Spotted leaf11, a negative regulator of plant cell death and defense, encodes a U-box/armadillo repeat protein endowed with E3 ubiquitin ligase activity

Comparison of early transcriptome responses to copper and cadmium in rice roots

The phytotoxic effects of copper (Cu) and cadmium (Cd) on plant growth are well documented. However, Cu and Cd toxicity targets and the cellular systems contributing to acquisition of tolerance are not fully understood at the molecular level. We aimed to identify genes and pathways that discriminate the actions of Cu and Cd in rice roots (Oryza sativa L. cv. TN67). The transcripts of 1,450 and 1,172 genes were regulated after Cu and Cd treatments, respectively. We identified 882 genes specifically respond to Cu treatment, and 604 unique genes as Cd-responsive by comparison of expression profiles of these two regulated gene groups. Gene ontology analysis for 538 genes involved in primary metabolism, oxidation reduction and response to stimulus was changed in response to both metals. In the individual aspect, Cu specifically altered levels of genes involved in vesicle trafficking transport, fatty acid metabolism and cellular component biogenesis. Cd-regulated genes related to unfolded protein binding and sulfate assimilation. To further characterize the functions of vesicle trafficking transport under Cu stress, interference of excytosis in root tissues was conducted by inhibitors and silencing of Exo70 genes. It was demonstrated that vesicle-trafficking is required for mediation of Cu-induced reactive oxygen species (ROS) production in root tissues. These results may provide new insights into understanding the molecular basis of the early metal stress response in plants.

Proteomic analysis of a disease-resistance-enhanced lesion mimic mutant spotted leaf 5 in rice

BACKGROUND

A lesion-mimic mutant in rice (Oryza sativa L.), spotted leaf 5 (spl5), displays a disease-resistance-enhanced phenotype, indicating that SPL5 negatively regulates cell death and resistance responses. To understand the molecular mechanisms of SPL5 mutation-induced cell death and resistance responses, a proteomics-based approach was used to identify differentially accumulated proteins between the spl5 mutant and wild type (WT).

RESULTS

Proteomic data from two-dimensional gel electrophoresis showed that 14 candidate proteins were significantly up- or down-regulated in the spl5 mutant compared with WT. These proteins are involved in diverse biological processes including pre-mRNA splicing, amino acid metabolism, photosynthesis, glycolysis, reactive oxygen species (ROS) metabolism, and defense responses. Two candidate proteins with a significant up-regulation in spl5 - APX7, a key ROS metabolism enzyme and Chia2a, a pathogenesis-related protein - were further analyzed by qPCR and enzyme activity assays. Consistent with the proteomic results, both transcript levels and enzyme activities of APX7 and Chia2a were significantly induced during the course of lesion formation in spl5 leaves.

CONCLUSIONS

Many functional proteins involving various metabolisms were likely to be responsible for the lesion formation of spl5 mutant. Generally, in spl5, the up-regulated proteins involve in defense response or PCD, and the down-regulated ones involve in amino acid metabolism and photosynthesis. These results may help to gain new insight into the molecular mechanism underlying spl5-induced cell death and disease resistance in plants.

Sugars, the clock and transition to flowering

Sugars do not only act as source of energy, but they also act as signals in plants. This mini review summarizes the emerging links between sucrose-mediated signaling and the cellular networks involved in flowering time control and defense. Cross-talks with gibberellin and jasmonate signaling pathways are highlighted. The circadian clock fulfills a crucial role at the heart of cellular networks and the bilateral relation between sugar signaling and the clock is discussed. It is proposed that important factors controlling plant growth (DELLAs, PHYTOCHROME INTERACTING FACTORS, invertases, and trehalose-6-phosphate) might fulfill central roles in the transition to flowering as well. The emerging concept of "sweet immunity," modulated by the clock, might at least partly rely on a sucrose-specific signaling pathway that needs further exploration.

