Calmodulin interacts with MLO protein to regulate defence against mildew in barley

Breaking the code: Ca2+ sensors in plant signalling

Ca2+ ions play a vital role as second messengers in plant cells during various developmental processes and in response to environmental stimuli. Plants have evolved a diversity of unique proteins that bind Ca2+ using the evolutionarily conserved EF-hand motif. The currently held hypothesis is that these proteins function as Ca2+ sensors by undergoing conformational changes in response to Ca2+-binding that facilitate their regulation of target proteins and thereby co-ordinate various signalling pathways. The three main classes of these EF-hand Ca2+sensors in plants are CaMs [calmodulins; including CMLs (CaM-like proteins)], CDPKs (calcium-dependent protein kinases) and CBLs (calcineurin B-like proteins). In the plant species examined to date, each of these classes is represented by a large family of proteins, most of which have not been characterized biochemically and whose physiological roles remain unclear. In the present review, we discuss recent advances in research on CaMs and CMLs, CDPKs and CBLs, and we attempt to integrate the current knowledge on the different sensor classes into common physiological themes.

Calcium signaling in pathogenic and beneficial plant microbe interactions: what can we learn from the interaction between Piriformospora indica and Arabidopsis thaliana

Elevation of intracellular calcium levels in a plant cell is an early signaling event in many mutualistic and pathogenic plant/microbe interactions. In pathogenic plant/fungus interactions, receptor-mediated cytoplasmic calcium elevations induce defense genes via the activation of ion fluxes at the plasma membrane, an oxidative burst and MAPK activation. Mycorrhizal and beneficial endophytic plant/fungus interactions result in a better plant performance through sequencial cytoplasmic and nuclear calcium elevations. The specificity of the calcium responses depends on the calcium signature, its amplitude, duration, frequency and location, a selective activation of calcium channels in the diverse cellular membranes and the stimulation of calcium-dependent signaling components. Arabidopsis contains more than 100 genes for calcium-binding proteins and channels and the response to pathogens and beneficial fungi relies on a highly specific activation of individual members of these protein families. Genetic tools are required to understand this complex response patterns and the cross talks between the individual calcium-dependent signaling pathways. The beneficial interaction of Arabidopsis with the growth-promoting endophyte Piriformospora indica provides a nice model system to unravel signaling events leading to mutualistic or pathogenic plant/fungus interactions.

Two seven-transmembrane domain MILDEW RESISTANCE LOCUS O proteins cofunction in Arabidopsis root thigmomorphogenesis

Directional root expansion is governed by nutrient gradients, positive gravitropism and hydrotropism, negative phototropism and thigmotropism, as well as endogenous oscillations in the growth trajectory (circumnutation). Null mutations in phylogenetically related Arabidopsis thaliana genes MILDEW RESISTANCE LOCUS O 4 (MLO4) and MLO11, encoding heptahelical, plasma membrane-localized proteins predominantly expressed in the root tip, result in aberrant root thigmomorphogenesis. mlo4 and mlo11 mutant plants show anisotropic, chiral root expansion manifesting as tightly curled root patterns upon contact with solid surfaces. The defect in mlo4 and mlo11 mutants is nonadditive and dependent on light and nutrients. Genetic epistasis experiments demonstrate that the mutant phenotype is independently modulated by the Gbeta subunit of the heterotrimeric G-protein complex. Analysis of expressed chimeric MLO4/MLO2 proteins revealed that the C-terminal domain of MLO4 is necessary but not sufficient for MLO4 action in root thigmomorphogenesis. The expression of the auxin efflux carrier fusion, PIN1-green fluorescent protein, the pattern of auxin-induced gene expression, and acropetal as well as basipetal auxin transport are altered at the root tip of mlo4 mutant seedlings. Moreover, addition of auxin transport inhibitors or the loss of EIR1/AGR1/PIN2 function abolishes root curling of mlo4, mlo11, and wild-type seedlings. These results demonstrate that the exaggerated root curling phenotypes of the mlo4 and mlo11 mutants depend on auxin gradients and suggest that MLO4 and MLO11 cofunction as modulators of touch-induced root tropism.

Lipid determinants of cell death

Lipids produced by the epidermis serve a number of protective functions, and also act as messengers which activate plant defense responses. The fatty acid elongases which catalyze the formation of very long-chain fatty acids, may be instrumental in the remodeling of the various classes of epidermal lipids, and they also provide a means with which to further investigate the defense mechanisms. In a recent publication, we reported that the epidermal mis-expression of FATTY ACID ELONGASE1 (FAE1) in the Arabidopsis plant both increased the levels of very long-chain fatty acids in various lipid classes, and unexpectedly induced a cell-type specific cell death program in trichome cells, giving the plants a glabrous appearance. Using these plants as a model system for a fatty acid-induced cell death (lipoapoptosis), and a platform for the chemical genetic screen, we identified trichome death inhibitors in the glycerophospholipid fatty acyl remodeling pathway: phospholipase A2 inhibitors, aristolochic acid and bromoenol lactone, as well as the putative lysophospholipid acyltransferase inhibitor, clofibrate. Herein, and due to space limitations, we will briefly discuss these results and the different ways in which the appearance of increasing chain-length fatty acids is likely to regulate the cellular life-or-death switch. The death receptor hypothesis implies the existence of a bioactive lipid ligand(s), the functionality of which is determined by phosphorylation, acyl chain length and saturation.

Grapevine MLO candidates required for powdery mildew pathogenicity?

MLOs belong to the largest family of seven-transmembrane (7TM) domain proteins found in plants. The Arabidopsis and rice genomes contain 15 and 12 MLO family members, respectively. Although the biological function of most MLO family members remains elusive, a select group of MLO proteins have been demonstrated to negatively regulate defence responses to the obligate biotrophic pathogen, powdery mildew, thereby acting as “susceptibility” genes. Recently we identified a family of 17 putative VvMLO genes in the genome of the cultivated winegrape species, Vitis vinifera . Expression analysis indicated that the VvMLO family members respond differently to biotic and abiotic stimuli. Infection of V. vinifera by grape powdery mildew (Erysiphe necator ) specifically upregulates four VvMLO genes that are orthologous to the Arabidopsis and tomato MLOs previously demonstrated to be required for powdery mildew susceptibility. We postulate that one or more of these E. necator responsive VvMLOs may have a role in the powdery mildew susceptibility of grapevine.

The calmodulin-binding transcription factor OsCBT suppresses defense responses to pathogens in rice

We previously isolated the OsCBT gene, which encodes a calmodulin (CaM)-binding protein, from a rice expression library constructed from fungal elicitor-treated rice suspension cells. In order to understand the function of OsCBT in rice, we isolated and characterized a T-DNA insertion mutant allele named oscbt-1. The oscbt-1 mutant exhibits reduced levels of OsCBT transcripts and no significant morphological changes compared to wild-type plant although the growth of the mutant is stunted. However, oscbt-1 mutants showed significant resistance to two major rice pathogens. The growth of the rice blast fungus Magnaporthe grisea, as well as the bacterial pathogen Xanthomonas oryzae pv. oryzae was significantly suppressed in oscbt-1 plants. Histochemical analysis indicated that the hypersensitive-response was induced in the oscbt-1 mutant in response to compatible strains of fungal pathogens. OsCBT expression was induced upon challenge with fungal elicitor. We also observed significant increase in the level of pathogenesis-related genes in the oscbt-1 mutant even under pathogen-free condition. Taken together, the results support an idea that OsCBT might act as a negative regulator on plant defense.

