Identification of Arabidopsis mutants exhibiting an altered hypersensitive response in gene-for-gene disease resistance

TOUCH 3 and CALMODULIN 1/4/6 cooperate with calcium-dependent protein kinases to trigger calcium-dependent activation of CAM-BINDING PROTEIN 60-LIKE G and regulate fungal resistance in plants

Plants utilize localized cell-surface and intracellular receptors to sense microbes and activate the influx of calcium, which serves as an important second messenger in eukaryotes to regulate cellular responses. However, the mechanisms through which plants decipher calcium influx to activate immune responses remain largely unknown. Here, we show that pathogen-associated molecular patterns (PAMPs) trigger calcium-dependent phosphorylation of CAM-BINDING PROTEIN 60-LIKE G (CBP60g) in Arabidopsis (Arabidopsis thaliana). CALCIUM-DEPENDENT PROTEIN KINASE5 (CPK5) phosphorylates CBP60g directly, thereby enhancing its transcription factor activity. TOUCH 3 (TCH3) and its homologs CALMODULIN (CAM) 1/4/6 and CPK4/5/6/11 are required for PAMP-induced CBP60g phosphorylation. TCH3 interferes with the auto-inhibitory region of CPK5 and promotes CPK5-mediated CBP60g phosphorylation. Furthermore, CPKs-mediated CBP60g phosphorylation positively regulates plant resistance to soil-borne fungal pathogens. These lines of evidence uncover a novel calcium signal decoding mechanism during plant immunity through which TCH3 relieves auto-inhibition of CPK5 to phosphorylate and activate CBP60g. The findings reveal cooperative interconnections between different types of calcium sensors in eukaryotes.

Identification of Candidate Susceptibility Genes to Puccinia graminis f. sp. tritici in Wheat

Wheat stem rust disease caused by Puccinia graminis f. sp. tritici (Pgt) is a global threat to wheat production. Fast evolving populations of Pgt limit the efficacy of plant genetic resistance and constrain disease management strategies. Understanding molecular mechanisms that lead to rust infection and disease susceptibility could deliver novel strategies to deploy crop resistance through genetic loss of disease susceptibility. We used comparative transcriptome-based and orthology-guided approaches to characterize gene expression changes associated with Pgt infection in susceptible and resistant Triticum aestivum genotypes as well as the non-host Brachypodium distachyon. We targeted our analysis to genes with differential expression in T. aestivum and genes suppressed or not affected in B. distachyon and report several processes potentially linked to susceptibility to Pgt, such as cell death suppression and impairment of photosynthesis. We complemented our approach with a gene co-expression network analysis to identify wheat targets to deliver resistance to Pgt through removal or modification of putative susceptibility genes.

Regulatory role of receptor-like cytoplasmic kinases in early immune signaling events in plants

Receptor-like cytoplasmic kinases (RLCKs) play crucial roles in regulating plant development and immunity. Conserved pathogen-associated molecular patterns (PAMPs) derived from microbes are recognized by plant pattern recognition receptors to activate PAMP-triggered immunity (PTI). Microbial effectors, whose initial function is to promote virulence, are recognized by plant intracellular nucleotide-binding domain and leucine-rich repeat receptors (NLRs) to initiate effector-triggered immunity (ETI). Both PTI and ETI trigger early immune signaling events including the production of reactive oxygen species, induction of calcium influx and activation of mitogen-activated protein kinases. Research progress has revealed the important roles of RLCKs in the regulation of early PTI signaling. Accordingly, RLCKs are often targeted by microbial effectors that are evolved to evade PTI via diverse modulations. In some cases, modulation of RLCKs by microbial effectors triggers the activation of NLRs. This review covers the mechanisms by which RLCKs engage diverse substrates to regulate early PTI signaling and the regulatory roles of RLCKs in triggering NLR activation. Accumulating evidence suggests evolutionary links and close connections between PAMP- and effector-triggered early immune signaling that are mediated by RLCKs. As key immune regulators, RLCKs can be considered targets with broad prospects for the improvement of plant resistance via genetic engineering.

Cognitive and Memory Functions in Plant Immunity

From the time of Thucydides in the 5th century BC, it has been known that specific recognition of pathogens and memory formation are critical components of immune functions. In contrast to the immune system of jawed vertebrates, such as humans and mice, plants lack a circulatory system with mobile immune cells and a repertoire of clonally distributed antigen receptors with almost unlimited specificities. However, without these systems and mechanisms, plants can live and survive in the same hostile environment faced by other organisms. In fact, they achieve specific pathogen recognition and elimination, with limited self-reactivity, and generate immunological memory, sometimes with transgenerational characteristics. Thus, the plant immune system satisfies minimal conditions for constituting an immune system, namely, the recognition of signals in the milieu, integration of that information, subsequent efficient reaction based on the integrated information, and memorization of the experience. In the previous report, this set of elements was proposed as an example of minimal cognitive functions. In this essay, I will first review current understanding of plant immunity and then discuss the unique features of cognitive activities, including recognition of signals from external as well as internal environments, autoimmunity, and memory formation. In doing so, I hope to reach a deeper understanding of the significance of immunity omnipresent in the realm of living organisms.

Plant Membrane Transport Research in the Post-genomic Era

Membrane transport processes are indispensable for many aspects of plant physiology including mineral nutrition, solute storage, cell metabolism, cell signaling, osmoregulation, cell growth, and stress responses. Completion of genome sequencing in diverse plant species and the development of multiple genomic tools have marked a new era in understanding plant membrane transport at the mechanistic level. Genes coding for a galaxy of pumps, channels, and carriers that facilitate various membrane transport processes have been identified while multiple approaches are developed to dissect the physiological roles as well as to define the transport capacities of these transport systems. Furthermore, signaling networks dictating the membrane transport processes are established to fully understand the regulatory mechanisms. Here, we review recent research progress in the discovery and characterization of the components in plant membrane transport that take advantage of plant genomic resources and other experimental tools. We also provide our perspectives for future studies in the field.

