A proxitome-RNA-capture approach reveals that processing bodies repress coregulated hub genes

Cellular condensates are usually ribonucleoprotein assemblies with liquid- or solid-like properties. Because these subcellular structures lack a delineating membrane, determining their compositions is difficult. Here we describe a proximity-biotinylation approach for capturing the RNAs of the condensates known as processing bodies (PBs) in Arabidopsis (Arabidopsis thaliana). By combining this approach with RNA detection, in silico, and high-resolution imaging approaches, we studied PBs under normal conditions and heat stress. PBs showed a much more dynamic RNA composition than the total transcriptome. RNAs involved in cell wall development and regeneration, plant hormonal signaling, secondary metabolism/defense, and RNA metabolism were enriched in PBs. RNA-binding proteins and the liquidity of PBs modulated RNA recruitment, while RNAs were frequently recruited together with their encoded proteins. In PBs, RNAs follow distinct fates: in small liquid-like PBs, RNAs get degraded while in more solid-like larger ones, they are stored. PB properties can be regulated by the actin-polymerizing SCAR (suppressor of the cyclic AMP)-WAVE (WASP family verprolin homologous) complex. SCAR/WAVE modulates the shuttling of RNAs between PBs and the translational machinery, thereby adjusting ethylene signaling. In summary, we provide an approach to identify RNAs in condensates that allowed us to reveal a mechanism for regulating RNA fate.

Plant condensates: no longer membrane-less?

Cellular condensation is a reinvigorated area of study in biology, with scientific discussions focusing mainly on the forces that drive condensate formation, properties, and functions. Usually, condensates are called 'membrane-less' to highlight the absence of a surrounding membrane and the lack of associated contacts. In this opinion article we take a different direction, focusing on condensates that may be interfacing with membranes and their possible functions. We also highlight changes in condensate material properties brought about by condensate-membrane interactions, proposing how condensates-membrane interfaces could potentially affect interorganellar communication, development, and growth, but also adaptation in an evolutionary context. We would thus like to stimulate research in this area, which is much less understood in plants compared with the animal field.

Stress-related biomolecular condensates in plants

Biomolecular condensates are membraneless organelle-like structures that can concentrate molecules and often form through liquid-liquid phase separation. Biomolecular condensate assembly is tightly regulated by developmental and environmental cues. Although research on biomolecular condensates has intensified in the past 10 years, our current understanding of the molecular mechanisms and components underlying their formation remains in its infancy, especially in plants. However, recent studies have shown that the formation of biomolecular condensates may be central to plant acclimation to stress conditions. Here, we describe the mechanism, regulation, and properties of stress-related condensates in plants, focusing on stress granules and processing bodies, 2 of the most well-characterized biomolecular condensates. In this regard, we showcase the proteomes of stress granules and processing bodies in an attempt to suggest methods for elucidating the composition and function of biomolecular condensates. Finally, we discuss how biomolecular condensates modulate stress responses and how they might be used as targets for biotechnological efforts to improve stress tolerance.

An actin remodeling role for Arabidopsis processing bodies revealed by their proximity interactome

Cellular condensates can comprise membrane-less ribonucleoprotein assemblies with liquid-like properties. These cellular condensates influence various biological outcomes, but their liquidity hampers their isolation and characterization. Here, we investigated the composition of the condensates known as processing bodies (PBs) in the model plant Arabidopsis thaliana through a proximity-biotinylation proteomics approach. Using in situ protein-protein interaction approaches, genetics and high-resolution dynamic imaging, we show that processing bodies comprise networks that interface with membranes. Surprisingly, the conserved component of PBs, DECAPPING PROTEIN 1 (DCP1), can localize to unique plasma membrane subdomains including cell edges and vertices. We characterized these plasma membrane interfaces and discovered a developmental module that can control cell shape. This module is regulated by DCP1, independently from its role in decapping, and the actin-nucleating SCAR-WAVE complex, whereby the DCP1-SCAR-WAVE interaction confines and enhances actin nucleation. This study reveals an unexpected function for a conserved condensate at unique membrane interfaces.

Deep inside the epigenetic memories of stressed plants

Recent evidence sheds light on the peculiar type of plant intelligence. Plants have developed complex molecular networks that allow them to remember, choose, and make decisions depending on the stress stimulus, although they lack a nervous system. Being sessile, plants can exploit these networks to optimize their resources cost-effectively and maximize their fitness in response to multiple environmental stresses. Even more interesting is the capability to transmit this experience to the next generation(s) through epigenetic modifications that add to the classical genetic inheritance. In this opinion article, we present concepts and perspectives regarding the capabilities of plants to sense, perceive, remember, re-elaborate, respond, and to some extent transmit to their progeny information to adapt more efficiently to climate change.

The host exocyst complex is targeted by a conserved bacterial type-III effector that promotes virulence

For most Gram-negative bacteria, pathogenicity largely depends on the type-III secretion system that delivers virulence effectors into eukaryotic host cells. The subcellular targets for the majority of these effectors remain unknown. Xanthomonas campestris, the causal agent of black rot disease of crucifers such as Brassica spp., radish, and turnip, delivers XopP, a highly conserved core-effector protein produced by X. campestris, which is essential for virulence. Here, we show that XopP inhibits the function of the host-plant exocyst complex by direct targeting of Exo70B, a subunit of the exocyst complex, which plays a significant role in plant immunity. XopP interferes with exocyst-dependent exocytosis and can do this without activating a plant NOD-like receptor that guards Exo70B in Arabidopsis. In this way, Xanthomonas efficiently inhibits the host's pathogen-associated molecular pattern (PAMP)-triggered immunity by blocking exocytosis of pathogenesis-related protein-1A, callose deposition, and localization of the FLAGELLIN SENSITIVE2 (FLS2) immune receptor to the plasma membrane, thus promoting successful infection. Inhibition of exocyst function without activating the related defenses represents an effective virulence strategy, indicating the ability of pathogens to adapt to host defenses by avoiding host immunity responses.

An Epigenetic Alphabet of Crop Adaptation to Climate Change

Crop adaptation to climate change is in a part attributed to epigenetic mechanisms which are related to response to abiotic and biotic stresses. Although recent studies increased our knowledge on the nature of these mechanisms, epigenetics remains under-investigated and still poorly understood in many, especially non-model, plants, Epigenetic modifications are traditionally divided into two main groups, DNA methylation and histone modifications that lead to chromatin remodeling and the regulation of genome functioning. In this review, we outline the most recent and interesting findings on crop epigenetic responses to the environmental cues that are most relevant to climate change. In addition, we discuss a speculative point of view, in which we try to decipher the “epigenetic alphabet” that underlies crop adaptation mechanisms to climate change. The understanding of these mechanisms will pave the way to new strategies to design and implement the next generation of cultivars with a broad range of tolerance/resistance to stresses as well as balanced agronomic traits, with a limited loss of (epi)genetic variability.

