Salicylic acid stimulates secretion of the normally symplastic enzyme mannitol dehydrogenase: a possible defense against mannitol-secreting fungal pathogens

Beyond the surface: the plant secretome as a bridge between the cell and its environment

MAIN CONCLUSIONS

We describe the biological importance of proteins secreted in plants under different conditions and biological processes, the secretion mechanisms, methodologies for obtaining and identifying these proteins, and future perspectives. Molecule secretion inside and outside the cell is relevant to all areas of plant biology. Protein secretion, in particular, has intriguing possibilities due to the different secretion pathways that the cell uses to send biochemical messages. The secretion of proteins-secretomes-into extracellular space in response to different stimuli or stress situations, in vitro or in planta conditions, has been studied in plants and plant tissues. Proteomics has allowed the quantitative and qualitative analysis of this process and the measurement of proteins associated with the cellular development of different tissues. This has provided the means of evaluating a more precise biochemical state of the cells and the changes that occur during their growth. With the development of new techniques in proteomics, such as mass spectrometry, sequencing, and bioinformatics, it is now possible to elucidate the main proteins secreted, with all their posttranslational modifications, in different plant species and under different specific conditions. This review presents the different pathways of protein secretion in plants, highlighting the well-known importance of signal peptides. The advances and disadvantages of in planta and in vitro systems used for proteomic purposes are discussed. The principal proteomic techniques to better understand the biological function of the secretome are summarized.

Mannitol as a Growth Substrate for Facultative Methylotroph Methylobrevis pamukkalensis PK2

Biochemistry of carbon assimilation in aerobic methylotrophs growing on reduced C1 compounds has been intensively studied due to the vital role of these microorganisms in nature. The biochemical pathways of carbon assimilation in methylotrophs growing on multi-carbon substrates are insufficiently explored. Here we elucidated the metabolic route of mannitol assimilation in the alphaproteobacterial facultative methylotroph Methylobrevis pamukkalensis PK2. Two key enzymes of mannitol metabolism, mannitol-2-dehydrogenase (MTD) and fructokinase (FruK), were obtained as His-tagged proteins by cloning and expression of mtd and fruK genes in Escherichia coli and characterized. Genomic analysis revealed that further transformation of fructose-6-phosphate proceeds via the Entner-Doudoroff pathway. During growth on mannitol + methanol mixture, the strain PK2 consumed both substrates simultaneously demonstrating independence of C1 and C6 metabolic pathways. Genome screening showed that genes for mannitol utilization enzymes are present in other alphaproteobacterial methylotrophs predominantly capable of living in association with plants. The capability to utilize a variety of carbohydrates (sorbitol, glucose, fructose, arabinose and xylose) suggests a broad adaptability of the strain PK2 to live in environments where availability of carbon substrate dynamically changes.

Paraphoma chrysanthemicola Affects the Carbohydrate and Lobetyolin Metabolism Regulated by Salicylic Acid in the Soilless Cultivation of Codonopsis pilosula

Paraphoma chrysanthemicola, an endophytic fungus isolated from the roots of Codonopsis pilosula, influences salicylic acid (SA) levels. The interaction mechanism between SA and P. chrysanthemicola within C. pilosula remains elusive. To elucidate this, an experiment was conducted with four treatments: sterile water (CK), P. chrysanthemicola (FG), SA, and a combination of P. chrysanthemicola with salicylic acid (FG+SA). Results indicated that P. chrysanthemicola enhanced plant growth and counteracted the growth inhibition caused by exogenous SA. Physiological analysis showed that P. chrysanthemicola reduced carbohydrate content and enzymatic activity in C. pilosula without affecting total chlorophyll concentration and attenuated the increase in these parameters induced by exogenous SA. Secondary metabolite profiling showed a decrease in soluble proteins and lobetyolin levels in the FG group, whereas SA treatment led to an increase. Both P. chrysanthemicola and SA treatments decreased antioxidase-like activity. Notably, the FG group exhibited higher nitric oxide (NO) levels, and the SA group exhibited higher hydrogen peroxide (H2O2) levels in the stems. This study elucidated the intricate context of the symbiotic dynamics between the plant species P. chrysanthemicola and C. pilosula, where an antagonistic interaction involving salicylic acid was prominently observed. This antagonism was observed in the equilibrium between carbohydrate metabolism and secondary metabolism. This equilibrium had the potential to engage reactive oxygen species (ROS) and nitric oxide (NO).

Evaluation of Hydrangea Cultivars for Tolerance Against Root Rot Caused by Fusarium oxysporum

Root rot caused by Fusarium oxysporum Schltdl. is a newly identified disease in oakleaf hydrangea. Some cultivars such as Pee Wee and Queen of Hearts grown in pot-in-pot container systems showed root rot symptoms after late spring frost in May 2018 with 40 and 60% incidence in the infected nursery, respectively. This experiment was carried out to evaluate the tolerance among different hydrangea cultivars against root rot caused by F. oxysporum. Fifteen hydrangea cultivars from four different species were selected, and rooted cuttings were prepared from new spring flushes. Twelve plants from each cultivar were transplanted in a 1-gallon pot. Half of transplanted plants (six single plants) were inoculated by drenching 150 ml of F. oxysporum conidial suspension to maintain the concentration of 1 × 106 conidia/ml. Half of the plants remain noninoculated (control) and were drenched with sterile water. After 4 months, root rot was assessed using a scale of 0 to 100% root area affected, and recovery of F. oxysporum was recorded by plating 1-cm root sections in Fusarium selective medium. Fusaric acid (FA) and mannitol were extracted from the roots of inoculated and noninoculated plants to see the effect and role on pathogenesis. Further, mannitol concentration was analyzed using absorption wavelength in a spectrophotometer, and FA was analyzed using high-performance liquid chromatography (HPLC). Results indicated that no cultivars were resistant to F. oxysporum. Cultivars from Hydrangea arborescens, H. macrophylla, and H. paniculata were more tolerant to F. oxysporum compared to cultivars from H. quercifolia. Among H. quercifolia, cultivars Snowflake, John Wayne, and Alice were more tolerant to F. oxysporum.

Traffic Control: Subversion of Plant Membrane Trafficking by Pathogens

Membrane trafficking pathways play a prominent role in plant immunity. The endomembrane transport system coordinates membrane-bound cellular organelles to ensure that immunological components are utilized effectively during pathogen resistance. Adapted pathogens and pests have evolved to interfere with aspects of membrane transport systems to subvert plant immunity. To do this, they secrete virulence factors known as effectors, many of which converge on host membrane trafficking routes. The emerging paradigm is that effectors redundantly target every step of membrane trafficking from vesicle budding to trafficking and membrane fusion. In this review, we focus on the mechanisms adopted by plant pathogens to reprogram host plant vesicle trafficking, providing examples of effector-targeted transport pathways and highlighting key questions for the field to answer moving forward.

