ERECTA, an LRR receptor-like kinase protein controlling development pleiotropically affects resistance to bacterial wilt

Pathogenomics of the Ralstonia solanacearum species complex

Ralstonia solanacearum is a major phytopathogen that attacks many crops and other plants over a broad geographical range. The extensive genetic diversity of strains responsible for the various bacterial wilt diseases has in recent years led to the concept of an R. solanacearum species complex. Genome sequencing of more than 10 strains representative of the main phylogenetic groups has broadened our knowledge of the evolution and speciation of this pathogen and led to the identification of novel virulence-associated functions. Comparative genomic analyses are now opening the way for refined functional studies. The many molecular determinants involved in pathogenicity and host-range specificity are described, and we also summarize current understanding of their roles in pathogenesis and how their expression is tightly controlled by an intricate virulence regulatory network.

The presence of multiple introns is essential for ERECTA expression in Arabidopsis

Gene expression in eukaryotes is often enhanced by the presence of introns. Depending on the specific gene, this enhancement can be minor or very large and occurs at both the transcriptional and post-transcriptional levels. The Arabidopsis ERECTA gene contains 27 exons encoding a receptor-like kinase that promotes cell proliferation and inhibits cell differentiation in above-ground plant organs. The expression of ERECTA very strongly depends on the presence of introns. The intronless ERECTA gene does not rescue the phenotype of erecta mutant plants and produces about 500-900 times less protein compared with the identical construct containing introns. This result is somewhat surprising as the region upstream of the ERECTA coding sequence effectively promotes the expression of extraneous genes. Here, we demonstrate that introns are essential for ERECTA mRNA accumulation and, to a lesser extent, for mRNA utilization in translation. Since mRNA produced by intronless ERECTA is degraded at the 3' end, we speculate that introns increase mRNA accumulation through increasing its stability at least in part. No individual intron is absolutely necessary for ERECTA expression, but rather multiple introns in specific locations increase ERECTA expression in an additive manner. The ability of introns to promote ERECTA expression might be linked to the process of splicing and not to a particular intron sequence.

PdERECTA, a leucine-rich repeat receptor-like kinase of poplar, confers enhanced water use efficiency in Arabidopsis

Water deficiency causes a dramatic reduction in crop production globally. Breeding crop varieties that are more efficient in their water use is one strategy to overcome this predicament. In this study, a member of the LRR-RLKs family, the Populus nigra × (Populus deltoides × Populus nigra) ERECTA (PdERECTA) gene was cloned. To study the biological functions of PdERECTA, transgenic Arabidopsis plants (35S:PdERECTA) that constitutively expressed the PdERECTA gene were constructed. Overexpression of PdERECTA resulted in early seedling establishment, longer primary roots, and larger leaf areas. Notably, transgenic Arabidopsis overexpressing PdERECTA resulted in enhanced long-term water use efficiency (WUEl), as estimated by the analysis of carbon isotopic discrimination. The WUEl results were supported by the physiological and anatomical results, which included improved photosynthetic rate, decreased transpiration rate, and stomatal density. The transgenic lines have significantly more dry-biomass as compared to the wild type. Since the overexpression of PdERECTA can strongly enhance the water use efficiency in transgenic Arabidopsis plants, PdERECTA could potentially be used in transgenic breeding to improve the water use efficiency.

Generation of signaling specificity in Arabidopsis by spatially restricted buffering of ligand-receptor interactions

Core signaling pathways function in multiple programs during multicellular development. The mechanisms that compartmentalize pathway function or confer process specificity, however, remain largely unknown. In Arabidopsis thaliana, ERECTA (ER) family receptors have major roles in many growth and cell fate decisions. The ER family acts with receptor TOO MANY MOUTHS (TMM) and several ligands of the EPIDERMAL PATTERNING FACTOR LIKE (EPFL) family, which play distinct yet overlapping roles in patterning of epidermal stomata. Here, our examination of EPFL genes EPFL6/CHALLAH (CHAL), EPFL5/CHALLAH-LIKE1, and EPFL4/CHALLAH-LIKE2 (CLL2) reveals that this family may mediate additional ER-dependent processes. chal cll2 mutants display growth phenotypes characteristic of er mutants, and genetic interactions are consistent with CHAL family molecules acting as ER family ligands. We propose that different classes of EPFL genes regulate different aspects of ER family function and introduce a TMM-based discriminatory mechanism that permits simultaneous, yet compartmentalized and distinct, function of the ER family receptors in growth and epidermal patterning.

The ERECTA gene controls spatial and temporal patterns of epidermal cell number and size in successive developing leaves of Arabidopsis thaliana

BACKGROUND AND AIMS

ERECTA has been identified as a pleiotropic regulator of developmental and physiological processes in Arabidopsis thaliana. Previous work demonstrated a role for ERECTA in the control of compensation between epidermal cell expansion and division in leaves.

METHODS

In this work, spatial and temporal analyses of epidermal cell division and expansion were performed on successive developing vegetative leaves of Arabidopsis thaliana in both ERECTA and erecta lines, LER and Ler, respectively, to understand how the ERECTA gene regulates compensation between these two processes.

KEY RESULTS

The loss of ERECTA function leads to a low cell expansion rate in all zones of a leaf and in all successive leaves of a plant. This low cell expansion rate is counterbalanced by an increase in the duration of cell division. As a consequence, the ERECTA mutation eliminates the tip to base cellular gradient generally observed in the leaf epidermis and also flattens the heteroblastic changes in epidermal cell area and number within a rosette. Ablation of floral buds eliminates the heteroblastic changes in cellular patterns in an ERECTA-dependent manner.

CONCLUSIONS

The results provide a detailed description of changes in leaf growth dynamics and cellular variables in both LER and Ler. Altogether they suggest that ERECTA influences leaf cellular development in relation to whole plant ontogeny.

Structure-function analysis of STRUBBELIG, an Arabidopsis atypical receptor-like kinase involved in tissue morphogenesis

Tissue morphogenesis in plants requires the coordination of cellular behavior across clonally distinct histogenic layers. The underlying signaling mechanisms are presently being unraveled and are known to include the cell surface leucine-rich repeat receptor-like kinase STRUBBELIG in Arabidopsis. To understand better its mode of action an extensive structure-function analysis of STRUBBELIG was performed. The phenotypes of 20 EMS and T-DNA-induced strubbelig alleles were assessed and homology modeling was applied to rationalize their possible effects on STRUBBELIG protein structure. The analysis was complemented by phenotypic, cell biological, and pharmacological investigations of a strubbelig null allele carrying genomic rescue constructs encoding fusions between various mutated STRUBBELIG proteins and GFP. The results indicate that STRUBBELIG accepts quite some sequence variation, reveal the biological importance for the STRUBBELIG N-capping domain, and reinforce the notion that kinase activity is not essential for its function in vivo. Furthermore, individual protein domains of STRUBBELIG cannot be related to specific STRUBBELIG-dependent biological processes suggesting that process specificity is mediated by factors acting together with or downstream of STRUBBELIG. In addition, the evidence indicates that biogenesis of a functional STRUBBELIG receptor is subject to endoplasmic reticulum-mediated quality control, and that an MG132-sensitive process regulates its stability. Finally, STRUBBELIG and the receptor-like kinase gene ERECTA interact synergistically in the control of internode length. The data provide genetic and molecular insight into how STRUBBELIG regulates intercellular communication in tissue morphogenesis.