The Armadillo repeat gene ZAK IXIK promotes Arabidopsis early embryo and endosperm development through a distinctive gametophytic maternal effect

The proper balance of parental genomic contributions to the fertilized embryo and endosperm is essential for their normal growth and development. The characterization of many gametophytic maternal effect (GME) mutants affecting seed development indicates that there are certain classes of genes with a predominant maternal contribution. We present a detailed analysis of the GME mutant zak ixik (zix), which displays delayed and arrested growth at the earliest stages of embryo and endosperm development. ZIX encodes an Armadillo repeat (Arm) protein highly conserved across eukaryotes. Expression studies revealed that ZIX manifests a GME through preferential maternal expression in the early embryo and endosperm. This parent-of-origin-dependent expression is regulated by neither the histone and DNA methylation nor the DNA demethylation pathways known to regulate some other GME mutants. The ZIX protein is localized in the cytoplasm and nucleus of cells in reproductive tissues and actively dividing root zones. The maternal ZIX allele is required for the maternal expression of miniseed3. Collectively, our results reveal a reproductive function of plant Arm proteins in promoting early seed growth, which is achieved through a distinct GME of ZIX that involves mechanisms for maternal allele-specific expression that are independent of the well-established pathways.

Characterization, fine mapping and expression profiling of Ragged leaves1 in maize

The Ragged leaves1 (Rg1) maize mutant frequently develops lesions on leaves, leaf sheaths, and ear bracts. Lesion formation is independent of biotic stress. High-level accumulation of H(2)O(2) revealed by staining Rg1 leaves, with 3',3'-diaminobenzidine and trypan blue, suggested that lesion formation appeared to be due to cell death. Rg1 was initially mapped to an interval around 70.5 Mb in bin 3.04 on the short arm of chromosome 3. Utilizing 15 newly developed markers, Rg1 was delimitated to an interval around 17 kb using 16,356 individuals of a BC1 segregating population. There was only one gene, rp3, predicted in this region according to the B73 genome. Analysis of transcriptome data revealed that 441 genes significantly up-regulated in Rg1 leaves were functionally over-represented. Among those genes, several were involved in the production of reactive oxygen species (ROS). Our results suggested that lesions of Rg1 maize arose probably due to an aberrant rust resistance allele of Rp3, which elicited the accumulation of ROS independent of biotic stress.

The ubiquitin-proteasome system: central modifier of plant signalling

Ubiquitin is well established as a major modifier of signalling in eukaryotes. However, the extent to which plants rely on ubiquitin for regulating their lifecycle is only recently becoming apparent. This is underlined by the over-representation of genes encoding ubiquitin-metabolizing enzymes in Arabidopsis when compared with other model eukaryotes. The main characteristic of ubiquitination is the conjugation of ubiquitin onto lysine residues of acceptor proteins. In most cases the targeted protein is rapidly degraded by the 26S proteasome, the major proteolysis machinery in eukaryotic cells. The ubiquitin-proteasome system is responsible for removing most abnormal peptides and short-lived cellular regulators, which, in turn, control many processes. This allows cells to respond rapidly to intracellular signals and changing environmental conditions. This review maps out the roles of the components of the ubiquitin-proteasome system with emphasis on areas where future research is urgently needed. We provide a flavour of the diverse aspects of plant lifecycle where the ubiquitin-proteasome system is implicated. We aim to highlight common themes using key examples that reiterate the importance of the ubiquitin-proteasome system to plants. The future challenge in plant biology is to define the targets for ubiquitination, their interactors and their molecular function within the regulatory context.

Roles of four Arabidopsis U-box E3 ubiquitin ligases in negative regulation of abscisic acid-mediated drought stress responses

AtPUB18 and AtPUB19 are homologous U-box E3 ubiquitin ligases in Arabidopsis (Arabidopsis thaliana). AtPUB19 is a negative regulator of abscisic acid (ABA)-mediated drought responses, whereas the role of AtPUB18 in drought responses is unknown. Here, loss-of-function and overexpression tests identified AtPUB18 as a negative regulator in ABA-mediated stomatal closure and water stress responses. The atpub18-2atpub19-3 double mutant line displayed more sensitivity to ABA and enhanced drought tolerance than each single mutant plant; therefore, AtPUB18 and AtPUB19 are agonistic. Stomatal closure of the atpub18-2atpub19-3 mutant was hypersensitive to hydrogen peroxide (H(2)O(2)) but not to calcium, suggesting that AtPUB18 and AtPUB19 exert negative effects on the ABA signaling pathway downstream of H(2)O(2) and upstream of calcium. AtPUB22 and AtPUB23 are other U-box E3 negative regulators of drought responses. Although atpub22atpub23 was more tolerant to drought stress relative to wild-type plants, its ABA-mediated stomatal movements were highly similar to those of wild-type plants. The atpub18-2atpub19-3atpub22atpub23 quadruple mutant exhibited enhanced tolerance to drought stress as compared with each atpub18-2atpub19-3 and atpub22atpub23 double mutant progeny; however, its stomatal behavior was almost identical to the atpub18-2atpub19-3 double mutant in the presence of ABA, H(2)O(2), and calcium. Overexpression of AtPUB18 and AtPUB19 in atpub22atpub23 effectively hindered ABA-dependent stomatal closure, but overexpression of AtPUB22 and AtPUB23 in atpub18-2atpub19-3 did not inhibit ABA-enhanced stomatal closure, highlighting their ABA-independent roles. Overall, these results suggest that AtPUB18 has a linked function with AtPUB19, but is independent from AtPUB22 and AtPUB23, in negative regulation of ABA-mediated drought stress responses.