Identification of differentially expressed proteins in poplar leaves induced by Marssonina brunnea f. sp. Multigermtubi

Black spot disease in poplar is a disease of the leaf caused by fungus. The major pathogen is Marssonina brunnea f. sp. multigermtubi. To date, little is known about the molecular mechanism of poplar (M. brunnea) interaction. In order to identify the proteins related to disease resistance and understand its molecular basis, the clone "NL895" (P. euramericana CL"NL895"), which is highly resistant to M. brunnea f. sp. multigermtubi, was used in this study. We used two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS) to identify the proteins in poplar leaves that were differentially expressed in response to black spot disease pathogen, M. brunnea f. sp. multigermtubi. Proteins extracted from poplar leaves at 0, 12, 24, 48, and 72 h after pathogen-inoculation were separated by 2-DE. About 500 reproducible protein spots were detected, of which 40 protein spots displayed differential expression in levels and were subjected to Matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) followed by database searching. According to the function, the identified proteins were sorted into five categories, that is, protein synthesis, metabolism, defense response and unclassified proteins.

Functional association of cell death suppressor, Arabidopsis Bax inhibitor-1, with fatty acid 2-hydroxylation through cytochrome b₅

Bax inhibitor-1 (BI-1) is a widely conserved cytoprotective protein localized in the endoplasmic reticulum (ER) membrane. We identified Arabidopsis cytochrome b(5) (AtCb5) as an interactor of Arabidopsis BI-1 (AtBI-1) by screening the Arabidopsis cDNA library with the split-ubiquitin yeast two-hybrid (suY2H) system. Cb5 is an electron transfer protein localized mainly in the ER membrane. In addition, a bimolecular fluorescence complementation (BiFC) assay and fluorescence resonance energy transfer (FRET) analysis confirmed that AtBI-1 interacted with AtCb5 in plants. On the other hand, we found that the AtBI-1-mediated suppression of cell death in yeast requires Saccharomyces cerevisiae fatty acid hydroxylase 1 (ScFAH1), which had a Cb5-like domain at the N terminus and interacted with AtBI-1. ScFAH1 is a sphingolipid fatty acid 2-hydroxylase localized in the ER membrane. In contrast, AtFAH1 and AtFAH2, which are functional ScFAH1 homologues in Arabidopsis, had no Cb5-like domain, and instead interacted with AtCb5 in plants. These results suggest that AtBI-1 interacts with AtFAHs via AtCb5 in plant cells. Furthermore, the overexpression of AtBI-1 increased the level of 2-hydroxy fatty acids in Arabidopsis, indicating that AtBI-1 is involved in fatty acid 2-hydroxylation.

Functional contribution of chorismate synthase, anthranilate synthase, and chorismate mutase to penetration resistance in barley-powdery mildew interactions

Plant processes resulting from primary or secondary metabolism have been hypothesized to contribute to defense against microbial attack. Barley chorismate synthase (HvCS), anthranilate synthase alpha subunit 2 (HvASa2), and chorismate mutase 1 (HvCM1) occupy pivotal branch points downstream of the shikimate pathway leading to the synthesis of aromatic amino acids. Here, we provide functional evidence that these genes contribute to penetration resistance to Blumeria graminis f. sp. hordei, the causal agent of powdery mildew disease. Single-cell transient-induced gene silencing of HvCS and HvCM1 in mildew resistance locus a (Mla) compromised cells resulted in increased susceptibility. Correspondingly, overexpression of HvCS, HvASa2, and HvCM1 in lines carrying mildew resistance locus o (Mlo), a negative regulator of penetration resistance, significantly decreased susceptibility. Barley stripe mosaic virus-induced gene silencing of HvCS, HvASa2, and HvCM1 significantly increased B. graminis f. sp. hordei penetration into epidermal cells, followed by formation of haustoria and secondary hyphae. However, sporulation of B. graminis f. sp. hordei was not detected on the silenced host plants up to 3 weeks after inoculation. Taken together, these results establish a previously unrecognized role for the influence of HvCS, HvASa2, and HvCM1 on penetration resistance and on the rate of B. graminis f. sp. hordei development in Mla-mediated, barley-powdery mildew interactions.

Temporal global expression data reveal known and novel salicylate-impacted processes and regulators mediating powdery mildew growth and reproduction on Arabidopsis

Salicylic acid (SA) is a critical mediator of plant innate immunity. It plays an important role in limiting the growth and reproduction of the virulent powdery mildew (PM) Golovinomyces orontii on Arabidopsis (Arabidopsis thaliana). To investigate this later phase of the PM interaction and the role played by SA, we performed replicated global expression profiling for wild-type and SA biosynthetic mutant isochorismate synthase1 (ics1) Arabidopsis from 0 to 7 d after infection. We found that ICS1-impacted genes constitute 3.8% of profiled genes, with known molecular markers of Arabidopsis defense ranked very highly by the multivariate empirical Bayes statistic (T(2) statistic). Functional analyses of T(2)-selected genes identified statistically significant PM-impacted processes, including photosynthesis, cell wall modification, and alkaloid metabolism, that are ICS1 independent. ICS1-impacted processes include redox, vacuolar transport/secretion, and signaling. Our data also support a role for ICS1 (SA) in iron and calcium homeostasis and identify components of SA cross talk with other phytohormones. Through our analysis, 39 novel PM-impacted transcriptional regulators were identified. Insertion mutants in one of these regulators, PUX2 (for plant ubiquitin regulatory X domain-containing protein 2), results in significantly reduced reproduction of the PM in a cell death-independent manner. Although little is known about PUX2, PUX1 acts as a negative regulator of Arabidopsis CDC48, an essential AAA-ATPase chaperone that mediates diverse cellular activities, including homotypic fusion of endoplasmic reticulum and Golgi membranes, endoplasmic reticulum-associated protein degradation, cell cycle progression, and apoptosis. Future work will elucidate the functional role of the novel regulator PUX2 in PM resistance.

Ca(2+)/calmodulin regulates salicylic-acid-mediated plant immunity

Intracellular calcium transients during plant-pathogen interactions are necessary early events leading to local and systemic acquired resistance. Salicylic acid, a critical messenger, is also required for both of these responses, but whether and how salicylic acid level is regulated by Ca(2+) signalling during plant-pathogen interaction is unclear. Here we report a mechanism connecting Ca(2+) signal to salicylic-acid-mediated immune response through calmodulin, AtSR1 (also known as CAMTA3), a Ca(2+)/calmodulin-binding transcription factor, and EDS1, an established regulator of salicylic acid level. Constitutive disease resistance and elevated levels of salicylic acid in loss-of-function alleles of Arabidopsis AtSR1 suggest that AtSR1 is a negative regulator of plant immunity. This was confirmed by epistasis analysis with mutants of compromised salicylic acid accumulation and disease resistance. We show that AtSR1 interacts with the promoter of EDS1 and represses its expression. Furthermore, Ca(2+)/calmodulin-binding to AtSR1 is required for suppression of plant defence, indicating a direct role for Ca(2+)/calmodulin in regulating the function of AtSR1. These results reveal a previously unknown regulatory mechanism linking Ca(2+) signalling to salicylic acid level.