Versatile roles of sorbitol in higher plants: luxury resource, effective defender or something else?

Sorbitol metabolism plays multiple roles in many plants, including energy and carbon enrichment, effective defence against various stresses and other emerging specific roles. The underlying mechanisms are, however, incompletely understood. This review provides the current state-of-the-art, highlights missing knowledge and poses several remaining questions. The basic properties of sugar alcohols are summarised and pathways of sorbitol metabolism, including biosynthesis, degradation and key enzymes are described. Sorbitol transport within the plant body is discussed and individual roles of sorbitol in different organs, specific cells or even cellular compartments, are elaborated, clarifying the critical importance of sorbitol allocation and distribution. In addition to plants that accumulate and transport significant quantities of sorbitol (usual producers), there are some that synthesize small amounts of sorbitol or only possess sorbitol metabolising enzymes (non-usual producers). Modern analytical methods have recently enabled large amounts of data to be acquired on this topic, although numerous uncertainties and questions remain. For a long time, it has been clear that enriching carbohydrate metabolism with a sorbitol branch improves plant fitness under stress. Nevertheless, this is probably valid only when appropriate growth and defence trade-offs are ensured. Information on the ectopic expression of sorbitol metabolism genes has contributed substantially to our understanding of the sorbitol roles and raises new questions regarding sorbitol signalling potential. We finally examine strategies in plants producing sorbitol compared with those producing mannitol. Providing an in-depth understanding of sugar alcohol metabolism is essential for the progress in plant physiology as well as in targeted, knowledge-based crop breeding.

Unconventional Pathways of Protein Secretion: Mammals vs. Plants

In eukaryotes, many proteins contain an N-terminal signal peptide that allows their translocation into the endoplasmic reticulum followed by secretion outside the cell according to the classical secretory system. However, an increasing number of secreted proteins lacking the signal peptide sequence are emerging. These proteins, secreted in several alternative ways collectively known as unconventional protein secretion (UPS) pathways, exert extracellular functions including cell signaling, immune modulation, as well as moonlighting activities different from their well-described intracellular functions. Pathways for UPS include direct transfer across the plasma membrane, secretion from endosomal/multivesicular body-related components, release within plasma membrane-derived microvesicles, or use of elements of autophagy. In this review we describe the mammals and plants UPS pathways identified so far highlighting commonalities and differences.

Plant Cell Wall Proteomes: The Core of Conserved Protein Families and the Case of Non-Canonical Proteins

Plant cell wall proteins (CWPs) play critical roles during plant development and in response to stresses. Proteomics has revealed their great diversity. With nearly 1000 identified CWPs, the Arabidopsis thaliana cell wall proteome is the best described to date and it covers the main plant organs and cell suspension cultures. Other monocot and dicot plants have been studied as well as bryophytes, such as Physcomitrella patens and Marchantia polymorpha. Although these proteomes were obtained using various flowcharts, they can be searched for the presence of members of a given protein family. Thereby, a core cell wall proteome which does not pretend to be exhaustive, yet could be defined. It comprises: (i) glycoside hydrolases and pectin methyl esterases, (ii) class III peroxidases, (iii) Asp, Ser and Cys proteases, (iv) non-specific lipid transfer proteins, (v) fasciclin arabinogalactan proteins, (vi) purple acid phosphatases and (vii) thaumatins. All the conserved CWP families could represent a set of house-keeping CWPs critical for either the maintenance of the basic cell wall functions, allowing immediate response to environmental stresses or both. Besides, the presence of non-canonical proteins devoid of a predicted signal peptide in cell wall proteomes is discussed in relation to the possible existence of alternative secretion pathways.

COPII Sec23 proteins form isoform-specific endoplasmic reticulum exit sites with differential effects on polarized growth

Coat Protein complex II (COPII), a coat protein complex that forms vesicles on the endoplasmic reticulum (ER), mediates trafficking to the Golgi. While metazoans have few genes encoding each COPII component, plants have expanded these gene families, leading to the hypothesis that plant COPII has functionally diversified. In the moss Physcomitrium (Physcomitrella) patens, the Sec23/24 gene families are each composed of seven genes. Silencing Sec23/24 revealed isoform-specific contributions to polarized growth, with the closely related Sec23D/E and Sec24C/D essential for protonemal development. Focusing on Sec23, we discovered that Sec23D/E mediate ER-to Golgi transport and are essential for tip growth, with Sec23D localizing to presumptive ER exit sites. In contrast, Sec23A, B, C, F, and G are dispensable and do not quantitatively affect ER-to-Golgi trafficking. However, Δsec23abcfg plants exhibited reduced secretion of plasma membrane cargo. Of the four highly expressed protonemal Sec23 genes, Sec23F/G are members of a divergent Sec23 clade specifically retained in land plants. Notably, Sec23G accumulates on ER-associated foci that are significantly larger, do not overlap with, and are independent of Sec23D. While Sec23D/E form ER exit sites and function as bona fide COPII components essential for tip-growing protonemata, Sec23G and the closely related Sec23F have likely functionally diversified, forming separate and independent ER exit sites and participating in Golgi-independent trafficking pathways.

Identification of multiple proteins whose interaction with mannitol dehydrogenase is induced by salicylic acid: Implications for unconventional secretion

Although protein secretion was previously believed to be solely via ER/Golgi pathways, Golgi-independent secretion has now been described in both animals and plants. Secretion of the mannitol catabolic enzyme mannitol dehydrogenase (MTD) in response to the endogenous pathogen response signal salicylic acid (SA) was one of the first reports of unconventional protein secretion in plants. To begin assessing potential secretion-associated MTD protein interactors, we present here high-quality databases describing changes in MTD-interacting proteins following SA treatment of Arabidopsis thaliana cells expressing MTD.