Characterisation of the legume SERK-NIK gene superfamily including splice variants: implications for development and defence

BACKGROUND

SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) genes are part of the regulation of diverse signalling events in plants. Current evidence shows SERK proteins function both in developmental and defence signalling pathways, which occur in response to both peptide and steroid ligands. SERKs are generally present as small gene families in plants, with five SERK genes in Arabidopsis. Knowledge gained primarily through work on Arabidopsis SERKs indicates that these proteins probably interact with a wide range of other receptor kinases and form a fundamental part of many essential signalling pathways. The SERK1 gene of the model legume, Medicago truncatula functions in somatic and zygotic embryogenesis, and during many phases of plant development, including nodule and lateral root formation. However, other SERK genes in M. truncatula and other legumes are largely unidentified and their functions unknown.

RESULTS

To aid the understanding of signalling pathways in M. truncatula, we have identified and annotated the SERK genes in this species. Using degenerate PCR and database mining, eight more SERK-like genes have been identified and these have been shown to be expressed. The amplification and sequencing of several different PCR products from one of these genes is consistent with the presence of splice variants. Four of the eight additional genes identified are upregulated in cultured leaf tissue grown on embryogenic medium. The sequence information obtained from M. truncatula was used to identify SERK family genes in the recently sequenced soybean (Glycine max) genome.

CONCLUSIONS

A total of nine SERK or SERK-like genes have been identified in M. truncatula and potentially 17 in soybean. Five M. truncatula SERK genes arose from duplication events not evident in soybean and Lotus. The presence of splice variants has not been previously reported in a SERK gene. Upregulation of four newly identified SERK genes (in addition to the previously described MtSERK1) in embryogenic tissue cultures suggests these genes also play a role in the process of somatic embryogenesis. The phylogenetic relationship of members of the SERK gene family to closely related genes, and to development and defence function is discussed.

Genome-wide identification, characterization and phylogenetic analysis of the rice LRR-kinases

LRR-kinases constitute the largest subfamily of receptor-like kinases in plants and regulate a wide variety of processes related to development and defense. Through a reiterative process of sequence analysis and re-annotation, we identified 309 LRR-kinase genes in the rice genome (Nipponbare). Among them, 127 genes in the Rice Annotation Project Database and 85 in Refseq of NCBI were amended (in addition, 62 LRR-kinase genes were not annotated in Refseq). The complete set of LRR-kinases was characterized. These LRR-kinases were classified into five groups according to phylogenetic analysis, and the genes in groups 1, 2, 3 and 4 usually have fewer introns than those in group 5. The introns in the LRR domain, which are highly conserved in regards to their positions and configurations, split the first Leu or other amino residues at this position of the ‘xxLxLxx’ motif with phase 2 and usually separate one or more LRR repeats exactly. Tandemly repeated LRR motifs have evolved from exon duplication, mutation and exon shuffling. The extensive distribution and diversity of the LRR-kinase genes have been mainly generated by tandem duplication and mutation after whole genome duplication. Positive selection has made a limited contribution to the sequence diversity after duplication, but positively selected sites located in the LRR domain are thought to involve in the protein-protein interaction.

Genetic and environmental control of the Verticillium syndrome in Arabidopsis thaliana

BACKGROUND

Verticillium spp. are major pathogens of dicotyledonous plants such as cotton, tomato, olive or oilseed rape. Verticillium symptoms are often ambiguous and influenced by development and environment. The aim of the present study was to define disease and resistance traits of the complex Verticillium longisporum syndrome in Arabidopsis thaliana (L.) Heynh. A genetic approach was used to determine genetic, developmental and environmental factors controlling specific disease and resistance traits and to study their interrelations.

RESULTS

A segregating F2/F3 population originating from ecotypes 'Burren' (Bur) and 'Landsberg erecta' (Ler) was established. Plants were root-dip inoculated and tested under greenhouse conditions. The Verticillium syndrome was dissected into components like systemic spread, stunting, development time and axillary branching. Systemic spread of V. longisporum via colonisation of the shoot was extensive in Ler; Bur showed a high degree of resistance against systemic spread. Fungal colonisation of the shoot apex was determined by (a) determining the percentage of plants from which the fungus could be re-isolated and (b) measuring fungal DNA content with quantitative real-time PCR (qPCR). Four quantitative trait loci (QTL) controlling systemic spread were identified for the percentage of plants showing fungal outgrowth, two of these QTL were confirmed with qPCR data. The degree of colonisation by V. longisporum was negatively correlated with development time. QTL controlling development time showed some overlap with QTL for resistance to systemic spread. Stunting depended on host genotype, development time and seasonal effects. Five QTL controlling this trait were identified which did not co-localize with QTL controlling systemic spread. V. longisporum induced increased axillary branching in Bur; two QTL controlling this reaction were found.

CONCLUSIONS

Systemic spread of V. longisporum in the host as well as resistance to this major disease trait are described for the first time in natural A. thaliana accessions. This creates the possibility to study a major resistance mechanism against vascular pathogens in this model plant and to clone relevant genes of the involved pathways. Stunting resistance and resistance to systemic spread were controlled by different QTL and should be treated as separate traits. Developmental and environmental effects on pathogenesis and resistance need to be considered when designing and interpreting experiments in research and breeding.

Regulatory network identification by genetical genomics: signaling downstream of the Arabidopsis receptor-like kinase ERECTA

Gene expression differences between individuals within a species can be largely explained by differences in genetic background. The effect of genetic variants (alleles) of genes on expression can be studied in a multifactorial way by the application of genetical genomics or expression quantitative trait locus mapping. In this paper, we present a strategy to construct regulatory networks by the application of genetical genomics in combination with transcript profiling of mutants that are disrupted in single genes. We describe the network identification downstream of the receptor-like kinase ERECTA in Arabidopsis (Arabidopsis thaliana). Extending genetical genomics on the Landsberg erecta/Cape Verde Islands (Ler/Cvi) recombinant inbred population with expression profiling of monogenic mutants enabled the identification of regulatory networks in the so far elusive ERECTA signal transduction cascade. We provide evidence that ERECTA is the causal gene for the major hotspot for transcript regulation in the Arabidopsis Ler/Cvi recombinant inbred population. We further propose additional genetic variation between Ler and Cvi in loci of the signaling pathway downstream of ERECTA and suggest candidate genes underlying these loci. Integration of publicly available microarray expression data of other monogenic mutants allowed us to link ERECTA to a downstream mitogen-activated protein kinase signaling cascade. Our study shows that microarray data of monogenic mutants can be effectively used in combination with genetical genomics data to enhance the identification of genetic regulatory networks.

Balanced nuclear and cytoplasmic activities of EDS1 are required for a complete plant innate immune response

An important layer of plant innate immunity to host-adapted pathogens is conferred by intracellular nucleotide-binding/oligomerization domain-leucine rich repeat (NB-LRR) receptors recognizing specific microbial effectors. Signaling from activated receptors of the TIR (Toll/Interleukin-1 Receptor)-NB-LRR class converges on the nucleo-cytoplasmic immune regulator EDS1 (Enhanced Disease Susceptibility1). In this report we show that a receptor-stimulated increase in accumulation of nuclear EDS1 precedes or coincides with the EDS1-dependent induction and repression of defense-related genes. EDS1 is capable of nuclear transport receptor-mediated shuttling between the cytoplasm and nucleus. By enhancing EDS1 export from inside nuclei (through attachment of an additional nuclear export sequence (NES)) or conditionally releasing EDS1 to the nucleus (by fusion to a glucocorticoid receptor (GR)) in transgenic Arabidopsis we establish that the EDS1 nuclear pool is essential for resistance to biotrophic and hemi-biotrophic pathogens and for transcriptional reprogramming. Evidence points to post-transcriptional processes regulating receptor-triggered accumulation of EDS1 in nuclei. Changes in nuclear EDS1 levels become equilibrated with the cytoplasmic EDS1 pool and cytoplasmic EDS1 is needed for complete resistance and restriction of host cell death at infection sites. We propose that coordinated nuclear and cytoplasmic activities of EDS1 enable the plant to mount an appropriately balanced immune response to pathogen attack.