Analyses of Protein Turnover at the Cell Plate by Fluorescence Recovery After Photobleaching During Cytokinesis

Membrane trafficking is central to cell plate construction during plant cytokinesis. Studies on cell plate formation can provide answers to basic biology questions including molecular mechanisms of membrane trafficking, tissue patterning, and cytoskeletal dynamics. Consequently, a detailed understanding of cytokinesis depends on the characterization of molecules that function in the formation, transport, targeting, and fusion of membrane vesicles and delivery of proteins to the developing and maturing plate. This chapter describes a pipeline based on fluorescence recovery after photobleaching (FRAP) to measure and analyze turnover of peripheral or transmembrane proteins on the cell plate. The approach described here can also be applied in other biological contexts.

Tudor staphylococcal nuclease is a docking platform for stress granule components and is essential for SnRK1 activation in Arabidopsis

Tudor staphylococcal nuclease (TSN; also known as Tudor-SN, p100, or SND1) is a multifunctional, evolutionarily conserved regulator of gene expression, exhibiting cytoprotective activity in animals and plants and oncogenic activity in mammals. During stress, TSN stably associates with stress granules (SGs), in a poorly understood process. Here, we show that in the model plant Arabidopsis thaliana, TSN is an intrinsically disordered protein (IDP) acting as a scaffold for a large pool of other IDPs, enriched for conserved stress granule components as well as novel or plant-specific SG-localized proteins. While approximately 30% of TSN interactors are recruited to stress granules de novo upon stress perception, 70% form a protein-protein interaction network present before the onset of stress. Finally, we demonstrate that TSN and stress granule formation promote heat-induced activation of the evolutionarily conserved energy-sensing SNF1-related protein kinase 1 (SnRK1), the plant orthologue of mammalian AMP-activated protein kinase (AMPK). Our results establish TSN as a docking platform for stress granule proteins, with an important role in stress signalling.

Do Microplastics Enter Our Food Chain Via Root Vegetables? A Raman Based Spectroscopic Study on Raphanus sativus

The outburst of plastic pollution in terrestrial ecosystems poses a potential threat to agriculture and food safety. Studies have already provided evidence for the uptake of plastic microparticles by several plant species, accompanied by numerous developmental effects, using fluorescence labelling techniques. Here, we introduce the implementation of confocal Raman spectroscopy, a label-free method, for the effective detection of microplastics (MPs) accumulation in the roots of a common edible root vegetable plant, Raphanus sativus, after treatment with acrylonitrile butadiene styrene (ABS) powder. We also demonstrate the concomitant occurrence of phenotypic defects in the polymer-treated plants. We anticipate that this work can provide new insights not only into the extent of the impact this widespread phenomenon has on crop plants but also on the methodological requirements to address it.

The RsRlpA Effector Is a Protease Inhibitor Promoting Rhizoctonia solani Virulence through Suppression of the Hypersensitive Response

Rhizoctonia solani (Rs) is a soil-borne pathogen with a broad host range. This pathogen incites a wide range of disease symptoms. Knowledge regarding its infection process is fragmented, a typical feature for basidiomycetes. In this study, we aimed at identifying potential fungal effectors and their function. From a group of 11 predicted single gene effectors, a rare lipoprotein A (RsRlpA), from a strain attacking sugar beet was analyzed. The RsRlpA gene was highly induced upon early-stage infection of sugar beet seedlings, and heterologous expression in Cercospora beticola demonstrated involvement in virulence. It was also able to suppress the hypersensitive response (HR) induced by the Avr4/Cf4 complex in transgenic Nicotiana benthamiana plants and functioned as an active protease inhibitor able to suppress Reactive Oxygen Species (ROS) burst. This effector contains a double-psi beta-barrel (DPBB) fold domain, and a conserved serine at position 120 in the DPBB fold domain was found to be crucial for HR suppression. Overall, R. solani seems to be capable of inducing an initial biotrophic stage upon infection, suppressing basal immune responses, followed by a switch to necrotrophic growth. However, regulatory mechanisms between the different lifestyles are still unknown.

Proteolytic Proteoforms: Elusive Components of Hormonal Pathways?

Hormonal pathways often converge on transcriptional repressors that can be degraded by the proteasome to initiate a response. We wish to draw attention to developments in a less-explored proteolytic branch called 'limited proteolysis' that, in addition to the classical proteolytic pathways, seems to regulate auxin and ethylene signaling.

Proteolysis and nitrogen: emerging insights

Nitrogen (N) is a core component of fertilizers used in modern agriculture to increase yields and thus to help feed a growing global population. However, this comes at a cost to the environment, through run-off of excess N as a result of poor N-use efficiency (NUE) by crops. An obvious remedy to this problem would therefore be the improvement of NUE, which requires advancing our understanding on N homeostasis, sensing, and uptake. Proteolytic pathways are linked to N homeostasis as they recycle proteins that contain N and carbon; however, emerging data suggest that their functions extend beyond this simple recycling. Here, we highlight roles of proteolytic pathways in non-symbiotic and symbiotic N uptake and in systemic N sensing. We also offer a novel view in which we suggest that proteolytic pathways have roles in N homeostasis that differ from their accepted function in recycling.

Distinct modes of derepression of an Arabidopsis immune receptor complex by two different bacterial effectors

Plant intracellular nucleotide-binding leucine-rich repeat (NLR) immune receptors often function in pairs to detect pathogen effectors and activate defense. The Arabidopsis RRS1-R-RPS4 NLR pair recognizes the bacterial effectors AvrRps4 and PopP2 via an integrated WRKY transcription factor domain in RRS1-R that mimics the effector's authentic targets. How the complex activates defense upon effector recognition is unknown. Deletion of the WRKY domain results in an RRS1 allele that triggers constitutive RPS4-dependent defense activation, suggesting that in the absence of effector, the WRKY domain contributes to maintaining the complex in an inactive state. We show the WRKY domain interacts with the adjacent domain 4, and that the inactive state of RRS1 is maintained by WRKY-domain 4 interactions before ligand detection. AvrRps4 interaction with the WRKY domain disrupts WRKY-domain 4 association, thus derepressing the complex. PopP2-triggered activation is less easily explained by such disruption and involves the longer C-terminal extension of RRS1-R. Furthermore, some mutations in RPS4 and RRS1 compromise PopP2 but not AvrRps4 recognition, suggesting that AvrRps4 and PopP2 derepress the complex differently. Consistent with this, a "reversibly closed" conformation of RRS1-R, engineered in a method exploiting the high affinity of colicin E9 and Im9 domains, reversibly loses AvrRps4, but not PopP2 responsiveness. Following RRS1 derepression, interactions between domain 4 and the RPS4 C-terminal domain likely contribute to activation. Simultaneous relief of autoinhibition and activation may contribute to defense activation in many immune receptors.

