Calcium interacts with antifreeze proteins and chitinase from cold-acclimated winter rye

Phosphorus availability and planting patterns regulate soil microbial effects on plant performance in a semiarid steppe

BACKGROUND AND AIMS

Growing evidence has suggested that plant responses to model soil microorganisms are context dependent; however, few studies have investigated the effects of whole soil microbial communities on plant performance in different abiotic and biotic conditions. To address this, we examined how soil phosphorus (P) availability and different planting patterns regulate soil microbial effects on the growth of two native plant species in a semiarid steppe.

METHODS

We carried out a glasshouse experiment to explore the effects of the whole indigenous soil microbiota on the growth and performance of Leymus chinensis and Cleistogenes squarrosa using soil sterilization with different soil P availabilities and planting patterns (monoculture and mixture). Transcriptome sequencing (RNA-seq) was used to explain the potential molecular mechanisms of the soil microbial effects on C. squarrosa.

KEY RESULTS

The soil sterilization treatment significantly increased the biomass of L. chinensis and C. squarrosa in both monoculture and mixture conditions, which indicated that the soil microbiota had negative growth effects on both plants. The addition of P neutralized the negative microbial effects for both L. chinensis and C. squarrosa, whereas the mixture treatment amplified the negative microbial effects on L. chinensis but alleviated them on C. squarrosa. Transcriptomic analysis from C. squarrosa roots underscored that the negative soil microbial effects were induced by the upregulation of defence genes. The P addition treatment resulted in significant decreases in the number of differentially expressed genes attributable to the soil microbiota, and some defence genes were downregulated.

CONCLUSIONS

Our results underline that indigenous soil microbiota have negative effects on the growth of two dominant plant species from a semiarid steppe, but their effects are highly dependent on the soil P availability and planting patterns. They also indicate that defence genes might play a key role in controlling plant growth responses to the soil microbiota.

Unraveling molecular mechanisms underlying low-temperature adaptation in Laguncularia racemosa

Laguncularia racemosa (L.) C.F. Gaertn is a controversial species in China, in terms of being a pioneer species for mangrove restoration and a putative invasive species occupying natural habitats. The tolerance to chilling stress allows L. racemosa to adapt to extreme climate change. However, little is known about the molecular-level chilling resistance mechanisms in L. racemosa, which restricts our understanding of its biological features and invasion potential. In this study, L. racemosa seedlings were treated with freezing temperature (0 °C) at four durations (0 h, 3 h, 12 h and 24 h of recovery after treatment), and both physiological and transcriptional regulations underlying chilling stress resistance were investigated. Chilling stress caused damage to the cell membrane system and reduced photosynthesis efficiency of L. racemosa seedlings. To combat the adverse impacts, plasma membrane biosynthesis and antioxidant processes were substantially enhanced. After 24 h of recovery, the seedlings nearly recovered to normal growth condition, except for the processes related to photosynthesis, indicating their vigorous adaptation to short-term chilling stress. Importantly, the individuals from higher latitude displayed better adaptation to chilling injury than those from lower latitude, highlighting the role of long-term heredity × environment interactions in promoting the chilling resistance capacity of L. racemosa. These features allow L. racemosa to survive in extremely cold weather, but may also increase its risk of invasion into intertidal ecosystems. Together, our findings present a comprehensive view of the chilling-adaptative mechanisms of L. racemosa, which provide clues for better evaluating the invasive potential of L. racemosa.

Transcriptomic and Metabolomic Analyses Reveal a Potential Mechanism to Improve Soybean Resistance to Anthracnose

Anthracnose, caused by Colletotrichum truncatum, leads to large-scale reduction in quality and yield in soybean production. Limited information is available regarding the molecular mechanisms of resistance to anthracnose in soybean. We conducted a transcriptomic and targeted metabolomic analysis of pods from two soybean lines, "Zhechun No. 3" (ZC3) and ZC-2, in response to C. truncatum infection. Factors contributing to the enhanced resistance of ZC-2 to anthracnose compared with that of ZC3, included signal transduction (jasmonic acid, auxin, mitogen-activated protein kinase, and Ca²⁺ signaling), transcription factors (WRKY and bHLH), resistance genes (PTI1, RPP13, RGA2, RPS6, and ULP2B), pathogenesis-related genes (chitinase and lipid transfer protein), and terpenoid metabolism. Targeted metabolomic analysis revealed that terpenoid metabolism responded more promptly and more intensely to C. truncatum infection in ZC-2 than in ZC3. In vitro antifungal activity and resistance induction test confirmed that jasmonic acid, auxin signaling and terpenoids played important roles in soybean resistance to anthracnose. This research is the first study to explore the molecular mechanisms of soybean resistance to anthracnose. The findings are important for in-depth analysis of molecular resistance mechanisms, discovery of resistance genes, and to expedite the breeding of anthracnose-resistant soybean cultivars.

