Purification and partial characterization of a glycoprotein from pea (Pisum sativum) with receptor activity for rhicadhesin, an attachment protein of Rhizobiaceae

OsGLP8-7 interacts with OsPRX111 to detoxify excess copper in rice

Lignin is a phenolic biopolymer generated from phenylpropanoid pathway in the secondary cell wall and is required for defense of plants against various stress. Although the fact of stress-induced lignin deposition has been clearly demonstrated, it remains largely elusive how the formation of lignin is promoted under Cu stress. The present study showed that OsGLP8-7, an extracellular glycoprotein of rice (Oryza sativa L.), plays an important function against Cu stress. The loss function of OsGLP8-7 results in Cu sensitivity whereas overexpression of OsGLP8-7 scavenges Cu-induced superoxide anion (O2•-). OsGLP8-7 interacts with apoplastic peroxidase111 (OsPRX111) and elevates OsPRX111 stability when exposed to excess Cu. In OsGLP8-7 overexpressing (OE) lines, the retention of Cu within cell wall limiting Cu uptake into cytoplasm is attributed to the enhanced lignification required for Cu tolerance. Exogenous application of a lignin inhibitor can impair the Cu tolerance of transgenic Arabidopsis lines overexpressing OsGLP8-7. In addition, co-expression of OsGLP8-7 and OsPRX111 genes in tobacco leaves leads to an improved lignin deposition compared to leaves expressing each gene individually or the empty vector. Taken together, our findings provided the convincing evidences that the interaction between OsGLP8-7 and OsPRX111 facilitates effectively lignin polymerization, thereby contributing to Cu tolerance in rice.

Unveiling the Secrets of Calcium-Dependent Proteins in Plant Growth-Promoting Rhizobacteria: An Abundance of Discoveries Awaits

The role of Calcium ions (Ca2+) is extensively documented and comprehensively understood in eukaryotic organisms. Nevertheless, emerging insights, primarily derived from studies on human pathogenic bacteria, suggest that this ion also plays a pivotal role in prokaryotes. In this review, our primary focus will be on unraveling the intricate Ca2+ toolkit within prokaryotic organisms, with particular emphasis on its implications for plant growth-promoting rhizobacteria (PGPR). We undertook an in silico exploration to pinpoint and identify some of the proteins described in the existing literature, including prokaryotic Ca2+ channels, pumps, and exchangers that are responsible for regulating intracellular Calcium concentration ([Ca2+]i), along with the Calcium-binding proteins (CaBPs) that play a pivotal role in sensing and transducing this essential cation. These investigations were conducted in four distinct PGPR strains: Pseudomonas chlororaphis subsp. aurantiaca SMMP3, P. donghuensis SVBP6, Pseudomonas sp. BP01, and Methylobacterium sp. 2A, which have been isolated and characterized within our research laboratories. We also present preliminary experimental data to evaluate the influence of exogenous Ca2+ concentrations ([Ca2+]ex) on the growth dynamics of these strains.

A Germin-Like Protein GLP1 of Legumes Mediates Symbiotic Nodulation by Interacting with an Outer Membrane Protein of Rhizobia

Rhizobia can infect legumes and induce the coordinated expression of symbiosis and defense genes for the establishment of mutualistic symbiosis. Numerous studies have elucidated the molecular interactions between rhizobia and host plants, which are associated with Nod factor, exopolysaccharide, and T3SS effector proteins. However, there have been relatively few reports about how the host plant recognizes the outer membrane proteins (OMPs) of rhizobia to mediate symbiotic nodulation. In our previous work, a gene (Mhopa22) encoding an OMP was identified in Mesorhizobium huakuii 7653R, whose homologous genes are widely distributed in Rhizobiales. In this study, a germin-like protein GLP1 interacting with Mhopa22 was identified in Astragalus sinicus. RNA interference of AsGLP1 resulted in a decrease in nodule number, whereas overexpression of AsGLP1 increased the number of nodules in the hairy roots of A. sinicus. Consistent symbiotic phenotypes were identified in Medicago truncatula with MtGLPx (refer to medtr7g111240.1, the isogeny of AsGLP1) overexpression or Tnt1 mutant (glpx-1) in symbiosis with Sinorhizobium meliloti 1021. The glpx-1 mutant displayed hyperinfection and the formation of more infection threads but a decrease in root nodules. RNA sequencing analysis showed that many differentially expressed genes were involved in hormone signaling and symbiosis. Taken together, AsGLP1 and its homology play an essential role in mediating the early symbiotic process through interacting with the OMPs of rhizobia. IMPORTANCE This study is the first report to characterize a legume host plant protein to sense and interact with an outer membrane protein (OMP) of rhizobia. It can be speculated that GLP1 plays an essential role to mediate early symbiotic process through interacting with OMPs of rhizobia. The results provide deeper understanding and novel insights into the molecular interactive mechanism of a legume symbiosis signaling pathway in recognition with rhizobial OMPs. Our findings may also provide a new perspective to improve the symbiotic compatibility and nodulation of legume.