Plants have evolved a multilayered innate immune system to recognize and respond to potentially destructive microbes in the environment. Resistance to invasive biotrophic and hemi-biotrophic pathogens often involves transcriptional mobilization of defenses and programmed death of host cells at infection sites. However, these processes disturb normal metabolism and growth and therefore have to be tightly controlled. In this study, we examine resistance signaling events inside Arabidopsis cells after pathogen activation of intracellular immune receptors. We show that the nucleo-cytoplasmic protein EDS1 acts as an important regulator of transcriptional reprogramming in the immune response by allowing the induction and repression of particular defense-related genes. We provide evidence that EDS1 accomplishes its role as a defense signaling ‘hub’ through coordinated activities in the cytoplasm and nucleus. Maintaining a balance between these two EDS1 pools is probably important for resistance and cell death to a range of infectious microbes and to not ‘overshoot’ defense activation which would be detrimental for the plant.

Identification of Nicotiana benthamiana genes involved in pathogen-associated molecular pattern-triggered immunity

In order to identify components of pathogen-associated molecular pattern-triggered immunity (PTI) pathways in Nicotiana benthamiana, we conducted a large-scale forward-genetics screen using virus-induced gene silencing and a cell-death-based assay for assessing PTI. The assay relied on four combinations of PTI-inducing nonpathogens and cell-death-causing challenger pathogens and was first validated in plants silenced for FLS2 or BAK1. Over 3,200 genes were screened and 14 genes were identified that, when silenced, compromised PTI as judged by the cell-death-based assay. Further analysis indicated that the 14 genes were not involved in a general cell death response. A subset of the genes was found to act downstream of FLS2-mediated PTI induction, and silencing of three genes compromised production of reactive oxygen species in leaves exposed to flg22. The 14 genes encode proteins with potential functions in defense and hormone signaling, protein stability and degradation, energy and secondary metabolism, and cell wall biosynthesis and provide a new resource to explore the molecular basis for the involvement of these processes in PTI.

Intersections between immune responses and morphological regulation in plants

Successful plant pathogens have developed strategies to interfere with the defence mechanisms of their host plants through evolution. Conversely, host plants have evolved systems to counteract pathogen attack. Some pathogens induce pathogenic symptoms on plants that include morphological changes in addition to interference with plant growth. Recent studies, based on molecular biology and genetics using Arabidopsis thaliana, have revealed that factors derived from pathogens can modulate host systems and/or host factors that play important roles in the morphological regulation of host plants. Other reports, meanwhile, have shown that factors known to have roles in plant morphology also function in plant immune responses. Evolutionary conservation of these factors and systems implies that host-pathogen interactions and the evolution they drive have yielded tight links between morphological processes and immune responses. In this review, recent findings about these topics are introduced and discussed.

Genome-wide cloning and sequence analysis of leucine-rich repeat receptor-like protein kinase genes in Arabidopsis thaliana

BACKGROUND

Transmembrane receptor kinases play critical roles in both animal and plant signaling pathways regulating growth, development, differentiation, cell death, and pathogenic defense responses. In Arabidopsis thaliana, there are at least 223 Leucine-rich repeat receptor-like kinases (LRR-RLKs), representing one of the largest protein families. Although functional roles for a handful of LRR-RLKs have been revealed, the functions of the majority of members in this protein family have not been elucidated.

RESULTS

As a resource for the in-depth analysis of this important protein family, the complementary DNA sequences (cDNAs) of 194 LRR-RLKs were cloned into the Gateway donor vector pDONR/Zeo and analyzed by DNA sequencing. Among them, 157 clones showed sequences identical to the predictions in the Arabidopsis sequence resource, TAIR8. The other 37 cDNAs showed gene structures distinct from the predictions of TAIR8, which was mainly caused by alternative splicing of pre-mRNA. Most of the genes have been further cloned into Gateway destination vectors with GFP or FLAG epitope tags and have been transformed into Arabidopsis for in planta functional analysis. All clones from this study have been submitted to the Arabidopsis Biological Resource Center (ABRC) at Ohio State University for full accessibility by the Arabidopsis research community.

CONCLUSIONS

Most of the Arabidopsis LRR-RLK genes have been isolated and the sequence analysis showed a number of alternatively spliced variants. The generated resources, including cDNA entry clones, expression constructs and transgenic plants, will facilitate further functional analysis of the members of this important gene family.

Genome-wide cloning and sequence analysis of leucine-rich repeat receptor-like protein kinase genes in Arabidopsis thaliana

Background

Transmembrane receptor kinases play critical roles in both animal and plant signaling pathways regulating growth, development, differentiation, cell death, and pathogenic defense responses. In Arabidopsis thaliana, there are at least 223 Leucine-rich repeat receptor-like kinases (LRR-RLKs), representing one of the largest protein families. Although functional roles for a handful of LRR-RLKs have been revealed, the functions of the majority of members in this protein family have not been elucidated.

Results

As a resource for the in-depth analysis of this important protein family, the complementary DNA sequences (cDNAs) of 194 LRR-RLKs were cloned into the Gateway^R donor vector pDONR/Zeo^R and analyzed by DNA sequencing. Among them, 157 clones showed sequences identical to the predictions in the Arabidopsis sequence resource, TAIR8. The other 37 cDNAs showed gene structures distinct from the predictions of TAIR8, which was mainly caused by alternative splicing of pre-mRNA. Most of the genes have been further cloned into Gateway^R destination vectors with GFP or FLAG epitope tags and have been transformed into Arabidopsis for in planta functional analysis. All clones from this study have been submitted to the Arabidopsis Biological Resource Center (ABRC) at Ohio State University for full accessibility by the Arabidopsis research community.

Conclusions

Most of the Arabidopsis LRR-RLK genes have been isolated and the sequence analysis showed a number of alternatively spliced variants. The generated resources, including cDNA entry clones, expression constructs and transgenic plants, will facilitate further functional analysis of the members of this important gene family.

Ethylene-induced hyponastic growth in Arabidopsis thaliana is controlled by ERECTA

Plants can respond quickly and profoundly to detrimental changes in their environment. For example, Arabidopsis thaliana can induce an upward leaf movement response through differential petiole growth (hyponastic growth) to outgrow complete submergence. This response is induced by accumulation of the phytohormone ethylene in the plant. Currently, only limited information is available on how this response is molecularly controlled. In this study, we utilized quantitative trait loci (QTL) analysis of natural genetic variation among Arabidopsis accessions to isolate novel factors controlling constitutive petiole angles and ethylene-induced hyponastic growth. Analysis of mutants in various backgrounds and complementation analysis of naturally occurring mutant accessions provided evidence that the leucin-rich repeat receptor-like Ser/Thr kinase gene, ERECTA, controls ethylene-induced hyponastic growth. Moreover, ERECTA controls leaf positioning in the absence of ethylene treatment. Our data demonstrate that this is not due to altered ethylene production or sensitivity.

QTL analysis of leaf architecture

Leaf size and shape define the photosynthetic capability of a plant and have a significant impact on important agronomic traits, such as yield, quality, disease resistance and stress responses. Cultivated varieties of many plant species show remarkable variations in leaf morphology. Such variation usually exhibits a continuous phenotypic distribution and is controlled by the interaction of multiple genes known as quantitative trait loci (QTL). Here, we review several studies that evaluate natural variations in the leaf morphologies of crop species, as well as in the model plant Arabidopsis thaliana. The use of high-throughput, genome-wide approaches such as transcriptomics and metabolomics is helping to identify the nucleotide polymorphism(s) responsible for the phenotypic differences attributed to some of these QTL.