Blufensin1 negatively impacts basal defense in response to barley powdery mildew

Plants have evolved complex regulatory mechanisms to control the defense response against microbial attack. Both temporal and spatial gene expression are tightly regulated in response to pathogen ingress, modulating both positive and negative control of defense. BLUFENSIN1 (BLN1), a small peptide belonging to a novel family of proteins in barley (Hordeum vulgare), is highly induced by attack from the obligate biotrophic fungus Blumeria graminis f. sp. hordei (Bgh), casual agent of powdery mildew disease. Computational interrogation of the Bln1 gene family determined that members reside solely in the BEP clade of the Poaceae family, specifically, barley, rice (Oryza sativa), and wheat (Triticum aestivum). Barley stripe mosaic virus-induced gene silencing of Bln1 enhanced plant resistance in compatible interactions, regardless of the presence or absence of functional Mla coiled-coil, nucleotide-binding site, Leu-rich repeat alleles, indicating that BLN1 can function in an R-gene-independent manner. Likewise, transient overexpression of Bln1 significantly increased accessibility toward virulent Bgh. Moreover, silencing in plants harboring the Mlo susceptibility factor decreased accessibility to Bgh, suggesting that BLN1 functions in parallel with or upstream of MLO to modulate penetration resistance. Collectively, these data suggest that the grass-specific Bln1 negatively impacts basal defense against Bgh.

Identification of grapevine MLO gene candidates involved in susceptibility to powdery mildew

The European cultivated grapevine, Vitis vinifera L., is a host for the powdery mildew pathogen Erisyphe necator, which is the most economically important fungal disease of viticulture. MLO proteins mediate powdery mildew susceptibility in the model plant species Arabidopsis and the crop plants barley and tomato. Seven VvMLO cDNA sequences were isolated from grapevine and were subsequently identified as part of a 17 member VvMLO gene family within the V. vinifera genome. Phylogenetic analysis of the 17 VvMLO genes in the grape genome indicated that the proteins they encode fall into six distinct clades. The expression of representative VvMLOs from each clade were analysed in a range of grape tissues, as well as in response to a range of biotic and abiotic factors. The VvMLOs investigated have unique, but overlapping tissue expression patterns. Expression analysis of VvMLO genes following E. necator infection identified four upregulated VvMLOs which are orthologous to the Arabidopsis AtMLO2, AtMLO6 and AtMLO12 and tomato SlMLO1 genes required for powdery mildew susceptibility. This suggests a degree of functional redundancy between the proteins encoded by these genes in terms of susceptibility to powdery mildew, and, as such, represent potential targets for modification to generate powdery mildew resistant grapevines.

Combination of novel green fluorescent protein mutant TSapphire and DsRed variant mOrange to set up a versatile in planta FRET-FLIM assay

Förster resonance energy transfer (FRET) measurements based on fluorescence lifetime imaging microscopy (FLIM) are increasingly being used to assess molecular conformations and associations in living systems. Reduction in the excited-state lifetime of the donor fluorophore in the presence of an appropriately positioned acceptor is taken as strong evidence of FRET. Traditionally, cyan fluorescent protein has been widely used as a donor fluorophore in FRET experiments. However, given its photolabile nature, low quantum yield, and multiexponential lifetime, cyan fluorescent protein is far from an ideal donor in FRET imaging. Here, we report the application and use of the TSapphire mutant of green fluorescent protein as an efficient donor to mOrange in FLIM-based FRET imaging in intact plant cells. Using time-correlated single photon counting-FLIM, we show that TSapphire expressed in living plant cells decays with lifetime of 2.93 +/- 0.09 ns. Chimerically linked TSapphire and mOrange (with 16-amino acid linker in between) exhibit substantial energy transfer based on the reduction in the lifetime of TSapphire in the presence of the acceptor mOrange. Experiments performed with various genetically and/or biochemically known interacting plant proteins demonstrate the versatility of the FRET-FLIM system presented here in different subcellular compartments tested (cytosol, nucleus, and at plasma membrane). The better spectral overlap with red monomers, higher photostability, and monoexponential lifetime of TSapphire makes it an ideal FRET-FLIM donor to study protein-protein interactions in diverse eukaryotic systems overcoming, in particular, many technical challenges encountered (like autofluorescence of cell walls and fluorescence of pigments associated with photosynthetic apparatus) while studying plant protein dynamics and interactions.

Regulation of MAPK phosphatase 1 (AtMKP1) by calmodulin in Arabidopsis

The mitogen-activated protein kinases (MAPKs) are key signal transduction molecules, which respond to various external stimuli. The MAPK phosphatases (MKPs) are known to be negative regulators of MAPKs in eukaryotes. We screened an Arabidopsis cDNA library using horseradish peroxidase-conjugated calmodulin (CaM), and isolated AtMKP1 as a CaM-binding protein. Recently, tobacco NtMKP1 and rice OsMKP1, two orthologs of Arabidopsis AtMKP1, were reported to bind CaM via a single putative CaM binding domain (CaMBD). However, little is known about the regulation of phosphatase activity of plant MKP1s by CaM binding. In this study, we identified two Ca(2+)-dependent CaMBDs within AtMKP1. Specific binding of CaM to two different CaMBDs was verified using a gel mobility shift assay, a competition assay with a Ca(2+)/CaM-dependent enzyme, and a split-ubiquitin assay. The peptides for two CaMBDs, CaMBDI and CaMBDII, bound CaM in a Ca(2+)-dependent manner, and the binding affinity of CaMBDII was found to be higher than that of CaMBDI. CaM overlay assays using mutated CaMBDs showed that four amino acids, Trp(453) and Leu(456) in CaMBDI and Trp(678) and Ile(684) in CaMBDII, play a pivotal role in CaM binding. Moreover, the phosphatase activity of AtMKP1 was increased by CaM in a Ca(2+)-dependent manner. Our results suggest that two important signaling pathways, Ca(2+) signaling and the MAPK signaling cascade, are connected in plants via the regulation of AtMKP1 activity. To our knowledge, this is the first report to show that the biochemical activity of MKP1 in plants is regulated by CaM.

RACK1 functions in rice innate immunity by interacting with the Rac1 immune complex

A small GTPase, Rac1, plays a key role in rice (Oryza sativa) innate immunity as part of a complex of regulatory proteins. Here, we used affinity column chromatography to identify rice RACK1 (for Receptor for Activated C-Kinase 1) as an interactor with Rac1. RACK1 functions in various mammalian signaling pathways and is involved in hormone signaling and development in plants. Rice contains two RACK1 genes, RACK1A and RACK1B, and the RACK1A protein interacts with the GTP form of Rac1. Rac1 positively regulates RACK1A at both the transcriptional and posttranscriptional levels. RACK1A transcription was also induced by a fungal elicitor and by abscisic acid, jasmonate, and auxin. Analysis of transgenic rice plants and cell cultures indicates that RACK1A plays a role in the production of reactive oxygen species (ROS) and in resistance against rice blast infection. Overexpression of RACK1A enhances ROS production in rice seedlings. RACK1A was shown to interact with the N terminus of NADPH oxidase, RAR1, and SGT1, key regulators of plant disease resistance. These results suggest that RACK1A functions in rice innate immunity by interacting with multiple proteins in the Rac1 immune complex.