Seasonal and temporal variations of ambient aerosols in a deciduous broadleaf forest from northern Japan: Contributions of biomass burning and biological particles

Total suspended particulate (TSP) samples were collected in a deciduous broadleaf forest in Sapporo, Hokkaido, Japan, from January to December 2010 to understand the molecular composition and abundance of sugar compounds (SCs) in atmospheric aerosols. We analyzed the samples for anhydrosugars, primary sugars, and sugar alcohols using a gas chromatograph-mass spectrometer. The annual mean concentrations of total SCs ranged from 16.1 to 1748 ng m^-3 (avg. 311 ng m^-3) with maxima in spring (avg. 484 ng m^-3) and minima in winter (avg. 28.2 ng m^-3). Primary sugars and sugar alcohols followed the seasonal pattern of total SCs. High levels of anhydrosugars in winter (avg. 22.9 ng m^-3) suggest a contribution of biomass burning from domestic heating due to lower ambient temperature. The high levels of arabitol and mannitol in spring followed by summer and autumn denote the contribution from multiple sources, i.e., growing vegetation and fungal spores in Sapporo forest. We observed an enhanced contribution of bioaerosols emitted from plant blossoms in spring and leaf decomposition in autumn. The identical seasonal trends of glucose and trehalose implied their similar sources in forest aerosols. Conversely, the highest concentration of sucrose in spring was due to the pollen emissions by blooming plants. Positive matrix factorization (PMF) analyses of the SCs suggested that organic aerosols in the deciduous forest are associated with the emissions from multiple sources, including vegetation, microbes, pollens, and wintertime biomass burning. The PMF analysis also suggested that vegetation is the primary carbon source in the forest atmosphere. The diagnostic mass ratios of levoglucosan to mannosan demonstrated the dominance of softwood burning. We noted that the meteorological parameters substantially affect the emission sources and seasonal concentrations of SCs in the deciduous forest.

Sclerotinia sclerotiorum Infection Triggers Changes in Primary and Secondary Metabolism in Arabidopsis thaliana

Sclerotinia sclerotiorum is a devastating plant pathogen that causes substantial losses in various agricultural crops. Although plants have developed some well-known defense mechanisms against invasive fungi, much remains to be learned about plant responses to fungal pathogens. In this study, we investigated how S. sclerotiorum infection affects plant primary and secondary metabolism in the model plant Arabidopsis thaliana. Our results showed that soluble sugar and amino acid content changed significantly in A. thaliana leaves upon fungal colonization, with a decrease in sucrose and an increase in mannitol, attributed to fungal biosynthesis. Furthermore, the jasmonate signaling pathway was rapidly activated by S. sclerotiorum infection, and there was a striking accumulation of antifungal metabolites such as camalexin, p-coumaroyl agmatine, feruloyl agmatine, and N^δ-acetylornithine. On the other hand, the characteristic defense compounds of the Brassicaceae, the glucosinolates, were not induced in A. thaliana infected by S. sclerotiorum. Our study provides a better understanding of how A. thaliana primary and secondary metabolism is modified during infection by a fungal pathogen like S. sclerotiorum that has both hemibiotrophic and necrotrophic stages.

Wheat Encodes Small, Secreted Proteins That Contribute to Resistance to Septoria Tritici Blotch

During plant-pathogen interactions, pathogens secrete many rapidly evolving, small secreted proteins (SSPs) that can modify plant defense and permit pathogens to colonize plant tissue. The fungal pathogen Zymoseptoria tritici is the causal agent of Septoria tritici blotch (STB), one of the most important foliar diseases of wheat, globally. Z. tritici is a strictly apoplastic pathogen that can secrete numerous proteins into the apoplast of wheat leaves to promote infection. We sought to determine if, during STB infection, wheat also secretes small proteins into the apoplast to mediate the recognition of pathogen proteins and/or induce defense responses. To explore this, we developed an SSP-discovery pipeline to identify small, secreted proteins from wheat genomic data. Using this pipeline, we identified 6,998 SSPs, representing 2.3% of all proteins encoded by the wheat genome. We then mined a microarray dataset, detailing a resistant and susceptible host response to STB, and identified 141 Z. tritici- responsive SSPs, representing 4.7% of all proteins encoded by Z. tritici - responsive genes. We demonstrate that a subset of these SSPs have a functional signal peptide and can interact with Z. tritici SSPs. Transiently silencing two of these wheat SSPs using virus-induced gene silencing (VIGS) shows an increase in susceptibility to STB, confirming their role in defense against Z. tritici.

The ArathEULS3 Lectin Ends up in Stress Granules and Can Follow an Unconventional Route for Secretion

Stress granules are cytoplasmic compartments, which serve as mRNA storage units during stress, therefore regulating translation. The Arabidopsis thaliana lectin ArathEULS3 has been widely described as a stress inducible gene. This study aimed to examine in detail the localization of ArathEULS3 lectin in normal and stressed cells. Colocalization experiments revealed that the nucleo-cytoplasmic lectin ArathEULS3 relocates to stress granules after stress. The ArathEULS3 sequence encodes a protein with a EUL lectin domain and an N-terminal domain with unknown structure and function. Bioinformatics analyses showed that the N-terminal domain sequence contains intrinsically disordered regions and likely does not exhibit a stable protein fold. Plasmolysis experiments indicated that ArathEULS3 also localizes to the apoplast, suggesting that this protein might follow an unconventional route for secretion. As part of our efforts we also investigated the interactome of ArathEULS3 and identified several putative interaction partners important for the protein translation process.

Merging roads: chemical tools and cell biology to study unconventional protein secretion

The endomembrane trafficking network is highly complex and dynamic, with both conventional and so-called unconventional routes which are in essence recently discovered pathways that are poorly understood in plants. One approach to dissecting endomembrane pathways that we have pioneered is the use of chemical biology. Classical genetic manipulations often deal with indirect pleiotropic phenotypes resulting from the perturbation of key players of the trafficking routes. Many of these difficulties can be circumvented using small molecules to modify or disrupt the function or localization of key proteins regulating these pathways. In this review, we summarize how small molecules have been used as probes to define these pathways, and how they could be used to increase current knowledge of unconventional protein secretion pathways.

Protein secretion in plants: conventional and unconventional pathways and new techniques

Protein secretion is an essential process in all eukaryotic cells and its mechanisms have been extensively studied. Proteins with an N-terminal leading sequence or transmembrane domain are delivered through the conventional protein secretion (CPS) pathway from the endoplasmic reticulum (ER) to the Golgi apparatus. This feature is conserved in yeast, animals, and plants. In contrast, the transport of leaderless secretory proteins (LSPs) from the cytosol to the cell exterior is accomplished via the unconventional protein secretion (UPS) pathway. So far, the CPS pathway has been well characterized in plants, with several recent studies providing new information about the regulatory mechanisms involved. On the other hand, studies on UPS pathways in plants remain descriptive, although a connection between UPS and the plant defense response is becoming more and more apparent. In this review, we present an update on CPS and UPS. With the emergence of new techniques, a more comprehensive understanding of protein secretion in plants can be expected in the future.