Proteins, Small Peptides and Other Signaling Molecules Identified as Inconspicuous but Possibly Important Players in Microspores Reprogramming Toward Embryogenesis

In this review, we describe and integrate the latest knowledge on the signaling role of proteins and peptides in the stress-induced microspore embryogenesis (ME) in some crop plants with agricultural importance (i.e., oilseed rape, tobacco, barley, wheat, rice, triticale, rye). Based on the results received from the most advanced omix analyses, we have selected some inconspicuous but possibly important players in microspores reprogramming toward embryogenic development. We provide an overview of the roles and downstream effect of stress-related proteins (e.g., β-1,3-glucanases, chitinases) and small signaling peptides, especially cysteine—(e.g., glutathione, γ-thionins, rapid alkalinization factor, lipid transfer, phytosulfokine) and glycine-rich peptides and other proteins (e.g., fasciclin-like arabinogalactan protein) on acclimation ability of microspores and the cell wall reconstruction in a context of ME induction and haploids/doubled haploids (DHs) production. Application of these molecules, stimulating the induction and proper development of embryo-like structures and green plant regeneration, brings significant improvement of the effectiveness of DHs procedures and could result in its wider incorporation on a commercial scale. Recent advances in the design and construction of synthetic peptides–mainly cysteine-rich peptides and their derivatives–have accelerated the development of new DNA-free genome-editing techniques. These new systems are evolving incredibly fast and soon will find application in many areas of plant science and breeding.

The Risk of Wheat Blast in Rice-Wheat Co-Planting Regions in China: MoO Strains of Pyricularia oryzae Cause Typical Symptom and Host Reaction on Both Wheat Leaves and Spikes

The Triticum pathotype of Magnaporthe oryzae (syn. Pyricularia oryzae) causes wheat blast, which has recently spread to Asia. To assess the potential risk of wheat blast in rice-wheat growing regions, we investigated the pathogenicity of 14 isolates of P. oryzae on 32 wheat cultivars, among which Oryzae pathotype of P. oryzae (MoO) isolates were completely avirulent on the wheat cultivars at 22°C but caused various degrees of infection 25°C. These reactions at 25°C were isolate and cultivar dependent, like race-cultivar specificity, which was also recognized at the heading stage and caused typical blast symptoms on spikes. Microscopic analyses indicated that a compatible MoO isolate produced appressoria and infection hyphae on wheat as on rice. When we compared transcriptomes in wheat-MoO interactions, the bulk of pathogen-related genes were upregulated or downregulated in compatible and incompatible patterns, but changes in gene transcription were more significant in a compatible pattern. These results indicate that temperature could influence the infection ratio of wheat with MoO, and some MoO strains could be potential pathogens that increase the risk of wheat blast outbreaks in wheat-rice growing regions with global warming. In addition, certain wheat cultivars exhibited resistance and are assumed to carry resistance-promoting genes to the MoO strains.

Genome-Wide Identification and Analysis of Chitinase GH18 Gene Family in Mycogone perniciosa

Mycogone perniciosa causes wet bubble disease in Agaricus bisporus and various Agaricomycetes species. In a previous work, we identified 41 GH18 chitinase genes and other pathogenicity-related genes in the genome of M. perniciosa Hp10. Chitinases are enzymes that degrade chitin, and they have diverse functions in nutrition, morphogenesis, and pathogenesis. However, these important genes in M. perniciosa have not been fully characterized, and their functions remain unclear. Here, we performed a genome-wide analysis of M. perniciosa GH18 genes and analyzed the transcriptome profiles and GH18 expression patterns in M. perniciosa during the time course of infection in A. bisporus. Phylogenetic analysis of the 41 GH18 genes with those of 15 other species showed that the genes were clustered into three groups and eight subgroups based on their conserved domains. The GH18 genes clustered in the same group shared different gene structures but had the same protein motifs. All GH18 genes were localized in different organelles, were unevenly distributed on 11 contigs, and had orthologs in the other 13 species. Twelve duplication events were identified, and these had undergone both positive and purifying selection. The transcriptome analyses revealed that numerous genes, including transporters, cell wall degrading enzymes (CWDEs), cytochrome P450, pathogenicity-related genes, secondary metabolites, and transcription factors, were significantly upregulated at different stages of M. perniciosa Hp10 infection of A. bisporus. Twenty-three out of the 41 GH18 genes were differentially expressed. The expression patterns of the 23 GH18 genes were different and were significantly expressed from 3 days post-inoculation of M. perniciosa Hp10 in A. bisporus. Five differentially expressed GH18 genes were selected for RT-PCR and gene cloning to verify RNA-seq data accuracy. The results showed that those genes were successively expressed in different infection stages, consistent with the previous sequencing results. Our study provides a comprehensive analysis of pathogenicity-related and GH18 chitinase genes’ influence on M. perniciosa mycoparasitism of A. bisporus. Our findings may serve as a basis for further studies of M. perniciosa mycoparasitism, and the results have potential value for improving resistance in A. bisporus and developing efficient disease-management strategies to mitigate wet bubble disease.