SlCNGC1 and SlCNGC14 Suppress Xanthomonas oryzae pv. oryzicola-Induced Hypersensitive Response and Non-host Resistance in Tomato

Mechanisms underlying plant non-host resistance to Xanthomonas oryzae pv. oryzicola (Xoc), the pathogen causing rice leaf streak disease, are largely unknown. Cyclic nucleotide-gated ion channels (CNGCs) are calcium-permeable channels that are involved in various biological processes including plant resistance. In this study, functions of two tomato CNGC genes SlCNGC1 and SlCNGC14 in non-host resistance to Xoc were analyzed. Silencing of SlCNGC1 and SlCNGC14 in tomato significantly enhanced Xoc-induced hypersensitive response (HR) and non-host resistance, demonstrating that both SlCNGC1 and SlCNGC14 negatively regulate non-host resistance related HR and non-host resistance to Xoc in tomato. Silencing of SlCNGC1 and SlCNGC14 strikingly increased Xoc-induced callose deposition and strongly promoted both Xoc-induced and flg22-elicited H₂O₂, indicating that these two SlCNGCs repress callose deposition and ROS accumulation to attenuate non-host resistance and PAMP-triggered immunity (PTI). Importantly, silencing of SlCNGC1 and SlCNGC14 apparently compromised cytosolic Ca²⁺ accumulation, implying that SlCNGC1 and SlCNGC14 function as Ca²⁺ channels and negatively regulate non-host resistance and PTI-related responses through modulating cytosolic Ca²⁺ accumulation. SlCNGC14 seemed to play a stronger regulatory role in the non-host resistance and PTI compared to SlCNGC1. Our results reveal the contribution of CNGCs and probably also Ca²⁺ signaling pathway to non-host resistance and PTI.

Silencing of DND1 in potato and tomato impedes conidial germination, attachment and hyphal growth of Botrytis cinerea

BACKGROUND

Botrytis cinerea, a necrotrophic pathogenic fungus, attacks many crops including potato and tomato. Major genes for complete resistance to B. cinerea are not known in plants, but a few quantitative trait loci have been described in tomato. Loss of function of particular susceptibility (S) genes appears to provide a new source of resistance to B. cinerea in Arabidopsis.

RESULTS

In this study, orthologs of Arabidopsis S genes (DND1, DMR6, DMR1 and PMR4) were silenced by RNAi in potato and tomato (only for DND1). DND1 well-silenced potato and tomato plants showed significantly reduced diameters of B. cinerea lesions as compared to control plants, at all-time points analysed. Reduced lesion diameter was also observed on leaves of DMR6 silenced potato plants but only at 3 days post inoculation (dpi). The DMR1 and PMR4 silenced potato transformants were as susceptible as the control cv Desiree. Microscopic analysis was performed to observe B. cinerea infection progress in DND1 well-silenced potato and tomato leaves. A significantly lower number of B. cinerea conidia remained attached to the leaf surface of DND1 well-silenced potato and tomato plants and the hyphal growth of germlings was hampered.

CONCLUSIONS

This is the first report of a cytological investigation of Botrytis development on DND1-silenced crop plants. Silencing of DND1 led to reduced susceptibility to Botrytis, which was associated with impediment of conidial germination and attachment as well as hyphal growth. Our results provide new insights regarding the use of S genes in resistance breeding.

Down-regulation of Arabidopsis DND1 orthologs in potato and tomato leads to broad-spectrum resistance to late blight and powdery mildew

Multiple susceptibility genes (S), identified in Arabidopsis, have been shown to be functionally conserved in crop plants. Mutations in these S genes result in resistance to different pathogens, opening a new way to achieve plant disease resistance. The aim of this study was to investigate the role of Defense No Death1 (DND1) in susceptibility of tomato and potato to late blight (Phytophthora infestans). In Arabidopsis, the dnd1 mutant has broad-spectrum resistance against several fungal, bacterial, and viral pathogens. However this mutation is also associated with a dwarfed phenotype. Using an RNAi approach, we silenced AtDND1 orthologs in potato and tomato. Our results showed that silencing of the DND1 ortholog in both crops resulted in resistance to the pathogenic oomycete P. infestans and to two powdery mildew species, Oidium neolycopersici and Golovinomyces orontii. The resistance to P. infestans in potato was effective to four different isolates although the level of resistance (complete or partial) was dependent on the aggressiveness of the isolate. In tomato, DND1-silenced plants showed a severe dwarf phenotype and autonecrosis, whereas DND1-silenced potato plants were not dwarfed and showed a less pronounced autonecrosis. Our results indicate that S gene function of DND1 is conserved in tomato and potato. We discuss the possibilities of using RNAi silencing or loss-of-function mutations of DND1 orthologs, as well as additional S gene orthologs from Arabidopsis, to breed for resistance to pathogens in crop plants.

An IQ domain mediates the interaction with calmodulin in a plant cyclic nucleotide-gated channel

Cyclic nucleotide-gated channels (CNGCs) form non-selective cation entry pathways regulated by calmodulin (CaM), a universal Ca(2+) sensor in eukaryotes. Although CaM binding has been shown to be important for Ca(2+)-dependent feedback regulation of CNGC activity, the CaM-binding properties of these channels have been investigated in a few cases only. We show that CNGC20 from Arabidopsis thaliana binds CaM in a Ca(2+)-dependent manner and interacts with all AtCaM isoforms but not with the CaM-like proteins CML8 and CML9. CaM interaction with the full-length channel was demonstrated in planta, using bimolecular fluorescence complementation. This interaction occurred at the plasma membrane, in accordance with our localization data of green fluorescent protein (GFP)-fused CNGC20 proteins. The CaM-binding site was mapped to an isoleucine glutamine (IQ) motif, which has not been characterized in plant CNGCs so far. Our results show that compared with the overlapping binding sites for cyclic nucleotides and CaM in CNGCs studied so far, they are sequentially organized in CNGC20. The presence of two alternative CaM-binding modes indicates that ligand regulation of plant CNGCs is more complex than previously expected. Since the IQ domain is conserved among plant CNGCs, this domain adds to the variability of Ca(2+)-dependent channel control mechanisms underlining the functional diversity within this multigene family.