An Overview of Protein Secretion in Plant Cells

The delivery of proteins to the apoplast or protein secretion is an essential process in plant cells. Proteins are secreted to perform various biological functions such as cell wall modification and defense response. Conserved from yeast to mammals, both conventional and unconventional protein secretion pathways have been demonstrated in plants. In the conventional protein secretion pathway, secretory proteins with an N-terminal signal peptide are transported to the extracellular region via the endoplasmic reticulum-Golgi apparatus and the subsequent endomembrane system. By contrast, multiple unconventional protein secretion pathways are proposed to mediate the secretion of the leaderless secretory proteins. In this review, we summarize the recent findings and provide a comprehensive overview of protein secretion pathways in plant cells.

Ice-binding proteins confer freezing tolerance in transgenic Arabidopsis thaliana

Lolium perenne is a freeze-tolerant perennial ryegrass capable of withstanding temperatures below -13 °C. Ice-binding proteins (IBPs) presumably help prevent damage associated with freezing by restricting the growth of ice crystals in the apoplast. We have investigated the expression, localization and in planta freezing protection capabilities of two L. perenne IBP isoforms, LpIRI2 and LpIRI3, as well as a processed IBP (LpAFP). One of these isoforms, LpIRI2, lacks a conventional signal peptide and was assumed to be a pseudogene. Nevertheless, both LpIRI2 and LpIRI3 transcripts were up-regulated following cold acclimation. LpIRI2 also demonstrated ice-binding activity when produced recombinantly in Escherichia coli. Both the LpIRI3 and LpIRI2 isoforms appeared to accumulate in the apoplast of transgenic Arabidopsis thaliana plants. In contrast, the fully processed isoform, LpAFP, remained intracellular. Transgenic plants expressing either LpIRI2 or LpIRI3 showed reduced ion leakage (12%-39%) after low-temperature treatments, and significantly improved freezing survival, while transgenic LpAFP-expressing lines did not confer substantial subzero protection. Freeze protection was further enhanced by with the introduction of more than one IBP isoform; ion leakage was reduced 26%-35% and 10% of plants survived temperatures as low as -8 °C. Our results demonstrate that apoplastic expression of multiple L. perenne IBP isoforms shows promise for providing protection to crops susceptible to freeze-induced damage.

Mannitol in Plants, Fungi, and Plant-Fungal Interactions

Although the presence of mannitol in organisms as diverse as plants and fungi clearly suggests that this compound has important roles, our understanding of fungal mannitol metabolism and its interaction with mannitol metabolism in plants is far from complete. Despite recent inroads into understanding the importance of mannitol and its metabolic roles in salt, osmotic, and oxidative stress tolerance in plants and fungi, our current understanding of exactly how mannitol protects against reactive oxygen is also still incomplete. In this opinion, we propose a new model of the interface between mannitol metabolism in plants and fungi and how it impacts plant-pathogen interactions.

Quantitative proteomics of the tobacco pollen tube secretome identifies novel pollen tube guidance proteins important for fertilization

BACKGROUND

As in animals, cell-cell communication plays a pivotal role in male-female recognition during plant sexual reproduction. Prelaid peptides secreted from the female reproductive tissues guide pollen tubes towards ovules for fertilization. However, the elaborate mechanisms for this dialogue have remained elusive, particularly from the male perspective.

RESULTS

We performed genome-wide quantitative liquid chromatography-tandem mass spectrometry analysis of a pistil-stimulated pollen tube secretome and identified 801 pollen tube-secreted proteins. Interestingly, in silico analysis reveals that the pollen tube secretome is dominated by proteins that are secreted unconventionally, representing 57 % of the total secretome. In support, we show that an unconventionally secreted protein, translationally controlled tumor protein, is secreted to the apoplast. Remarkably, we discovered that this protein could be secreted by infiltrating through the initial phases of the conventional secretory pathway and could reach the apoplast via exosomes, as demonstrated by co-localization with Oleisin1 exosome marker. We demonstrate that translationally controlled tumor protein-knockdown Arabidopsis thaliana plants produce pollen tubes that navigate poorly to the target ovule and that the mutant allele is poorly transmitted through the male. Further, we show that regulators of the endoplasmic reticulum-trans-Golgi network protein secretory pathway control secretion of Nicotiana tabacum Pollen tube-secreted cysteine-rich protein 2 and Lorelei-like GPI-anchor protein 3 and that a regulator of endoplasmic reticulum-trans-Golgi protein translocation is essential for pollen tube growth, pollen tube guidance and ovule-targeting competence.

CONCLUSIONS

This work, the first study on the pollen tube secretome, identifies novel genome-wide pollen tube-secreted proteins with potential functions in pollen tube guidance towards ovules for sexual reproduction. Functional analysis highlights a potential mechanism for unconventional secretion of pollen tube proteins and reveals likely regulators of conventional pollen tube protein secretion. The association of pollen tube-secreted proteins with marker proteins shown to be secreted via exosomes in other species suggests exosome secretion is a possible mechanism for cell-cell communication between the pollen tube and female reproductive cells.

Unconventional Protein Secretion in Plants

Unconventional protein secretion (UPS) describes secretion pathways that bypass one or several of the canonical secretion pit-stops on the way to the plasma membrane, and/or involve the secretion of leaderless proteins. So far, alternatives to conventional secretion were primarily observed and studied in yeast and animal cells. The sessile lifestyle of plants brings with it unique restraints on how they adapt to adverse conditions and environmental challenges. Recently, attention towards unconventional secretion pathways in plant cells has substantially increased, with the large number of leaderless proteins identified through proteomic studies. While UPS pathways in plants are certainly not yet exhaustively researched, an emerging notion is that induction of UPS pathways is correlated with pathogenesis and stress responses. Given the multitude UPS events observed, comprehensively organizing the routes proteins take to the apoplast in defined UPS categories is challenging. With the establishment of a larger collection of studied plant proteins taking these UPS pathways, a clearer picture of endomembrane trafficking as a whole will emerge. There are several novel enabling technologies, such as vesicle proteomics and chemical genomics, with great potential for dissecting secretion pathways, providing information about the cargo that travels along them and the conditions that induce them.