The Roots of Plant Frost Hardiness and Tolerance

Frost stress severely affects agriculture and agroforestry worldwide. Although many studies about frost hardening and resistance have been published, most of them focused on the aboveground organs and only a minority specifically targets the roots. However, roots and aboveground tissues have different physiologies and stress response mechanisms. Climate models predict an increase in the magnitude and frequency of late-frost events, which, together with an observed loss of soil insulation, will greatly decrease plant primary production due to damage at the root level. Molecular and metabolic responses inducing root cold hardiness are complex. They involve a variety of processes related to modifications in cell wall composition, maintenance of the cellular homeostasis and the synthesis of primary and secondary metabolites. After a summary of the current climatic models, this review details the specificity of freezing stress at the root level and explores the strategies roots developed to cope with freezing stress. We then describe the level to which roots can be frost hardy, depending on their age, size category and species. After that, we compare the environmental signals inducing cold acclimation and frost hardening in the roots and aboveground organs. Subsequently, we discuss how roots sense cold at a cellular level and briefly describe the following signal transduction pathway, which leads to molecular and metabolic responses associated with frost hardening. Finally, the current options available to increase root frost tolerance are explored and promising lines of future research are discussed.

A cold responsive ethylene responsive factor from Medicago falcata confers cold tolerance by up-regulation of polyamine turnover, antioxidant protection, and proline accumulation

Ethylene responsive factor (ERF) subfamily transcription factors play an important role in plant abiotic and biotic stress tolerance. A cold responsive ERF, MfERF1, was isolated from Medicago falcata, an important forage legume that has great cold tolerance. Overexpression of MfERF1 resulted in an increased tolerance to freezing and chilling in transgenic tobacco plants, whereas down-regulation of the ortholog of MfERF1 in Medicago truncatula resulted in reduced freezing tolerance in RNAi plants. Higher transcript levels of some stress responsive genes (CHN50, OSM, ERD10C, and SAMS) and those involved in spermidine (Spd) and spermine (Spm) synthesis (SAMDC1, SAMDC2, SPDS1, SPDS2, and SPMS) and catabolism (PAO) were observed in transgenic plants than in wild type. However, neither Spd nor Spm level was accumulated in transgenic plants as a result of promoted polyamine oxidase activity. Transgenic plants had higher activities of antioxidants associated with the induced encoding genes including Cu, Zn-SOD, CAT1, CAT2, CAT3, and cpAPX and accumulated more proline associated with induced P5CS and reduced PROX2 transcription as compared with wild type. The results suggest that MfERF1 confers cold tolerance through promoted polyamine turnover, antioxidant protection, and proline accumulation.

Proteomic Analysis of Differentially Accumulated Proteins in Cucumber (Cucumis sativus) Fruit Peel in Response to Pre-storage Cold Acclimation

Harvested fruits are still living organs and respond to environmental stimuli. Low temperature storage is effective in extending life of harvested fruit, but it may also cause chilling injury. Cold acclimation has been shown to induce chilling tolerance in plants, but what proteomic changes caused by cold acclimation are related to defense against chilling stress remains largely unclear. Here, 3 d of pre-storage cold acclimation (PsCA) at 10°C reduced chilling injury and secondary disease severity in cucumber stored at 5°C by 51 and 94%, respectively, compared with the control which was directly stored at 5°C. Proteomic analysis of cucumber peel identified 21 significant differentially-accumulated proteins (SDAPs) right after PsCA treatment and 23 after the following cold storage (PsCA+CS). These proteins are mainly related to stress response and defense (SRD), energy metabolism, protein metabolism, signal transduction, primary metabolism, and transcription. The SRD proteins, which made up 37% of the 21 and 47% of the 23, respectively, represented the largest class of SDAPs, and all but one protein were up-regulated, suggesting accumulation of proteins involved in defense response is central feature of proteomic profile changes brought about by PsCA. In fruit just after PsCA treatment, the identified SDAPs are related to responses to various stresses, including chilling, salt stress, dehydration, fungi, bacteria, insects, and DNA damage. However, after prolonged cold storage, the targeted proteins in acclimated fruit were narrowed down in scope to those involved in defense against chilling and pathogens. The change patterns at the transcription level of the majority of the up-regulated differentially-accumulated proteins were highly consistent with those at protein level. Taken all, the results suggest that the short-time cold acclimation initiated comprehensive defense responses in cucumber fruit at first, while the long term storage thereafter altered the responses more specifically to chilling. These findings add to the understanding of plants' molecular responses to cold acclimation.

