we aimed to corroborate the previous suggested role for RADICAL-INDUCED CELL DEATH1 (RCD1) in UV-B signalling under full spectrum sunlight. Wild-type Arabidopsis thaliana and the rcd1-1 mutant were used in an experimental design outdoors where UV-B and UV-A irradiances were manipulated using plastic films, and gene expression, PYRIDOXINE BIOSYNTHESIS1 (PDX1) accumulation and metabolite profiles were analysed in the leaves. At the level of transcription, RCD1 was not directly involved in the solar UV-B regulation of genes with functions in UV acclimation, hormone signalling and stress-related markers. Furthermore, RCD1 had no role on PDX1 accumulation but modulated the UV-B induction of flavonoid accumulation in leaves of Arabidopsis exposed to solar UV. We conclude that RCD1 does not play an active role in UV-B signalling but rather modulates UV-B responses under full spectrum sunlight.

Epidermal UV-A absorbance and whole-leaf flavonoid composition in pea respond more to solar blue light than to solar UV radiation.

Plants synthesize phenolic compounds in response to certain environmental signals or stresses. One large group of phenolics, flavonoids, is considered particularly responsive to ultraviolet (UV) radiation. However, here we demonstrate that solar blue light stimulates flavonoid biosynthesis in the absence of UV-A and UV-B radiation. We grew pea plants (Pisum sativum cv. Meteor) outdoors, in Finland during the summer, under five types of filters differing in their spectral transmittance. These filters were used to (1) attenuate UV-B; (2) attenuate UV-B and UV-A < 370 nm; (3) attenuate UV-B and UV-A; (4) attenuate UV-B, UV-A and blue light; and (5) as a control not attenuating these wavebands. Attenuation of blue light significantly reduced the flavonoid content in leaf adaxial epidermis and reduced the whole-leaf concentrations of quercetin derivatives relative to kaempferol derivatives. In contrast, UV-B responses were not significant. These results show that pea plants regulate epidermal UV-A absorbance and accumulation of individual flavonoids by perceiving complex radiation signals that extend into the visible region of the solar spectrum. Furthermore, solar blue light instead of solar UV-B radiation can be the main regulator of phenolic compound accumulation in plants that germinate and develop outdoors.

Multiple roles for UV RESISTANCE LOCUS8 in regulating gene expression and metabolite accumulation in Arabidopsis under solar ultraviolet radiation.

Photomorphogenic responses triggered by low fluence rates of ultraviolet B radiation (UV-B; 280-315 nm) are mediated by the UV-B photoreceptor UV RESISTANCE LOCUS8 (UVR8). Beyond our understanding of the molecular mechanisms of UV-B perception by UVR8, there is still limited information on how the UVR8 pathway functions under natural sunlight. Here, wild-type Arabidopsis (Arabidopsis thaliana) and the uvr8-2 mutant were used in an experiment outdoors where UV-A (315-400 nm) and UV-B irradiances were attenuated using plastic films. Gene expression, PYRIDOXINE BIOSYNTHESIS1 (PDX1) accumulation, and leaf metabolite signatures were analyzed. The results show that UVR8 is required for transcript accumulation of genes involved in UV protection, oxidative stress, hormone signal transduction, and defense against herbivores under solar UV. Under natural UV-A irradiance, UVR8 is likely to interact with UV-A/blue light signaling pathways to moderate UV-B-driven transcript and PDX1 accumulation. UVR8 both positively and negatively affects UV-A-regulated gene expression and metabolite accumulation but is required for the UV-B induction of phenolics. Moreover, UVR8-dependent UV-B acclimation during the early stages of plant development may enhance normal growth under long-term exposure to solar UV.

5-Aminothiazoles Reveal a New Ligand-Binding Site on Prolyl Oligopeptidase Which is Important for Modulation of Its Protein-Protein Interaction-Derived Functions.

A series of novel 5-aminothiazole-based ligands for prolyl oligopeptidase (PREP) comprise selective, potent modulators of the protein-protein interaction (PPI)-mediated functions of PREP, although they are only weak inhibitors of the proteolytic activity of PREP. The disconnected structure-activity relationships are significantly more pronounced for the 5-aminothiazole-based ligands than for the earlier published 5-aminooxazole-based ligands. Furthermore, the stability of the 5-aminothiazole scaffold allowed exploration of wider substitution patterns than that was possible with the 5-aminooxazole scaffold. The intriguing structure-activity relationships for the modulation of the proteolytic activity and PPI-derived functions of PREP were elaborated by presenting a new binding site for PPI modulating PREP ligands, which was initially discovered using molecular modeling and later confirmed through point mutation studies. Our results suggest that this new binding site on PREP is clearly more important than the active site of PREP for the modulation of its PPI-mediated functions.

Regulation of ABA dependent wound induced spreading cell death by MYB108.