The wheat secreted root proteome: Implications for phosphorus mobilisation and biotic interactions

Root secreted acid phosphatases and organic anions are widely perceived as major players of plant phosphorus (P) mobilisation from the rhizosphere under P limiting growth conditions. Previous research indicated that other mechanisms play a role, especially in species with fine roots, such as wheat. In this study we characterised the plant-derived extracellular proteome of wheat roots by profiling root tip mucilage, soluble root secreted and root tip proteomes. Extracellular acid phosphatases and enzymes of the central carbon metabolism were targeted using selected reaction monitoring. More than 140 proteins with extracellular localisation prediction were identified in mucilage. P starvation induced proteins predicted to be localised to the apoplast which are related to cell wall modification and defence in both, root tip and soluble root-secreted proteomes. Glycolytic enzymes were strongly increased in abundance by P limitation in root tips, as were PEPC and plastidial MDH. Soluble acid phosphatases were not identified in extracellular protein samples. Our results indicate that root tip mucilage contains proteins with the functional potential to actively shape their immediate environment by modification of plant structural components and biotic interactions. Wheat acid phosphatases appear to play a minor role in P mobilisation beyond the immediate root surface. SIGNIFICANCE: Phosphorus (P) is a plant growth limiting nutrient in many agricultural situations and the development of phosphorus efficient crops is of paramount importance for future agricultural management practices. As P is relatively immobile in soils, processes occurring at the root-soil interface, the rhizosphere, are suspected to play a key role in plant-induced P mobilisation. According to the current view, the secretion of extracellular acid phosphatases and organic anions enhances P mobilisation within several millimetres beyond the root surface, either directly or indirectly through the selection and appropriate soil microbes. However, the mechanisms of P mobilisation in species with fine roots, such as wheat, and the role of other secreted root proteins are poorly understood. Here, we carried out the profiling of wheat root tip mucilage, soluble root secreted and root tip proteomes. We analysed proteome changes in response to P starvation. We found that proteins with a predicted localisation to the apoplast made up a major proportion of stress-responsive proteins. Acid phosphatases were not identified within extracellular protein samples, which were enriched in proteins with predicted extracellular localisation. The absence of extracellular APases was further validated by multiple reaction monitoring. Our data indicates that wheat acid phosphatases play a minor role in P mobilisation beyond the immediate root surface and provides a resource for breeding strategies and further investigations of the functional roles of root tip-released proteins in the rhizosphere under P limitation.

The rice germin-like protein OsGLP1 participates in acclimation to UV-B radiation

Exposure to ultraviolet B radiation (UV-B) stress can have serious effects on the growth and development of plants. Germin-like proteins (GLPs) may be involved in different abiotic and biotic stress responses in different plants, but little is known about the role of GLPs in UV-B stress response and acclimation in plants. In the present study, knockout of GLP 8-14 (OsGLP1) using the CRISPR/Cas9 system resulted in mutant rice (Oryza sativa L.) plants (herein called glp1) that exhibited UV-B-dependent formation of lesion mimic in leaves. Moreover, glp1 grown under solar radiation (including UV-B) showed decreased plant height and increased leaf angle, but we observed no significant differences in phenotypes between wild-type (WT) plants and glp1 grown under artificial light lacking UV-B. Fv/Fm, Y (II) and the expression of many genes, based on RNA-seq analysis, related to photosynthesis were also only reduced in glp1, but not in WT, after transfer from a growth cabinet illuminated with artificial white light lacking UV-B to growth under natural sunlight. The genes-associated with flavonoid metabolism as well as UV resistance locus 8 (OsUVR8), phytochrome interacting factor-like 15-like (OsPIF3), pyridoxal 5'-phosphate synthase subunit PDX1.2 (OsPDX1.2), deoxyribodipyrimidine photolyase (OsPHR), and deoxyribodipyrimidine photolyase family protein-like (OsPHRL) exhibited lower expression levels, while higher expression levels of mitogen-activated protein kinase 5-like (OsMPK3), mitogen-activated protein kinase 13-like (OsMPK13), and transcription factor MYB4-like (OsMYB4) were observed in glp1 than in WT after transfer from a growth cabinet illuminated with artificial white light to growth under natural sunlight. Therefore, mutations in OsGLP1 resulted in rice plants more sensitive to UV-B and reduced expression of some genes for UV-B protection, suggesting that OsGLP1 is involved in acclimation to UV-B radiation.

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

The intracellular infection thread initiated in a root hair cell is a unique structure associated with Rhizobium-legume symbiosis. It is characterized by inverted tip growth of the plant cell wall, resulting in a tunnel that allows invasion of host cells by bacteria during the formation of the nitrogen-fixing root nodule. Regulation of the plant-microbial interface is essential for infection thread growth. This involves targeted deposition of the cell wall and extracellular matrix and tight control of cell wall remodeling. This review describes the potential role of different actors such as transcription factors, receptors, and enzymes in the rearrangement of the plant-microbial interface and control of polar infection thread growth. It also focuses on the composition of the main polymers of the infection thread wall and matrix and the participation of reactive oxygen species (ROS) in the development of the infection thread. Mutant analysis has helped to gain insight into the development of host defense reactions. The available data raise many new questions about the structure, function, and development of infection threads.