A secreted chitinase-like protein (OsCLP) supports root growth through calcium signaling in Oryza sativa

Chitinases belong to a conserved protein family and play multiple roles in defense, development and growth regulation in plants. Here, we identified a secreted chitinase-like protein, OsCLP, which functions in rice growth. A T-DNA insertion mutant of OsCLP (osclp) showed significant retardation of root and shoot growth. A comparative proteomic analysis was carried out using root tissue of wild-type and the osclp mutant to understand the OsCLP-mediated rice growth retardation. Results obtained revealed that proteins related to glycolysis (phosphoglycerate kinase), stress adaption (chaperonin) and calcium signaling (calreticulin and CDPK1) were differentially regulated in osclp roots. Fura-2 molecular probe staining, which is an intracellular calcium indicator, and inductively coupled plasma-mass spectrometry (ICP-MS) analysis suggested that the intracellular calcium content was significantly lower in roots of osclp as compared with the wild-type. Exogenous application of Ca²⁺ resulted in successful recovery of both primary and lateral root growth in osclp. Moreover, overexpression of OsCLP resulted in improved growth with modified seed shape and starch structure; however, the overall yield remained unaffected. Taken together, our results highlight the involvement of OsCLP in rice growth by regulating the intracellular calcium concentrations.

Molecular modelling, dynamics simulation and characterization of antifungal chitinase from Sechium edule

Chitinases are varied sized proteins which have the ability to degrade chitin and are present in a huge range of organisms like fungi, yeasts, arthropods, humans etc. and have been getting increased attention due to their biocontrol properties. In silico analysis sheds light on the extensive properties of this plant protein. In this paper, a particular antifungal protein Chitinase sourced from Sechium edule from East Khasi Hills, Meghalaya was characterized using an array of bioinformatics tools. The modelled protein showed conserved domains characteristic to glycosyl hydrolase, family 18 superfamily. Likewise, a part of the conserved domain area fits in with xylanase inhibitor Xip-1 and the class ΙΙΙ plant chitinases, for example, concanavalin B, hevamine, which have a GH18 area. The modelled wild type protein exhibited secondary characteristics comprising of 48.8% helix, 62.2% sheets and 13.8% turns, displaying an aliphatic index of 80.53 and instability index of 48.88 inferring upon the fact that the protein is relatively unstable without its appropriate environment. The paper functions as the first attempt to portray molecular dynamics simulation of Chitinase from Sechium edule reinforced by modelling and thorough characteristic analysis of the protein by employing parameters like Ramachandran Plot, Chou and Fasman Secondary Structure prediction, ProtParam etc. Further approaches like protein engineering and activity analysis suggested.

Ice-Binding Proteins in Plants

Sub-zero temperatures put plants at risk of damage associated with the formation of ice crystals in the apoplast. Some freeze-tolerant plants mitigate this risk by expressing ice-binding proteins (IBPs), that adsorb to ice crystals and modify their growth. IBPs are found across several biological kingdoms, with their ice-binding activity and function uniquely suited to the lifestyle they have evolved to protect, be it in fishes, insects or plants. While IBPs from freeze-avoidant species significantly depress the freezing point, plant IBPs typically have a reduced ability to lower the freezing temperature. Nevertheless, they have a superior ability to inhibit the recrystallization of formed ice. This latter activity prevents ice crystals from growing larger at temperatures close to melting. Attempts to engineer frost-hardy plants by the controlled transfer of IBPs from freeze-avoiding fish and insects have been largely unsuccessful. In contrast, the expression of recombinant IBP sequences from freeze-tolerant plants significantly reduced electrolyte leakage and enhanced freezing survival in freeze-sensitive plants. These promising results have spurred additional investigations into plant IBP localization and post-translational modifications, as well as a re-evaluation of IBPs as part of the anti-stress and anti-pathogen axis of freeze-tolerant plants. Here we present an overview of plant freezing stress and adaptation mechanisms and discuss the potential utility of IBPs for the generation of freeze-tolerant crops.