Paranoid potato: phytophthora-resistant genotype shows constitutively activated defense

Phytophthora is the most devastating pathogen of dicot plants. There is a need for resistance sources with different modes of action to counteract the fast evolution of this pathogen. In order to better understand mechanisms of defense against P. infestans, we analyzed several clones of potato. Two of the genotypes tested, Sarpo Mira and SW93-1015, exhibited strong resistance against P. infestans in field trials, whole plant assays and detached leaf assays. The resistant genotypes developed different sizes of hypersensitive response (HR)-related lesions. HR lesions in SW93-1015 were restricted to very small areas, whereas those in Sarpo Mira were similar to those in Solanum demissum, the main source of classical resistance genes. SW93-1015 can be characterized as a cpr (constitutive expressor of PR genes) genotype without spontaneous microscopic or macroscopic HR lesions. This is indicated by constitutive hydrogen peroxide (H₂O₂) production and PR1 (pathogenesis-related protein 1) secretion. SW93-1015 is one of the first plants identified as having classical protein-based induced defense expressed constitutively without any obvious metabolic costs or spontaneous cell death lesions.

How do plants achieve immunity? Defence without specialized immune cells

Vertebrates have evolved a sophisticated adaptive immune system that relies on an almost infinite diversity of antigen receptors that are clonally expressed by specialized immune cells that roam the circulatory system. These immune cells provide vertebrates with extraordinary antigen-specific immune capacity and memory, while minimizing self-reactivity. Plants, however, lack specialized mobile immune cells. Instead, every plant cell is thought to be capable of launching an effective immune response. So how do plants achieve specific, self-tolerant immunity and establish immune memory? Recent developments point towards a multilayered plant innate immune system comprised of self-surveillance, systemic signalling and chromosomal changes that together establish effective immunity.

Salicylic acid beyond defence: its role in plant growth and development

In recent years salicylic acid (SA) has been the focus of intensive research due to its function as an endogenous signal mediating local and systemic plant defence responses against pathogens. It has also been found that SA plays a role during the plant response to abiotic stresses such as drought, chilling, heavy metal toxicity, heat, and osmotic stress. In this sense, SA appears to be, just like in mammals, an 'effective therapeutic agent' for plants. Besides this function during biotic and abiotic stress, SA plays a crucial role in the regulation of physiological and biochemical processes during the entire lifespan of the plant. The discovery of its targets and the understanding of its molecular modes of action in physiological processes could help in the dissection of the complex SA signalling network, confirming its important role in both plant health and disease. Here, the evidence that supports the role of SA during plant growth and development is reviewed by comparing experiments performed by exogenous application of SA with analysis of genotypes affected by SA levels and/or perception.

The role of cyclic nucleotide-gated ion channels in plant immunity

Since the first plant cyclic nucleotide-gated ion channel (CNGC), HvCBT1, was identified as a calmodulin binding protein, more than a decade has passed and a substantial amount of work has been done to understand the molecular nature and function of these channel proteins. Based on electrophysiological and heterologous expression analyses, plant CNGCs function as non-selective cation channels and, so far, their biological roles have been reported in defense responses, development, and ion homeostasis. Forward genetic approaches identified four AtCNGCs (AtCNGC2, 4, 11, and 12) to be involved in plant immunity, as null mutants for AtCNGC2, 4, 11, and 12 as well as a gain-of- function mutant for AtCNGC11 and 12 exhibited alterations in defense responses. Since ion flux changes have been reported as one of the early events upon pathogen recognition and also are an essential component for the activation of defense responses, the involvement of CNGCs in these ion flux changes has been suggested. However, the recent detailed characterization of null mutants suggested a more complex involvement of this channel family. In this review, we focus on the discoveries and characterization of these CNGC mutants and discuss possible roles of CNGCs as components in plant immunity.

Phenotypic characterization of potato late blight resistance mediated by the broad-spectrum resistance gene RB

The potato gene RB, cloned from the wild potato species Solanum bulbocastanum, confers partial resistance to late blight, caused by the oomycete pathogen Phytophthora infestans. In order to better characterize this partial resistance phenotype, we have compared host resistance responses mediated by RB with those mediated by the S. demissum-derived R gene R9, which confers immunity to P. infestans carrying the corresponding avirulence gene avrR9. We found that both RB and R9 genes were capable of eliciting a hypersensitive cell death response (HR). However, in RB plants, the pathogen escaped HR lesions and continued to grow beyond the inoculation sites. We also found that callose deposition was negatively correlated with resistance levels in tested plants. Transcription patterns of pathogenesis-related (PR) genes PR-1 basic, PR-2 acidic, and PR-5 indicated that P. infestans inoculation induced transcription of these defense-related genes regardless of the host genotype; however, transcription was reduced in both the susceptible and partially resistant plants later in the infection process but remained elevated in the immune host. Most interestingly, transcription of the HR-associated gene Hin1 was suppressed in both Katahdin and RB-transgenic Katahdin but not in R9 4 days after inoculation. Together, this suggests that suppression of certain defense-related genes may allow P. infestans to spread beyond the site of infection in the partially resistant host despite elicitation of hypersensitive cell death.

SGT1 positively regulates the process of plant cell death during both compatible and incompatible plant-pathogen interactions

SGT1 (suppressor of G2 allele of Skp1), an interactor of SCF (Skp1-Cullin-F-box) ubiquitin ligase complexes that mediate protein degradation, plays an important role at both G1-S and G2-M cell cycle transitions in yeast, and is highly conserved throughout eukaryotes. Plant SGT1 is required for both resistance (R) gene-mediated disease resistance and nonhost resistance to certain pathogens. Using virus-induced gene silencing (VIGS) in Nicotiana benthamiana, we demonstrate that SGT1 positively regulates the process of cell death during both host and nonhost interactions with various pathovars of Pseudomonas syringae. Silencing of NbSGT1 in N. benthamiana plants delays the induction of hypersensitive response (HR)-mediated cell death against nonhost pathogens and the development of disease-associated cell death caused by the host pathogen P. syringae pv. tabaci. Our results further demonstrate that NbSGT1 is required for Erwinia carotovora- and Sclerotinia sclerotiorum-induced disease-associated cell death. Overexpression of NbSGT1 in N. benthamiana accelerates the development of HR during R gene-mediated disease resistance and nonhost resistance. Our data also indicate that SGT1 is required for pathogen-induced cell death, but is not always necessary for the restriction of bacterial multiplication in planta. Therefore, we conclude that SGT1 is an essential component affecting the process of cell death during both compatible and incompatible plant-pathogen interactions.