Cutting in the middleman: hidden substrates at the interface between proteases and plant development

Contents Summary 916 I. Introduction 916 II. DEK1: towards identification of protease substrates 917 III. Separases: when proteolytic modules attain nonproteolytic functions 918 IV. The peculiar case of a nonredundant subtilisin 919 V. Towards a solution to the protease redundancy problem 920 VI. Matters arising and closing remarks 921 Acknowledgements 921 References 921 SUMMARY: Proteases are integral components of proteome remodelling networks that regulate turnover of proteins and expand their functional diversity. Accumulating evidence highlights the importance of proteases as being central hubs of developmental programs. Yet the molecular pathways that many proteases act on, their natural substrates and their putative nonproteolytic functions remain largely elusive. Here, we discuss recent findings on proteases with functions that converge into plant development regulation, such as DEFECTIVE KERNEL 1 (DEK1), separase and subtilisins, to highlight conspicuous but unexplored aspects of protease biology. We also suggest an exploratory framework for addressing protease functions.

Transcriptional stimulation of rate-limiting components of the autophagic pathway improves plant fitness

Autophagy is a major catabolic process whereby autophagosomes deliver cytoplasmic content to the lytic compartment for recycling. Autophagosome formation requires two ubiquitin-like systems conjugating Atg12 with Atg5, and Atg8 with lipid phosphatidylethanolamine (PE), respectively. Genetic suppression of these systems causes autophagy-deficient phenotypes with reduced fitness and longevity. We show that Atg5 and the E1-like enzyme, Atg7, are rate-limiting components of Atg8-PE conjugation in Arabidopsis. Overexpression of ATG5 or ATG7 stimulates Atg8 lipidation, autophagosome formation, and autophagic flux. It also induces transcriptional changes opposite to those observed in atg5 and atg7 mutants, favoring stress resistance and growth. As a result, ATG5- or ATG7-overexpressing plants exhibit increased resistance to necrotrophic pathogens and oxidative stress, delayed aging and enhanced growth, seed set, and seed oil content. This work provides an experimental paradigm and mechanistic insight into genetic stimulation of autophagy in planta and shows its efficiency for improving plant productivity.

Determination of di-/Polyamine Oxidase Activity in Plants by an In-Gel Spermidine Oxidation Assay

Diamine and polyamine catabolism controls plant development, resistance to pathogens and stress responses. Diamine and polyamine oxidases control the catabolism of diamines and polyamines, respectively. Two major routes of di-/polyamine catabolism exist: the terminal and the interconverting. The in vitro activity of each route is assayed by the colorimetric or chemiluminescent determination of hydrogen peroxide produced by oxidation of di-/polyamine substrates. However, these assays fail to estimate activity of individual di-/polyamine oxidase isoenzymes. Herein, I describe an assay for the simultaneous in-gel determination of terminal and interconverting di-/polyamine oxidase isoenzyme activities.

Limited and digestive proteolysis: crosstalk between evolutionary conserved pathways

Contents 958 I. 958 II. 959 III. 960 IV. 962 V. 962 962 References 963 SUMMARY: Proteases can either digest target proteins or perform the so-called 'limited proteolysis' by cleaving polypeptide chains at specific site(s). Autophagy and the ubiquitin-proteasome system (UPS) are two main mechanisms carrying out digestive proteolysis. While the net outcome of digestive proteolysis is the loss of function of protein substrates, limited proteolysis can additionally lead to gain or switch of function. Recent evidence of crosstalk between autophagy, UPS and limited proteolysis indicates that these pathways are parts of the same proteolytic nexus. Here, we focus on three emerging themes within this area: limited proteolysis as a mechanism modulating autophagy; interplay between autophagy and UPS, including autophagic degradation of proteasomes (proteophagy); and specificity of protein degradation during bulk autophagy.

Separase Promotes Microtubule Polymerization by Activating CENP-E-Related Kinesin Kin7

Microtubules play an essential role in breaking cellular symmetry. We have previously shown that separase associates with microtubules and regulates microtubule-dependent establishment of cell polarity in Arabidopsis. However, separase lacks microtubule-binding activity, raising questions about mechanisms underlying this phenomenon. Here we report that the N-terminal non-catalytic domain of separase binds to the C-terminal tail domain of three homologs of the centromeric protein CENP-E Kinesin 7 (Kin7). Conformational changes of Kin7 induced upon binding to separase facilitate recruitment of Kin7/separase complex (KISC) onto microtubules. KISC operates independently of proteolytic activity of separase in promoting microtubule rescue and pauses, as well as in suppressing catastrophes. Genetic complementation experiments in conditional separase mutant rsw4 background demonstrate the importance of KISC for the establishment of cell polarity and for plant development. Our study establishes a mechanism governing microtubule dynamics via the separase-dependent activation of CENP-E-related kinesins.

The Arabidopsis homolog of Scc4/MAU2 is essential for embryogenesis

Factors regulating dynamics of chromatin structure have direct impact on expression of genetic information. Cohesin is a multi-subunit protein complex that is crucial for pairing sister chromatids during cell division, DNA repair and regulation of gene transcription and silencing. In non-plant species, cohesin is loaded on chromatin by the Scc2-Scc4 complex (also known as the NIBPL-MAU2 complex). Here, we identify the Arabidopsis homolog of Scc4, which we denote Arabidopsis thaliana (At)SCC4, and show that it forms a functional complex with AtSCC2, the homolog of Scc2. We demonstrate that AtSCC2 and AtSCC4 act in the same pathway, and that both proteins are indispensable for cell fate determination during early stages of embryo development. Mutant embryos lacking either of these proteins develop only up to the globular stage, and show the suspensor overproliferation phenotype preceded by ectopic auxin maxima distribution. We further establish a new assay to reveal the AtSCC4-dependent dynamics of cohesin loading on chromatin in vivo Our findings define the Scc2-Scc4 complex as an evolutionary conserved machinery controlling cohesin loading and chromatin structure maintenance, and provide new insight into the plant-specific role of this complex in controlling cell fate during embryogenesis.