Influence of water deficit on the molecular responses of Pinus contorta × Pinus banksiana mature trees to infection by the mountain pine beetle fungal associate, Grosmannia clavigera

Conifers exhibit a number of constitutive and induced mechanisms to defend against attack by pests and pathogens such as mountain pine beetle (Dendroctonus ponderosae Hopkins) and their fungal associates. Ecological studies have demonstrated that stressed trees are more susceptible to attack by mountain pine beetle than their healthy counterparts. In this study, we tested the hypothesis that water deficit affects constitutive and induced responses of mature lodgepole pine × jack pine hybrids (Pinus contorta Dougl. ex Loud. var. latifolia Engelm. ex S. Wats. × Pinus banksiana Lamb.) to inoculation with the mountain pine beetle fungal associate Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield. The degree of stress induced by the imposed water-deficit treatment was sufficient to reduce photosynthesis. Grosmannia clavigera-induced lesions exhibited significantly reduced dimensions in water-deficit trees relative to well-watered trees at 5 weeks after inoculation. Treatment-associated cellular-level changes in secondary phloem were also observed. Quantitative RT-PCR was used to analyze transcript abundance profiles of 18 genes belonging to four families classically associated with biotic and abiotic stress responses: aquaporins (AQPs), dehydration-responsive element binding (DREB), terpene synthases (TPSs) and chitinases (CHIs). Transcript abundance profiles of a TIP2 AQP and a TINY-like DREB decreased significantly in fungus-inoculated trees, but not in response to water deficit. One TPS, Pcb(+)-3-carene synthase, and the Class II CHIs PcbCHI2.1 and PcbCHI2.2 showed increased expression under water-deficit conditions in the absence of fungal inoculation, while another TPS, Pcb(E)-β-farnesene synthase-like, and two CHIs, PcbCHI1.1 and PcbCHI4.1, showed attenuated expression under water-deficit conditions in the presence of fungal inoculation. The effects were observed both locally and systemically. These results demonstrate that both constitutive and induced carbon- and nitrogen-based defenses are affected by water deficit, suggesting potential consequences for mountain pine beetle dynamics, particularly in novel environments.

Refolding of β-stranded class I chitinases of Hippophae rhamnoides enhances the antifreeze activity during cold acclimation

Class I chitinases hydrolyse the β-1,4-linkage of chitin and also acquire antifreeze activity in some of the overwintering plants during cold stress. Two chitinases, HrCHT1a of 31 kDa and HrCHT1b of 34 kDa, were purified from cold acclimated and non-acclimated seabuckthorn seedlings using chitin affinity chromatography. 2-D gels of HrCHT1a and HrCHT1b showed single spots with pIs 7.0 and 4.6 respectively. N-terminal sequence of HrCHT1b matched with the class I chitinase of rice and antifreeze proteins while HrCHT1a could not be sequenced as it was N-terminally blocked. Unlike previous reports, where antifreeze activity of chitinase was cold inducible, our results showed that antifreeze activity is constitutive property of class I chitinase as both HrCHT1a and HrCHT1b isolated even from non-acclimated seedlings, exhibited antifreeze activity. Interestingly, HrCHT1a and HrCHT1b purified from cold acclimated seedlings, exhibited 4 and 2 times higher antifreeze activities than those purified from non-acclimated seedlings, suggesting that antifreeze activity increased during cold acclimation. HrCHT1b exhibited 23–33% higher hydrolytic activity and 2–4 times lower antifreeze activity than HrCHT1a did. HrCHT1b was found to be a glycoprotein; however, its antifreeze activity was independent of glycosylation as even deglycosylated HrCHT1b exhibited antifreeze activity. Circular dichroism (CD) analysis showed that both these chitinases were rich in unusual β-stranded conformation (36–43%) and the content of β-strand increased (∼11%) during cold acclimation. Surprisingly, calcium decreased both the activities of HrCHT1b while in case of HrCHT1a, a decrease in the hydrolytic activity and enhancement in its antifreeze activity was observed. CD results showed that addition of calcium also increased the β-stranded conformation of HrCHT1a and HrCHT1b. This is the first report, which shows that antifreeze activity is constitutive property of class I chitinase and cold acclimation and calcium regulate these activities of chitinases by changing the secondary structure.