Wounding results in the controlled cell death of a few rows of cells adjacent to disrupted cells resulting in physical wound closure, which combined with phenolic compound deposition, prevents water loss and pathogen entry. The control of these processes remains uncharacterized. Cell death in a mutant of Arabidopsis thaliana lacking BOTRYTIS SENSITIVE1/MYB108 (BOS1/MYB108) function was characterized utilizing physiological, cell biological and genetic methods. The bos1 mutant has a wound induced runaway cell death that includes enhanced reactive oxygen species (ROS) production that followed the extent of enhanced cell death. Exogenous abscisic acid (ABA) enhanced wound induced cell death in Col-0 plants and was sufficient to trigger cell death in bos1. Uncontrolled cell death was dependent of the production and perception of ABA. Furthermore, bos1 had altered sensitivity to and accumulation of ABA. Arabidopsis possesses a genetic program controlling the extent of wound inducible cell death. BOS1 acts as a negative regulator of ABA induced cell death, which functions in the control of this wound sealing program. This program is distinct from other known cell death programs in that it is ABA dependent, but independent of salicylate biosynthesis, ethylene, jasmonate, metacaspases and ROS derived from RBOHD and RBOHF.

Defense-related transcription factors WRKY70 and WRKY54 modulate osmotic stress tolerance by regulating stomatal aperture in Arabidopsis.

WRKY transcription factors (TFs) have been mainly associated with plant defense, but recent studies have suggested additional roles in the regulation of other physiological processes. Here, we explored the possible contribution of two related group III WRKY TFs, WRKY70 and WRKY54, to osmotic stress tolerance. These TFs are positive regulators of plant defense, and co-operate as negative regulators of salicylic acid (SA) biosynthesis and senescence. We employed single and double mutants of wrky54 and wrky70, as well as a WRKY70 overexpressor line, to explore the role of these TFs in osmotic stress (polyethylene glycol) responses. Their effect on gene expression was characterized by microarrays and verified by quantitative PCR. Stomatal phenotypes were assessed by water retention and stomatal conductance measurements. The wrky54wrky70 double mutants exhibited clearly enhanced tolerance to osmotic stress. However, gene expression analysis showed reduced induction of osmotic stress-responsive genes in addition to reduced accumulation of the osmoprotectant proline. By contrast, the enhanced tolerance was correlated with improved water retention and enhanced stomatal closure. These findings demonstrate that WRKY70 and WRKY54 co-operate as negative regulators of stomatal closure and, consequently, osmotic stress tolerance in Arabidopsis, suggesting that they have an important role, not only in plant defense, but also in abiotic stress signaling.

Petunia dihydroflavonol 4-reductase is only a few amino acids away from producing orange pelargonidin-based anthocyanins.

Anthocyanins are responsible for the color spectrum of both ornamental and natural flowers. However, not all plant species produce all colors. For example, roses are not blue because they do not naturally possess a hydroxylase that opens the pathway for delphinidin and its derivatives. It is more intriguing why some plants do not carry orange or scarlet red flowers with anthocyanins based on pelargonidin, because the precursor for these anthocyanins should be available if anthocyanins are made at all. The key to this is the substrate specificity of dihydroflavonol 4-reductase (DFR), an enzyme located at the branch point between flavonols and anthocyanins. The most common example is petunia, which does not bear orange flowers unless the enzyme is complemented by biotechnology. We changed a few amino acids in the active site of the enzyme and showed that the mutated petunia DFR started to favor dihydrokaempferol, the precursor to orange pelargonidin, in vitro. When transferred to petunia, it produced an orange hue and dramatically more pelargonidin-based anthocyanins in the flowers.

Enzymatic assay for UDP-GlcNAc and its application in the parallel assessment of substrate availability and protein O-GlcNAcylation.

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a ubiquitous and dynamic non-canonical glycosylation of intracellular proteins. Several branches of metabolism converge at the hexosamine biosynthetic pathway (HBP) to produce the substrate for protein O-GlcNAcylation, the uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). Availability of UDP-GlcNAc is considered a key regulator of O-GlcNAcylation. Yet UDP-GlcNAc concentrations are rarely reported in studies exploring the HBP and O-GlcNAcylation, most likely because the methods to measure it are restricted to specialized chromatographic procedures. Here, we introduce an enzymatic method to quantify cellular and tissue UDP-GlcNAc. The method is based on O-GlcNAcylation of a substrate peptide by O-linked N-acetylglucosamine transferase (OGT) and subsequent immunodetection of the modification. The assay can be performed in dot-blot or microplate format. We apply it to quantify UDP-GlcNAc concentrations in several mouse tissues and cell lines. Furthermore, we show how changes in UDP-GlcNAc levels correlate with O-GlcNAcylation and the expression of OGT and O-GlcNAcase (OGA).

Mitochondrial complex III deficiency drives c-MYC overexpression and illicit cell cycle entry leading to senescence and segmental progeria.