Whole proteome identification of plant candidate G-protein coupled receptors in Arabidopsis, rice, and poplar: computational prediction and in-vivo protein coupling

Computational prediction and in vivo protein coupling experiments identify candidate plant G-protein coupled receptors in Arabidopsis, rice and poplar.

Background

The classic paradigm of heterotrimeric G-protein signaling describes a heptahelical, membrane-spanning G-protein coupled receptor that physically interacts with an intracellular Gα subunit of the G-protein heterotrimer to transduce signals. G-protein coupled receptors comprise the largest protein superfamily in metazoa and are physiologically important as they sense highly diverse stimuli and play key roles in human disease. The heterotrimeric G-protein signaling mechanism is conserved across metazoa, and also readily identifiable in plants, but the low sequence conservation of G-protein coupled receptors hampers the identification of novel ones. Using diverse computational methods, we performed whole-proteome analyses of the three dominant model plant species, the herbaceous dicot Arabidopsis thaliana (mouse-eared cress), the monocot Oryza sativa (rice), and the woody dicot Populus trichocarpa (poplar), to identify plant protein sequences most likely to be GPCRs.

Results

Our stringent bioinformatic pipeline allowed the high confidence identification of candidate G-protein coupled receptors within the Arabidopsis, Oryza, and Populus proteomes. We extended these computational results through actual wet-bench experiments where we tested over half of our highest ranking Arabidopsis candidate G-protein coupled receptors for the ability to physically couple with GPA1, the sole Gα in Arabidopsis. We found that seven out of eight tested candidate G-protein coupled receptors do in fact interact with GPA1. We show through G-protein coupled receptor classification and molecular evolutionary analyses that both individual G-protein coupled receptor candidates and candidate G-protein coupled receptor families are conserved across plant species and that, in some cases, this conservation extends to metazoans.

Conclusion

Our computational and wet-bench results provide the first step toward understanding the diversity, conservation, and functional roles of plant candidate G-protein coupled receptors.

Adult plant resistance-related gene expression in 'Camp Remy' wheat inoculated with Puccinia striiformis

The French wheat variety 'Camp Remy' (CR) possesses a durable, adult plant resistance to yellow rust (YR), caused by the pathogen Puccinia striiformis. Using cDNA-AFLP on different sets of heterogeneous inbred families (HIFs) derived from the cross CR x Récital, we compared gene expression profiles during one seedling and two adult developmental stages following inoculation with P. striiformis. Transcripts differentially expressed in response to YR infection were isolated and cloned. Sequence analysis of the resultant clones revealed several classes of putative genes, including those related to resistance/defence responses, transcription and signal transduction, and primary metabolism. The expression profiles of seven selected genes were obtained using real-time PCR in CR leaves at the same three stages of development. The results confirmed the stage-specific expression of the genes at one or two specific stages in response to P. striiformis infection and demonstrated that CR modifies the expression of some resistance/defence-related genes during its transition from the seedling to adult growth stages. These results provided the first clue to understand the molecular basis of quantitative trait loci for adult plant resistance to YR and connect it with durability.

The Powdery Mildew Disease of Arabidopsis: A Paradigm for the Interaction between Plants and Biotrophic Fungi

The powdery mildew diseases, caused by fungal species of the Erysiphales, have an important economic impact on a variety of plant species and have driven basic and applied research efforts in the field of phytopathology for many years. Although the first taxonomic reports on the Erysiphales date back to the 1850's, advances into the molecular biology of these fungal species have been hampered by their obligate biotrophic nature and difficulties associated with their cultivation and genetic manipulation in the laboratory. The discovery in the 1990's of a few species of powdery mildew fungi that cause disease on Arabidopsis has opened a new chapter in this research field. The great advantages of working with a model plant species have translated into remarkable progress in our understanding of these complex pathogens and their interaction with the plant host. Herein we summarize advances in the study of Arabidopsis-powdery mildew interactions and discuss their implications for the general field of plant pathology. We provide an overview of the life cycle of the pathogens on Arabidopsis and describe the structural and functional changes that occur during infection in the host and fungus in compatible and incompatible interactions, with special emphasis on defense signaling, resistance pathways, and compatibility factors. Finally, we discuss the future of powdery mildew research in anticipation of the sequencing of multiple powdery mildew genomes. The cumulative body of knowledge on powdery mildews of Arabidopsis provides a valuable tool for the study and understanding of disease associated with many other obligate biotrophic pathogen species.

Programmed cell death in plants: new insights into redox regulation and the role of hydrogen peroxide

Programmed cell death (PCD), the highly regulated dismantling of cells, is essential for plant growth and survival. PCD plays key roles in embryo development, formation and maturation of many cell types and tissues, and plant reaction/adaptation to environmental conditions. Reactive oxygen species (ROS) are not only toxic by products of aerobic metabolism with strictly controlled cellular levels, but they also function as signaling agents regulating many biological processes and producing pleiotropic effects. Over the last decade, ROS have become recognized as important modulators of plant PCD. Molecular genetic approaches using plant mutants and transcriptome studies related to ROS-mediated PCD have revealed a wide array of plant-specific cell death regulators and have contributed to unraveling the elaborate redox signaling network. This review summarizes the biological processes, in which plant PCD participates and discusses the signaling functions of ROS with emphasis on hydrogen peroxide.

Disturbance of the Ca(2+)/calmodulin-dependent signalling pathway is responsible for the resistance of Arabidopsis dnd1 against Pectobacterium carotovorum infection

SUMMARY Arabidopsis thaliana wild-type Col-0 and its mutant, 'defence, no death' (dnd) 1-1, were infected with biotrophic Pseudomonas syringae pv. tomato strain DC3000 and necrotrophic Pectobacterium carotovorum strain KACC 10228, and cellular and molecular responses among them were then analysed. Col-0 wild-type was susceptible to both pathogens. By contrast, neither DC3000 nor KACC 10228 infected dnd1-1 (Yu et al., 1998. Proc. Natl. Acad. Sci. USA 95: 7819-7824). Neither of the pathogens triggered cell death or accumulation of active oxygen species in dnd1-1. KACC 10228 induced accelerated transcriptions of PDF1.2 and AtEBP genes in wild-type Col-0, while DC3000-induced transcriptions of them were relatively retarded. Neither of the pathogens modified the constitutive transcription of PR1 in dnd1-1. PDF1.2 and AtEBP transcriptions were not induced by the same treatments. Hydrogen peroxide scavengers, catalase and ascorbic acid, and LaCl(3), an inhibitor of Ca(2+) influx, diminished cell death and protected the wild-type plant from KACC 10228 infection, while EGTA inhibited cell death and pathogen growth. Exogenous Ca(2+) nullified resistance against KACC 10228 challenge in dnd1-1. W-7 and chloropromazine, two calmodulin antagonists, also triggered cell death in dnd1-1 and abolished resistance against KACC 10228. In summary, cell death is correlated with KACC 10228 infection and disease development. Furthermore, the resistance of dnd1-1 against P. carotovorum is dependent on calmodulin and inhibition of cytosolic Ca(2+) increment.