AGB1 and PMR5 contribute to PEN2-mediated preinvasion resistance to Magnaporthe oryzae in Arabidopsis thaliana

Rice blast, caused by Magnaporthe oryzae, is a devastating disease of rice (Oryza sativa). The mechanisms involved in resistance of rice to blast have been studied extensively and the rice-M. oryzae pathosystem has become a model for plant-microbe interaction studies. However, the mechanisms involved in nonhost resistance (NHR) of other plants to rice blast are still poorly understood. Here, we investigated interactions between Arabidopsis thaliana and M. oryzae to identify the genetic basis of NHR. In A. thaliana accessions, preinvasion resistance to M. oryzae in Col-0 was stronger than that of Ler. To examine the genetic basis underlying the natural variation in the responses, we used a well-established set of recombinant inbred lines (RIL) derived from a Col x Ler cross and identified three quantitative trait loci that govern the expression of NHR in A. thaliana against M. oryzae. Among the penetration (pen) mutants, only the pen2 mutant allowed increased penetration into epidermal cells by M. oryzae. Double mutant analysis indicated that AGB1 and PMR5 contribute to PEN2-mediated preinvasion resistance to M. oryzae in A. thaliana, suggesting a complex genetic network regulating the resistance. Our results demonstrate that A. thaliana can be used to study mechanisms of NHR to M. oryzae.

Over-expression of the rice LRK1 gene improves quantitative yield components

In rice (Oryza sativa L.), the number of panicles, spikelets per panicle and grain weight are important components of grain yield. These characteristics are controlled by quantitative trait loci (QTLs) and are derived from variation inherent in crops. As a result of the complex genetic basis of these traits, only a few genes involved in their control have been cloned and characterized. We have previously map-cloned a gene cluster including eight leucine-rich repeat receptor-like kinase (LRK) genes in Dongxiang wild rice (Oryza rufipogon Griff.), which increased the grain yield by 16%. In the present study, we characterized the LRK1 gene, which was contained in the donor parent (Dongxiang wild rice) genome and absent from the recurrent parent genome (Guichao2, Oryza sativa L. ssp. indica). Our data showed that rice LRK1 is a plasma membrane protein expressed constitutively in leaves, young panicles, roots and culms. The over-expression of rice LRK1 results in increased panicles, spikelets per panicle, weight per grain and enhanced cellular proliferation, leading to a 27.09% increase in total grain yield per plant. The increased number of panicles and spikelets per panicle are associated with increased branch number. Our data suggest that rice LRK1 regulates rice branch number by enhancing cellular proliferation. The functional characterization of rice LRK1 facilitates an understanding of the mechanisms involved in cereal crop yield, and may have utility in improving grain yield in cereal crops.

Genetic technologies for the identification of plant genes controlling environmental stress responses

Abiotic conditions such as light, temperature, water availability and soil parameters determine plant growth and development. The adaptation of plants to extreme environments or to sudden changes in their growth conditions is controlled by a well balanced, genetically determined signalling system, which is still far from being understood. The identification and characterisation of plant genes which control responses to environmental stresses is an essential step to elucidate the complex regulatory network, which determines stress tolerance. Here, we review the genetic approaches, which have been used with success to identify plant genes which control responses to different abiotic stress factors. We describe strategies and concepts for forward and reverse genetic screens, conventional and insertion mutagenesis, TILLING, gene tagging, promoter trapping, activation mutagenesis and cDNA library transfer. The utility of the various genetic approaches in plant stress research we review is illustrated by several published examples.

The ERECTA Receptor-Like Kinase Regulates Cell Wall-Mediated Resistance to Pathogens in Arabidopsis thaliana

Some receptor-like kinases (RLK) control plant development while others regulate immunity. The Arabidopsis ERECTA (ER) RLK regulates both biological processes. To discover specific components of ER-mediated immunity, a genetic screen was conducted to identify suppressors of erecta (ser) susceptibility to Plectosphaerella cucumerina fungus. The ser1 and ser2 mutations restored disease resistance to this pathogen to wild-type levels in the er-1 background but failed to suppress er-associated developmental phenotypes. The deposition of callose upon P. cucumerina inoculation, which was impaired in the er-1 plants, was also restored to near wild-type levels in the ser er-1 mutants. Analyses of er cell walls revealed that total neutral sugars were reduced and uronic acids increased relative to those of wild-type walls. Interestingly, in the ser er-1 walls, neutral sugars were elevated and uronic acids were reduced relative to both er-1 and wild-type plants. The cell-wall changes found in er-1 and the ser er-1 mutants are unlikely to contribute to their developmental alterations. However, they may influence disease resistance, as a positive correlation was found between uronic acids content and resistance to P. cucumerina. We propose a specific function for ER in regulating cell wall-mediated disease resistance that is distinct from its role in development.

Polymorphic genes of major effect: consequences for variation, selection and evolution in Arabidopsis thaliana

The importance of genes of major effect for evolutionary trajectories within and among natural populations has long been the subject of intense debate. For example, if allelic variation at a major-effect locus fundamentally alters the structure of quantitative trait variation, then fixation of a single locus can have rapid and profound effects on the rate or direction of subsequent evolutionary change. Using an Arabidopsis thaliana RIL mapping population, we compare G-matrix structure between lines possessing different alleles at ERECTA, a locus known to affect ecologically relevant variation in plant architecture. We find that the allele present at ERECTA significantly alters G-matrix structure-in particular the genetic correlations between branch number and flowering time traits-and may also modulate the strength of natural selection on these traits. Despite these differences, however, when we extend our analysis to determine how evolution might differ depending on the ERECTA allele, we find that predicted responses to selection are similar. To compare responses to selection between allele classes, we developed a resampling strategy that incorporates uncertainty in estimates of selection that can also be used for statistical comparisons of G matrices.

The many functions of ERECTA

The Arabidopsis thaliana accession Landsberg erecta contains an induced mutation in the leucine-rich repeat receptor-like Ser/Thr kinase gene ERECTA. Landsberg erecta is commonly used as a genetic background in mutant screens and in natural variation studies. Therefore, the erecta mutation is present in many loss-of-function mutants and recombinant inbred lines. Information on how the absence of functional ERECTA affects the interpretation of obtained phenotypic results is scattered. In this report we inventoried ERECTA functions and highlight ERECTA as a pleiotropic regulator of developmental and physiological processes, as well as a modulator of responses to environmental stimuli.

A rice kinase-protein interaction map

Plants uniquely contain large numbers of protein kinases, and for the vast majority of the 1,429 kinases predicted in the rice (Oryza sativa) genome, little is known of their functions. Genetic approaches often fail to produce observable phenotypes; thus, new strategies are needed to delineate kinase function. We previously developed a cost-effective high-throughput yeast two-hybrid system. Using this system, we have generated a protein interaction map of 116 representative rice kinases and 254 of their interacting proteins. Overall, the resulting interaction map supports a large number of known or predicted kinase-protein interactions from both plants and animals and reveals many new functional insights. Notably, we found a potential widespread role for E3 ubiquitin ligases in pathogen defense signaling mediated by receptor-like kinases, particularly by the kinases that may have evolved from recently expanded kinase subfamilies in rice. We anticipate that the data provided here will serve as a foundation for targeted functional studies in rice and other plants. The application of yeast two-hybrid and TAPtag analyses for large-scale plant protein interaction studies is also discussed.