Ubiquitination in NB-LRR-mediated immunity

As a common protein modification, ubiquitination is used for regulating the fate of protein targets, notably in terms of stability. In recent years, it has emerged to play key roles in the regulation of plant defense responses. Given its flexibility and critical roles in signaling, primarily in the control of protein turnover, ubiquitination is probably targeting many major immune regulators for modification or degradation. In this review, we summarize the latest findings on how different components of the ubiquitination pathway are involved in NB-LRR R protein-mediated immunity.

PUB13, a U-box/ARM E3 ligase, regulates plant defense, cell death, and flowering time

The ubiquitination pathway is involved in a variety of cellular processes in plant growth, development, and immune responses. However, the function of this pathway in connecting plant development and innate immunity is still largely unknown. Recently, we characterized the U-box/ARM E3 ubiquitin ligase PUB13, which regulates both immune responses and flowering time in Arabidopsis. Here, we show that the rice Spl11 gene can complement the cell death and flowering functions of PUB13 in the pub13 mutant. In addition, HFR1, which functions mainly in photomorphogenesis, was identified as one of the PUB13-interacting proteins through yeast two-hybrid screening and pull-down assays. Because the flowering phenotype of pub13 depends on photoperiod, we propose that PUB13 may regulate HFR1 to fine-tune photomorphogenesis and flowering time in Arabidopsis.

Early senescence and cell death in Arabidopsis saul1 mutants involves the PAD4-dependent salicylic acid pathway

Age-dependent leaf senescence and cell death in Arabidopsis (Arabidopsis thaliana) requires activation of the transcription factor ORESARA1 (ORE1) and is not initiated prior to a leaf age of 28 d. Here, we investigate the conditional execution of events that regulate early senescence and cell death in senescence-associated ubiquitin ligase1 (saul1) mutants, deficient in the PLANT U-BOX-ARMADILLO E3 ubiquitin ligase SAUL1. In saul1 mutants challenged with low light, the switch of age-dependent cell death was turned on prematurely, as indicated by the accumulation of ORE1 transcripts, induction of the senescence marker gene SENESCENCE-ASSOCIATED GENE12, and cell death. However, ORE1 accumulation by itself was not sufficient to cause saul1 phenotypes, as demonstrated by double mutant analysis. Exposure of saul1 mutants to low light for only 24 h did not result in visible symptoms of senescence; however, the senescence-promoting transcription factor genes WRKY53, WRKY6, and NAC-LIKE ACTIVATED BY AP3/PI were up-regulated, indicating that senescence in saul1 seedlings was already initiated. To resolve the time course of gene expression, microarray experiments were performed at narrow intervals. Differential expression of the genes involved in salicylic acid and defense mechanisms were the earliest events detected, suggesting a central role for salicylic acid in saul1 senescence and cell death. The salicylic acid content increased in low-light-treated saul1 mutants, and application of exogenous salicylic acid was indeed sufficient to trigger saul1 senescence in permissive light conditions. Double mutant analyses showed that PHYTOALEXIN DEFICIENT4 (PAD4) but not NONEXPRESSER OF PR GENES1 (NPR1) is essential for saul1 phenotypes. Our results indicate that saul1 senescence depends on the PAD4-dependent salicylic acid pathway but does not require NPR1 signaling.