An NADPH-Oxidase/Polyamine Oxidase Feedback Loop Controls Oxidative Burst Under Salinity

The apoplastic polyamine oxidase (PAO) catalyzes the oxidation of the higher polyamines spermidine and spermine, contributing to hydrogen peroxide (H2O2) accumulation. However, it is yet unclear whether apoplastic PAO is part of a network that coordinates the accumulation of reactive oxygen species (ROS) under salinity or if it acts independently. Here, we unravel that NADPH oxidase and apoplastic PAO cooperate to control the accumulation of H2O2 and superoxides (O2·-) in tobacco (Nicotiana tabacum). To examine to what extent apoplastic PAO constitutes part of a ROS-generating network, we examined ROS accumulation in guard cells of plants overexpressing or down-regulating apoplastic PAO (lines S2.2 and A2, respectively) or down-regulating NADPH oxidase (line AS-NtRbohD/F). The H2O2-specific probe benzene sulfonyl-H2O2 showed that, under salinity, H2O2 increased in S2.2 and decreased in A2 compared with the wild type. Surprisingly, the O2·--specific probe benzene sulfonyl-So showed that O2·- levels correlated positively with that of apoplastic PAO (i.e. showed high and low levels in S2.2 and A2, respectively). By using AS-NtRbohD/F lines and a pharmacological approach, we could show that H2O2 and O2·- accumulation at the onset of salinity stress was dependent on NADPH oxidase, indicating that NADPH oxidase is upstream of apoplastic PAO. Our results suggest that NADPH oxidase and the apoplastic PAO form a feed-forward ROS amplification loop, which impinges on oxidative state and culminates in the execution of programmed cell death. We propose that the PAO/NADPH oxidase loop is a central hub in the plethora of responses controlling salt stress tolerance, with potential functions extending beyond stress tolerance.

EXTRA SPINDLE POLES (Separase) controls anisotropic cell expansion in Norway spruce (Picea abies) embryos independently of its role in anaphase progression

The caspase-related protease separase (EXTRA SPINDLE POLES, ESP) plays a major role in chromatid disjunction and cell expansion in Arabidopsis thaliana. Whether the expansion phenotypes are linked to defects in cell division in Arabidopsis ESP mutants remains elusive. Here we present the identification, cloning and characterization of the gymnosperm Norway spruce (Picea abies, Pa) ESP. We used the P. abies somatic embryo system and a combination of reverse genetics and microscopy to explore the roles of Pa ESP during embryogenesis. Pa ESP was expressed in the proliferating embryonal mass, while it was absent in the suspensor cells. Pa ESP associated with kinetochore microtubules in metaphase and then with anaphase spindle midzone. During cytokinesis, it localized on the phragmoplast microtubules and on the cell plate. Pa ESP deficiency perturbed anisotropic expansion and reduced mitotic divisions in cotyledonary embryos. Furthermore, whilst Pa ESP can rescue the chromatid nondisjunction phenotype of Arabidopsis ESP mutants, it cannot rescue anisotropic cell expansion. Our data demonstrate that the roles of ESP in daughter chromatid separation and cell expansion are conserved between gymnosperms and angiosperms. However, the mechanisms of ESP-mediated regulation of cell expansion seem to be lineage-specific.

Silencing S-Adenosyl-L-Methionine Decarboxylase (SAMDC) in Nicotiana tabacum Points at a Polyamine-Dependent Trade-Off between Growth and Tolerance Responses

Polyamines (PAs) are nitrogenous molecules that are indispensable for cell viability and with an agreed-on role in the modulation of stress responses. Tobacco plants with downregulated SAMDC (AS-SAMDC) exhibit reduced PAs synthesis but normal levels of PA catabolism. We used AS-SAMDC to increase our understanding on the role of PAs in stress responses. Surprisingly, at control conditions AS-SAMDC plants showed increased biomass and altered developmental characteristics, such as increased height and leaf number. On the contrary, during salt stress AS-SAMDC plants showed reduced vigor when compared to the WT. During salt stress, the AS-SAMDC plants although showing compensatory readjustments of the antioxidant machinery and of photosynthetic apparatus, they failed to sustain their vigor. AS-SAMDC sensitivity was accompanied by inability to effectively control H2O2 levels and concentrations of monovalent and divalent cations. In accordance with these findings, we suggest that PAs may regulate the trade-off between growth and tolerance responses.

WUSCHEL-RELATED HOMEOBOX 2 is important for protoderm and suspensor development in the gymnosperm Norway spruce

BACKGROUND

Distinct expression domains of WUSCHEL-RELATED HOMEOBOX (WOX) gene family members are involved in patterning and morphogenesis of the early embryo in Arabidopsis. However, the role of WOX genes in other taxa, including gymnosperms, remains elusive. Here, we use somatic embryos and reverse genetics for studying expression and function of PaWOX2, the corresponding homolog of AtWOX2 in the gymnosperm Picea abies (Pa; Norway spruce).

RESULTS

The mRNA level of PaWOX2 was transiently up-regulated during early and late embryogeny. PaWOX2 mRNA in early and early late embryos was detected both in the embryonal mass and in the upper part of the suspensor. Down-regulation of PaWOX2 during development of early embryos resulted in aberrant early embryos, which failed to form a proper protoderm. Cells on the surface layer of the embryonal mass became vacuolated, and new embryogenic tissue differentiated from the embryonal mass. In addition, the aberrant early embryos lacked a distinct border between the embryonal mass, and the suspensor and the length of the suspensor cells was reduced. Down-regulation of PaWOX2 in the beginning of embryo development, before late embryos were formed, caused a significant decrease in the yield of mature embryos. On the contrary, down-regulation of PaWOX2 after late embryos were formed had no effect on further embryo development and maturation.

CONCLUSIONS

Our data suggest an evolutionarily conserved function of WOX2 in protoderm formation early during embryo development among seed plants. In addition, PaWOX2 might exert a unique function in suspensor expansion in gymnosperms.

The WUSCHEL-RELATED HOMEOBOX 3 gene PaWOX3 regulates lateral organ formation in Norway spruce

In angiosperms, WUSCHEL-RELATED HOMEOBOX 3 (WOX3) genes are required for the recruitment of founder cells from the lateral domains of shoot meristems that form lateral regions of leaves. However, the regulation of the formation of lateral organs in gymnosperms remains unknown. By using somatic embryos of Norway spruce (Picea abies) we have studied the expression and function of PaWOX3 during embryo development. The mRNA abundance of PaWOX3 was determined by quantitative real-time PCR, and the spatial expression of PaWOX3 was analysed by histochemical β-glucuronidase (GUS) assays and in situ mRNA hybridization. To investigate the function of PaWOX3, we analysed how downregulation of PaWOX3 in RNA interference lines affected embryo development and morphology. PaWOX3 was highly expressed in mature embryos at the base of each cotyledon close to the junction between the cotyledons, and in the lateral margins of cotyledons and needles, separating them into an adaxial and an abaxial side. Downregulation of the expression of PaWOX3 caused defects in lateral margin outgrowth in cotyledons and needles, and reduced root elongation. Our data suggest that the WOX3 function in margin outgrowth in lateral organs is conserved among the seed plants, whereas its function in root elongation may be unique to gymnosperms.