Physiology and biophysics of plant ligand-gated ion channels

Small molecules and metabolites often act as intra- or extracellular messengers in signal transduction pathways. Ligand-gated ion channels provide a mean to transduce those biochemical signals at the membrane into electrical events and ion fluxes. In plants, cyclic nucleotides and glutamate represent intra- and extracellular signalling ligands, respectively. While the former have been shown to regulate voltage-dependent ion channels and are supposed to activate cyclic nucleotide gated (CNG) channels, the latter are perceived by ionotropic glutamate receptors (GLRs). This review summarises our current knowledge about CNG channels and glutamate receptors in plants and their proposed roles in plant development and adaptation to biotic and abiotic stresses.

The first set of EST resource for gene discovery and marker development in pigeonpea (Cajanus cajan L.)

BACKGROUND

Pigeonpea (Cajanus cajan (L.) Millsp) is one of the major grain legume crops of the tropics and subtropics, but biotic stresses [Fusarium wilt (FW), sterility mosaic disease (SMD), etc.] are serious challenges for sustainable crop production. Modern genomic tools such as molecular markers and candidate genes associated with resistance to these stresses offer the possibility of facilitating pigeonpea breeding for improving biotic stress resistance. Availability of limited genomic resources, however, is a serious bottleneck to undertake molecular breeding in pigeonpea to develop superior genotypes with enhanced resistance to above mentioned biotic stresses. With an objective of enhancing genomic resources in pigeonpea, this study reports generation and analysis of comprehensive resource of FW- and SMD- responsive expressed sequence tags (ESTs).

RESULTS

A total of 16 cDNA libraries were constructed from four pigeonpea genotypes that are resistant and susceptible to FW ('ICPL 20102' and 'ICP 2376') and SMD ('ICP 7035' and 'TTB 7') and a total of 9,888 (9,468 high quality) ESTs were generated and deposited in dbEST of GenBank under accession numbers GR463974 to GR473857 and GR958228 to GR958231. Clustering and assembly analyses of these ESTs resulted into 4,557 unique sequences (unigenes) including 697 contigs and 3,860 singletons. BLASTN analysis of 4,557 unigenes showed a significant identity with ESTs of different legumes (23.2-60.3%), rice (28.3%), Arabidopsis (33.7%) and poplar (35.4%). As expected, pigeonpea ESTs are more closely related to soybean (60.3%) and cowpea ESTs (43.6%) than other plant ESTs. Similarly, BLASTX similarity results showed that only 1,603 (35.1%) out of 4,557 total unigenes correspond to known proteins in the UniProt database (or= 5 sequences detected 102 single nucleotide polymorphisms (SNPs) in 37 contigs. As an example, a set of 10 contigs were used for confirming in silico predicted SNPs in a set of four genotypes using wet lab experiments. Occurrence of SNPs were confirmed for all the 6 contigs for which scorable and sequenceable amplicons were generated. PCR amplicons were not obtained in case of 4 contigs. Recognition sites for restriction enzymes were identified for 102 SNPs in 37 contigs that indicates possibility of assaying SNPs in 37 genes using cleaved amplified polymorphic sequences (CAPS) assay.

CONCLUSION

The pigeonpea EST dataset generated here provides a transcriptomic resource for gene discovery and development of functional markers associated with biotic stress resistance. Sequence analyses of this dataset have showed conservation of a considerable number of pigeonpea transcripts across legume and model plant species analysed as well as some putative pigeonpea specific genes. Validation of identified biotic stress responsive genes should provide candidate genes for allele mining as well as candidate markers for molecular breeding.

Loss of calmodulin binding to Bax inhibitor-1 affects Pseudomonas-mediated hypersensitive response-associated cell death in Arabidopsis thaliana

Bax inhibitor-1 (BI-1) is a cell death suppressor protein conserved across a variety of organisms. The Arabidopsis atbi1-1 plant is a mutant in which the C-terminal 6 amino acids of the expressed BI-1 protein have been replaced by T-DNA insertion. This mutant BI-1 protein (AtBI-CM) produced in Escherichia coli can no longer bind to calmodulin. A promoter-reporter assay demonstrated compartmentalized expression of BI-1 during hypersensitive response, introduced by the inoculation of Pseudomonas syringae possessing the avrRTP2 gene, Pst(avrRPT2). In addition, both BI-1 knockdown plants and atbi1-1 showed increased sensitivity to Pst(avrRPT2)-induced cell death. The results indicated that the loss of calmodulin binding reduces the cell death suppressor activity of BI-1 in planta.

Transcript-based cloning of RRP46, a regulator of rRNA processing and R gene-independent cell death in barley-powdery mildew interactions

Programmed cell death (PCD) plays a pivotal role in plant development and defense. To investigate the interaction between PCD and R gene-mediated defense, we used the 22K Barley1 GeneChip to compare and contrast time-course expression profiles of Blumeria graminis f. sp hordei (Bgh) challenged barley (Hordeum vulgare) cultivar C.I. 16151 (harboring the Mla6 powdery mildew resistance allele) and its fast neutron-derived Bgh-induced tip cell death1 mutant, bcd1. Mixed linear model analysis identified genes associated with the cell death phenotype as opposed to R gene-mediated resistance. One-hundred fifty genes were found at the threshold P value < 0.0001 and a false discovery rate <0.6%. Of these, 124 were constitutively overexpressed in the bcd1 mutant. Gene Ontology and rice (Oryza sativa) alignment-based annotation indicated that 68 of the 124 overexpressed genes encode ribosomal proteins. A deletion harboring six genes on chromosome 5H cosegregates with bcd1-specified cell death and is associated with misprocessing of rRNAs but segregates independent of R gene-mediated resistance. Barley stripe mosaic virus-induced gene silencing of one of the six deleted genes, RRP46 (rRNA-processing protein 46), phenocopied bcd1-mediated tip cell death. These findings suggest that RRP46, a critical component of the exosome core, mediates RNA processing and degradation involved in cell death initiation as a result of attempted penetration by Bgh during the barley-powdery mildew interaction but is independent of gene-for-gene resistance.