Antagonistic control of powdery mildew host cell entry by barley calcium-dependent protein kinases (CDPKs)

Calcium-dependent protein kinases (CDPKs) are known to play pivotal roles in intracellular signaling during abiotic and biotic stress responses. To unravel potential functions of CDPKs in the course of barley (Hordeum vulgare)-powdery mildew (Blumeria graminis) interactions, we systematically analyzed the HvCDPK gene family. We found that, according to the existence of respective expressed sequence tags, at least nine paralogs are expressed in the barley leaf epidermis, the sole target tissue of powdery mildew fungi. We exemplarily selected two HvCDPKs with known full-length coding sequence for functional analysis. Transient expression of a putative constitutive active variant of one of these (HvCDPK4) in Nicotiana benthamiana triggered kinase-dependent mesophyll cell death in tobacco leaves. In a barley mlo mutant genotype, a constitutive active variant of the second paralog, HvCDPK3, partially compromised the highly effective resistance to B. graminis f. sp. hordei. A similar break of mlo resistance was seen upon expression of the junction domain of HvCDPK4, supposed to act as a dominant inhibitor of CDPK activity. Expression of a constitutive active HvCDPK3 or HvCDPK4 form also compromised penetration resistance to the inappropriate wheat powdery mildew fungus. Collectively, our data provide evidence for antagonistic roles of individual CDPK paralogs in the control of host cell entry during the early phase of powdery mildew pathogenesis.

Barley MLO modulates actin-dependent and actin-independent antifungal defense pathways at the cell periphery

Cell polarization is a crucial process during plant development, as well as in plant-microbe interactions, and is frequently associated with extensive cytoskeletal rearrangements. In interactions of plants with inappropriate fungal pathogens (so-called non-host interactions), the actin cytoskeleton is thought to contribute to the establishment of effective barriers at the cell periphery against fungal ingress. Here, we impeded actin cytoskeleton function in various types of disease resistance using pharmacological inhibitors and genetic interference via ectopic expression of an actin-depolymerizing factor-encoding gene, ADF. We demonstrate that barley (Hordeum vulgare) epidermal cells require actin cytoskeleton function for basal defense to the appropriate powdery mildew pathogen Blumeria graminis f. sp. hordei and for mlo-mediated resistance at the cell wall, but not for several tested race-specific immune responses. Analysis of non-host resistance to two tested inappropriate powdery mildews, Erysiphe pisi and B. graminis f. sp. tritici, revealed the existence of actin-dependent and actin-independent resistance pathways acting at the cell periphery. These pathways act synergistically and appear to be under negative control by the plasma membrane-resident MLO protein.

Pathogen-induced calmodulin isoforms in basal resistance against bacterial and fungal pathogens in tobacco

Thirteen tobacco calmodulin (CaM) genes fall into three distinct amino acid homology types. Wound-inducible type I isoforms NtCaM1 and 2 were moderately induced by tobacco mosaic virus (TMV)-mediated hypersensitive reaction, and the type III isoform NtCaM13 was highly induced, while the type II isoforms NtCaM3-NtCaM12 showed little response. Type I and III knockdown tobacco lines were generated using inverted repeat sequences from NtCaM1 and 13, respectively, to evaluate the contribution of pathogen-induced calmodulins (CaMs) to disease resistance. After specific reduction of type I and III CaM gene expression was confirmed in both transgenic lines, we analyzed the response to TMV infection, and found that TMV susceptibility was slightly enhanced in type III CaM knockdown lines compared with the control line. Resistance to a compatible strain of the bacterial pathogen Ralstonia solanacearum, and fungal pathogens Rhizoctonia solani and Pythium aphanidermatum was significantly lower in type III but not in type I CaM knockdown plants. Expression of jasmonic acid (JA)- and/or ethylene-inducible basic PR genes was not affected in these lines, suggesting that type III CaM isoforms are probably involved in basal defense against necrotrophic pathogens in a manner that is independent of JA and ethylene signaling.

Cell death suppressor Arabidopsis bax inhibitor-1 is associated with calmodulin binding and ion homeostasis

Cell death suppressor Bax inhibitor-1 (BI-1), an endoplasmic reticulum membrane protein, exists in a wide range of organisms. The split-ubiquitin system, overlay assay, and bimolecular fluorescence complementation analysis demonstrated that Arabidopsis (Arabidopsis thaliana) BI-1 (AtBI-1) interacted with calmodulin in yeast (Saccharomyces cerevisiae) and in plant cells. Furthermore, AtBI-1 failed to rescue yeast mutants lacking Ca2+ ATPase (Pmr1 or Spf1) from Bax-induced cell death. Pmr1 and Spf1, p-type ATPases localized at the inner membrane, are believed to be involved in transmembrane movement of calcium ions in yeast. Thus, the presence of intact Ca2+ ATPases was essential for AtBI-1-mediated cell death suppression in yeast. To investigate the effect of AtBI-1 on calcium homeostasis, we evaluated sensitivity against cyclopiazonic acid (CPA), an inhibitor of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase in AtBI-1-overexpressing or knock-down transgenic Arabidopsis plants. These plants demonstrated altered CPA or ion stress sensitivity. Furthermore, AtBI-1-overexpressing cells demonstrated an attenuated rise in cytosolic calcium following CPA or H2O2 treatment, suggesting that AtBI-1 affects ion homeostasis in plant cell death regulation.

Cell wall-associated mechanisms of disease resistance and susceptibility

The plant cuticle and cell wall separate microbial pathogens from the products of plant metabolism. While microbial pathogens try to breach these barriers for colonization, plants respond to attempted penetration by a battery of wall-associated defense reactions. Successful pathogens circumvent or suppress plant nonself recognition and basal defense during penetration and during microbial reproduction. Additionally, accommodation of fungal infection structures within intact cells requires host reprogramming. Recent data highlight that both early plant defense to fungal penetration and host reprogramming for susceptibility can function at the host cell periphery. Genetic evidence has also widened our understanding of how fungal pathogens are restricted during penetration at the plant cell wall. This review summarizes the current view of how plants monitor and model their cell periphery during interaction with microbial invaders.

Resistance to cereal rusts at the plant cell wall—what can we learn from other host-pathogen systems?