Mannitol metabolism during pathogenic fungal-host interactions under stressed conditions

Numerous plants and fungi produce mannitol, which may serve as an osmolyte or metabolic store; furthermore, mannitol also acts as a powerful quencher of reactive oxygen species (ROS). Some phytopathogenic fungi use mannitol to stifle ROS-mediated plant resistance. Mannitol is essential in pathogenesis to balance cell reinforcements produced by both plants and animals. Mannitol likewise serves as a source of reducing power, managing coenzymes, and controlling cytoplasmic pH by going about as a sink or hotspot for protons. The metabolic pathways for mannitol biosynthesis and catabolism have been characterized in filamentous fungi by direct diminishment of fructose-6-phosphate into mannitol-1-phosphate including a mannitol-1-phosphate phosphatase catalyst. In plants mannitol is integrated from mannose-6-phosphate to mannitol-1-phosphate, which then dephosphorylates to mannitol. The enzyme mannitol dehydrogenase plays a key role in host-pathogen interactions and must be co-localized with pathogen-secreted mannitol to resist the infection.

WallProtDB, a database resource for plant cell wall proteomics

BACKGROUND

During the last fifteen years, cell wall proteomics has become a major research field with the publication of more than 50 articles describing plant cell wall proteomes. The WallProtDB database has been designed as a tool to facilitate the inventory, the interpretation of cell wall proteomics data and the comparisons between cell wall proteomes.

RESULTS

WallProtDB (http://www.polebio.lrsv.ups-tlse.fr/WallProtDB/) presently contains 2170 proteins and ESTs identified experimentally in 36 cell wall proteomics studies performed on 11 different plant species. Two criteria have to be met for entering WallProtDB. First one is related to the identification of proteins. Only proteins identified in plant with available genomic or ESTs data are considered to ensure unambiguous identification. Second criterion is related to the difficulty to obtain clean cell wall fractions. Indeed, since cell walls constitute an open compartment difficult to isolate, numerous proteins predicted to be intracellular and/or having functions inside the cell have been identified in cell wall extracts. Then, except proteins predicted to be plasma membrane proteins, only proteins having a predicted signal peptide and no known intracellular retention signal are included in the database. In addition, WallProtDB contains information about the strategies used to obtain cell wall protein extracts and to identify proteins by mass spectrometry and bioinformatics. Mass spectrometry data are included when available. All the proteins of WallProtDB are linked to ProtAnnDB, another database, which contains structural and functional bioinformatics annotations of proteins as well as links to other databases (Aramemnon, CAZy, Planet, Phytozome). A list of references in the cell wall proteomics field is also provided.

CONCLUSIONS

WallProtDB aims at becoming a cell wall proteome reference database. It can be updated at any time on request and provide a support for sharing cell wall proteomics data and literature references with researchers interested in plant cell wall biology.

Arabinogalactan glycosyltransferases target to a unique subcellular compartment that may function in unconventional secretion in plants

We report that fluorescently tagged arabinogalactan glycosyltransferases target not only the Golgi apparatus but also uncharacterized smaller compartments when transiently expressed in Nicotiana benthamiana. Approximately 80% of AtGALT31A [Arabidopsis thaliana galactosyltransferase from family 31 (At1g32930)] was found in the small compartments, of which, 45 and 40% of AtGALT29A [Arabidopsis thaliana galactosyltransferase from family 29 (At1g08280)] and AtGlcAT14A [Arabidopsis thaliana glucuronosyltransferase from family 14 (At5g39990)] colocalized with AtGALT31A, respectively; in contrast, N-glycosylation enzymes rarely colocalized (3-18%), implicating a role of the small compartments in a part of arabinogalactan (O-glycan) biosynthesis rather than N-glycan processing. The dual localization of AtGALT31A was also observed for fluorescently tagged AtGALT31A stably expressed in an Arabidopsis atgalt31a mutant background. Further, site-directed mutagenesis of a phosphorylation site of AtGALT29A (Y144) increased the frequency of the protein being targeted to the AtGALT31A-localized small compartments, suggesting a role of Y144 in subcellular targeting. The AtGALT31A localized to the small compartments were colocalized with neither SYP61 (syntaxin of plants 61), a marker for trans-Golgi network (TGN), nor FM4-64-stained endosomes. However, 41% colocalized with EXO70E2 (Arabidopsis thaliana exocyst protein Exo70 homolog 2), a marker for exocyst-positive organelles, and least affected by Brefeldin A and Wortmannin. Taken together, AtGALT31A localized to small compartments that are distinct from the Golgi apparatus, the SYP61-localized TGN, FM4-64-stained endosomes and Wortmannin-vacuolated prevacuolar compartments, but may be part of an unconventional protein secretory pathway represented by EXO70E2 in plants.

A stress responsive gene of Fortunella crassifolia FcSISP functions in salt stress resistance

Exploration of genes functioning in salt tolerance is crucial for generating transgenic plants with enhanced salt tolerance. In this study, we report the isolation and functional characterization of a stress-responsive gene FcSISP from Meiwa kumquat (Fortunella crassifolia). FcSISP encodes a putative protein of 47 amino acids, with a calculated molecular mass of 4.94 kDa and theoretical isoelectric point of 3.76, and was localized in the nucleus. Transcript levels of FcSISP were induced by dehydration, cold, salt and bacterium causing citrus canker, and hormones (salicylic acid and abscisic acid), with the greatest induction under salt treatment. Overexpression of FcSISP in tobacco (Nicotiana nudicaulis) conferred enhanced salt tolerance. The transgenic lines accumulated lower Na(+) contents, leading to reduced Na/K ratio, but accumulated more proline than the wild type (WT). Steady state mRNA levels of genes involved in Na(+) exchange (three SOS genes and three NHX genes) and proline synthesis (P5CS and P5CR) were higher in the transgenic lines in comparison with WT. Moreover, overexpression of FcSISP in trifoliate orange [Poncirus trifoliata (L.) Raf.], a widely-used and salt-sensitive citrus rootstock, led to elevated salt tolerance. Taken together, the data demonstrate that FcSISP plays a positive role in salt tolerance and that it holds a great potential for engineering salt tolerance in crops.

Unconventional protein secretion (UPS) pathways in plants

As in yeast and mammalian cells, novel unconventional protein secretion (UPS) or unconventional membrane trafficking pathways are now known to operate in plants. UPS in plants is generally associated with stress conditions such as pathogen attack, but little is known about its underlying mechanism and function. Here, we present an update on the current knowledge of UPS in the plants in terms of its transport pathways, possible functions and its relationship to autophagy.