Accumulating evidence suggests mitochondria as key modulators of normal and premature aging, yet whether primary oxidative phosphorylation (OXPHOS) deficiency can cause progeroid disease remains unclear. Here, we show that mice with severe isolated respiratory complex III (CIII) deficiency display nuclear DNA damage, cell cycle arrest, aberrant mitoses, and cellular senescence in the affected organs such as liver and kidney, and a systemic phenotype resembling juvenile-onset progeroid syndromes. Mechanistically, CIII deficiency triggers presymptomatic cancer-like c-MYC upregulation followed by excessive anabolic metabolism and illicit cell proliferation against lack of energy and biosynthetic precursors. Transgenic alternative oxidase dampens mitochondrial integrated stress response and the c-MYC induction, suppresses the illicit proliferation, and prevents juvenile lethality despite that canonical OXPHOS-linked functions remain uncorrected. Inhibition of c-MYC with the dominant-negative Omomyc protein relieves the DNA damage in CIII-deficient hepatocytes in vivo. Our results connect primary OXPHOS deficiency to genomic instability and progeroid pathogenesis and suggest that targeting c-MYC and aberrant cell proliferation may be therapeutic in mitochondrial diseases.

Nonpeptidic Oxazole-Based Prolyl Oligopeptidase Ligands with Disease-Modifying Effects on α-Synuclein Mouse Models of Parkinson's Disease.

Prolyl oligopeptidase (PREP) is a widely distributed serine protease in the human body cleaving proline-containing peptides; however, recent studies suggest that its effects on pathogenic processes underlying neurodegeneration are derived from direct protein-protein interactions (PPIs) and not from its regulation of certain neuropeptide levels. We discovered novel nonpeptidic oxazole-based PREP inhibitors, which deviate from the known structure-activity relationship for PREP inhibitors. These new compounds are effective modulators of the PPIs of PREP, reducing α-synuclein (αSyn) dimerization and enhancing protein phosphatase 2A activity in a concentration-response manner, as well as reducing reactive oxygen species production. From the best performing oxazoles, HUP-55 was selected for in vivo studies. Its brain penetration was evaluated, and it was tested in αSyn virus vector-based and αSyn transgenic mouse models of Parkinson's disease, where it restored motor impairment and reduced levels of oligomerized αSyn in the striatum and substantia nigra.

Apoplastic reactive oxygen species transiently decrease auxin signaling and cause stress-induced morphogenic response in Arabidopsis.

Reactive oxygen species (ROS) are ubiquitous signaling molecules in plant stress and development. To gain further insight into the plant transcriptional response to apoplastic ROS, the phytotoxic atmospheric pollutant ozone was used as a model ROS inducer in Arabidopsis (Arabidopsis thaliana) and gene expression was analyzed with microarrays. In contrast to the increase in signaling via the stress hormones salicylic acid, abscisic acid, jasmonic acid (JA), and ethylene, ROS treatment caused auxin signaling to be transiently suppressed, which was confirmed with a DR5-uidA auxin reporter construct. Transcriptomic data revealed that various aspects of auxin homeostasis and signaling were modified by apoplastic ROS. Furthermore, a detailed analysis of auxin signaling showed that transcripts of several auxin receptors and Auxin/Indole-3-Acetic Acid (Aux/IAA) transcriptional repressors were reduced in response to apoplastic ROS. The ROS-derived changes in the expression of auxin signaling genes partially overlapped with abiotic stress, pathogen responses, and salicylic acid signaling. Several mechanisms known to suppress auxin signaling during biotic stress were excluded, indicating that ROS regulated auxin responses via a novel mechanism. Using mutants defective in various auxin (axr1, nit1, aux1, tir1 afb2, iaa28-1, iaa28-2) and JA (axr1, coi1-16) responses, ROS-induced cell death was found to be regulated by JA but not by auxin. Chronic ROS treatment resulted in altered leaf morphology, a stress response known as "stress-induced morphogenic response." Altered leaf shape of tir1 afb2 suggests that auxin was a negative regulator of stress-induced morphogenic response in the rosette.

Regulatory subunit B'gamma of protein phosphatase 2A prevents unnecessary defense reactions under low light in Arabidopsis.

Light is an important environmental factor that modulates acclimation strategies and defense responses in plants. We explored the functional role of the regulatory subunit B'γ (B'γ) of protein phosphatase 2A (PP2A) in light-dependent stress responses of Arabidopsis (Arabidopsis thaliana). The predominant form of PP2A consists of catalytic subunit C, scaffold subunit A, and highly variable regulatory subunit B, which determines the substrate specificity of PP2A holoenzymes. Mutant leaves of knockdown pp2a-b'γ plants show disintegration of chloroplasts and premature yellowing conditionally under moderate light intensity. The cell-death phenotype is accompanied by the accumulation of hydrogen peroxide through a pathway that requires CONSTITUTIVE EXPRESSION OF PR GENES5 (CPR5). Moreover, the pp2a-b'γ cpr5 double mutant additionally displays growth suppression and malformed trichomes. Similar to cpr5, the pp2a-b'γ mutant shows constitutive activation of both salicylic acid- and jasmonic acid-dependent defense pathways. In contrast to cpr5, however, pp2a-b'γ leaves do not contain increased levels of salicylic acid or jasmonic acid. Rather, the constitutive defense response associates with hypomethylation of DNA and increased levels of methionine-salvage pathway components in pp2a-b'γ leaves. We suggest that the specific B'γ subunit of PP2A is functionally connected to CPR5 and operates in the basal repression of defense responses under low irradiance.