The ability of plant cells to resist invasion by pathogenic fungi at the cell periphery (pre-invasion resistance) differs from other types of resistance that are generally triggered after parasite entry and during differentiation of specialised intracellular feeding structures. Genetic sources of pre-invasion resistance such as mlo for barley powdery mildew and Lr34 for resistance to wheat leaf rust have proven to be broad-spectrum in effect and durable in the field. Continued breeding for this type of resistance (often quantitative in effect) is therefore considered an important strategy to protect cereal crops long-term against potentially devastating fungal diseases such as rusts. Considerable progress has been made in characterising genes and processes underlying pre-invasion resistance using mutant analysis, molecular genetics, gene cloning, and the model plant Arabidopsis, as well as comparative functional analysis of genes in Arabidopsis and cereals. This review summarises the current knowledge in this field, and discusses several aspects of pre-invasion resistance potentially pertinent to use in breeding; namely, biological cost of the resistance and effectiveness of individual resistance genes against multiple pathogen types. We show that mutations in Mlo, Ror1, and Ror2 genes known to affect powdery mildew pre-invasion resistance have no detectable effect on partial resistance to barley leaf rust as measured by latency period.

Inducible cell death in plant immunity

Programmed cell death (PCD) occurs during vegetative and reproductive plant growth, as typified by autumnal leaf senescence and the terminal differentiation of the endosperm of cereals which provide our major source of food. PCD also occurs in response to environmental stress and pathogen attack, and these inducible PCD forms are intensively studied due their experimental tractability. In general, evidence exists for plant cell death pathways which have similarities to the apoptotic, autophagic and necrotic forms described in yeast and metazoans. Recent research aiming to understand these pathways and their molecular components in plants are reviewed here.

Protein polyubiquitination plays a role in basal host resistance of barley

To study protein ubiquitination pathways in the interaction of barley (Hordeum vulgare) with the powdery mildew fungus (Blumeria graminis), we measured protein turnover and performed transient-induced gene silencing (TIGS) of ubiquitin and 26S proteasome subunit encoding genes in epidermal cells. Attack by B. graminis hyperdestabilized a novel unstable green fluorescent protein fusion that contains a destabilization domain of a putative barley 1-aminocyclopropane-1-carboxylate synthase, suggesting enhanced protein turnover. Partial depletion of cellular ubiquitin levels by TIGS induced extreme susceptibility of transformed cells toward the appropriate host pathogen B. graminis f. sp hordei, whereas papilla-based resistance to the nonhost pathogen B. graminis f. sp tritici and host resistance mediated by the mlo gene (for mildew resistance locus O) remained unaffected. Cells were rescued from TIGS-induced ubiquitin depletion by synthetic genes encoding wild-type or mutant barley monoubiquitin proteins. The strongest rescue was from a gene encoding a K63R mutant form of ubiquitin blocked in several ubiquitination pathways while still allowing Lys-48-dependent polyubiquitination required for proteasomal protein degradation. Systematic RNA interference of 40 genes encoding all 17 subunits of the proteasome 19S regulatory particle failed to induce hypersusceptibility against B. graminis f. sp hordei. This suggests a role for Lys-48-linked protein polyubiquitination, which is independent from the proteasome pathway, in basal host defense of barley.

Sinorhizobium meliloti nifA mutant induces different gene expression profile from wild type in Alfalfa nodules

Several studies have demonstrated that the Rhizobium nifA gene is an activator of nitrogen fixation acting in nodule bacteria. To understand the effects of the Sinorhizobium meliloti nifA gene on Alfalfa, the cDNA-AFLP technique was employed to study the changes in gene expression in nifA mutant nodules. Among the approximately 3,000 transcript-derived fragments, 37 had differential expression levels. These expression levels were subsequently confirmed by reverse Northern blot and RT-polymerase chain reaction. Sequence analyses revealed that 21 cDNA fragments corresponded to genes involved in signal communication, protein degradation, nutrient metabolism, cell growth and development.

Loss of Necrotic Spotted Lesions 1 associates with cell death and defense responses in Arabidopsis thaliana

We isolated a lesion mimic mutant, necrotic spotted lesions 1 (nsl1), from Ds-tagged Arabidopsis thaliana accession No-0. The nsl1 mutant exhibits a growth retardation phenotype and develops spotted necrotic lesions on its rosette and cauline leaves. These phenotypes occur in the absence of pathogens indicating that nsl1 mutants may constitutively express defense responses. Consistent with this idea, nsl1 accumulates high levels of callose and autofluorescent phenolic compounds localized to the necrotic lesions. Furthermore RNA gel blot analysis revealed that genes associated with disease resistance activation are upregulated in the nsl1 mutants and these plants contain elevated levels of salicylic acid (SA). Crossing nsl1 with an SA deficient mutant, eds16-1, revealed that the nsl1 lesions and growth retardation are dependent upon SA. The nsl1 phenotypes are not suppressed under either the rar1-10 or sgt1b-1 genetic background. NSL1 encodes a novel 612aa protein which contains a membrane-attack complex/perforin (MACPF) domain, which is conserved in bacteria, fungi, mammals and plants. The possible modes of action of NSL1 protein in negative regulation of cell death programs and defense responses are discussed.

RMo1 confers blast resistance in barley and is located within the complex of resistance genes containing Mla, a powdery mildew resistance gene

Isolates of Magnaporthe oryzae (the causal agent of rice blast disease) can infect a range of grass species, including barley. We report that barley Hordeum vulgare cv. Baronesse and an experimental line, BCD47, show a range of resistance reactions to infection with two rice blast isolates. The complete resistance of Baronesse to the isolate Ken 54-20 is controlled by a single dominant gene, designated RMo1. RMo1 mapped to the same linkage map position on chromosome 1H as the powdery mildew resistance locus Mla and an expressed sequence tag (k04320) that corresponds to the barley gene 711N16.16. A resistance quantitative trait locus (QTL), at which Baronesse contributed the resistance allele, to the isolate Ken 53-33 also mapped at the same position as RMo1. Synteny analysis revealed that a corresponding region on rice chromosome 5 includes the bacterial blight resistance gene xa5. These results indicate that a defined region on the short arm of barley chromosome 1H, including RMo1 and Mla, harbors genes conferring qualitative and quantitative resistance to multiple pathogens. The partial resistance of BCD47 to Ken53-33 is determined by alleles at three QTL, two of which coincide with the linkage map positions of the mildew resistance genes mlo and Mlf.

Show and tell: cell biology of pathogen invasion

Because the initial stages of pathogen invasion are often confined to a limited number of host cells, measures of host responses that are averaged over attacked and non-attacked cells provide an unsatisfactory view of these events. To identify the earliest and often transient responses to pathogen attack, there is considerable interest in monitoring the subcellular events that occur specifically in living host cells. Recent improvements in live-cell imaging using fluorescent-tagged markers have expanded the scope of the experiments that can be performed. Changes in the subcellular distribution of organelles and of fluorescently tagged proteins can be monitored in real time in living tissues during pathogen attack, and the dynamic nature of such changes across space and over time can be determined. The application of these sensitive imaging methods has extended earlier observations, made with Nomarski microscopy or inferred from static transmission electron micrographs, about the focal accumulation of subcellular organelles at sites of pathogen attack. In addition, recent experiments have demonstrated the focused accumulation and interaction of specific plant proteins at penetration sites, opening a new window on early host responses and raising questions about the underlying plant processes that sense and direct this marshalling of host resources to block pathogen entry.

Endophyte or parasite--what decides?