A novel leucine-rich repeat receptor-like kinase gene in potato, StLRPK1, is involved in response to diverse stresses

A potato gene, StLRPK1 (Solanum tuberosum L. leucine-rich-repeat receptor-like protein kinases 1), encoding a protein belonging to leucine-rich repeat receptor-like kinases (LRR-RLKs) was identified. It encodes 796 amino acids with 88% of identity to SRF3 of Arabidopsis thaliana and contains a signal peptide, five LRR motifs, a transmembrane domain, two proline-rich regions and a serine/threonine protein kinase domain. The transcripts were present at high levels in flowers and young leaves, while low in other tested organs. The mRNA of StLRPK1 was inducible in potato leaves by Phytophthora infestans, a pathogen causing late blight disease, and showed different profiles after treatment with salicylic acid, methyl jasmonate, ethylene, abscissic acid, wounding, 40 degrees C, 4 degrees C and a salinity stress. The results suggest that StLRPK1 may participate in the responses against environmental stresses and disease resistance in potato.

The genomic architecture of disease resistance in lettuce

Genbank and The Compositae Genome Project database, containing over 42,000 lettuce unigenes from Lactuca sativa cv. Salinas and L. serriola accession UC96US23 were mined to identify 702 candidate genes involved in pathogen recognition (RGCs), resistance signal transduction, defense responses, and disease susceptibility. In addition, to identify sequences representing additional sub-families of nucleotide binding site (NBS)-leucine-rich repeat encoding genes; the major classes of resistance genes (R-genes), NBS-encoding sequences were amplified by PCR using degenerate oligonucleotides designed to NBS sub-families specific to the subclass Asteridae, which includes the Compositae family. These products were cloned and sequenced resulting in 18 novel NBS sequences from cv. Salinas and 15 novel NBS sequences from UC96US23. Using a variety of marker technologies, 294 of the 735 candidate disease resistance genes were mapped in our primary mapping population, which consisted of 119 F7 recombinant inbred lines derived from an interspecific cross between cv. Salinas and UC96US23. Using markers shared across multiple genetic maps, 36 resistance phenotypic loci, including two new loci for resistance to downy mildew and two quantitative trait loci for resistance to anthracnose were positioned onto the reference map to provide a global view of the genomic architecture of disease resistance in lettuce and to identify candidate genes for resistance phenotypes. The majority but not all of the resistance phenotypes were genetically associated with RGCs.

Using Arabidopsis genetic sequences to teach bioinformatics

This article describes a new approach to teaching bioinformatics using Arabidopsis genetic sequences. Several open-ended and inquiry-based laboratory exercises have been designed to help students grasp key concepts and gain practical skills in bioinformatics, using Arabidopsis leucine-rich repeat receptor-like kinase (LRR RLK) genetic sequences. In these exercises, students are asked to explore the intimate relationship between the structure and functions of LRR RLKs and to understand the diverse signaling roles of LRR RLKs in Arabidopsis. Students are also required to identify LRR RLK paralogs and orthologs and to analyze their evolutionary and functional relationships using bioinformatics software. Through these exercises, students are expected to gain the ability to search databases to retrieve genetic data and analyze them using a variety of bioinformatics tools.

Molecular analyses of the rice tubby-like protein gene family and their response to bacterial infection

Tubby-like protein family has been identified in various multicellular organisms, indicating its fundamental functions in the organisms. However, the roles of plant tubby-like proteins are unknown. In this study, we have defined the tubby-like protein gene (OsTLP) family with 14 members in rice. Most of the OsTLPs harbor a tubby domain in their carboxyl terminus and an F-box domain in the amino terminus. The expression of all the OsTLPs was induced on infection of Xanthomonas oryzae pv. oryzae, which causes bacterial blight, one of the most devastating diseases of rice worldwide. The maximal expression levels were observed at 2-8 h after infection for all the genes. Eight of the 14 OsTLPs were also responsive to wounding. All the OsTLPs showed differential expression in different tissues at different developmental stages. However, four pairs of the 14 OsTLPs, with each pair having high sequence similarity and distributing on the similar position of different chromosomes, showed similar expression pattern in different tissues, indicating their direct relationship in evolution. These results suggest that the OsTLP family is involved in host-pathogen interaction and it may be also associated with other physiological and developmental activities.

Combined genetic and modeling approaches reveal that epidermal cell area and number in leaves are controlled by leaf and plant developmental processes in Arabidopsis

Both leaf production and leaf expansion are tightly linked to cell expansion and cell division, but the functional relationships between all these variables are not clearly established. To get insight into these relationships, a quantitative genetic analysis was performed in 118 recombinant inbred lines derived from a cross between the Landsberg erecta and Antwerp accessions and was combined with a structural equation modeling approach. Main effects and epistatic interactions at the quantitative trait locus (QTL) level were detected for rosette area, rosette leaf number, leaf 6 area, epidermal cell area and number. A QTL at ERECTA marker (ER) controlled cell expansion and cell division, in interaction with two other QTLs at SNP295 and SNP21 markers. Moreover, both the screening for marker association involved in the variation of the relationships between leaf growth variables and the test of alternative functional models by structural equation modeling revealed that the allelic value at ER controlled epidermal cell area and epidermal cell number in a leaf. These effects are driven both by a whole plant mechanism associated with leaf production and by a single leaf mechanism associated with leaf expansion. The complex effects of the QTL at ER were validated in selected heterogeneous inbred families. The ERECTA gene, which is mutated in the Landsberg erecta parental line, was found to be a putative candidate responsible for these mapped effects by phenotyping mutants of this gene at the cellular level. Together, these results give insight into the complex determination of leaf epidermal cell number and area.

Transposon mutagenesis reveals differential pathogenesis of Ralstonia solanacearum on tomato and Arabidopsis

Ralstonia solanacearum causes a deadly wilting disease on a wide range of crops. To elucidate pathogenesis of this bacterium in different host plants, we set out to identify R. solanacearum genes involved in pathogenesis by screening random transposon insertion mutants of a highly virulent strain, Pss190, on tomato and Arabidopsis thaliana. Mutants exhibiting various decreased virulence levels on these two hosts were identified. Sequence analysis showed that most, but not all, of the identified pathogenesis genes are conserved among distinct R. solanacearum strains. A few of the disrupted loci were not reported previously as being involved in R. solanacearum pathogenesis. Notably, a group of mutants exhibited differential pathogenesis on tomato and Arabidopsis. These results were confirmed by characterizing allelic mutants in one other R. solanacearum strain of the same phylotype. The significantly decreased mutants' colonization in Arabidopsis was found to be correlated with differential pathogenesis on these two plants. Differential requirement of virulence genes suggests adaptation of this bacterium in different host environments. Together, this study reveals commonalities and differences of R. solanacearum pathogenesis on single solanaceous and nonsolanaceous hosts, and provides important new insights into interactions between R. solanacearum and different host plants.

Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR KINASE proteins serve brassinosteroid-dependent and -independent signaling pathways

The Arabidopsis (Arabidopsis thaliana) SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) genes belong to a small family of five plant receptor kinases that are involved in at least five different signaling pathways. One member of this family, BRASSINOSTEROID INSENSITIVE1 (BRI1)-ASSOCIATED KINASE1 (BAK1), also known as SERK3, is the coreceptor of the brassinolide (BR)-perceiving receptor BRI1, a function that is BR dependent and partially redundant with SERK1. BAK1 (SERK3) alone controls plant innate immunity, is also the coreceptor of the flagellin receptor FLS2, and, together with SERK4, can mediate cell death control, all three in a BR-independent fashion. SERK1 and SERK2 are essential for male microsporogenesis, again independent from BR. SERK5 does not appear to have any function under the conditions tested. Here, we show that the different SERK members are only redundant in pairs, whereas higher order mutant combinations only show additive phenotypes. Surprisingly, SERK members that are redundant within one are not redundant in another pathway. We also show that this evolution of functional pairs occurred by a change in protein function and not by differences in spatial expression. We propose that, in plants, closely related receptor kinases have a minimal homo- or heterodimeric configuration to achieve specificity.