Tudor staphylococcal nuclease links formation of stress granules and processing bodies with mRNA catabolism in Arabidopsis

Tudor Staphylococcal Nuclease (TSN or Tudor-SN; also known as SND1) is an evolutionarily conserved protein involved in the transcriptional and posttranscriptional regulation of gene expression in animals. Although TSN was found to be indispensable for normal plant development and stress tolerance, the molecular mechanisms underlying these functions remain elusive. Here, we show that Arabidopsis thaliana TSN is essential for the integrity and function of cytoplasmic messenger ribonucleoprotein (mRNP) complexes called stress granules (SGs) and processing bodies (PBs), sites of posttranscriptional gene regulation during stress. TSN associates with SGs following their microtubule-dependent assembly and plays a scaffolding role in both SGs and PBs. The enzymatically active tandem repeat of four SN domains is crucial for targeting TSN to the cytoplasmic mRNA complexes and is sufficient for the cytoprotective function of TSN during stress. Furthermore, our work connects the cytoprotective function of TSN with its positive role in stress-induced mRNA decapping. While stress led to a pronounced increase in the accumulation of uncapped mRNAs in wild-type plants, this increase was abrogated in TSN knockout plants. Taken together, our results establish TSN as a key enzymatic component of the catabolic machinery responsible for the processing of mRNAs in the cytoplasmic mRNP complexes during stress.

Tudor Staphylococcal Nuclease plays two antagonistic roles in RNA metabolism under stress

Adaptation to stress entails a repertoire of molecular pathways that remodel the proteome, thereby promoting selective translation of pro-survival proteins. Yet, translation of other proteins, especially those which are harmful for stress adaptation is, on the contrary, transiently suppressed through mRNA decay or storage. Proteome remodeling under stress is intimately associated with the cytoplasmic ribonucleoprotein (RNP) complexes called stress granules (SGs) and processing bodies (PBs). The molecular composition and regulation of SGs and PBs in plants remain largely unknown. Recently, we identified the Arabidopsis Tudor Staphylococcal Nuclease (TSN, Tudor-SN or SND1) as a SG- and PB-associated protein required for mRNA decapping under stress conditions. Here we show that SGs localize in close proximity to PBs within plant cells that enable the exchange of molecular components. Furthermore, we provide a meta-analysis of mRNA degradome of TSN-deficient plants suggesting that TSN might inhibit the degradation of mRNAs which are involved in stress adaptation. Our results establish TSN as a versatile mRNA regulator during stress.

Wuschel-related homeobox 8/9 is important for proper embryo patterning in the gymnosperm Norway spruce

Proper embryo development is crucial as that is when the primary body axes are established. In Arabidopsis, AtWOX8 and AtWOX9, members of the Wuschel-related homeobox (WOX) gene family, are critical for embryo development. In Norway spruce, PaWOX8/9, which is expressed in embryos, is the homologue of AtWOX8 and AtWOX9. In this work, it is shown that the transcript abundance of PaWOX8/9 is high during early and late embryogeny and that it decreases when the maturation phase starts. To address the function of PaWOX8/9 during embryo development, RNAi lines were established to down-regulate the transcript level of PaWOX8/9, using both constitutive and inducible promoters. Embryos in the PaWOX8/9 RNAi lines show an aberrant morphology caused by disturbed orientation of the cell division plane at the basal part of the embryonal mass during early and late embryogeny. In addition, the transcript level of several key cell-cycle-regulating genes, for example, PaE2FAB-like and PaCYCLIN B-like, are affected in the PaWOX8/9 RNAi lines. Taken together, our results suggest that PaWOX8/9 may perform an evolutionarily conserved function as a regulator of the establishment of the apical-basal embryo pattern.

Polyamines and programmed cell death

Polyamines (PAs) have been considered as important molecules for survival. However, evidence reinforces that PAs are also implicated, directly or indirectly, in pathways regulating programmed cell death (PCD). Direct correlation of PAs with cell death refers to their association with particular biological processes, and their physical contact with molecules or structures involved in cell death. Indirectly, PAs regulate PCD through their metabolic derivatives, such as catabolic and interconversion products. Cytotoxic products of PA metabolism are involved in PCD cascades, whereas it remains largely elusive how PAs directly control pathways leading to PCD. In this review, we present and compare advances in PA-dependent PCD in animals and plants.

Peroxisomal polyamine oxidase and NADPH-oxidase cross-talk for ROS homeostasis which affects respiration rate in Arabidopsis thaliana

Homeostasis of reactive oxygen species (ROS) in the intracellular compartments is of critical importance as ROS have been linked with nearly all cellular processes and more importantly with diseases and aging. PAs are nitrogenous molecules with an evolutionary conserved role in the regulation of metabolic and energetic status of cells. Recent evidence also suggests that polyamines (PA) are major regulators of ROS homeostasis. In Arabidopsis the backconversion of the PAs spermidine (Spd) and spermine to putrescine and Spd, respectively, is catalyzed by two peroxisomal PA oxidases (AtPAO). However, the physiological role of this pathway remains largely elusive. Here we explore the role of peroxisomal PA backconversion and in particular that catalyzed by the highly expressed AtPAO3 in the regulation of ROS homeostasis and mitochondrial respiratory burst. Exogenous PAs exert an NADPH-oxidase dependent stimulation of oxygen consumption, with Spd exerting the strongest effect. This increase is attenuated by treatment with the NADPH-oxidase blocker diphenyleneiodonium iodide (DPI). Loss-of-function of AtPAO3 gene results to increased NADPH-oxidase-dependent production of superoxide anions ([Formula: see text] ), but not H2O2, which activate the mitochondrial alternative oxidase pathway (AOX). On the contrary, overexpression of AtPAO3 results to an increased but balanced production of both H2O2 and [Formula: see text] . These results suggest that the ratio of [Formula: see text] /H2O2 regulates respiratory chain in mitochondria, with PA-dependent production of [Formula: see text] by NADPH-oxidase tilting the balance of electron transfer chain in favor of the AOX pathway. In addition, AtPAO3 seems to be an important component in the regulating module of ROS homeostasis, while a conserved role for PA backconversion and ROS across kingdoms is discussed.