A novel motif in the NaTrxh N-terminus promotes its secretion, whereas the C-terminus participates in its interaction with S-RNase in vitro

BACKGROUND

NaTrxh, a thioredoxin type h, shows differential expression between self-incompatible and self-compatible Nicotiana species. NaTrxh interacts in vitro with S-RNase and co-localizes with it in the extracellular matrix of the stylar transmitting tissue. NaTrxh contains N- and C-terminal extensions, a feature shared by thioredoxin h proteins of subgroup 2. To ascertain the function of these extensions in NaTrxh secretion and protein-protein interaction, we performed a deletion analysis on NaTrxh and fused the resulting variants to GFP.

RESULTS

We found an internal domain in the N-terminal extension, called Nβ, that is essential for NaTrxh secretion but is not hydrophobic, a canonical feature of a signal peptide. The lack of hydrophobicity as well as the location of the secretion signal within the NaTrxh primary structure, suggest an unorthodox secretion route for NaTrxh. Notably, we found that the fusion protein NaTrxh-GFP(KDEL) is retained in the endoplasmic reticulum and that treatment of NaTrxh-GFP-expressing cells with Brefeldin A leads to its retention in the Golgi, which indicates that NaTrxh uses, to some extent, the endoplasmic reticulum and Golgi apparatus for secretion. Furthermore, we found that Nβ contributes to NaTrxh tertiary structure stabilization and that the C-terminus functions in the protein-protein interaction with S-RNase.

CONCLUSIONS

The extensions contained in NaTrxh sequence have specific functions on the protein. While the C-terminus directly participates in protein-protein interaction, particularly on its interaction with S-RNase in vitro; the N-terminal extension contains two structurally different motifs: Nα and Nβ. Nβ, the inner domain (Ala-17 to Pro-27), is essential and enough to target NaTrxh towards the apoplast. Interestingly, when it was fused to GFP, this protein was also found in the cell wall of the onion cells. Although the biochemical features of the N-terminus suggested a non-classical secretion pathway, our results provided evidence that NaTrxh at least uses the endoplasmic reticulum, Golgi apparatus and also vesicles for secretion. Therefore, the Nβ domain sequence is suggested to be a novel signal peptide.

Homeologous genes involved in mannitol synthesis reveal unequal contributions in response to abiotic stress in Coffea arabica

Polyploid plants can exhibit transcriptional modulation in homeologous genes in response to abiotic stresses. Coffea arabica, an allotetraploid, accounts for 75% of the world's coffee production. Extreme temperatures, salinity and drought limit crop productivity, which includes coffee plants. Mannitol is known to be involved in abiotic stress tolerance in higher plants. This study aimed to investigate the transcriptional responses of genes involved in mannitol biosynthesis and catabolism in C. arabica leaves under water deficit, salt stress and high temperature. Mannitol concentration was significantly increased in leaves of plants under drought and salinity, but reduced by heat stress. Fructose content followed the level of mannitol only in heat-stressed plants, suggesting the partitioning of the former into other metabolites during drought and salt stress conditions. Transcripts of the key enzymes involved in mannitol biosynthesis, CaM6PR, CaPMI and CaMTD, were modulated in distinct ways depending on the abiotic stress. Our data suggest that changes in mannitol accumulation during drought and salt stress in leaves of C. arabica are due, at least in part, to the increased expression of the key genes involved in mannitol biosynthesis. In addition, the homeologs of the Coffea canephora subgenome did not present the same pattern of overall transcriptional response, indicating differential regulation of these genes by the same stimulus. In this way, this study adds new information on the differential expression of C. arabica homeologous genes under adverse environmental conditions showing that abiotic stresses can influence the homeologous gene regulation pattern, in this case, mainly on those involved in mannitol pathway.

The plant secretory pathway: an essential factory for building the plant cell wall

For building and maintaining the complex structure of the surrounding wall throughout their life, plant cells rely on the endomembrane system, which functions as the main provider and transporter of cell wall constituents. Efforts to understand the mechanisms of synthesis and transport of cell wall materials have been generating valuable information for diverse practical applications. Nonetheless, the identity of the endomembrane components necessary for the transport of cell wall enzymes and polysaccharides is not well known. Evidence indicates that plant cells can accomplish secretion of cell wall constituents through multiple pathways during development or under stress conditions and, that compared with other eukaryotes, they rely on a highly diversified toolkit of proteins for membrane traffic. This suggests that production of the cell wall in plants consists of intricate and highly regulated pathways. In this review, we summarize important discoveries that have allowed the activities of the plant secretory pathway to be linked to the production and deposition of cell wall-synthesizing enzymes and polysaccharides.

A pair of receptor-like kinases is responsible for natural variation in shoot growth response to mannitol treatment in Arabidopsis thaliana

Growth is a complex trait that adapts to the prevailing conditions by integrating many internal and external signals. Understanding the molecular origin of this variation remains a challenging issue. In this study, natural variation of shoot growth under mannitol-induced stress was analyzed by standard quantitative trait locus mapping methods in a recombinant inbred line population derived from a cross between the Col-0 and Cvi-0 Arabidopsis thaliana accessions. Cloning of a major QTL specific to mannitol-induced stress condition led to identification of EGM1 and EGM2, a pair of tandem-duplicated genes encoding receptor-like kinases that are potentially involved in signaling of mannitol-associated stress responses. Using various genetic approaches, we identified two non-synonymous mutations in the EGM2[Cvi] allele that are shared by at least ten accessions from various origins and are probably responsible for a specific tolerance to mannitol. We have shown that the enhanced shoot growth phenotype contributed by the Cvi allele is not linked to generic osmotic properties but instead to a specific chemical property of mannitol itself. This result raises the question of the function of such a gene in A. thaliana, a species that does not synthesize mannitol. Our findings suggest that the receptor-like kinases encoded by EGM genes may be activated by mannitol produced by pathogens such as fungi, and may contribute to plant defense responses whenever mannitol is present.

Plant secretome - from cellular process to biological activity

Recent studies suggest that plants secrete a large number of proteins and peptides into the extracellular space. Secreted proteins play a crucial role in stress response, communication and development of organisms. Here we review the current knowledge of the secretome of more than ten plant species, studied in natural conditions or during (a)biotic stress. This review not only deals with the classical secretory route via endoplasmic reticulum and Golgi followed by proteins containing a known N-terminal signal peptide, but also covers new findings about unconventional secretion of leaderless proteins. We describe alternative secretion pathways and the involved compartments like the recently discovered EXPO. The well characterized secreted peptides that function as ligands of receptor proteins exemplify the biological significance and activity of the secretome. This article is part of a Special Issue entitled: An Updated Secretome.