Finding a missing link in MAP kinase cascade

Mitogen-activated protein kinase (MAPK) cascade is one of the major signaling systems in eukaryotes. External signals are tranduced through three protein kinases, which successively relay phosphorylation to finally activate target genes/proteins. However, few information on targets of MAPK have so far been available. In this study, we identified a novel transcription factor, NtWIF, which is directly phosphorylated by a wound-induced protein kinase (WIPK), a typical MAPK from tobacco plants. Phosphorylated NtWIF recognizes the auxin responsive element (ARE), and transcriptionally activates ARE-driven Luciferase-reporter genes. Transgenic tobacco plants, in which NtWIF was overexpressed or suppressed, showed distinct features not only in pathogen resistance, but also in seed development and root growth. Micro-array assay using transgenic lines identified 178 differentially expressed genes, among which nearly half was related to defense and development. Screening of the available promoter regions revealed that multiple genes encoding such as pathogenesis-related protein Q (PR-Q), beta-1,3-glucanase, aminocyclopropane carboxylic acid synthase 2, P-450 and WIPK itself possess the ARE motif. Upon coexpression in cultured cells, NtWIF transcriptionally activated the Luciferase-reporter gene driven by intact promoters of PR-Q and WIPK. Since ARE is commonly found in auxin-responsive genes, NtWIF possibly targets diverse genes for defense and development by sharing processes that involve auxins. Transcriptional activation of WIPK by NtWIF suggests that WIPK is produced through a feed-back controlling system. It was thus concluded that NtWIF is a missing link between WIPK and its down-stream proteins, and that WIPK cascade is auto-regulated through a self-amplifying circuit.

A novel pepper membrane-located receptor-like protein gene CaMRP1 is required for disease susceptibility, methyl jasmonate insensitivity and salt tolerance

Plant receptor proteins are involved in the signaling networks required for defense against pathogens. The novel pepper pathogen-induced gene CaMRP1 was isolated from pepper leaves infected with Xanthomonas campestris pv. vesicatoria (Xcv). This gene is predicted to encode a membrane-located receptor-like protein that has an N-terminal signal peptide and a C-terminal transmembrane helix. A CaMRP1-GFP fusion protein localized primarily to the plasma membrane of plant cells. Strong and early induction of CaMRP1 expression occurred following exposure of pepper plants to Xcv, Colletotricum coccodes, methyl jasmonate (MeJA) and wounding stress. Virus-induced gene silencing (VIGS) of CaMRP1 in pepper conferred enhanced basal resistance to Xcv infection, accompanied by induction of genes encoding basic PR1 (CaBPR1), defensin (CaDEF1) and SAR8.2 (CaSAR82A). In contrast, CaMRP1 overexpression (OX) in transgenic Arabidopsis plants resulted in increased disease susceptibility to Hyaloperonospora parasitica infection. Arabidopsis plants overexpressing CaMRP1 exhibited insensitivity to MeJA by causing reduced expression of MeJA-responsive genes. Overexpression also resulted in tolerance to NaCl and during salt stress, the expression of several abscisic acid-responsive genes was induced. Together, these results suggest that pepper CaMRP1 may belong to a new subfamily of membrane-located receptor-like proteins that regulate disease susceptibility, MeJA-insensitivity and salt tolerance.

Regulation of Arabidopsis early anther development by the mitogen-activated protein kinases, MPK3 and MPK6, and the ERECTA and related receptor-like kinases

Mitogen-activated protein kinase (MAPK) and leucine-rich repeat receptor-like kinase (LRR-RLK) signaling pathways have been shown to regulate diverse aspects of plant growth and development. In Arabidopsis, proper anther development relies on intercellular communication to coordinate cell proliferation and differentiation. Two closely related genes encoding MAPKs, MPK3 and MPK6, function redundantly in regulating stomatal patterning. Although the mpk6 mutant has reduced fertility, the function of MPK3 and MPK6 in anther development has not been characterized. Similarly, the ERECTA (ER), ERECTA-LIKE1 (ERL1) and ERL2 genes encoding LRR-RLKs function together to direct stomatal cell fate specification and the er-105 erl1-2 erl2-1 triple mutant is sterile. Because the mpk3 mpk6 double null mutant is embryo lethal, anther development was characterized in the viable mpk3/+ mpk6/- and er-105 erl1-2 erl2-1 mutants. We found that both mutant anthers usually fail to form one or more of the four anther lobes, with the er-105 erl1-2 erl2-1 triple mutant exhibiting more severe phenotypes than those of the mpk3/+ mpk6/- mutant. The somatic cell layers of the differentiated mutant lobes appeared larger and more disorganized than that of wild-type. In addition, the er-105 erl1-2 erl2-1 triple mutant has a reduced number of stamens, the majority of which possess completely undifferentiated or under-differentiated anthers. Furthermore, sometimes, the mpk3/+ mpk6/- mutant anthers do not dehisce, and the er-105 erl1-2 erl2-1 anthers were not observed to dehisce. Therefore, our results indicate that both ER/ERL1/ERL2 and MPK3/MPK6 play important roles in normal anther lobe formation and anther cell differentiation. The close functional relationship between these genes in other developmental processes and the similarities in anther developmental phenotypes of the two types of mutants reported here further suggest the possibility that these genes might also function in the same pathway to regulate anther cell division and differentiation.

A genome-wide functional investigation into the roles of receptor-like proteins in Arabidopsis

Receptor-like proteins (RLPs) are cell surface receptors that typically consist of an extracellular leucine-rich repeat domain, a transmembrane domain, and a short cytoplasmatic tail. In several plant species, RLPs have been found to play a role in disease resistance, such as the tomato (Solanum lycopersicum) Cf and Ve proteins and the apple (Malus domestica) HcrVf2 protein that mediate resistance against the fungal pathogens Cladosporium fulvum, Verticillium spp., and Venturia inaequalis, respectively. In addition, RLPs play a role in plant development; Arabidopsis (Arabidopsis thaliana) TOO MANY MOUTHS (TMM) regulates stomatal distribution, while Arabidopsis CLAVATA2 (CLV2) and its functional maize (Zea mays) ortholog FASCINATED EAR2 regulate meristem maintenance. In total, 57 RLP genes have been identified in the Arabidopsis genome and a genome-wide collection of T-DNA insertion lines was assembled. This collection was functionally analyzed with respect to plant growth and development and sensitivity to various stress responses, including susceptibility toward pathogens. A number of novel developmental phenotypes were revealed for our CLV2 and TMM insertion mutants. In addition, one AtRLP gene was found to mediate abscisic acid sensitivity and another AtRLP gene was found to influence nonhost resistance toward Pseudomonas syringae pv phaseolicola. This genome-wide collection of Arabidopsis RLP gene T-DNA insertion mutants provides a tool for future investigations into the biological roles of RLPs.