Chitinase III in pomegranate seeds (Punica granatum Linn.): a high-capacity calcium-binding protein in amyloplasts

Chitinases are a class of ubiquitous proteins that are widely distributed in plants. Defense is the major natural role for chitinases, primarily against fungal pathogens. Little is known regarding their non-defensive roles in seeds. In this study, a new class III chitinase from pomegranate seeds (pomegranate seed chitinase, PSC) was isolated and purified to homogeneity. The native state of PSC is a monomer with a molecular weight of approximately 30 kDa. This chitinase naturally binds calcium ions with high capacity and low affinity, suggesting that PSC is a calcium storage protein. Consistent with this idea, its amino acid sequence (inferred from cDNA) is rich in acidic amino acid residues, especially Asp, similar to reported calcium storage proteins. The presence of calcium considerably improves the stability of the protein but has little effect on its enzymatic activity. Transmission electron microscopy analyses indicate that, similar to phytoferritin, this enzyme is widely distributed in the stroma of amyloplasts of the embryonic cells, suggesting that amyloplasts in seeds could serve as an alternative plastid for calcium storage. Indeed, the transmission electron microscopy results showed that, within the embryonic cells, calcium ions are mainly distributed in the stroma of the amyloplasts, consistent with a role for PSC in calcium storage. Thus, the plant appears to have evolved a new plastid for calcium storage in seeds. During seed germination, the content of this enzyme decreases with time, suggesting that it is involved in the germination process.

The message of nitric oxide in cadmium challenged plants

During the last decade it has been found that cadmium (Cd), one of the most toxic elements occurring in polluted environments, interferes with nitric oxide (NO), a multifunctional signaling molecule in living organisms. The formation of NO has been demonstrated in vivo in various plant tissues exposed to Cd stress, but unfortunately, the time and intensity of NO generation, relatively frequently shows conflicting data. What is more, there is still limited information regarding the functional role of endogenously produced NO in plants challenged with heavy metals. The first pharmacological approaches revealed that exogenously applied NO can alleviate cadmium toxicity in plants, promoting the direct scavenging of reactive oxygen species (ROS) or activating antioxidant enzymes. However, recent reports have indicated that NO even contributes to Cd toxicity by promoting Cd uptake and participates in metal-induced reduction of root growth. In view of this heterogeneous knowledge, much more puzzling if we consider results first obtained using exogenous NO sources, this review is focused mainly on the implication of endogenous NO in plant response to Cd exposure. Furthermore, a basic draft for NO mode of action during cadmium stress is proposed.

Identification of NaCl stress-responsive apoplastic proteins in rice shoot stems by 2D-DIGE

Plants have evolved sophisticated systems to cope with adverse environmental conditions such as cold, drought, and salinity. Although a number of stress response networks have been proposed, the role of plant apoplast in plant stress response has been ignored. To investigate the role of apoplastic proteins in the salt stress response, 10-day old rice plants were treated with 200mM NaCl for 1, 6 or 12h, and the soluble apoplast proteins of rice shoot stems were extracted for differential analysis, compared with untreated controls, by 2-D DIGE saturation labeling techniques. One hundred twenty-two significantly changed spots were identified by LC-MS/MS, and 117 spots representing 69 proteins have been identified. Of these proteins, 37 are apoplastic proteins according to the bioinformatic analysis. These proteins are mainly involved in the processes of carbohydrate metabolism, oxido-reduction, and protein processing and degradation. According to their functional categories and cluster analysis, a stress response model of apoplastic proteins has been proposed. These data indicate that the apoplast is important in plant stress signal reception and response.

Cold activation of a plasma membrane-tethered NAC transcription factor induces a pathogen resistance response in Arabidopsis

Cold signals interact with other environmental cues to modulate plant developmental processes. Recent studies have shown that many Pathogenesis-Related (PR) genes are induced and disease resistance is enhanced after exposure to low temperatures, linking cold signals with pathogenesis in plants. However, the underlying molecular mechanisms and signaling schemes are largely unknown. Here, we demonstrate that cold stimulates proteolytic activation of a plasma membrane-tethered NAC (NAM/ATAF1/2/CUC2) transcription factor NTL6. The transcriptionally active NTL6 protein enters the nucleus, where it induces a subset of PR genes by directly binding to a conserved sequence in the promoters of cold-responsive PR genes, such as PR1, PR2, and PR5. While transgenic plants overexpressing an active NTL6 form exhibited enhanced disease resistance, RNAi plants with reduced NTL6 activity were more susceptible to pathogen infection at low temperatures. Accordingly, cold induction of PR1 disappeared in the RNAi plants. Consistent with the close relationship between cold and pathogenesis, cold-acclimated plants showed enhanced resistance to pathogen infection. In this signaling cascade, controlled activation of the membrane-tethered, dormant NTL6 transcription factor serves as a molecular link that incorporates cold signals into pathogen resistance responses. However, the NTL6-mediated cold induction of the PR genes is independent of salicylic acid (SA). The PR genes were still induced by SA in the NTL6 RNAi plants. Cold regulation of the PR genes through the membrane-mediated transcriptional control is thought to be an adaptive process that ensures quick plant responses to incoming pathogens that frequently occur during cold seasons.