Signaling pathways that regulate the enhanced disease resistance of Arabidopsis "defense, no death" mutants

Arabidopsis dnd1 and dnd2 mutants lack cyclic nucleotide-gated ion channel proteins and carry out avirulence or resistance gene-mediated defense with a greatly reduced hypersensitive response (HR). They also exhibit elevated broad-spectrum disease resistance and constitutively elevated salicylic acid (SA) levels. We examined the contributions of NPR1, SID2 (EDS16), NDR1, and EIN2 to dnd phenotypes. Mutations that affect SA accumulation or signaling (sid2, npr1, and ndr1) abolished the enhanced resistance of dnd mutants against Pseudomonas syringae pv. tomato and Hyaloperonospora parasitica but not Botrytis cinerea. When SA-associated pathways were disrupted, the constitutive activation of NPR1-dependent and NPR1-independent and SA-dependent pathways was redirected toward PDF1.2-associated pathways. This PDF1.2 overexpression was downregulated after infection by P. syringae. Disruption of ethylene signaling abolished the enhanced resistance to B. cinerea but not P. syringae or H. parasitica. However, loss of NPR1, SID2, NDR1, or EIN2 did not detectably alter the reduced HR in dnd mutants. The susceptibility of dnd ein2 plants to B. cinerea despite their reduced-HR phenotype suggests that cell death repression is not the primary cause of dnd resistance to necrotrophic pathogens. The partial restoration of resistance to B. cinerea in dnd1 npr1 ein2 triple mutants indicated that this resistance is not entirely EIN2 dependent. The above findings indicate that the broad-spectrum resistance of dnd mutants occurs due to activation or sensitization of multiple defense pathways, yet none of the investigated pathways are required for the reduced-HR phenotype.

In silico comparison of transcript abundances during Arabidopsis thaliana and Glycine max resistance to Fusarium virguliforme

BACKGROUND

Sudden death syndrome (SDS) of soybean (Glycine max L. Merr.) is an economically important disease, caused by the semi-biotrophic fungus Fusarium solani f. sp. glycines, recently renamed Fusarium virguliforme (Fv). Due to the complexity and length of the soybean-Fusarium interaction, the molecular mechanisms underlying plant resistance and susceptibility to the pathogen are not fully understood. F. virguliforme has a very wide host range for the ability to cause root rot and a very narrow host range for the ability to cause a leaf scorch. Arabidopsis thaliana is a host for many types of phytopathogens including bacteria, fungi, viruses and nematodes. Deciphering the variations among transcript abundances (TAs) of functional orthologous genes of soybean and A. thaliana involved in the interaction will provide insights into plant resistance to F. viguliforme.

RESULTS

In this study, we reported the analyses of microarrays measuring TA in whole plants after A. thaliana cv 'Columbia' was challenged with fungal pathogen F. virguliforme. Infection caused significant variations in TAs. The total number of increased transcripts was nearly four times more than that of decreased transcripts in abundance. A putative resistance pathway involved in responding to the pathogen infection in A. thaliana was identified and compared to that reported in soybean.

CONCLUSION

Microarray experiments allow the interrogation of tens of thousands of transcripts simultaneously and thus, the identification of plant pathways is likely to be involved in plant resistance to Fusarial pathogens. Dissection of the set functional orthologous genes between soybean and A. thaliana enabled a broad view of the functional relationships and molecular interactions among plant genes involved in F. virguliforme resistance.

30-year progress of membrane transport in plants

In the past 30 years enormous progress was made in plant membrane biology and transport physiology, a fact reflected in the appearance of textbooks. The first book dedicated to 'Membrane Transport in Plants' was published on the occasion of the 'International Workshop on Membrane Transport in Plants' held at the Nuclear Research Center, Jülich, Germany [Zimmermann and Dainty (eds) 1974] and was followed in 1976 by a related volume 'Transport in plants II' in the 'Encyclopedia of plant physiology' [Lüttge and Pitman (eds) 1976]. A broad spectrum of topics including thermodynamics of transport processes, water relations, primary reactions of photosynthesis, as well as more conventional aspects of membrane transport was presented. The aim of the editors of the first book was to bring advanced thermodynamical concepts to the attention of biologists and to show physical chemists and biophysicist what the more complex biological systems were like. To bundle known data on membrane transport in plants and relevant fields for mutual understanding, interdisciplinary research and clarification of problems were considered highly important for further progress in this scientific area of plant physiology. The present review will critically evaluate the progress in research in membrane transport in plants that was achieved during the past. How did 'Membrane Transport in Plants' progress within the 30 years between the publication of the first book about this topic (Zimmermann and Dainty 1974), a recent one with the same title (Blatt 2004), and today?