Molecular characterization of an elicitor-responsive Armadillo repeat gene (GhARM) from cotton (Gossypium hirsutum)

Only a few Armadillo (ARM) repeat proteins have been characterized in plants where they appear to have diverse functions, including the regulation of defence responses. In this study, the identification, cloning and characterization of a gene, encoding an ARM repeat protein (GhARM), is described. GhARM exists as multiple copies in cotton, with an 1713 bp ORF encoding 570 amino acids. The predicted protein contains three consecutive ARM repeats within an Armadillo-type fold, with no other distinguishing domains. Sequence alignments and phylogenetic analysis revealed that GhARM has a high homology with other ARM proteins in plants. The predicted three dimensional model of GhARM displayed a characteristic right-handed superhelical twist. In silico analysis of the promoter sequence revealed that it contains several defence- and hormone-responsive cis-regulatory elements. Expression of GhARM was significantly down-regulated in response to treatment with a V. dahliae elicitor suggesting that GhARM may function as a negative-regulator of cotton defence signalling against V. dahliae. To date, GhARM is the only ARM repeat gene that has been completely sequenced and characterized in cotton.

The U-Box/ARM E3 ligase PUB13 regulates cell death, defense, and flowering time in Arabidopsis

The components in plant signal transduction pathways are intertwined and affect each other to coordinate plant growth, development, and defenses to stresses. The role of ubiquitination in connecting these pathways, particularly plant innate immunity and flowering, is largely unknown. Here, we report the dual roles for the Arabidopsis (Arabidopsis thaliana) Plant U-box protein13 (PUB13) in defense and flowering time control. In vitro ubiquitination assays indicated that PUB13 is an active E3 ubiquitin ligase and that the intact U-box domain is required for the E3 ligase activity. Disruption of the PUB13 gene by T-DNA insertion results in spontaneous cell death, the accumulation of hydrogen peroxide and salicylic acid (SA), and elevated resistance to biotrophic pathogens but increased susceptibility to necrotrophic pathogens. The cell death, hydrogen peroxide accumulation, and resistance to necrotrophic pathogens in pub13 are enhanced when plants are pretreated with high humidity. Importantly, pub13 also shows early flowering under middle- and long-day conditions, in which the expression of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 and FLOWERING LOCUS T is induced while FLOWERING LOCUS C expression is suppressed. Finally, we found that two components involved in the SA-mediated signaling pathway, SID2 and PAD4, are required for the defense and flowering-time phenotypes caused by the loss of function of PUB13. Taken together, our data demonstrate that PUB13 acts as an important node connecting SA-dependent defense signaling and flowering time regulation in Arabidopsis.

A common plant cell-wall protein HyPRP1 has dual roles as a positive regulator of cell death and a negative regulator of basal defense against pathogens

Although hybrid proline-rich proteins (HyPRPs) are ubiquitous in plants, little is known about their roles other than as cell-wall structural proteins. We identified the gene HyPRP1 in Capsicum annuum and Nicotiana benthamiana, which encodes a protein containing proline-rich domain and eight-cysteine motif (8CM) that is constitutively expressed in various organs, mostly in the root, but is down-regulated upon inoculation with either incompatible or compatible pathogens. Ectopic expression of HyPRP1 in plants accelerated cell death, showing developmental abnormality with down-regulation of ROS-scavenging genes, and enhanced pathogen susceptibility suppressing expression of defense-related genes. Conversely, silencing of HyPRP1 suppressed pathogen-induced cell death, but enhanced disease resistance, with up-regulation of defense-related genes and inhibition of in planta growth of bacterial pathogens independently of signal molecule-mediated pathways. Furthermore, the secreted 8CM was sufficient for these HyPRP1 functions. Together, our results suggest that a common plant cell-wall structural protein, HyPRP1, performs distinct dual roles in positive regulation of cell death and negative regulation of basal defense against pathogen.

The F-box protein CPR1/CPR30 negatively regulates R protein SNC1 accumulation

Disease resistance (R) proteins, as central regulators of plant immunity, are tightly regulated for effective defense responses and to prevent constitutive defense activation under non-pathogenic conditions. Here we report the identification of an F-box protein CPR1/CPR30 as a negative regulator of an R protein SNC1 likely through SCF (Skp1-cullin-F-box) mediated protein degradation. The cpr1-2 (cpr30-1) loss-of-function mutant has constitutive defense responses, and it interacts synergistically with a gain-of function mutant snc1-1 and a bon1-1 mutant where SNC1 is upregulated. The loss of SNC1 function suppresses the mutant phenotypes of cpr1-2 and cpr1-2 bon1-1, while overexpression of CPR1 rescues mutant phenotypes of both bon1-1 and snc1-1. Furthermore, the amount of SNC1 protein is upregulated in the cpr1-2 mutant and down-regulated when CPR1 is overexpressed. The regulation of SNC1 by CPR1 is dependent on the 26S proteosome as a protease inhibitor MG132 stabilizes SNC1 and reverses the effect of CPR1 on SNC1. Interestingly, CPR1 is induced after infection of both virulent and avirulent pathogens similarly to the other negative defense regulator BON1. Thus, this study reveals a new mechanism in R protein regulation likely through protein degradation and suggests negative regulation as a critical component in fine control of plant immunity.