Cell wall proteome of wheat roots under flooding stress using gel-based and LC MS/MS-based proteomics approaches

Cell wall proteins (CWPs) are important both for maintenance of cell structure and for responses to abiotic and biotic stresses. In this study, a destructive CWP purification procedure was adopted using wheat seedling roots and the purity of the CWP extract was confirmed by minimizing the activity of glucose-6-phosphate dehydrogenase, a cytoplasmic marker enzyme. To determine differentially expressed CWPs under flooding stress, gel-based proteomic and LC-MS/MS-based proteomic techniques were applied. Eighteen proteins were found to be significantly regulated in response to flood by gel-based proteomics and 15 proteins by LC MS/MS-based proteomics. Among the flooding down-regulated proteins, most were related to the glycolysis pathway and cell wall structure and modification. However, the most highly up-regulated proteins in response to flooding belong to the category of defense and disease response proteins. Among these differentially expressed proteins, only methionine synthase, beta-1,3-glucanases, and beta-glucosidase were consistently identified by both techniques. The down-regulation of these three proteins suggested that wheat seedlings respond to flooding stress by restricting cell growth to avoid energy consumption; by coordinating methionine assimilation and cell wall hydrolysis, CWPs played critical roles in flooding responsiveness.

Cellular response of pea plants to cadmium toxicity: cross talk between reactive oxygen species, nitric oxide, and calcium

Cadmium (Cd) toxicity has been widely studied in different plant species; however, the mechanism involved in its toxicity as well as the cell response against the metal have not been well established. In this work, using pea (Pisum sativum) plants, we studied the effect of Cd on antioxidants, reactive oxygen species (ROS), and nitric oxide (NO) metabolism of leaves using different cellular, molecular, and biochemical approaches. The growth of pea plants with 50 mum CdCl(2) affected differentially the expression of superoxide dismutase (SOD) isozymes at both transcriptional and posttranscriptional levels, giving rise to a SOD activity reduction. The copper/zinc-SOD down-regulation was apparently due to the calcium (Ca) deficiency induced by the heavy metal. In these circumstances, the overproduction of the ROS hydrogen peroxide and superoxide could be observed in vivo by confocal laser microscopy, mainly associated with vascular tissue, epidermis, and mesophyll cells, and the production of superoxide radicals was prevented by exogenous Ca. On the other hand, the NO synthase-dependent NO production was strongly depressed by Cd, and treatment with Ca prevented this effect. Under these conditions, the pathogen-related proteins PrP4A and chitinase and the heat shock protein 71.2, were up-regulated, probably to protect cells against damages induced by Cd. The regulation of these proteins could be mediated by jasmonic acid and ethylene, whose contents increased by Cd treatment. A model is proposed for the cellular response to long-term Cd exposure consisting of cross talk between Ca, ROS, and NO.

Characterization of cold-responsive extracellular chitinase in bromegrass cell cultures and its relationship to antifreeze activity

A cold-responsive chitinase gene, BiCHT1, was isolated from bromegrass (Bromus inermis) 'Manchar' suspension cells. BiCHT1 messenger RNA was detected at low levels in nonstressed bromegrass cells, whereas its accumulation was induced by incubation at 10 degrees C and 4 degrees C as detected by northern- and western-blot analyses. BiCHT1 was highly homologous to rye CHT9, known to encode an antifreeze protein. BiCHT1 was overexpressed in Escherichia coli and bromegrass cells using genetic transformation procedures. BiCHT1 products expressed in both systems had chitinase activity, but the expressed proteins did not affect the growth of ice crystals in any conditions tested. Besides cold stress, the expression of the BiCHT1 gene was up-regulated by exposure to 35 degrees C, but not by salt or osmotic stress, abscisic acid, or ethephon. BiCHT1 messenger RNA did not accumulate in response to methyl jasmonate and salicylic acid, but was slightly increased by prolonged culture at 25 degrees C and only transiently by chitin. Antifreeze activity detected in the culture medium was induced at 4 degrees C but only slightly at 10 degrees C. It was also induced by ethephon treatment, but not by abscisic acid, chitin, or prolonged incubation at 25 degrees C. The results of transgenics and expression analyses suggest that the BiCHT1 product is a major protein with chitinase activity secreted in the medium of cold-treated cells and is unlikely to be responsible for the antifreeze activity detected in the culture medium.