Protein secretion: how many secretory routes does a plant cell have?

Conventional protein secretion is mediated by the endomembrane system. Secreted proteins are inserted into the endomembrane system through a N-terminal signal peptide and follow the endoplasmic reticulum to the Golgi trafficking pathway en route to the plasma membrane or the extracellular apoplastic space. In mammalian and yeast cells, unconventional secretion has been identified and relatively well studied. Also in plants, evidence of unconventional secretion mechanisms is accumulating. The ever-increasing number of leaderless proteins identified in proteomic studies indicates the importance of unconventional protein secretion in plants. Novel approaches, such as chemical genomics and vesicle proteomics might be able to provide new insights into unconventional protein secretion in plants.

Influences of natural emission sources (wildfires and Saharan dust) on the urban organic aerosol in Barcelona (Western Mediterranean Basis) during a PM event

The urban air quality in Barcelona in the Western Mediterranean Basin is characterized by overall high particulate matter (PM) concentrations, due to intensive local anthropogenic emissions and specific meteorological conditions. Moreover, on several days, especially in summer, natural PM sources, such as long-range transported Saharan dust from Northern Africa or wildfires on the Iberian Peninsula and around the Mediterranean Basin, may influence the levels and composition of the organic aerosol. In the second half of July 2009, daily collected PM(10) filter samples in an urban background site in Barcelona were analyzed on organic tracer compounds representing several emission sources. During this period, an important PM peak event was observed. Individual organic compound concentrations increased two to five times during this event. Although highest increase was observed for the organic tracer of biomass burning, the contribution to the organic aerosol was estimated to be around 6 %. Organic tracers that could be related to Saharan dust showed no correlation with the PM and OC levels, while this was the case for those related to fossil fuel combustion from traffic emissions. Moreover, a change in the meteorological conditions gave way to an overall increase of the urban background contamination. Long-range atmospheric transport of organic compounds from primary emissions sources (i.e., wildfires and Saharan dust) has a relatively moderate impact on the organic aerosol in an urban area where the local emissions are dominating.

Unconventional protein secretion

It is generally believed that protein secretion or exocytosis is achieved via a conventional ER (endoplasmic reticulum)-Golgi-TGN (trans-Golgi network)-PM (plasma membrane) pathway in the plant endomembrane system. However, such signal peptide (SP)-dependent protein secretion cannot explain the increasing number of SP-lacking proteins which are found outside of the PM in plant cells. The process by which such leaderless secretory proteins (LSPs) gain access to the cell exterior is termed unconventional protein secretion (UPS) and has been well-studied in animal and yeast cells, but largely ignored by the plant community. Here, we review the evidence for UPS in plants especially in regard to the recently discovered EXPO (exocyst-positive-organelle).

Apoplastic exosome-like vesicles: a new way of protein secretion in plants?

The presence of apoplastic proteins without predicted signal peptide in the gene sequence suggests the existence of protein secretion independent of the ER/Golgi classical route. In animals, one of the pathways proposed for alternative protein secretion involves the release of exosomes to the extracellular space. Although this pathway has not been dissected in plants some indirect evidence is emerging. We have reported that apoplastic fractions of sunflower seeds contain exosome-like vesicles. Besides, these vesicles are enriched in the lectin Helja, which is immunolocalized in the extracellular space even if it the protein has no predicted signal peptide. Here we show that Helja is not glycosylated and its secretion is insensitive to brefeldin A, two of the major characteristics to discard ER/Golgi-mediated protein transport. Moreover, the levels of Helja in sunflower extracellular vesicles are not affected by brefeldin A treatment. Our results suggest that Helja could be exported through an exosome-mediated pathway and point out that this mechanism may be responsible for the secretion of at least part of the leaderless proteins detected in the extracellular compartment of plants.

Comparison of salt stress resistance genes in transgenic Arabidopsis thaliana indicates that extent of transcriptomic change may not predict secondary phenotypic or fitness effects

Engineered abiotic stress resistance is an important target for increasing agricultural productivity. There are concerns, however, regarding possible ecological impacts of transgenic crops. In contrast to the first wave of transgenic crops, many abiotic stress resistance genes can initiate complex downstream changes. Transcriptome profiling has been suggested as a comprehensive non-targeted approach to examine the secondary effects. We compared phenotypic and transcriptomic effects of constitutive expression of genes intended to confer salt stress tolerance by three different mechanisms: a transcription factor, CBF3/DREB1a; a metabolic gene, M6PR, for mannitol biosynthesis; and the Na⁺/H⁺ antiporter, SOS1. Transgenic CBF3, M6PR and SOS1 Arabidopsis thaliana were grown together in the growth chamber, greenhouse and field. In the absence of salt, M6PR and SOS1 lines performed comparably with wild type; CBF3 lines exhibited dwarfing as reported previously. All three transgenes conferred fitness advantage when subjected to 100 mm NaCl in the growth chamber. CBF3 and M6PR affected transcription of numerous abiotic stress-related genes as measured by Affymetrix microarray analysis. M6PR additionally modified expression of biotic stress and oxidative stress genes. Transcriptional effects of SOS1 in the absence of salt were smaller and primarily limited to redox-related genes. The extent of transcriptome change, however, did not correlate with the effects on growth and reproduction. Thus, the magnitude of global transcriptome differences may not predict phenotypic differences upon which environment and selection act to influence fitness. These observations have implications for interpretation of transcriptome analyses in the context of risk assessment and emphasize the importance of evaluation within a phenotypic context.

Golgi apparatus-localized synaptotagmin 2 is required for unconventional secretion in Arabidopsis

BACKGROUND

Most secretory proteins contain signal peptides that direct their sorting to the ER and secreted via the conventional ER/Golgi transport pathway, while some signal-peptide-lacking proteins have been shown to export through ER/Golgi independent secretory pathways. Hygromycin B is an aminoglycoside antibiotic produced by Streptomyces hygroscopicus that is active against both prokaryotic and eukaryotic cells. The hygromycin phosphotransferase (HYG(R)) can phosphorylate and inactivate the hygromycin B, and has been widely used as a positive selective marker in the construction of transgenic plants. However, the localization and trafficking of HYG(R) in plant cells remain unknown. Synaptotagmins (SYTs) are involved in controlling vesicle endocytosis and exocytosis as calcium sensors in animal cells, while their functions in plant cells are largely unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We found Arabidopsis synaptotagmin SYT2 was localized on the Golgi apparatus by immunofluorescence and immunogold labeling. Surprisingly, co-expression of SYT2 and HYG(R) caused hypersensitivity of the transgenic Arabidopsis plants to hygromycin B. HYG(R), which lacks a signal sequence, was present in the cytoplasm as well as in the extracellular space in HYG(R)-GFP transgenic Arabidopsis plants and its secretion is not sensitive to brefeldin A treatment, suggesting it is not secreted via the conventional secretory pathway. Furthermore, we found that HYG(R)-GFP was truncated at carboxyl terminus of HYG(R) shortly after its synthesis, and the cells deficient SYT2 failed to efficiently truncate HYG(R)-GFP,resulting in HYG(R)-GFP accumulated in prevacuoles/vacuoles, indicating that SYT2 was involved in HYG(R)-GFP trafficking and secretion.