Both hypersensitive and non-hypersensitive responses are associated with resistance in Salix viminalis against the gall midge Dasineura marginemtorquens

Hypersensitivity responses (HR) play a major role in plant resistance to pathogens. It is often claimed that HR is also important in plant resistance to insects, although there is little unambiguous documentation. Large genotypic variation in resistance against the gall midge Dasineura marginemtorquens is found in Salix viminalis. Variation in larval performance and induced responses within a full-sib S. viminalis family is reported here; 36 sibling plants were completely resistant (larvae died within 48 h after egg hatch, no gall induction), 11 plants were totally susceptible, 25 plants were variable (living and dead larvae present on the same plant). Resistance was associated with HR, but to different degrees; 21 totally resistant genotypes showed typical HR symptoms (many distinct necrotic spots) whereas the remaining 15 genotypes showed no, or very few, such symptoms. Hydrogen peroxide, used as a marker for HR, was induced in genotypes expressing HR symptoms but not in resistant genotypes without symptoms, or in susceptible genotypes. These data suggest that production of hydrogen peroxide, and accompanying cell death, cannot explain larval mortality in the symptomless reaction. Another, as yet unknown, mechanism of resistance may be present. If so, then it is possible that this unknown mechanism also contributes to resistance in plants displaying HR. The apparent complexity observed in this interaction, with both visible and invisible plant responses associated with resistance against an adapted insect species, may have implications for the study of resistance factors in other plant-insect interactions.

Root response to Fusarium solani f. sp . glycines: temporal accumulation of transcripts in partially resistant and susceptible soybean

Sudden death syndrome (SDS) of soybean is a complex of root rot disease caused by the semi-biotrophic fungus Fusarium solani f. sp. glycines (Fsg) and a leaf scorch disease caused by toxins produced by the pathogen in the roots. Development of partial rate-reducing resistance in roots to SDS was studied. The recombinant inbred line 23 (RIL23) that carried resistance conferred by six quantitative trait loci (QTL) derived from cultivars 'Essex' x 'Forrest' was compared to the susceptible cultivar Essex. Roots of RIL23 and its susceptible parent Essex were inoculated with Fsg. Transcript abundance (TA) of 191 ESTs was studied at five time points after inoculation. For most of the genes, there was an initial decrease in TA in the inoculated roots of both genotypes. By days 7 and 10 the inoculated roots of Essex failed to increase expression of the transcripts of defense-related genes. In RIL23 inoculated roots, the TA of 81 genes was increased by at least two-fold at day 3 (P=0.004), 88 genes at day 7 (P=0.0023) and 129 genes at day 10 (P=0.0026). A set of 35 genes maintained at least a two-fold higher abundance at all three time points. The increase in TA in RIL23 was in contrast to that observed in Essex where most of the ESTs showed either no change or a decreased TA. The ESTs with an increased TA had homology to the genes involved in resistance (analogs), signal transduction, plant defense, cell wall synthesis and transport of metabolites. Pathways that responded included the protein phosphorylation cascade, the phospholipase cascade and the phenolic natural products pathways, including isoflavone and cell wall synthesis.

Plant defence responses: what have we learnt from Arabidopsis?

To overcome the attack of invading pathogens, a plant's defence system relies on preformed and induced responses. The induced responses are activated following detection of a pathogen, with the subsequent transmission of signals and orchestrated cellular events aimed at eliminating the pathogen and preventing its spread. Numerous studies are proving that the activated signalling pathways are not simply linear, but rather, form complex networks where considerable cross talk takes place. This review covers the recent application of powerful genetic and genomic approaches to identify key defence signalling pathways in the model plant Arabidopsis thaliana (L.) Heynh. The identification of key regulatory components of these pathways may offer new approaches to increase the defence capabilities of crop plants.

Arabidopsis DND2, a second cyclic nucleotide-gated ion channel gene for which mutation causes the "defense, no death" phenotype

A previous mutant screen identified Arabidopsis dnd1 and dnd2 "defense, no death" mutants, which exhibit loss of hypersensitive response (HR) cell death without loss of gene-for-gene resistance. The dnd1 phenotype is caused by mutation of the gene encoding cyclic nucleotide-gated (CNG) ion channel AtCNGC2. This study characterizes dnd2 plants. Even in the presence of high titers of Pseudomonas syringae expressing avrRpt2, most leaf mesophyll cells in the dnd2 mutant exhibited no HR. These plants retained strong RPS2-, RPM1-, or RPS4-mediated restriction of P. syringae pathogen growth. Mutant dnd2 plants also exhibited enhanced broad-spectrum resistance against virulent P. syringae and constitutively elevated levels of salicylic acid, and pathogenesis-related (PR) gene expression. Unlike the wild type, dnd2 plants responding to virulent and avirulent P. syringae exhibited elevated expression of both salicylate-dependent PR-1 and jasmonate and ethylene-dependent PDF1.2. Introduction of nahG+ (salicylate hydroxylase) into the dnd2 background, which removes salicylic acid and causes other defense alterations, eliminated constitutive disease resistance and PR gene expression but only weakly impacted the HR- phenotype. Map-based cloning revealed that dnd2 phenotypes are caused by mutation of a second CNG ion channel gene, AtCNGC4. Hence, loss of either of two functionally nonredundant CNG ion channels can cause dnd phenotypes. The dnd mutants provide a unique genetic background for dissection of defense signaling.

Characterisation of an Arabidopsis-Leptosphaeria maculans pathosystem: resistance partially requires camalexin biosynthesis and is independent of salicylic acid, ethylene and jasmonic acid signalling

Out of 168 Arabidopsis accessions screened with isolates of Leptosphaeria maculans, one (An-1) showed clear disease symptoms. In order to identify additional components involved in containment of L. maculans in Arabidopsis, a screen for L. maculans-susceptible (lms) mutants was performed. Eleven lms mutants were isolated, which displayed differential susceptibility responses to L. maculans. lms1 was crossed with Columbia (Col-0) and Ws-0, and mapping data for both populations showed the highest linkage to a region on chromosome 2. Reduced levels of PR-1 and PDF1.2 expression were found in lms1 compared to wild-type plants 48 h after pathogen inoculation. In contrast, the lms1 mutant displayed upregulation of either marker gene upon chemical treatment, possibly as an effect of an altered ethylene (ET) response. To assess the contribution of different defence pathways, genotypes implicated in salicylic acid (SA) signalling plants expressing the bacterial salicylate hydroxylase (nahG) gene, non-expressor of PR1 (npr1)-1 and phytoalexin-deficient (pad4-1), jasmonic acid (JA) signalling (coronatine insensitive (coi)1-16, enhanced disease susceptibility (eds)8-1 and jasmonic acid resistant (jar)1-1) and ET signalling (eds4-1, ethylene insensitive (ein)2, ein3-1 and ethylene resistant (etr)1-1) were screened. All the genotypes screened were as resistant as wild-type plants, demonstrating the dispensability of the pathways in L. maculans resistance. When mutants implicated in cell death responses were assayed, responsive to antagonist 1 (ran1)-1 exhibited a weak susceptible phenotype, whereas accelerated cell death (acd)1-20 showed a rapid lesion development. Camalexin is only partially responsible for L. maculans containment in Arabidopsis, as pad3-1 and enhanced susceptibility to Alternaria (esa)1 clearly showed a susceptible response while wild-type levels of camalexin were present in An-1 and lms1. The data presented point to the existence of multiple defence mechanisms controlling the containment of L. maculans in Arabidopsis.