The caspase-related protease separase (extra spindle poles) regulates cell polarity and cytokinesis in Arabidopsis

Vesicle trafficking plays an important role in cell division, establishment of cell polarity, and translation of environmental cues to developmental responses. However, the molecular mechanisms regulating vesicle trafficking remain poorly understood. Here, we report that the evolutionarily conserved caspase-related protease separase (extra spindle poles [ESP]) is required for the establishment of cell polarity and cytokinesis in Arabidopsis thaliana. At the cellular level, separase colocalizes with microtubules and RabA2a (for RAS genes from rat brainA2a) GTPase-positive structures. Separase facilitates polar targeting of the auxin efflux carrier PIN-formed2 (PIN2) to the rootward side of the root cortex cells. Plants with the radially swollen4 (rsw4) allele with compromised separase activity, in addition to mitotic failure, display isotropic cell growth, perturbation of auxin gradient formation, slower gravitropic response in roots, and cytokinetic failure. Measurements of the dynamics of vesicle markers on the cell plate revealed an overall reduction of the delivery rates of KNOLLE and RabA2a GTPase in separase-deficient roots. Furthermore, dissociation of the clathrin light chain, a protein that plays major role in the formation of coated vesicles, was slower in rsw4 than in the control. Our results demonstrate that separase is a key regulator of vesicle trafficking, which is indispensable for cytokinesis and the establishment of cell polarity.

Autophagy mediates caloric restriction-induced lifespan extension in Arabidopsis

Caloric restriction (CR) extends lifespan in various heterotrophic organisms ranging from yeasts to mammals, but whether a similar phenomenon occurs in plants remains unknown. Plants are autotrophs and use their photosynthetic machinery to convert light energy into the chemical energy of glucose and other organic compounds. As the rate of photosynthesis is proportional to the level of photosynthetically active radiation, the CR in plants can be modeled by lowering light intensity. Here, we report that low light intensity extends the lifespan in Arabidopsis through the mechanisms triggering autophagy, the major catabolic process that recycles damaged and potentially harmful cellular material. Knockout of autophagy-related genes results in the short lifespan and suppression of the lifespan-extending effect of the CR. Our data demonstrate that the autophagy-dependent mechanism of CR-induced lifespan extension is conserved between autotrophs and heterotrophs.

The polyamines and their catabolic products are significant players in the turnover of nitrogenous molecules in plants

Polyamines (PAs) are nitrogenous molecules which play a well-established role in most cellular processes during growth and development under physiological or biotic/abiotic stress conditions. The molecular mode(s) of PA action have only recently started to be unveiled, and comprehensive models for their molecular interactions have been proposed. Their multiple roles are exerted, at least partially, through signalling by hydrogen peroxide (H(2)O(2)), which is generated by the oxidation/back-conversion of PAs by copper amine oxidases and PA oxidases. Accumulating evidence suggests that in plants the cellular titres of PAs are affected by other nitrogenous compounds. Here, we discuss the state of the art on the possible nitrogen flow in PAs, their interconnection with nitrogen metabolism, as well as the signalling roles of PA-derived H(2)O(2) during some developmental processes and stress responses.

Separases: biochemistry and function

Tight regulation of cell cycle is of critical importance for eukaryotic biology and is achieved through a combined action of a large number of highly specialized proteins. Separases are evolutionarily conserved caspase-like proteases playing a crucial role in cell cycle regulation, as they execute sister chromatid separation at metaphase to anaphase transition. In contrast to extensively studied yeast and metazoan separases, very little is known about the role of separases in plant biology. Here we describe the molecular mechanisms of separase-mediated chromatid segregation in yeast and metazoan models, discuss new emerging but less-understood functions of separases and highlight major gaps in our knowledge about plant separases.

The members of Arabidopsis thaliana PAO gene family exhibit distinct tissue- and organ-specific expression pattern during seedling growth and flower development

Polyamine oxidases (PAOs) are FAD-dependent enzymes involved in polyamine catabolism. In Arabidopsis thaliana, five PAOs (AtPAO1-5) are present with cytosolic or peroxisomal localization. Here, we present a detailed study of the expression pattern of AtPAO1, AtPAO2, AtPAO3 and AtPAO5 during seedling and flower growth and development through analysis of promoter activity in AtPAO::β-glucuronidase (GUS) transgenic Arabidopsis plants. The results reveal distinct expression patterns for each studied member of the AtPAO gene family. AtPAO1 is mostly expressed in the transition region between the meristematic and the elongation zone of roots and anther tapetum, AtPAO2 in the quiescent center, columella initials and pollen, AtPAO3 in columella, guard cells and pollen, and AtPAO5 in the vascular system of roots and hypocotyls. Furthermore, treatment with the plant hormone abscisic acid induced expression of AtPAO1 in root tip and AtPAO2 in guard cells. These data suggest distinct physiological role(s) for each member of the AtPAO gene family.

Perturbation of polyamine catabolism can strongly affect root development and xylem differentiation

Spermidine (Spd) treatment inhibited root cell elongation, promoted deposition of phenolics in cell walls of rhizodermis, xylem elements, and vascular parenchyma, and resulted in a higher number of cells resting in G(1) and G(2) phases in the maize (Zea mays) primary root apex. Furthermore, Spd treatment induced nuclear condensation and DNA fragmentation as well as precocious differentiation and cell death in both early metaxylem and late metaxylem precursors. Treatment with either N-prenylagmatine, a selective inhibitor of polyamine oxidase (PAO) enzyme activity, or N,N(1)-dimethylthiourea, a hydrogen peroxide (H(2)O(2)) scavenger, reverted Spd-induced autofluorescence intensification, DNA fragmentation, inhibition of root cell elongation, as well as reduction of percentage of nuclei in S phase. Transmission electron microscopy showed that N-prenylagmatine inhibited the differentiation of the secondary wall of early and late metaxylem elements, and xylem parenchymal cells. Moreover, although root growth and xylem differentiation in antisense PAO tobacco (Nicotiana tabacum) plants were unaltered, overexpression of maize PAO (S-ZmPAO) as well as down-regulation of the gene encoding S-adenosyl-l-methionine decarboxylase via RNAi in tobacco plants promoted vascular cell differentiation and induced programmed cell death in root cap cells. Furthermore, following Spd treatment in maize and ZmPAO overexpression in tobacco, the in vivo H(2)O(2) production was enhanced in xylem tissues. Overall, our results suggest that, after Spd supply or PAO overexpression, H(2)O(2) derived from polyamine catabolism behaves as a signal for secondary wall deposition and for induction of developmental programmed cell death.