Potassium and sodium transporters of Pseudomonas aeruginosa regulate virulence to barley

We investigated the role of three uncharacterized cation transporters of Pseudomonas aeruginosa PAO1 as virulence factors for barley: PA1207, PA5021, and PA2647. PAO1 displayed reduced barley virulence with inactivated PA1207, PA5021, and PA2647 as well as with one known Na(+)/H(+) antiporter, PA1820. Using the Escherichia coli LB2003 mutant lacking three K(+) uptake systems, the expression of the PA5021 gene repressed LB2003 growth with low K(+), but the strain acquired tolerance to high K(+). In contrast, the expression of the PA1207 gene enhanced growth of LB2003 with low K(+) but repressed its growth with high K(+); therefore, the PA5021 protein exports K(+), while the PA1207 protein imports K(+). The PA5021 mutant of P. aeruginosa also showed impaired growth at 400 mM KCl and at 400 mM NaCl; therefore, the PA5021 protein may also export Na(+). The loss of the PA5021 protein also decreased production of the virulence factor pyocyanin; corroborating this result, pyocyanin production decreased in wild-type PAO1 under high salinity. Whole-genome transcriptome analysis showed that PAO1 induced more genes in barley upon infection compared to the PA5021 mutant. Additionally, PAO1 infection induced water stress-related genes in barley, which suggests that barley may undergo water deficit upon infection by this pathogen.

Plant receptor-like serine threonine kinases: roles in signaling and plant defense

Plants are hosts to a wide array of pathogens from all kingdoms of life. In the absence of an active immune system or combinatorial diversifications that lead to recombination-driven somatic gene flexibility, plants have evolved different strategies to combat both individual pathogen strains and changing pathogen populations. The receptor-like kinase (RLK) gene-family expansion in plants was hypothesized to have allowed accelerated evolution among domains implicated in signal reception, typically a leucine-rich repeat (LRR). Under that model, the gene-family expansion represents a plant-specific adaptation that leads to the production of numerous and variable cell surface and cytoplasmic receptors. More recently, it has emerged that the LRR domains of RLK interact with a diverse group of proteins leading to combinatorial variations in signal response specificity. Therefore, the RLK appear to play a central role in signaling during pathogen recognition, the subsequent activation of plant defense mechanisms, and developmental control. The future challenges will include determinations of RLK modes of action, the basis of recognition and specificity, which cellular responses each receptor mediates, and how both receptor and kinase domain interactions fit into the defense signaling cascades. These challenges will be complicated by the limited information that may be derived from the primary sequence of the LRR domain. The review focuses upon implications derived from recent studies of the secondary and tertiary structures of several plant RLK that change understanding of plant receptor function and signaling. In addition, the biological functions of plant and animal RLK-containing receptors were reviewed and commonalities among their signaling mechanisms identified. Further elucidated were the genomic and structural organizations of RLK gene families, with special emphasis on RLK implicated in resistance to disease and development.

Rice gene network inferred from expression profiling of plants overexpressing OsWRKY13, a positive regulator of disease resistance

Accumulating information indicates that plant disease resistance signaling pathways frequently interact with other pathways regulating developmental processes or abiotic stress responses. However, the molecular mechanisms of these types of crosstalk remain poorly understood in most cases. Here we report that OsWRKY13, an activator of rice resistance to both bacterial and fungal pathogens, appears to function as a convergent point for crosstalk among the pathogen-induced salicylate-dependent defense pathway and five other physiologic pathways. Genome-wide analysis of the expression profiles of OsWRKY13-overexpressing lines suggests that OsWRKY13 directly or indirectly regulates the expression of more than 500 genes that are potentially involved in different physiologic processes according to the classification of the Gene Ontology database. By comparing the expression patterns of genes functioning in known pathways or cellular processes of pathogen infection and the phenotypes between OsWRKY13-overexpressing and wild-type plants, our data suggest that OsWRKY13 is also a regulator of other physiologic processes during pathogen infection. The OsWRKY13-associated disease resistance pathway synergistically interacts via OsWRKY13 with the glutathione/glutaredoxin system and flavonoid biosynthesis pathway to monitor redox homeostasis and to putatively enhance the biosynthesis of antimicrobial flavonoid phytoalexins, respectively, in OsWRKY13-overexpressing lines. Meanwhile, the OsWRKY13-associated disease resistance pathway appears to interact antagonistically with the SNAC1-mediated abiotic stress defense pathway, jasmonic acid signaling pathway, and terpenoid metabolism pathway via OsWRKY13 to suppress salt and cold defense responses as well as to putatively retard rice growth and development.

GASSHO1 and GASSHO2 encoding a putative leucine-rich repeat transmembrane-type receptor kinase are essential for the normal development of the epidermal surface in Arabidopsis embryos

Receptor-like kinases (RLKs) containing leucine-rich repeats (LRRs) act as both signal receptor and signal transducer in ligand-mediated communication between cells. It is believed that many LRR-RLKs are present in the Arabidopsis genome, but the functions of most are unknown. We recently identified Bnms4D-82, an expressed sequence tag (EST) in Brassica napus that encodes an LRR-RLK and is expressed at an early stage of its microspore embryogenesis. To elucidate the function of this gene we used GASSHO1 (GSO1) and GSO2, two Arabidopsis genes with a high degree of homology with Bnms4D-82. The products of transcripts of GSO1 and GSO2 accumulate in parts of the embryo and in seedlings, but not in true leaves. Plants that lacked both GSO1 and GSO2 exhibited pleiotropy, including abnormal bending of embryos, ectopic adhesion between cotyledons, a highly permeable epidermal structure, and an abnormal pattern of distribution of stomata on cotyledons in embryos and seedlings. However, plants homozygous for either gso1-1 or gso2-1 had no visible abnormality. These results suggest that GASSHO genes are essential for the formation of a normal epidermal surface during embryogenesis.

Brassinosteroid-independent functions of the BRI1-associated kinase BAK1/SERK3

Eukaryotes have evolved programmed cell death (PCD) mechanisms that play important roles in both, development and immunity.1-3 We demonstrated a requirement for the Arabidopsis thaliana leucine-rich repeat receptor-like kinase (LRR-RLK), BAK1/SERK3 (BRI1-Associated receptor Kinase 1/Somatic Embryogenesis Receptor Kinase 3) in regulating the containment of microbial infection-induced necrosis. BAK1-deficient plants showed constitutive expression of defense-related genes and developed spreading cell death upon infection by necrotizing pathogens that result in enhanced susceptibility to necrotrophic pathogens. This reaction was not inducible by exposition of bak1 mutants to general stresses but appeared to be solely inducible by necrotizing pathogen infection. BAK1 is known to interact with the brassinosteroid receptor, BRI1, and thereby facilitates plant growth and development in a brassinolide (BL)-dependent manner.4,5 Surprisingly, the cell death-related phenotype in bak1 mutants is brassinolide-independent. In this addendum we want to present recent new data on BAK1 and discuss its role as a general regulator in plant processes being as diverse as brassinosteroid signaling in development, perception of pathogen associated molecular patterns (PAMPs), and cell-death control in innate immunity.

BAK1 and BKK1 regulate brassinosteroid-dependent growth and brassinosteroid-independent cell-death pathways

Brassinosteroids (BRs) are phytosteroid hormones controlling various physiological processes critical for normal growth and development. BRs are perceived by a protein complex containing two transmembrane receptor kinases, BRASSINOSTEROID INSENSITIVE 1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) [1-3]. BRI1 null mutants exhibit a dwarfed stature with epinastic leaves, delayed senescence, reduced male fertility, and altered light responses. BAK1 null mutants, however, only show a subtle phenotype, suggesting that functionally redundant proteins might be present in the Arabidopsis genome. Here we report that BAK1-LIKE 1 (BKK1) functions redundantly with BAK1 in regulating BR signaling. Surprisingly, rather than the expected bri1-like phenotype, bak1 bkk1 double mutants exhibit a seedling-lethality phenotype due to constitutive defense-gene expression, callose deposition, reactive oxygen species (ROS) accumulation, and spontaneous cell death even under sterile growing conditions. Our detailed analyses demonstrate that BAK1 and BKK1 have dual physiological roles: positively regulating a BR-dependent plant growth pathway, and negatively regulating a BR-independent cell-death pathway. Both BR signaling and developmentally controlled cell death are critical to optimal plant growth and development, but the mechanisms regulating early events in these pathways are poorly understood. This study provides novel insights into the initiation and crosstalk of the two signaling cascades.