Wheat germin-like protein: Studies on chitin/chitosan matrix for tissue engineering applications

Advances in tissue engineering require the development of new biomaterials with adequate properties of cell attachment and growth. The properties of biomaterials can be improved by incorporation of bioactive molecules to enhance in vitro and/or in vivo functions. In this work, we study the role of a wheat germin-like protease inhibitor (GLPI), free or immobilized in biocompatible matrices to improve cell-attachment ability on different mammalian cell lines. The phylogenetic relationships and functional diversity of the GLPI were analyzed among diverse genera to get insights into sequence motif conservations. The cytocompatibility effect of free GLPI on C2C12 premyoblastic cells and B16 cells as tumoral model has been tested. GLPI promoted proliferation and metabolic activity of both cell types on in vitro models, not showing cytotoxic effects. Furthermore, GLPI was immobilized in chitin microparticles and in chitosan films; we demonstrated an accelerated cell adhesion process in both biomaterials.

GhABP19, a Novel Germin-Like Protein From Gossypium hirsutum, Plays an Important Role in the Regulation of Resistance to Verticillium and Fusarium Wilt Pathogens

Germin-like proteins (GLPs) are water-soluble plant glycoproteins belonging to the cupin superfamily. The important role of GLPs in plant responses against various abiotic and biotic stresses, especially pathogens, is well validated. However, little is known about cotton GLPs in relation to fungal pathogens. Here, a novel GLP gene was isolated from Gossypium hirsutum and designated as GhABP19. The expression of GhABP19 was upregulated in cotton plants inoculated with Verticillium dahliae and Fusarium oxysporum and in response to treatment with jasmonic acid (JA) but was suppressed in response to salicylic acid treatment. A relatively small transient increase in GhABP19 was seen in H2O2 treated samples. The three-dimensional structure prediction of the GhABP19 protein indicated that the protein has three histidine and one glutamate residues responsible for metal ion binding and superoxide dismutase (SOD) activity. Purified recombinant GhABP19 exhibits SOD activity and could inhibit growth of V. dahliae, F. oxysporum, Rhizoctonia solani, Botrytis cinerea, and Valsa mali in vitro. To further verify the role of GhABP19 in fungal resistance, GhABP19-overexpressing Arabidopsis plants and GhABP19-silenced cotton plants were developed. GhABP19-transgenic Arabidopsis lines showed much stronger resistance to V. dahliae and F. oxysporum infection than control (empty vector) plants did. On the contrary, silencing of GhABP19 in cotton conferred enhanced susceptibility to fungal pathogens, which resulted in necrosis and wilt on leaves and vascular discoloration in GhABP19-silenced cotton plants. The H2O2 content and endogenous SOD activity were affected by GhABP19 expression levels in Arabidopsis and cotton plants after inoculation with V. dahliae and F. oxysporum, respectively. Furthermore, GhABP19 overexpression or silencing resulted in activation or suppression of JA-mediated signaling, respectively. Thus, GhABP19 plays important roles in the regulation of resistance to verticillium and fusarium wilt in plants. These modulatory roles were exerted by its SOD activity and ability to activate the JA pathway. All results suggest that GhABP19 was involved in plant disease resistance.

Improvement in Agrobacterium-mediated transformation of chickpea (Cicer arietinum L.) by the inhibition of polyphenolics released during wounding of cotyledonary node explants

Agrobacterium-mediated transformation of chickpea (Cicer arietinum L.) has been performed using cotyledonary node explants (CNs), which release phenolics upon excision that are detrimental to the viability of Agrobacterium tumefaciens and result in low transformation frequency. Twelve low molecular weight phenolic compounds and salicylic acid were identified in the exudates released upon excision during the preparation of cotyledonary nodes by reverse phase high-performance liquid chromatography (RP-HPLC). Zone inhibition assays performed with the explant exudates released at periodic intervals after excision showed the inhibition of A. tumefaciens. Agroinoculation of freshly excised cotyledonary nodes of chickpea showed 98-99 % inhibition of colony forming units (cfu). Osmium tetraoxide fixation of excised tissues showed enhanced accumulation of phenolics in the sub-epidermal regions causing enzymatic browning, affecting the viability and performance of A. tumefaciens for T-DNA delivery. The periodic analysis of exudates released from excised CNs showed enhanced levels of gallic acid (0.2945 ± 0.014 μg/g), chlorogenic acid (0.0978 ± 0.0046 μg/g), and quercetin (0.0971 ± 0.0046 μg/g) fresh weight, which were detrimental to A. tumefaciens. Quantitative assays and the elution profile showed the maximum leaching of phenolics, flavonoids, and salicylic acid immediately after the excision of explants and continued till 4 to 8 h post-excision. Pre-treatment of excised explants with inhibitors of polyphenol oxidase like L-cysteine, DTT, and sodium thiosulfate before co-cultivation showed the recovery of A. tumefaciens cfu, decreased the accumulation of phenolics, and improved transformation frequency. Our results show the hypersensitive response of excision stress for the expression of defense response-related genes and synthesis of metabolites in grain legume chickpea against pathogen infestation including Agrobacterium.