CONCLUSION/SIGNIFICANCE

These findings reveal for the first time that SYT2 is localized on the Golgi apparatus and regulates HYG(R)-GFP secretion via the unconventional protein transport from the cytosol to the extracelluar matrix in plant cells.

Mannitol transport and mannitol dehydrogenase activities are coordinated in Olea europaea under salt and osmotic stresses

The intracellular accumulation of organic compatible solutes functioning as osmoprotectants, such as polyols, is an important response mechanism of several plants to drought and salinity. In Olea europaea a mannitol transport system (OeMaT1) was previously characterized as a key player in plant response to salinity. In the present study, heterotrophic sink models, such as olive cell suspensions and fruit tissues, and source leaves were used for analytical, biochemical and molecular studies. The kinetic parameters of mannitol dehydrogenase (MTD) determined in cells growing in mannitol, at 25°C and pH 9.0, were as follows: K(m), 54.5 mM mannitol; and V(max), 0.47 μmol h⁻¹ mg⁻¹ protein. The corresponding cDNA was cloned and named OeMTD1. OeMTD1 expression was correlated with MTD activity, OeMaT1 expression and carrier-mediated mannitol transport in mannitol- and sucrose-grown cells. Furthermore, sucrose-grown cells displayed only residual OeMTD activity, even though high levels of OeMTD1 transcription were observed. There is evidence that OeMTD is regulated at both transcriptional and post-transcriptional levels. MTD activity and OeMTD1 expression were repressed after Na+, K+ and polyethylene glycol (PEG) treatments, in both mannitol- and sucrose-grown cells. In contrast, salt and drought significantly increased mannitol transport activity and OeMaT1 expression. Taken together, these studies support that olive trees cope with salinity and drought by coordinating mannitol transport with intracellular metabolism.

EXPO, an exocyst-positive organelle distinct from multivesicular endosomes and autophagosomes, mediates cytosol to cell wall exocytosis in Arabidopsis and tobacco cells

The exocyst protein complex mediates vesicle fusion with the plasma membrane. By expressing an (X)FP-tagged Arabidopsis thaliana homolog of the exocyst protein Exo70 in suspension-cultured Arabidopsis and tobacco (Nicotiana tabacum) BY-2 cells, and using antibodies specific for Exo70, we detected a compartment, which we term EXPO (for exocyst positive organelles). Standard markers for the Golgi apparatus, the trans-Golgi network/early endosome, and the multivesicular body/late endosome in plants do not colocalize with EXPO. Inhibitors of the secretory and endocytic pathways also do not affect EXPO. Exo70E2-(X)FP also locates to the plasma membrane (PM) as discrete punctae and is secreted outside of the cells. Immunogold labeling of sections cut from high-pressure frozen samples reveal EXPO to be spherical double membrane structures resembling autophagosomes. However, unlike autophagosomes, EXPOs are not induced by starvation and do not fuse with the lytic compartment or with endosomes. Instead, they fuse with the PM, releasing a single membrane vesicle into the cell wall. EXPOs are also found in other cell types, including root tips, root hair cells, and pollen grains. EXPOs therefore represent a form of unconventional secretion unique to plants.

Data-independent liquid chromatography/mass spectrometry (LC/MS(E)) detection and quantification of the secreted Apium graveolens pathogen defense protein mannitol dehydrogenase

Plant cells secrete a wide variety of defense-related proteins into the extracellular space or apoplast in response to pathogen attack. One of these, mannitol dehydrogenase (MTD), is normally a cytoplasmic enzyme whose primary role is the regulation of intracellular levels of the sugar alcohol mannitol in plants. Recent immunological and biochemical evidence, however, suggests that MTD is also secreted into the apoplast in response to pathogen attack, despite lacking a known peptide signal sequence for Golgi-mediated secretion. Because many plant pathogenic fungi secrete mannitol to overcome pathogen-induced generation of reactive oxygen species (ROS) by the plant, extracellular localization of MTD is hypothesized to have a defensive role of catabolizing pathogen-secreted mannitol. In the current study, LC/MS(E) was used to analyze proteins in the secretome of Apium graveolens (celery) following treatment with salicylic acid (SA), an endogenous elicitor of defense responses in plants. Levels of MTD in the secretome of SA-treated celery cell cultures were found to be induced at least 18-fold over secretome samples from cell cultures not exposed to SA. This value is in close agreement with published immunological and biochemical observations. Overall, this report provides the first mass spectrometry identification and quantification measurements supporting the hypothesis that MTD is secreted in response to simulated pathogen attack via a non-classical secretion mechanism. As demonstrated with MTD secretion, LC/MS(E) can be implemented as a discovery-driven MRM-based quantitative approach which can be used to reveal potential post-translational modifications, thus providing a new method in the area of gel-free and label-free proteomic analysis.

Is there leaderless protein secretion in plants?

The sugar alcohol mannitol and it’s catabolic enzyme mannitol dehydrogenase (MTD), in addition to welldocumented roles in metabolism and osmoprotection, may play roles in hostpathogen interactions. Research suggests that in response to the mannitol that pathogenic fungi secrete to suppress reactive oxygen-mediated host defenses, plants make MTD to catabolize fungal mannitol. Yet previous work suggested that pathogen-secreted mannitol is extracellular, while in healthy plants MTD is cytoplasmic. We have presented results showing that the normally cytoplasmic MTD is exported into the cell wall or extracellular space in response to the endogenous inducer of plant defense responses salicylic acid (SA). This SA-induced secretion is insensitive to brefeldin A, an inhibitor of Golgimediated protein transport. Together with the absence of MTD in Golgi stacks and the lack of a documented extracellular targeting sequence in the MTD protein, this suggests MTD is secreted by a non-Golgi, pathogen-activated secretion mechanism in plants. Here we discuss the potential significance of non-Golgi secretion in response to stress.