Functional diversity inside the Arabidopsis polyamine oxidase gene family

Polyamine oxidases (PAOs) are FAD-dependent enzymes involved in polyamine catabolism. All so far characterized PAOs from monocotyledonous plants, such as the apoplastic maize PAO, oxidize spermine (Spm) and spermidine (Spd) to produce 1,3-diaminopropane, H(2)O(2), and an aminoaldehyde, and are thus considered to be involved in a terminal catabolic pathway. Mammalian PAOs oxidize Spm or Spd (and/or their acetyl derivatives) differently from monocotyledonous PAOs, producing Spd or putrescine, respectively, in addition to H(2)O(2) and an aminoaldehyde, and are therefore involved in a polyamine back-conversion pathway. In Arabidopsis thaliana, five PAOs (AtPAO1-AtPAO5) are present with cytosolic or peroxisomal localization and three of them (the peroxisomal AtPAO2, AtPAO3, and AtPAO4) form a distinct PAO subfamily. Here, a comparative study of the catalytic properties of recombinant AtPAO1, AtPAO2, AtPAO3, and AtPAO4 is presented, which shows that all four enzymes strongly resemble their mammalian counterparts, being able to oxidize the common polyamines Spd and/or Spm through a polyamine back-conversion pathway. The existence of this pathway in Arabidopsis plants is also evidenced in vivo. These enzymes are also able to oxidize the naturally occurring uncommon polyamines norspermine and thermospermine, the latter being involved in important plant developmental processes. Furthermore, data herein reveal some important differences in substrate specificity among the various AtPAOs, which suggest functional diversity inside the AtPAO gene family. These results represent a new starting point for further understanding of the physiological role(s) of the polyamine catabolic pathways in plants.

Characterization, assay, and substrate specificity of plant polyamine oxidases

Polyamine oxidation is the main catabolic process of polyamines. This process is crucial because not only it participates in the regulation of the endogenous titers of polyamines but also it generates hydrogen peroxide, which can act as a signaling molecule. The recent identification of polyamine oxidases that differ in substrate specificity and mode of action in plants necessitates the use of additional techniques for their characterization based on the determination of the end-product. Herein, we describe the most widely used techniques as well as new techniques that can be used for analysis of the newly identified polyamine oxidases.

Spermidine oxidase-derived H₂O₂ regulates pollen plasma membrane hyperpolarization-activated Ca(2+) -permeable channels and pollen tube growth

Spermidine (Spd) has been correlated with various physiological and developmental processes in plants, including pollen tube growth. In this work, we show that Spd induces an increase in the cytosolic Ca(2+) concentration that accompanies pollen tube growth. Using the whole-cell patch clamp and outside-out single-channel patch clamp configurations, we show that exogenous Spd induces a hyperpolarization-activated Ca(2+) current: the addition of Spd cannot induce the channel open probability increase in excised outside-out patches, indicating that the effect of Spd in the induction of Ca(2+) currents is exerted via a second messenger. This messenger is hydrogen peroxide (H₂O₂), and is generated during Spd oxidation, a reaction mediated by polyamine oxidase (PAO). These reactive oxygen species trigger the opening of the hyperpolarization-activated Ca(2+) -permeable channels in pollen. To provide further evidence that PAO is in fact responsible for the effect of Spd on the Ca(2+) -permeable channels, two Arabidopsis mutants lacking expression of the peroxisomal-encoding AtPAO3 gene, were isolated and characterized. Pollen from these mutants was unable to induce the opening of the Ca(2+) -permeable channels in the presence of Spd, resulting in reduced pollen tube growth and seed number. However, a high Spd concentration triggers a Ca(2+) influx beyond the optimal, which has a deleterious effect. These findings strongly suggest that the Spd-derived H₂O₂ signals Ca(2+) influx, thereby regulating pollen tube growth.

Spermidine oxidase-derived H₂O₂ regulates pollen plasma membrane hyperpolarization-activated Ca(2+) -permeable channels and pollen tube growth

Spermidine (Spd) has been correlated with various physiological and developmental processes in plants, including pollen tube growth. In this work, we show that Spd induces an increase in the cytosolic Ca(2+) concentration that accompanies pollen tube growth. Using the whole-cell patch clamp and outside-out single-channel patch clamp configurations, we show that exogenous Spd induces a hyperpolarization-activated Ca(2+) current: the addition of Spd cannot induce the channel open probability increase in excised outside-out patches, indicating that the effect of Spd in the induction of Ca(2+) currents is exerted via a second messenger. This messenger is hydrogen peroxide (H₂O₂), and is generated during Spd oxidation, a reaction mediated by polyamine oxidase (PAO). These reactive oxygen species trigger the opening of the hyperpolarization-activated Ca(2+) -permeable channels in pollen. To provide further evidence that PAO is in fact responsible for the effect of Spd on the Ca(2+) -permeable channels, two Arabidopsis mutants lacking expression of the peroxisomal-encoding AtPAO3 gene, were isolated and characterized. Pollen from these mutants was unable to induce the opening of the Ca(2+) -permeable channels in the presence of Spd, resulting in reduced pollen tube growth and seed number. However, a high Spd concentration triggers a Ca(2+) influx beyond the optimal, which has a deleterious effect. These findings strongly suggest that the Spd-derived H₂O₂ signals Ca(2+) influx, thereby regulating pollen tube growth.

Abscisic acid signals reorientation of polyamine metabolism to orchestrate stress responses via the polyamine exodus pathway in grapevine

Polyamines (PAs) have been suggested to be implicated in plant responses to abiotic and biotic stress. Grapevine is a model perennial plant species whose cultivars respond differently to osmotic stress. In this study, we used two cultivars, one sensitive (S) and one tolerant (T) to drought. In adult vines subjected to drought under greenhouse conditions, total PAs were significantly lower in the control T- and higher in the control S-genotype and significantly increased or decreased, respectively, post-treatment. Soluble Put and Spd exhibited the greatest increase on d 8 post-treatment in the T- but not in the S-genotype, which accumulated soluble Spm. Abscisic acid (ABA) was differentially accumulated in T- and S-genotypes under drought conditions, and activated the PA biosynthetic pathway, which in turn was correlated with the differential increases in PA titers. In parallel, polyamine oxidases (PAOs) increased primarily in the S-genotype. ABA at least partially induced PA accumulation and exodus into the apoplast, where they were oxidized by the apoplastic amine oxidases (AOs), producing H2O2, which signaled secondary stress responses. The results here show that the ABA signaling pathway integrates PAs and AOs to regulate the generation of H2O2, which signals further stress responses or the PCD syndrome.