Laser Microdissection of Grapevine Leaves Reveals Site-Specific Regulation of Transcriptional Response to Plasmopara viticola

Grapevine is one of the most important fruit crops in the world, and it is highly susceptible to downy mildew caused by the biotrophic oomycete Plasmopara viticola. Gene expression profiling has been used extensively to investigate the regulation processes of grapevine-P. viticola interaction, but all studies to date have involved the use of whole leaves. However, only a small fraction of host cells is in contact with the pathogen, so highly localized transcriptional changes of infected cells may be masked by the large portion of non-infected cells when analyzing the whole leaf. In order to understand the transcriptional regulation of the plant reaction at the sites of pathogen infection, we optimized a laser microdissection protocol and analyzed the transcriptional changes in stomata cells and surrounding areas of grapevine leaves at early stages of P. viticola infection. The results indicate that the expression levels of seven P. viticola-responsive genes were greater in microdissected cells than in whole leaves, highlighting the site-specific transcriptional regulation of the host response. The gene modulation was restricted to the stomata cells and to the surrounding areas of infected tissues, indicating that the host response is mainly located at the infection sites and that short-distance signals are implicated. In addition, due to the high sensitivity of the laser microdissection technique, significant modulations of three genes that were completely masked in the whole tissue analysis were detected. The protocol validated in this study could greatly increase the sensitivity of further transcriptomic studies of the grapevine-P. viticola interaction.

Proteomics insights into the basis of interspecific facilitation for maize (Zea mays) in faba bean (Vicia faba)/maize intercropping

Faba bean/maize intercropping significantly promotes maize productivity in phosphorus-deficient soils. This has been attributed to the below-ground interactions including rhizosphere effects and spatial effects. Nevertheless, the molecular mechanisms underlying these interactions have been scarcely investigated. Here, three types of pots were used to distinguish the influences of rhizosphere effects vs. spatial effects. Phosphorus and nitrogen uptake of shoots, biomass, total root length, and root classification were evaluated between the three treatments. Quantitative RT-PCR and proteomics analyses were conducted to investigate the putative components in the molecular basis of these interactions. Quantitative RT-PCR results indicated that rhizosphere effects promoted maize phosphorus status at molecular levels. 66 differentially accumulated protein spots were successfully identified through proteomics analyses. Most of the protein species were found to be involved in phosphorus, nitrogen, and allelochemical metabolism, signal transduction, or stress resistance. The results suggest that rhizosphere effects promoted phosphorus and nitrogen assimilation in maize roots and thus enhanced maize growth and nutrient uptake. The reprogramming of proteome profiles suggests that rhizosphere effects can also enhance maize tolerance through regulating the metabolism of allelochemicals and eliciting systemic acquired resistance via the stimulation of a mitogen-activated protein kinase signal pathway.

BIOLOGICAL SIGNIFICANCE

The results obtained contribute to a comprehensive understanding of the response of maize to the changes of rhizosphere condition influenced by the below-ground interactions in faba bean/maize intercropping at molecular levels. The identified protein species involved in nutrient metabolisms and stress resistance reveal the molecular basis underlying the major advantages of effective nutrient utilization and higher stress tolerance in legume/cereal intercropping systems. This work provides essential new insights into the putative components in the molecular basis of interspecific facilitation for maize in faba bean/maize intercropping.

Developmental stimuli and stress factors affect expression of ClGLP1, an emerging allergen-related gene in Citrus limon

Germins and germin-like proteins (GLPs) constitute an ubiquitous family of plant proteins that seem to be involved in many developmental and stress related processes. A novel GLP cDNA was isolated from Citrus limon and structural features and genomic organization were investigated by in silico and Southern blots analysis. In lemon, the ClGLP1 encodes a 24.38 kDa which possesses a conserved motif of plant GLPs proteins. A phylogetic analysis mapped ClGLP1 as belonging to the GER3 subfamily into the GLP1 group of large GLP family. ClGLP1 was differentially expressed in the various organs and was highest in mature fruit. Moreover, expression in the fruit was tissue- and stage-related as well as dependent on agricultural practice (organic vs conventional). ClGLP1 transcripts increased during the transition from the green (180 days after blooming) to the yellow (240 days after blooming) mature fruit and were strongly enhanced in yellow mature fruit from organic compared with conventional culture. A sudden and systemic increase in ClGLP1 expression level was observed in leaves injured by wounding, together with an increase of endogenous H2O2 amount. Notably, an enhancement of H202 was observed in fruit peel during transition from green to yellow fruit stage. All together our data showed that ClGLP1 expression can be modulated in relation to both developmental stimuli and culture practices; evidence is also provided that through an oxidase activity this gene could play a role in fruit maturation as well as in stress responses.

Tackling heterogeneity: a leaf disc-based assay for the high-throughput screening of transient gene expression in tobacco

Transient Agrobacterium-mediated gene expression assays for Nicotiana tabacum (N. tabacum) are frequently used because they facilitate the comparison of multiple expression constructs regarding their capacity for maximum recombinant protein production. However, for three model proteins, we found that recombinant protein accumulation (rpa) was significantly influenced by leaf age and leaf position effects. The ratio between the highest and lowest amount of protein accumulation (max/min ratio) was found to be as high as 11. Therefore, construct-based impacts on the rpa level that are less than 11-fold will be masked by background noise. To address this problem, we developed a leaf disc-based screening assay and infiltration device that allows the rpa level in a whole tobacco plant to be reliably and reproducibly determined. The prototype of the leaf disc infiltration device allows 14 Agrobacterium-mediated infiltration events to be conducted in parallel. As shown for three model proteins, the average max/min rpa ratio was reduced to 1.4 using this method, which allows for a sensitive comparison of different genetic elements affecting recombinant protein expression.