Negative cross-talk between salicylate- and jasmonate-mediated pathways in the Wassilewskija ecotype of Arabidopsis thaliana

Plants often respond to attack by insect herbivores and necrotrophic pathogens with induction of jasmonate-dependent resistance traits, but respond to attack by biotrophic pathogens with induction of salicylate-dependent resistance traits. To assess the degree to which the jasmonate- and salicylate-dependent pathways interact, we compared pathogenesis-related protein activity and bacterial performance in four mutant Arabidopsis thaliana lines relative to their wild-type backgrounds. We found that two salicylate-dependent pathway mutants (cep1, nim1-1) exhibited strong effects on the growth of the generalist biotrophic pathogen, Pseudomonas syringae pv. tomato, whereas two jasmonate-dependent pathway mutants (fad3-2fad7-2fad8, jar1-1) did not. Leaf peroxidase and exochitinase activity were negatively correlated with bacterial growth, whereas leaf polyphenol oxidase activity and trypsin inhibitor concentration were not. Interestingly, leaf total glucosinolate concentration was positively correlated with bacterial growth. In the same experiment, we also found that application of jasmonic acid generally increased leaf peroxidase activity and trypsin inhibitor concentration in the mutant lines. However, the cep1 mutant, shown previously to overexpress salicylic acid, exhibited no detectable biological or chemical responses to jasmonic acid, suggesting that high levels of salicylic acid may have inhibited a plant response. In a second experiment, we compared the effect of jasmonic acid and/or salicylic acid on two ecotypes of A. thaliana. Application of salicylic acid to the Wassilewskija ecotype decreased bacterial growth. However, this effect was not observed when both salicylic acid and jasmonic acid were applied, suggesting that jasmonic acid negated the beneficial effect of salicylic acid. Collectively, our results confirm that the salicylate-dependent pathway is more important than the jasmonate-dependent pathway in determining growth of P. syringae pv. tomato in A. thaliana, and suggest important negative interactions between these two major defensive pathways in the Wassilewskija ecotype. In contrast, the Columbia ecotype exhibited little evidence of negative interactions between the two pathways, suggesting intraspecific variability in how these pathways interact in A. thaliana.

Preexisting systemic acquired resistance suppresses hypersensitive response-associated cell death in Arabidopsis hrl1 mutant

The hypersensitive response (HR) displayed by resistant plants against invading pathogens is a prominent feature of plant-pathogen interactions. The Arabidopsis hypersensitive response like lesions1 (hrl1) mutant is characterized by heightened defense responses that make it more resistant to virulent pathogens. However, hrl1 suppresses avirulent pathogen-induced HR cell death. Furthermore, the high PR-1 expression observed in hrl1 remains unaltered after avirulent and virulent pathogen infections. The suppressed HR phenotype in hrl1 is observed even when an elicitor is expressed endogenously from an inducible promoter, suggesting that an impaired transfer of avirulent factors is not the reason. Interestingly, the lack of HR phenotype in hrl1 is reversed if the constitutive defense responses are compromised either by a mutation in NON EXPRESSOR OF PR-1 (NPR1) or by depleting salicylic acid due to the expression of the nahG gene. The rescue of HR cell death in hrl1 npr1 and in hrl1 nahG depends on the extent to which the constitutive systemic acquired response (SAR) is compromised. Pretreating Arabidopsis wild-type plants with SAR-inducers, before pathogen infection resulted in a significant decrease in HR cell death. Together, these results demonstrate that the preexisting SAR may serve as one form of negative feedback loop to regulate HR-associated cell death in hrl1 mutant and in the wild-type plants.

Rds and Rih mediate hypersensitive cell death independent of gene-for-gene resistance to the oat crown rust pathogen Puccinia coronata f. sp. avenae

The Pca crown rust resistance cluster in the diploid Avena genus confers gene-for-gene specificity to numerous isolates of Puccinia coronata f. sp. avenae. Recombination breakpoint analysis indicates that specificities conferred by the Pca cluster are controlled by at least five distinct genes, designated Pc81, Pc82, Pc83, Pc84, and Pc85. Avena plants with the appropriate genotype frequently respond to P. coronata by undergoing hypersensitive cell death at the sites of fungal infection. Autofluorescence of host cells in response to P. coronata occurs in plants that develop visible necrotic lesions but not in plants that lack this phenotype. Two newly described, non-Pc loci were shown to control hypersensitive cell death. Rds (resistance-dependent suppressor of cell death) suppresses the hypersensitive response (HR), but not the resistance, mediated by the Pc82 resistance gene. In contrast, Rih (resistance-independent hypersensitive cell death) confers HR in both resistant and susceptible plants. Linkage analysis indicates that Rds is unlinked to the Pca cluster, whereas Rih is tightly linked to it. These results indicate that multiple synchronous pathways affect the development of hypersensitive cell death and that HR is not essential for resistance to crown rust. Further characterization of these genes will clarify the relationship between plant disease resistance and localized hypersensitive cell death.