Symbiosis between a fungus and a plant is a widespread phenomenon in nature. The outcome of such an interaction can vary in a seamless manner from mutualism to parasitism. In most cases, the host plant does not suffer, in fact it often gains an advantage from colonization by a fungus. This benefit is based on a fine-tuned balance between the demands of the invader and the plant response. If the interaction becomes unbalanced, disease symptoms appear or the fungus is excluded by induced host defence reactions. Symbioses of plants with beneficial or neutral endophytes share many common attributes with plant interactions with pathogens. Recent findings emerging from studies of compatible host-fungus interactions have enhanced our understanding of what determines whether the fungus behaves as an endophyte or a parasite and of how plants avoid exploitation by detrimental parasites but benefit from mutualistic endophytes.

The visible touch: in planta visualization of protein-protein interactions by fluorophore-based methods

Non-invasive fluorophore-based protein interaction assays like fluorescence resonance energy transfer (FRET) and bimolecular fluorescence complementation (BiFC, also referred to as "split YFP") have been proven invaluable tools to study protein-protein interactions in living cells. Both methods are now frequently used in the plant sciences and are likely to develop into standard techniques for the identification, verification and in-depth analysis of polypeptide interactions. In this review, we address the individual strengths and weaknesses of both approaches and provide an outlook about new directions and possible future developments for both techniques.

Genome-wide isolation of resistance gene analogs in maize (Zea mays L.)

Conserved domains or motifs shared by most known resistance (R) genes have been extensively exploited to identify unknown R-gene analogs (RGAs). In an attempt to isolate all potential RGAs from the maize genome, we adopted the following three methods: modified amplified fragment length polymorphism (AFLP), modified rapid amplification of cDNA ends (RACE), and data mining. The first two methods involved PCR-based isolations of RGAs with degenerate primers designed based on the conserved NBS domain; while the third method involved mining of RGAs from the maize EST database using full-length R-gene sequences. A total of 23 and 12 RGAs were obtained from the modified AFLP and RACE methods, respectively; while, as many as 109 unigenes and 77 singletons with high homology to known R-genes were recovered via data-mining. Moreover, R-gene-like ESTs (or RGAs) identified from the data-mining method could cover all RACE-derived RGAs and nearly half AFLP-derived RGAs. Totally, the three methods resulted in 199 non-redundant RGAs. Of them, at least 186 were derived from putative expressed R-genes. RGA-tagged markers were developed for 55 unique RGAs, including 16 STS and 39 CAPS markers.

Plant calmodulins and calmodulin-related proteins: multifaceted relays to decode calcium signals

The calmodulin (CaM) family is a major class of calcium sensor proteins which collectively play a crucial role in cellular signaling cascades through the regulation of numerous target proteins. Although CaM is one of the most conserved proteins in all eukaryotes, several features of CaM and its downstream effector proteins are unique to plants. The continuously growing repertoire of CaM-binding proteins includes several plant-specific proteins. Plants also possess a particular set of CaM isoforms and CaM-like proteins (CMLs) whose functions have just begun to be elucidated. This review summarizes recent insights that help to understand the role of this multigene family in plant development and adaptation to environmental stimuli.

AtBAG6, a novel calmodulin-binding protein, induces programmed cell death in yeast and plants

Calmodulin (CaM) influences many cellular processes by interacting with various proteins. Here, we isolated AtBAG6, an Arabidopsis CaM-binding protein that contains a central BCL-2-associated athanogene (BAG) domain. In yeast and plants, overexpression of AtBAG6 induced cell death phenotypes consistent with programmed cell death (PCD). Recombinant AtBAG6 had higher affinity for CaM in the absence of free Ca2 + than in its presence. An IQ motif (IQXXXRGXXXR, where X denotes any amino-acid) was required for Ca2 +-independent CaM complex formation and single amino-acid changes within this motif abrogated both AtBAG6-activated CaM-binding and cell death in yeast and plants. A 134-amino-acid stretch, encompassing both the IQ motif and BAG domain, was sufficient to induce cell death. Agents generating oxygen radicals, which are known to be involved in plant PCD, specifically induced the AtBAG6 transcript. Collectively, these results suggest that AtBAG6 is a stress-upregulated CaM-binding protein involved in plant PCD.

Expression analysis of the AtMLO gene family encoding plant-specific seven-transmembrane domain proteins

The Arabidopsis (Arabidopsis thaliana) genome contains 15 genes encoding protein homologs of the barley mildew resistance locus o (MLO) protein biochemically shown to have a seven-transmembrane domain topology and localize to the plasma membrane. Towards elucidating the functions of MLOs, the largest family of seven-transmembrane domain proteins specific to plants, we comprehensively determined AtMLO gene expression patterns by a combination of experimental and in silico studies. Experimentation comprised analyses of transgenic Arabidopsis lines bearing promoter::Beta-glucuronidase (GUS) transcriptional fusions as well as semi-quantitative determination of transcripts by reverse transcription coupled to polymerase chain reaction (RT-PCR). These results were combined with information extracted from public gene profiling databases, and compared to the expression patterns of genes encoding the heterotrimeric G-protein subunits. We found that each AtMLO gene has a unique expression pattern and is regulated differently by a variety of biotic and/or abiotic stimuli, suggesting that AtMLO proteins function in diverse developmental and response processes. The expression of several phylogenetically closely-related AtMLO genes showed similar or overlapping tissue specificity and analogous responsiveness to external stimuli, suggesting functional redundancy, co-function, or antagonistic function(s).

Targeting xa13, a recessive gene for bacterial blight resistance in rice

Bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most serious diseases of rice worldwide. Thirty bacterial blight resistance (R) genes (21 dominant genes and 9 recessive genes) in rice have been identified. They are the main sources for the genetic improvement of rice for resistance to Xoo. However, little is known about the recessive R genes. To clone and characterize the recessive R genes, we fine-mapped xa13, a fully recessive gene for Xoo resistance, to a DNA fragment of 14.8 kb using the map-based cloning strategy and a series of sequence-based molecular markers. Sequence analysis of this fragment indicated that this region contains only two apparently intact candidate genes (an extensin-like gene and a homologue of nodulin MtN3) and the 5' end of a predicted hypothetical gene. These results will greatly facilitate the isolation and characterization of xa13. Four PCR-based markers, E6a, SR6, ST9 and SR11 that were tightly linked to the xa13 locus, were also developed. These markers will be useful tools for the marker-assisted selection of xa13 in breeding programs.

A general model of G protein-coupled receptor sequences and its application to detect remote homologs

G protein-coupled receptors (GPCRs) constitute a large superfamily involved in various types of signal transduction pathways triggered by hormones, odorants, peptides, proteins, and other types of ligands. The superfamily is so diverse that many members lack sequence similarity, although they all span the cell membrane seven times with an extracellular N and a cytosolic C terminus. We analyzed a divergent set of GPCRs and found distinct loop length patterns and differences in amino acid composition between cytosolic loops, extracellular loops, and membrane regions. We configured GPCRHMM, a hidden Markov model, to fit those features and trained it on a large dataset representing the entire superfamily. GPCRHMM was benchmarked to profile HMMs and generic transmembrane detectors on sets of known GPCRs and non-GPCRs. In a cross-validation procedure, profile HMMs produced an error rate nearly twice as high as GPCRHMM. In a sensitivity-selectivity test, GPCRHMM's sensitivity was about 15% higher than that of the best transmembrane predictors, at comparable false positive rates. We used GPCRHMM to search for novel members of the GPCR superfamily in five proteomes. All in all we detected 120 sequences that lacked annotation and are potentially novel GPCRs. Out of those 102 were found in Caenorhabditis elegans, four in human, and seven in mouse. Many predictions (65) belonged to Pfam domains of unknown function. GPCRHMM strongly rejected a family of arthropod-specific odorant receptors believed to be GPCRs. A detailed analysis showed that these sequences are indeed very different from other GPCRs. GPCRHMM is available at http://gpcrhmm.cgb.ki.se.