Regulation of two germin-like protein genes during plum fruit development

Germin-like proteins (GLPs) have several proposed roles in plant development and defence. Two novel genes (Ps-GLP1 and 2) encoding germin-like protein were isolated from plum (Prunus salicina). Their regulation was studied throughout fruit development and during ripening of early and late cultivars. These two genes exhibited similar expression patterns throughout the various stages of fruit development excluding two important stages, pit hardening (S2) and fruit ripening (S4). During fruit development until the ripening phase, the accumulation of both Ps-GLPs is related to the evolution of auxin. However, during the S2 stage only Ps-GLP1 is induced and this could putatively be in a H(2)O(2)-dependent manner. On the other hand, the diversity in the Ps-GLPs accumulation profile during the ripening process seems to be putatively due to the variability of endogenous auxin levels among the two plum cultivars, which consequently change the levels of autocatalytic ethylene available for the fruit to co-ordinate ripening. The effect of auxin on stimulating ethylene production and in regulating Ps-GLPs transcripts was also investigated. These data, supported by their localization in the extracellular matrix, suggest that auxin is somehow involved in the regulation of both transcripts throughout fruit development and ripening.

Plant proteins involved in Agrobacterium-mediated genetic transformation

Agrobacterium species genetically transform plants by transferring a region of plasmid DNA, T-DNA, into host plant cells. The bacteria also transfer several virulence effector proteins. T-DNA and virulence proteins presumably form T-complexes within the plant cell. Super-T-complexes likely also form by interaction of plant-encoded proteins with T-complexes. These protein-nucleic acid complexes traffic through the plant cytoplasm, enter the nucleus, and eventually deliver T-DNA to plant chromatin. Integration of T-DNA into the plant genome establishes a permanent transformation event, permitting stable expression of T-DNA-encoded transgenes. The transformation process is complex and requires participation of numerous plant proteins. This review discusses our current knowledge of plant proteins that contribute to Agrobacterium-mediated transformation, the roles these proteins play in the transformation process, and the modern technologies that have been employed to elucidate the cell biology of transformation.

Role of vitronectin-like protein in Agrobacterium attachment and transformation of Arabidopsis cells

The role of plant vitronectin-like protein (Vn) in Agrobacterium-host plant interactions and receptor-specific bacterial attachment is unclear and still open to debate. Using a well-established Agrobacterium-mediated Arabidopsis transformation system, the marker gene beta-glucuronidase (GUS) of Escherichia coli, and biochemical and cytological methods, such as ELISA tests, immunoblots, immunolocalization, and functional in vitro binding assays, we have reassessed the role of Vn in receptor-specific bacterial attachment and transformation. We provide evidence that Vn is present in the host plant cells and anti-human vitronectin antibody cross-reacts with a 65-kDa protein from Arabidopsis cells. The specificity of the immunological cross-reactivity of anti-vitronectin antibodies was further demonstrated by ELISA competition experiments. Immunogold labeling showed that Vn is localized in the plant cell wall, and its level increased considerably after phytohormone treatment of the petiole explants. However, Agrobacterium attachment was unaffected, and no inhibition of petiole cell transformation was detected in the presence of human vitronectin and anti-vitronectin antibodies in the media. Additionally, no correlation between the occurrence of Vn, attachment of bacteria to the cells, and susceptibility to Agrobacterium-mediated transformation was observed. Taken together, our data do not support a functional role of plant Vn as the receptor for site-specific Agrobacterium attachment leading to the transformation of Arabidopsis cells.

Induction of a grapevine germin-like protein (VvGLP3) gene is closely linked to the site of Erysiphe necator infection: a possible role in defense?

Germin-like proteins (GLP) have various proposed roles in plant development and defense. Seven novel GLP cDNA clones were isolated from grapevine (Vitis vinifera cv. Chardonnay). Reverse transcriptase-polymerase chain reaction expression analysis revealed that the VvGLP genes exhibit diverse and highly specific patterns of expression in response to a variety of abiotic and biotic treatments, including challenge by Erysiphe necator, Plasmopara viticola, and Botrytis cinerea, suggesting a diversity of roles for each of the GLP family members. Significantly, one of the grapevine GLP genes, VvGLP3, is induced specifically by E. necator infection and expression is closely linked to the site of infection. Subcellular localization of VvGLP3 determined by transient expression of a VvGLP3:GFP fusion construct in onion cells indicated that the recombinant protein was targeted to the cell wall. Recombinant VvGLP3 was successfully expressed in Arabidopsis thaliana and the partially purified recombinant protein was demonstrated to have superoxide dismutase activity. This data has provided an insight into the diverse nature of the GLP family in grapevine and suggests that VvGLP3 may be involved in the defense response against E. necator.