Functional expression of Cf9 and Avr9 genes in Brassica napus induces enhanced resistance to Leptosphaeria maculans

The tomato Cf9 resistance gene induces an Avr9-dependent hypersensitive response (HR) in tomato and transgenic Solanaceae spp. We studied whether the Cf9 gene product responded functionally to the corresponding Avr9 gene product when introduced in a heterologous plant species. We successfully expressed the Cf9 gene under control of its own promoter and the Avr9 or Avr9R8K genes under control of the p35S1 promoter in transgenic oilseed rape. We demonstrated that the transgenic oilseed rape plants produced the Avr9 elicitor with the same specific necrosis-inducing activity as reported for Cladosporium fulvum. An Avr9-dependent HR was induced in Cf9 oilseed rape upon injection of intercellular fluid containing Avr9. We showed Avr9-specific induction of PR1, PR2, and Cxc750 defense genes in oilseed rape expressing CJ9. Cf9 x Avr9 oilseed rape did not result in seedling death of the F1 progeny, independent of the promoters used to express the genes. The F1 (Cf9 x Avr9) plants, however, were quantitatively more resistant to Leptosphaeria maculans. Phytopathological analyses revealed that disease development of L. maculans was delayed when the pathogen was applied on an Avr9-mediated HR site. We demonstrate that the CJ9 and Avr9 gene can be functionally expressed in a heterologous plant species and that the two components confer an increase in disease resistance.

The Arabidopsis aberrant growth and death2 mutant shows resistance to Pseudomonas syringae and reveals a role for NPR1 in suppressing hypersensitive cell death

A novel Arabidopsis mutant has been identified with constitutive expression of GST1-GUS using plants with a pathogen-responsive reporter transgene containing the beta-glucuronidase (GUS) coding region driven by the GST1 promoter. The recessive mutant, called agd2 (aberrant growth and death2), has salicylic acid (SA)-dependent increased resistance to virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae, elevated SA levels, a low level of spontaneous cell death, callose deposition, and enlarged cells in leaves. The enhanced resistance of agd2 to virulent P. syringae requires the SA signaling component NONEXPRESSOR OF PR1 (NPR1). However, agd2 renders the resistance response to P. syringae carrying avrRpt2 NPR1-independent. Thus agd2 affects both an SA- and NPR1-dependent general defense pathway and an SA-dependent, NPR1-independent pathway that is active during the recognition of avirulent P. syringae. agd2 plants also fail to show a hypersensitive cell death response (HR) unless NPR1 is removed. This novel function for NPR1 is also apparent in otherwise wild-type plants: npr1 mutants show a stronger HR, while NPR1-overproducing plants show a weaker HR when infected with P. syringae carrying the avrRpm1 gene. Spontaneous cell death in agd2 is partially suppressed by npr1, indicating that NPR1 can suppress or enhance cell death depending on the cellular context. agd2 plants depleted of SA show a dramatic exacerbation of the cell-growth phenotype and increased callose deposition, suggesting a role for SA in regulating growth and this cell-wall modification. AGD2 may function in cell death and/or growth control as well as the defense response, similarly to what has been described in animals for the functions of NFkappaB.

Genes controlling expression of defense responses in Arabidopsis--2001 status

In the past two years, the focus of studies of the genes controlling expression of defense responses in Arabidopsis has shifted from the identification of mutants to gene isolation and the ordering of genes within branches of the signal transduction networks. It is now clear that gene-for-gene resistance can be mediated through at least three genetically distinguishable pathways. Additional genes affecting salicylic-acid-dependent signaling have been identified, and double-mutant analyses have begun to reveal the order in which they act. Genes required for jasmonic-acid-dependent signaling and for induced systemic resistance have also been identified.

Mutational analysis of the Arabidopsis RPS2 disease resistance gene and the corresponding pseudomonas syringae avrRpt2 avirulence gene

Plants have evolved a large number of disease resistance genes that encode proteins containing conserved structural motifs that function to recognize pathogen signals and to initiate defense responses. The Arabidopsis RPS2 gene encodes a protein representative of the nucleotide-binding site-leucine-rich repeat (NBS-LRR) class of plant resistance proteins. RPS2 specifically recognizes Pseudomonas syringae pv. tomato strains expressing the avrRpt2 gene and initiates defense responses to bacteria carrying avrRpt2, including a hypersensitive cell death response (HR). We present an in planta mutagenesis experiment that resulted in the isolation of a series of rps2 and avrRpt2 alleles that disrupt the RPS2-avrRpt2 gene-for-gene interaction. Seven novel avrRpt2 alleles incapable of eliciting an RPS2-dependent HR all encode proteins with lesions in the C-terminal portion of AvrRpt2 previously shown to be sufficient for RPS2 recognition. Ten novel rps2 alleles were characterized with mutations in the NBS and the LRR. Several of these alleles code for point mutations in motifs that are conserved among NBS-LRR resistance genes, including the third LRR, which suggests the importance of these motifs for resistance gene function.

The Arabidopsis dnd1 "defense, no death" gene encodes a mutated cyclic nucleotide-gated ion channel

Gene-for-gene disease resistance typically includes a programmed cell death response known as the hypersensitive response (HR). The Arabidopsis thaliana dnd1 mutant was previously isolated as a line that failed to produce the HR in response to avirulent Pseudomonas syringae pathogens; plants homozygous for the recessive dnd1-1 mutation still carry out effective gene-for-gene resistance. The dnd1-1 mutation also causes constitutive systemic resistance and elevated levels of salicylic acid. In the present study, a positional cloning approach was used to isolate DND1. DND1 encodes the same protein as AtCNGC2, a cyclic nucleotide-gated ion channel of previously unknown organismal function that can allow passage of Ca(2+), K(+) and other cations [Leng, Q., Mercier, R. W., Yao, W. & Berkowitz, G. A. (1999) Plant Physiol. 121, 753-761]. By using a nahG transgene, we found that salicylic acid is required for the elevated resistance caused by the dnd1 mutation but that removal of salicylic acid did not completely eliminate the dwarf and loss-of-HR phenotypes of mutant dnd1 plants. A stop codon that would severely truncate the DND1 gene product was identified in the dnd1-1 allele. This demonstrates that broad-spectrum disease resistance and inhibition of the HR can be activated in plants by disruption of a cyclic nucleotide-gated ion channel.