Polyamine anabolic/catabolic regulation along the woody grapevine plant axis

The distribution of the endogenous PA fractions throughout the entire perennial woody grapevine (Vitis vinifera L.) plant was studied, along with the expression profiles of the PA anabolic and catabolic genes and their substrates and secondary metabolites. Putrescine fractions increased with increasing leaf age, although the expression of its biosynthetic enzymes Arg and Orn decarboxylases decreased. Orn transport from young organs dramatically enhanced putrescine biosynthesis in older tissues, via the Orn decarboxylase pathway. S-adenosylmethionine decarboxylase and spermidine synthase genes were down-regulated during development in a tissue/organ-specific manner, as were spermidine and spermine levels. In contrast, amine oxidases, peroxidases and phenolics increased from the youngest to the fully developed vascular tissues; they also increased from the peripheral regions of leaves to the petioles. Hydrogen peroxide generated by amine oxidases accumulated for the covalent linkage of proteins via peroxidases during lignification. These results could be valuable for addressing further questions on the role of PAs in plant development.

Engineered polyamine catabolism preinduces tolerance of tobacco to bacteria and oomycetes

Polyamine oxidase (PAO) catalyzes the oxidative catabolism of spermidine and spermine, generating hydrogen peroxide. In wild-type tobacco (Nicotiana tabacum 'Xanthi') plants, infection by the compatible pathogen Pseudomonas syringae pv tabaci resulted in increased PAO gene and corresponding PAO enzyme activities; polyamine homeostasis was maintained by induction of the arginine decarboxylase pathway and spermine was excreted into the apoplast, where it was oxidized by the enhanced apoplastic PAO, resulting in higher hydrogen peroxide accumulation. Moreover, plants overexpressing PAO showed preinduced disease tolerance against the biotrophic bacterium P. syringae pv tabaci and the hemibiotrophic oomycete Phytophthora parasitica var nicotianae but not against the Cucumber mosaic virus. Furthermore, in transgenic PAO-overexpressing plants, systemic acquired resistance marker genes as well as a pronounced increase in the cell wall-based defense were found before inoculation. These results reveal that PAO is a nodal point in a specific apoplast-localized plant-pathogen interaction, which also signals parallel defense responses, thus preventing pathogen colonization. This strategy presents a novel approach for producing transgenic plants resistant to a broad spectrum of plant pathogens.

Plant polyamine catabolism: The state of the art

Polyamines have long been implicated in plant growth and development, as well as adaptation to abiotic and biotic stress. As a general rule of thumb the higher the polyamine titers the better. However, their molecular roles in plant stress responses still remain obscure. It has been postulated that they could act through their catabolism, which generates molecules which may act as secondary messengers signalling networks of numerous developmental and stress adaptation processes. Recently it was shown that plant and mammalian polyamine catabolism share critical features, giving new insight in plant polyamine catabolism. In this review, the advances in genes and proteins of polyamine catabolism in plants is presented and compared to other models.

Bridging the gap between plant and mammalian polyamine catabolism: a novel peroxisomal polyamine oxidase responsible for a full back-conversion pathway in Arabidopsis

In contrast to animals, where polyamine (PA) catabolism efficiently converts spermine (Spm) to putrescine (Put), plants have been considered to possess a PA catabolic pathway producing 1,3-diaminopropane, Delta(1)-pyrroline, the corresponding aldehyde, and hydrogen peroxide but unable to back-convert Spm to Put. Arabidopsis (Arabidopsis thaliana) genome contains at least five putative PA oxidase (PAO) members with yet-unknown localization and physiological role(s). AtPAO1 was recently identified as an enzyme similar to the mammalian Spm oxidase, which converts Spm to spermidine (Spd). In this work, we have performed in silico analysis of the five Arabidopsis genes and have identified PAO3 (AtPAO3) as a nontypical PAO, in terms of homology, compared to other known PAOs. We have expressed the gene AtPAO3 and have purified a protein corresponding to it using the inducible heterologous expression system of Escherichia coli. AtPAO3 catalyzed the sequential conversion/oxidation of Spm to Spd, and of Spd to Put, thus exhibiting functional homology to the mammalian PAOs. The best substrate for this pathway was Spd, whereas the N(1)-acetyl-derivatives of Spm and Spd were oxidized less efficiently. On the other hand, no activity was detected when diamines (agmatine, cadaverine, and Put) were used as substrates. Moreover, although AtPAO3 does not exhibit significant similarity to the other known PAOs, it is efficiently inhibited by guazatine, a potent PAO inhibitor. AtPAO3 contains a peroxisomal targeting motif at the C terminus, and it targets green fluorescence protein to peroxisomes when fused at the N terminus but not at the C terminus. These results reveal that AtPAO3 is a peroxisomal protein and that the C terminus of the protein contains the sorting information. The overall data reinforce the view that plants and mammals possess a similar PA oxidation system, concerning both the subcellular localization and the mode of its action.

Spermidine exodus and oxidation in the apoplast induced by abiotic stress is responsible for H2O2 signatures that direct tolerance responses in tobacco

Polyamines (PAs) exert a protective effect against stress challenges, but their molecular role in this remains speculative. In order to detect the signaling role of apoplastic PA-derived hydrogen peroxide (H2O2) under abiotic stress, we developed a series of tobacco (Nicotiana tabacum cv Xanthi) transgenic plants overexpressing or downregulating apoplastic polyamine oxidase (PAO; S-pao and A-pao plants, respectively) or downregulating S-adenosyl-l-methionine decarboxylase (samdc plants). Upon salt stress, plants secreted spermidine (Spd) into the apoplast, where it was oxidized by the apoplastic PAO, generating H2O2. A-pao plants accumulated less H2O2 and exhibited less programmed cell death (PCD) than did wild-type plants, in contrast with S-pao and samdc downregulating plants. Induction of either stress-responsive genes or PCD was dependent on the level of Spd-derived apoplastic H2O2. Thus, in wild-type and A-pao plants, stress-responsive genes were efficiently induced, although in the latter at a lower rate, while S-pao plants, with higher H2O2 levels, failed to accumulate stress-responsive mRNAs, inducing PCD instead. Furthermore, decreasing intracellular PAs, while keeping normal apoplastic Spd oxidation, as in samdc downregulating transgenic plants, caused enhanced salinity-induced PCD. These results reveal that salinity induces the exodus of Spd into the apoplast, where it is catabolized by PAO, producing H2O2. The accumulated H2O2 results in the induction of either tolerance responses or PCD, depending also on the levels of intracellular PAs.