Biological systems of the host cell involved in Agrobacterium infection

Genetic transformation of plants by Agrobacterium, which in nature causes neoplastic growths, represents the only known case of trans-kingdom DNA transfer. Furthermore, under laboratory conditions, Agrobacterium can also transform a wide range of other eukaryotic species, from fungi to sea urchins to human cells. How can the Agrobacterium virulence machinery function in such a variety of evolutionarily distant and diverse species? The answer to this question lies in the ability of Agrobacterium to hijack fundamental cellular processes which are shared by most eukaryotic organisms. Our knowledge of these host cellular functions is critical for understanding the molecular mechanisms that underlie genetic transformation of eukaryotic cells. This review outlines the bacterial virulence machinery and provides a detailed discussion of seven major biological systems of the host cell-cell surface receptor arrays, cellular motors, nuclear import, chromatin targeting, targeted proteolysis, DNA repair, and plant immunity--thought to participate in the Agrobacterium-mediated genetic transformation.

Elevated atmospheric CO2 and strain of rhizobium alter freezing tolerance and cold-induced molecular changes in alfalfa (Medicago sativa)

BACKGROUND AND AIMS

The objective of the study was to assess the impact of elevated CO2 in interaction with rhizobial strains on freezing tolerance and cold-induced molecular changes in alfalfa.

METHODS

Alfalfa inoculated with two different strains of rhizobium (A2 and NRG34) was grown and cold acclimated (2 weeks at 2 degrees C) under either 400 (ambient) or 800 micromol mol(-1) (elevated) CO2.

KEY RESULTS

Plants acclimated under 400 micromol mol(-1) CO2 were more freezing tolerant than those maintained under 800 micromol mol(-1). Cryoprotective sugars typically linked with the acquisition of freezing tolerance such as sucrose, stachyose and raffinose increased in roots in response to low temperature but did not differ between CO2 treatments. Similarly high CO2 did not alter the expression of many cold-regulated (COR) genes although it significantly increased the level of transcripts encoding a COR gene homologous to glyceraldehyde-3-phosphate-dehydrogenase (GAPDH). A significant effect of rhizobial strain was observed on both freezing tolerance and gene expression. Plants of alfalfa inoculated with strain A2 were more freezing tolerant than those inoculated with strain NRG34. Transcripts of COR genes homologous to a pathogenesis-related protein (PR-10) and to a nuclear-targeted protein were markedly enhanced in roots of alfalfa inoculated with strain A2 as compared with strain NRG34. Transcripts encoding the vegetative storage proteins (VSPs) beta-amylase and chitinase were more abundant in roots of non-acclimated plants inoculated with strain NRG34 than with strain A2.

CONCLUSIONS

Taken together, the results suggest that elevated CO2 stimulates plant growth and reduces freezing tolerance. The acquisition of cold tolerance is also influenced by the rhizobial strain, as indicated by lower levels of expression of COR genes and sustained accumulation of VSP-encoding transcripts in alfalfa inoculated with strain NRG34 as compared with strain A2.

Unwounded plants elicit Agrobacterium vir gene induction and T-DNA transfer: transformed plant cells produce opines yet are tumour free

Agrobacterium tumefaciens is well known to cause crown gall tumours at plant wound sites and to benefit from this plant association by obtaining nutrients called opines that are produced by these tumours. Tumourigenesis requires expression of the vir regulon in response to chemical signals that are thought to be released from wound sites. Here, we examine chemical interactions between A. tumefaciens and unwounded plants. To determine whether unwounded plants can release significant amounts of vir gene inducers, we constructed an A. tumefaciens strain carrying a PvirB-gfp fusion. This fusion was strongly induced by co-culture with tobacco seedlings that have been germinated without any intentional wounding. The release of phenolic vir gene inducers was confirmed by GC/MS analysis. We also constructed a strain containing the gfp reporter located on an artificial T-DNA and expressed from a plant promoter. A. tumefaciens efficiently transferred this T-DNA into cells of unwounded plants in the absence of exogenous vir gene inducers. Many cells of seedlings colonized by the bacteria also produced octopine, which was detected using a Pocc-gfp reporter strain. This indicates transfer of the native T-DNA. However, these transformed plant cells did not form tumours. These results suggest that successful colonization of plants by A. tumefaciens, including T-DNA transfer and opine production, does not require wounding and does not necessarily cause cell proliferation. Transformation of plant cells without inciting tumours may represent a colonization strategy for this pathogen that has largely been overlooked.

QTLs controlling the production of transgenic and adventitious roots in Brassica oleracea following treatment with Agrobacterium rhizogenes

Brassica oleracea can be genetically engineered using Agrobacterium rhizogenes. The initial stage of this process is the production of transgenic ('hairy') roots; shoots are subsequently regenerated from these roots. Previous work using gus and gfp reporter genes has shown that genotypes of B. oleracea vary in their performance for transgenic root production. Quantitative trait loci (QTLs) controlling this trait have been located in one mapping population. The current study provides evidence that performance for transgenic root production is associated with performance for adventitious (non-transgenic) root production in B. oleracea across a second mapping population. This is shown by regression analyses between performance for the two traits and the demonstration that QTLs controlling the two traits map to the same positions within the genome. Since the rate of adventitious root production does not differ significantly in the presence and absence of A. rhizogenes, there is no evidence that the expression of Agrobacterium genes induces adventitious root production. It is apparent that genotypes exhibiting high adventitious root production in the absence of A. rhizogenes will also tend to show high transgenic root production, thereby allowing the selection of lines that are more efficiently transformed.

Detection of and response to signals involved in host-microbe interactions by plant-associated bacteria

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described.