Calcium in plant defence-signalling pathways

In plant cells, the calcium ion is a ubiquitous intracellular second messenger involved in numerous signalling pathways. Variations in the cytosolic concentration of Ca2+ ([Ca2+]cyt) couple a large array of signals and responses. Here we concentrate on calcium signalling in plant defence responses, particularly on the generation of the calcium signal and downstream calcium-dependent events participating in the establishment of defence responses with special reference to calcium-binding proteins.

Transient expression and insertional mutagenesis of Puccinia triticina using biolistics

The fungal genus Puccinia contains more than 4,000 species. Puccinia triticina, causal agent of wheat leaf rust, is an economically significant, biotrophic basidiomycete. Little is known about the molecular biology of this group, and tools for understanding gene function have not yet been established. A set of parameters was established for the transient transformation of urediniospores. The expression of three heterologous promoters (actin, elongation factor 1-alpha, and Hss1, Heat Shock 70 protein), derived from Puccinia graminis, was evaluated along with the potential for insertional mutagenesis. The UidA (GUS) gene was used as a marker for transient expression. When transferred into P. triticina urediniospores, transient expression was observed across four helium pressures using one size of gold and three sizes of tungsten microprojectiles. Each of the three promoters displayed strong transient expression in germinated urediniospores; however, higher numbers of GUS-positive urediniospores were observed when either the actin or Hss1 promoters were used. Possible concomitant insertional mutagenesis of several avirulence genes was selected in wheat cultivars harboring the cognate resistance genes. Using a linearized cloning plasmid, stable integration into the genome was achieved as demonstrated by PCR and sequencing analysis.

G proteins Go green: a plant G protein signaling FAQ sheet

Plants, like animals, use signal transduction pathways based on heterotrimeric guanine nucleotide-binding proteins (G proteins) to regulate many aspects of development and cell signaling. Some components of G protein signaling are highly conserved between plants and animals and some are not. This Viewpoint compares key aspects of G protein signal transduction in plants and animals and describes the current knowledge of this system in plants, the questions that still await exploration, and the value of research on plant G proteins to scientists who do not study plants. Pathways in Science's Signal Transduction Knowledge Environment Connections Maps database provide details about the emerging roles of G proteins in several cellular processes of plants.

Discovery of novel conserved peptide domains by ortholog comparison within plant multi-protein families

Assigning individual functions to the proteins encoded by the genome of the dicotyledonous reference species Arabidopsis thaliana is one of the major challenges in current plant molecular biology. Frequently, Arabidopsis protein families are biocomputationally analyzed by multiple amino acid sequence alignments of the respective family members for detection of conserved peptide motifs that might be of functional relevance. Mere sequence alignment of paralogous sequences may obscure amino acid patches that are highly conserved amongst orthologs and thus potentially relevant for isoform-specific protein function(s). Here I exemplarily illustrate this potential pitfall by amino acid sequence alignments of the heptahelical MLO proteins using either the suite of 15 isoforms (paralogs) encoded by the Arabidopsis genome or a collection of 13 ortholog sequences derived from a set of both monocotyledonous and dicotyledonous plant species. The findings are corroborated by an analogous analysis of the distinct plant multi-protein family of CONSTANS-like transcription regulators. The data reveal that the generally higher sequence similarity of orthologs versus paralogs is not uniformly distributed among the amino acid positions of the orthologs but at least partially clustered in distinct sites/domains, suggesting conservation of isoform-specific functional modules across taxa.

Isolation of a calmodulin-binding transcription factor from rice (Oryza sativa L.)

Calmodulin (CaM) regulates diverse cellular functions by modulating the activities of a variety of enzymes and proteins. However, direct modulation of transcription factors by CaM has been poorly understood. In this study, we isolated a putative transcription factor by screening a rice cDNA expression library by using CaM:horse-radish peroxidase as a probe. This factor, which we have designated OsCBT (Oryza sativa CaM-binding transcription factor), has structural features similar to Arabidopsis AtSRs/AtCAMTAs and encodes a 103-kDa protein because it contains a CG-1 homology DNA-binding domain, three ankyrin repeats, a putative transcriptional activation domain, and five putative CaM-binding motifs. By using a gel overlay assay, gel mobility shift assays, and site-directed mutagenesis, we showed that OsCBT has two different types of functional CaM-binding domains, an IQ motif, and a Ca(2+)-dependent motif. To determine the DNA binding specificity of OsCBT, we employed a random binding site selection method. This analysis showed that OsCBT preferentially binds to the sequence 5'-TWCG(C/T)GTKKKKTKCG-3' (W and K represent A or C and T or G, respectively). OsCBT was able to bind this sequence and activate beta-glucuronidase reporter gene expression driven by a minimal promoter containing tandem repeats of these sequences in Arabidopsis leaf protoplasts. Green fluorescent protein fusions of two putative nuclear localization signals of OsCBT, a bipartite and a SV40 type, were predominantly localized in the nucleus. Most interestingly, the transcriptional activation mediated by OsCBT was inhibited by co-transfection with a CaM gene. Taken together, our results suggest that OsCBT is a transcription activator modulated by CaM.

Calmodulin-like proteins from Arabidopsis and tomato are involved in host defense against Pseudomonas syringae pv. tomato

Complex signal transduction pathways underlie the myriad plant responses to attack by pathogens. Ca(2+) is a universal second messenger in eukaryotes that modulates various signal transduction pathways through stimulus-specific changes in its intracellular concentration. Ca(2+)-binding proteins such as calmodulin (CaM) detect Ca(2+) signals and regulate downstream targets as part of a coordinated cellular response to a given stimulus. Here we report the characterization of a tomato gene (APR134) encoding a CaM-related protein that is induced in disease-resistant leaves in response to attack by Pseudomonas syringae pv. tomato. We show that suppression of APR134 gene expression in tomato (Solanum lycopersicum), using virus-induced gene silencing (VIGS), compromises the plant's immune response. We isolated APR134-like genes from Arabidopsis, termed CML42 and CML43, to investigate whether they serve a functionally similar role. Gene expression analysis revealed that CML43 is rapidly induced in disease-resistant Arabidopsis leaves following inoculation with Pseudomonas syringae pv. tomato. Overexpression of CML43 in Arabidopsis accelerated the hypersensitive response. Recombinant APR134, CML42, and CML43 proteins all bind Ca(2+ )in vitro. Collectively, our data support a role for CML43, and APR134 as important mediators of Ca(2+)-dependent signals during the plant immune response to bacterial pathogens.