The developmental selector AS1 is an evolutionarily conserved regulator of the plant immune response

The MYB-related gene ASYMMETRIC LEAVES 1 (AS1) and its orthologs have an evolutionarily conserved role in specification of leaf cell identity. AS1 is expressed in leaf founder cells, where it functions as a heterodimer with the structurally unrelated AS2 proteins to repress activity of KNOTTED 1-like homeobox (KNOX) genes. AS1 therefore confines KNOX activity to the shoot apical meristem, where it promotes stem cell function through the regulation of phytohormone activities. Here, we show that loss-of-function mutations in AS1 unexpectedly convey heightened protection against necrotrophic fungi. AS1 operates as a negative regulator of inducible resistance against these pathogens by selectively binding to the promoters of genes controlled by the immune activator, jasmonic acid (JA), damping the defense response. In contrast, AS1 is a positive regulator of salicylic acid (SA)-independent extracellular defenses against bacterial pathogens. Neither the absence of AS2 nor ERECTA function, which enhances the morphological phenotype of as1, nor the conditional or constitutive expression of KNOX genes impacted disease resistance. Thus, the function of AS1 in responses to phytopathogens is independent of its AS2-associated role in development. Loss of function in the AS1 orthologs PHAN in Antirrhinum majus and NSPHAN in Nicotiana sylvestris produced pathogen-response phenotypes similar to as1 plants, and therefore the defense function of AS1 is evolutionarily conserved in plant species with a divergence time of approximately 125 million years.

Cell signalling: the merry lives of BAK1

Plant receptor-like kinases characterised by leucine-rich repeats have been shown to play dual roles in seemingly unrelated biological processes, inviting comparison with TOLL-like receptors of animals.

Molecular analysis of three new receptor-like kinase genes from hexaploid wheat and evidence for their participation in the wheat hypersensitive response to stripe rust fungus infection

Considerable progress has been made in understanding the function of receptor-like kinase (RLK) genes in model plants. However, much less is known about these genes in crop species. Here we report the characterization of three new wheat RLK genes (TaRLK-R1, 2 and 3). The primary structure of the putative proteins TaRLK-R1, 2 and 3 contained a signal peptide, a cysteine-rich extracellular domain, a transmembrane domain, and a predicted intracellular kinase domain. The fusions between TaRLK-R1, 2 or 3 and the green fluorescence protein (GFP) were targeted to the plasma membrane; such targeting required the signal peptide, extracellular domain and transmembrane domain. Transcription of TaRLK-R1, 2 and 3 was found mainly in the green organs, and was regulated by light. Transcript levels of TaRLK-R1, 2 and 3 increased during the hypersensitive reaction (HR) to stripe rust fungus. In addition, the TaRLK-R3 transcript level was also upregulated by abiotic stresses. Further experiments revealed that the recombinant kinase domain of TaRLK-R3 exhibited auto-phosphorylation activity in vitro. Knocking down the transcript levels of TaRLK-R1, 2 or 3 individually or all together by virus-induced gene silencing compromised the wheat HR to stripe rust fungus. The demonstration of TaRLK-R1, 2 and 3 as positive contributors in the wheat HR to stripe rust fungus suggests a new direction for further functional studies of this important family of RLK genes, and may facilitate the breeding of wheat varieties resistant to stripe rust disease.

MicroRNAs in loblolly pine (Pinus taeda L.) and their association with fusiform rust gall development

MicroRNAs (miRNAs) are endogenous small RNAs that can have large-scale regulatory effects on development and on stress responses in plants. The endemic rust fungus Cronartium quercuum f. sp. fusiforme causes fusiform rust disease in pines, resulting in the development of spindle-shaped galls (cankers) on branches or stems. This disease is the most destructive disease of pines in the southern USA. To test whether miRNAs play roles in fusiform rust gall development, we cloned and identified 26 miRNAs from stem xylem of loblolly pine (Pinus taeda), which belong to four conserved and seven loblolly pine-specific miRNA families. Forty-three targets for nine of these 11 families were experimentally validated in vivo. Sequence analysis suggested that the target cleavage site may be determined not only by the miRNA sequence but also by the target sequence. Members of three loblolly pine-specific miRNA families target a large number of non-protein coding transcripts, and one of these families could also initiate secondary phased production from its target of a putative trans-acting short interfering RNA (ta-siRNA). Expression of 10 of these 11 miRNA families was significantly repressed in the galled stem. PCR-based transcript quantification showed complex expression patterns of these miRNAs and their targets in the galled tissues and in tissues surrounding the gall. We further predict 82 plant disease-related transcripts that may also response to miRNA regulation in pine. These results reveal a new genetic basis for host-pathogen interactions in the development of fusiform rust gall.

The expression pattern of a rice disease resistance gene xa3/xa26 is differentially regulated by the genetic backgrounds and developmental stages that influence its function

Genetic background and developmental stage influence the function of some disease resistance (R) genes. The molecular mechanisms of these modifications remain elusive. Our results show that the two factors are associated with the expression of the R gene in rice Xa3 (also known as Xa26)-mediated resistance to Xanthomonas oryzae pv. oryzae (Xoo), which in turn influences the expression of defense-responsive genes. The background of japonica rice, one of the two major subspecies of Asian cultivated rice, facilitates the function of Xa3 more than the background of indica rice, another rice subspecies. Xa3 expression gradually increases from early seedling stage to adult stage. Japonica plants carrying Xa3 regulated by the native promoter showed an enlarged resistance spectrum (i.e., resistance to more Xoo races), an increased resistance level (i.e., further reduced lesion length), and whole-growth-stage resistance compared to the indica rice; this enhanced resistance was associated with an increased expression of Xa3 throughout the growth stages in the japonica plants, which resulted in enhanced expression of defense-responsive genes. Overexpressing Xa3 with a constitutive strong promoter further enhanced rice resistance due to further increased Xa3 transcripts in both indica and japonica backgrounds, whereas regulating Xa3 with a pathogen-induced weak promoter impaired rice resistance.

Genetics of microenvironmental canalization in Arabidopsis thaliana

Canalization is a fundamental feature of many developmental systems, yet the genetic basis for this property remains elusive. We examine the genetic basis of microenvironmental canalization in the model plant Arabidopsis thaliana, focusing on differential developmental stability between genotypes in one fitness and four quantitative morphological traits. We measured developmental stability in genetically identical replicates of two populations of recombinant inbred (RI) lines and one population of geographically widespread accessions of A. thaliana grown in two different photoperiod-controlled environments. We were able to map quantitative trait loci associated with developmental stability. We also identified a candidate gene, ERECTA, that may contribute to microenvironmental canalization in rosette leaf number under long-day photoperiods, and analysis of mutant lines indicates that the er-105 allele results in increased canalization for this trait. ERECTA, which encodes a signaling protein, appears to act as an ecological amplifier by transducing developmental noise (e.g., microenvironmental variation) into phenotypic differentiation. We also measured genotypic selection on four plant architecture traits and find evidence for selection for both increased and decreased canalization at various traits.