Germin-like protein gene family of a moss, Physcomitrella patens, phylogenetically falls into two characteristic new clades

We identified 77 EST clones encoding germin-like proteins (GLPs) from a moss, Physcomitrella patens in a database search. These Physcomitrella GLPs ( PpGLP s) were separated into seven groups based on DNA sequence homology. Phylogenetic analysis showed that these groups were divided into two novel clades clearly distinguishable from higher plant germins and GLPs, named bryophyte subfamilies 1 and 2. PpGLPs belonging to bryophyte subfamilies 1 lacked two cysteines at the conserved positions observed in higher plant germins or GLPs. PpGLPs belonging to bryophyte subfamily 2 contained two cysteines as observed in higher plant germins and GLPs. In bryophyte subfamily 1, 12 amino acids, in which one of two cysteines is included, were deleted between boxes A and B. Further, we determined the genomic structure of all of seven PpGLP genes. The sequences of PpGLP s of bryophyte subfamily 1 contained one or two introns, whereas those of bryophyte subfamily 2 contained no introns. Other GLPs from bryophytes, a liverwort GLP from Marchantia polymorpha , and two moss GLPs from Barbula unguiculata and Ceratodon purpureus also fell into bryophyte subfamily 1 and bryophyte subfamily 2, respectively. No higher plant germins and GLPs were grouped into the bryophyte subfamilies 1 and 2 by our analysis. Moreover, we revealed that PpGLP6 had manganese-containing extracellular superoxide dismutase activity. These results indicated that bryophyte possess characteristic GLPs, which phylogenetically are clearly distinguishable from higher plant GLPs.

Proteome analysis differentiates between two highly homologues germin-like proteins in Arabidopsis thaliana ecotypes Col-0 and Ws-2

A proteome approach based on 2-D gel electrophoresis (2-DE) was used to compare the protein patterns of the Arabidopsis ecotypes Col-0 and Ws-2. In leaf extracts a pair of protein spots were found to be diagnostic for each of the lines. Both pairs of spots were identified as closely related germin-like proteins differing in only one amino acid by using peptide mass finger printing of tryptic digests and by gaining additional data from post-source decay spectra in the MALDI-TOF analysis. Western blot analysis after separation of protein extracts by 2-DE confirmed results from Coomassie blue-stained gels and revealed additional immunoreactive spots for both ecotypes most likely representing dimers of the spots first identified. Western blot analysis and mass spectrometrical identification of the corresponding weakly stained protein in Coomassie blue-stained gels of the ecotype Col-0 also demonstrated for the first time the occurrence of AtGER3 protein in root extracts. Our results demonstrate the capacity of proteome analysis to analyse and distinguish closely related members of large protein families.

A member of the germin-like protein family is a highly conserved mycorrhiza-specific induced gene

A Medicago truncatula cDNA encoding a germin-like protein (GLP) was isolated from a suppression subtractive hybridization cDNA library enriched for arbuscular mycorrhiza (AM)-induced genes. The MtGLP1 amino acid sequence shows some striking differences to previously described plant GLP sequences and might therefore represent a new subgroup of this multigene family. The MtGlp1 mRNA was strongly induced in roots and root cultures colonized by the AM fungus Glomus intraradices. Whereas MtGlp1 is strongly induced in AM, no transcripts of the gene were detected in non-infected roots or in roots after infection with the oomycete root pathogen Aphanomyces euteiches or with Rhizobia. Increased phosphate levels during fertilization also could not stimulate MtGlp1 transcription. Hence, MtGlp1 induction seems to be an AM-specific phenomenon. In situ hybridization showed that MtGlp1 is localized in arbuscule containing cells. A putative orthologue of this AM-specific GLP gene could be localized in a second legume Lotus japonicus, indicating that the regulation of a member of the GLP family belongs to a conserved mechanism in AM regulation in different plant species.

Ring up the curtain on DING proteins

DING proteins have a characteristic DINGGG- or closely related N-terminal sequence. One is found in human synovial fluid, and may be associated with rheumatoid arthritis. Other examples have receptor or signalling roles in various human and animal cells, or are involved in biomineralisation, and several of them bind to phytochemicals. As plant DING proteins have recently been discovered, we hypothesise that the DING protein-phytochemical association may represent one aspect of a ubiquitous receptor-linked signalling system. Several microbial proteins related to DING proteins have phosphatase activity, which may relate to biomineralisation in eukaryotic systems. Plant DING proteins and their microbial relatives may elicit allergic responses leading to arthritic disease.

Partners-in-infection: host proteins involved in the transformation of plant cells by Agrobacterium

Genetic modification of plant cells by Agrobacterium is the only known natural example of DNA transport between kingdoms. While the bacterial factors involved in Agrobacterium infection have been relatively well characterized, studies of their host cellular partners are just beginning. Here, we describe the plant cell factors that might participate in Agrobacterium-mediated genetic transformation and discuss their possible roles in this process. Because Agrobacterium probably adapts existing cellular processes for its life cycle, identifying the host factors participating in Agrobacterium infection might contribute to a better understanding of such basic biological processes as cell communication, intracellular transport and DNA repair and recombination as well as help expand the host range of Agrobacterium as a genetic engineering tool.