Roles of salicylic acid-responsive cis-acting elements and W-boxes in salicylic acid induction of VCH3 promoter in transgenic tobaccos

A salicylic acid (SA)-inducible VCH3 promoter was recently identified from grapevine (Vitis amurensis) that contains two inverse SA-responsive cis-acting elements and four W-boxes. To further demonstrate the roles of these elements, four fragments with lengths from -1187, -892, -589, -276 to +7 bp were fused with the b-glucuronidase (GUS) reporter gene and transferred to Nicotiana tobacum, together with another four VCH3 promoter fragments with mutation in the two inverse SA-responsive elements. The functions of each promoter fragment were examined by analysis of GUS activity in the transgenic tobacco root treated with SA. Enhanced GUS activity was shown in the roots of transgenic tobaccos with the VCH3 (-1187)-GUS construct containing two SA-responsive cis-acting elements and four W-boxes. However, GUS activity directed by the VCH3 (-892)-GUS construct, containing one SA cis-acting element and four W-boxes, was reduced by up to 35% compared with that in tobaccos transformed with the VCH3 (-1187)-GUS construct, indicating that the SA cis-acting element plays an important role in SA induction of the VCH3 promoter. Neither the m2VCH3 (-1187)-GUS nor the mVCH3 (-892)-GUS construct, with mutation on the SA-responsive elements, abolished the expression of GUS activity, demonstrating that the W-boxes in the VCH3 promoter are also involved in SA induction. Histochemical analysis of GUS activity directed by each of the eight VCH3 promoter fragments showed that GUS was expressed specifically in vascular tissue. It was concluded that both the SA-responsive cis-acting elements and the W-boxes are important for the SA induction of the VCH3 promoter. This promoter might have a potential use in plant genetic engineering.

A hyperactive, Ca2+-dependent antifreeze protein in an Antarctic bacterium

In cold climates, some plants and bacteria that cannot avoid freezing use antifreeze proteins (AFPs) to lessen the destructive effects of ice recrystallization. These AFPs have weak freezing point depression activity, perhaps to avoid sudden, uncontrolled growth of ice. Here, we report on an uncharacteristically powerful bacterial AFP found in an Antarctic strain of the bacterium, Marinomonas primoryensis. It is Ca(2+)-dependent, shows evidence of cooperativity, and can produce over 2 degrees C of freezing point depression. Unlike most AFPs, it does not produce obvious crystal faceting during thermal hysteresis. This AFP might be capable of imparting freezing avoidance to M. primoryensis in ice-covered Antarctic lakes. A hyperactive bacterial AFP has not previously been reported.

Digging deeper into the plant cell wall proteome

The proteome of the plant cell wall/apoplast is less well characterized than those of other subcellular compartments. This largely reflects the many technical challenges involved in extracting and identifying extracellular proteins, many of which resist isolation and identification, and in capturing a population that is both comprehensive and relatively uncontaminated with intracellular proteins. However, a range of disruptive techniques, involving tissue homogenization and subsequent sequential extraction and non-disruptive approaches has been developed. These approaches have been complemented more recently by other genome-scale screens, such as secretion traps that reveal the genes encoding proteins with N-terminal signal peptides that are targeted to the secretory pathway, many of which are subsequently localized in the wall. While the size and complexity of the wall proteome is still unresolved, the combination of experimental tools and computational prediction is rapidly expanding the catalog of known wall-localized proteins, suggesting the unexpected extracellular localization of other polypeptides and providing the basis for further exploration of plant wall structure and function.

Antifreeze proteins in overwintering plants: a tale of two activities

Antifreeze proteins are found in a wide range of overwintering plants where they inhibit the growth and recrystallization of ice that forms in intercellular spaces. Unlike antifreeze proteins found in fish and insects, plant antifreeze proteins have multiple, hydrophilic ice-binding domains. Surprisingly, antifreeze proteins from plants are homologous to pathogenesis-related proteins and also provide protection against psychrophilic pathogens. In winter rye (Secale cereale), antifreeze proteins accumulate in response to cold, short daylength, dehydration and ethylene, but not pathogens. Transferring single genes encoding antifreeze proteins to freezing-sensitive plants lowered their freezing temperatures by approximately 1 degrees C. Genes encoding dual-function plant antifreeze proteins are excellent models for use in evolutionary studies to determine how genes acquire new expression patterns and how proteins acquire new activities.