Molecular determinants of rhizosphere colonization by Pseudomonas

Rhizosphere colonization is one of the first steps in the pathogenesis of soilborne microorganisms. It can also be crucial for the action of microbial inoculants used as biofertilizers, biopesticides, phytostimulators, and bioremediators. Pseudomonas, one of the best root colonizers, is therefore used as a model root colonizer. This review focuses on (a) the temporal-spatial description of root-colonizing bacteria as visualized by confocal laser scanning microscopal analysis of autofluorescent microorganisms, and (b) bacterial genes and traits involved in root colonization. The results show a strong parallel between traits used for the colonization of roots and of animal tissues, indicating the general importance of such a study. Finally, we identify several noteworthy areas for future research.

AGROBACTERIUM AND PLANT GENES INVOLVED IN T-DNA TRANSFER AND INTEGRATION

The phytopathogenic bacterium Agrobacterium tumefaciens genetically transforms plants by transferring a portion of the resident Ti-plasmid, the T-DNA, to the plant. Accompanying the T-DNA into the plant cell is a number of virulence (Vir) proteins. These proteins may aid in T-DNA transfer, nuclear targeting, and integration into the plant genome. Other virulence proteins on the bacterial surface form a pilus through which the T-DNA and the transferred proteins may translocate. Although the roles of these virulence proteins within the bacterium are relatively well understood, less is known about their roles in the plant cell. In addition, the role of plant-encoded proteins in the transformation process is virtually unknown. In this article, I review what is currently known about the functions of virulence and plant proteins in several aspects of the Agrobacterium transformation process.

Microbial relatives of the seed storage proteins of higher plants: conservation of structure and diversification of function during evolution of the cupin superfamily

This review summarizes the recent discovery of the cupin superfamily (from the Latin term "cupa," a small barrel) of functionally diverse proteins that initially were limited to several higher plant proteins such as seed storage proteins, germin (an oxalate oxidase), germin-like proteins, and auxin-binding protein. Knowledge of the three-dimensional structure of two vicilins, seed proteins with a characteristic beta-barrel core, led to the identification of a small number of conserved residues and thence to the discovery of several microbial proteins which share these key amino acids. In particular, there is a highly conserved pattern of two histidine-containing motifs with a varied intermotif spacing. This cupin signature is found as a central component of many microbial proteins including certain types of phosphomannose isomerase, polyketide synthase, epimerase, and dioxygenase. In addition, the signature has been identified within the N-terminal effector domain in a subgroup of bacterial AraC transcription factors. As well as these single-domain cupins, this survey has identified other classes of two-domain bicupins including bacterial gentisate 1, 2-dioxygenases and 1-hydroxy-2-naphthoate dioxygenases, fungal oxalate decarboxylases, and legume sucrose-binding proteins. Cupin evolution is discussed from the perspective of the structure-function relationships, using data from the genomes of several prokaryotes, especially Bacillus subtilis. Many of these functions involve aspects of sugar metabolism and cell wall synthesis and are concerned with responses to abiotic stress such as heat, desiccation, or starvation. Particular emphasis is also given to the oxalate-degrading enzymes from microbes, their biological significance, and their value in a range of medical and other applications.

Arabidopsis ecotypes and mutants that are recalcitrant to Agrobacterium root transformation are susceptible to germ-line transformation

Germ-line transformation (vacuum infiltration) is frequently used to transform Arabidopsis thaliana using Agrobacterium tumefaciens. We have recently identified several Arabidopsis ecotypes and T-DNA-tagged mutants that are recalcitrant to Agrobacterium-mediated transformation of cut root segments. Some of these ecotypes and mutants are deficient in their ability to bind bacteria. Some are deficient in T-DNA integration. We report here that using a germ-line transformation protocol we transformed these ecotypes and mutants, including attachment- and integration-defective Arabidopsis plants, with a frequency similar to that of highly susceptible wild-type plants. However, we could not transform otherwise highly susceptible Arabidopsis plants by germ-line or root transformation using several vir and attachment-deficient Agrobacterium mutants. These results indicate that certain plant factors important for transformation may exist in germ-line tissue but may be lacking in some somatic cells.

The genetic and biochemical basis for nodulation of legumes by rhizobia

Soil bacteria of the genera Azorhizobium, Bradyrhizobium, and Rhizobium are collectively termed rhizobia. They share the ability to penetrate legume roots and elicit morphological responses that lead to the appearance of nodules. Bacteria within these symbiotic structures fix atmosphere nitrogen and thus are of immense ecological and agricultural significance. Although modern genetic analysis of rhizobia began less than 20 years ago, dozens of nodulation genes have now been identified, some in multiple species of rhizobia. These genetic advances have led to the discovery of a host surveillance system encoded by nodD and to the identification of Nod factor signals. These derivatives of oligochitin are synthesized by the protein products of nodABC, nodFE, NodPQ, and other nodulation genes; they provoke symbiotic responses on the part of the host and have generated immense interest in recent years. The symbiotic functions of other nodulation genes are nonetheless uncertain, and there remain significant gaps in our knowledge of several large groups of rhizobia with interesting biological properties. This review focuses on the nodulation genes of rhizobia, with particular emphasis on the concept of biological specificity of symbiosis with legume host plants.