Regulation of plant immune receptors by ubiquitination

From pathogen perception and the activation of signal transduction cascades to the deployment of defense responses, protein ubiquitination plays a key role in the modulation of plant immunity. Ubiquitination is mediated by three enzymes, of which the E3 ubiquitin ligases, the substrate determinants, have been the major focus of attention. Accumulating evidence suggests that ubiquitination modulates signaling mediated by pattern recognition receptors and is important for the accumulation of nucleotide-binding leucine-rich repeat type intracellular immune sensors. Recent studies also indicate that ubiquitination directs vesicle trafficking, a function that has been clearly established for immune signaling in animals. In this mini review, we discuss these and other recent advances and highlight important open questions.

A novel protein RLS1 with NB-ARM domains is involved in chloroplast degradation during leaf senescence in rice

Leaf senescence, a type of programmed cell death (PCD) characterized by chlorophyll degradation, is important to plant growth and crop productivity. It emerges that autophagy is involved in chloroplast degradation during leaf senescence. However, the molecular mechanism(s) involved in the process is not well understood. In this study, the genetic and physiological characteristics of the rice rls1 (rapid leaf senescence 1) mutant were identified. The rls1 mutant developed small, yellow-brown lesions resembling disease scattered over the whole surfaces of leaves that displayed earlier senescence than those of wild-type plants. The rapid loss of chlorophyll content during senescence was the main cause of accelerated leaf senescence in rls1. Microscopic observation indicated that PCD was misregulated, probably resulting in the accelerated degradation of chloroplasts in rls1 leaves. Map-based cloning of the RLS1 gene revealed that it encodes a previously uncharacterized NB (nucleotide-binding site)-containing protein with an ARM (armadillo) domain at the carboxyl terminus. Consistent with its involvement in leaf senescence, RLS1 was up-regulated during dark-induced leaf senescence and down-regulated by cytokinin. Intriguingly, constitutive expression of RLS1 also slightly accelerated leaf senescence with decreased chlorophyll content in transgenic rice plants. Our study identified a previously uncharacterized NB-ARM protein involved in PCD during plant growth and development, providing a unique tool for dissecting possible autophagy-mediated PCD during senescence in plants.

Molecular cloning and characterization of OsUPS, a U-box containing E3 ligase gene that respond to phosphate starvation in rice (Oryza sativa)

The ubiquitin-26S proteasome system is important in the quality control of intracellular proteins. The ubiquitin-26S proteasome system includes the E1 (ubiquitin activating), E2 (ubiquitin conjugating), and E3 (ubiquitin ligase) enzymes. U-box proteins are a derived version of RING-finger domains, which have E3 enzyme activity. Here, we present the isolation of a novel U-box protein, U-box containing E3 ligase induced by phosphate starvation (OsUPS), from rice (Oryza sativa). The cDNA encoding the O. sativa U-box protein (OsUPS) comprises 1338 bp, with an open reading frame of 445 amino acids. The amino acid sequence of OsUPS cDNA shows 41-79% identity with other plant U-box homologous genes. The open reading frame of the OsUPS protein is comprised of notable domains: a single ~70-amino acid domain and a GKL domain that contains conserved glycine, lysine/arginine residues and leucine-rich feature. We found that full-length expression of OsUPS was up-regulated in both rice plants and cell culture in the absence of inorganic phosphate (P(i)). A self-ubiquitination assay indicated that the bacterially expressed OsUPS protein had E3 ligase activity, and subcellular localization results showed that OsUPS was located in the chloroplast. These results support the notion that OsUPS plays an important role in the P(i) signaling pathway through the ubiquitin-26S proteasome system.

Rice 14-3-3 protein (GF14e) negatively affects cell death and disease resistance

Plant 14-3-3 proteins regulate important cellular processes, including plant immune responses, through protein-protein interactions with a wide range of target proteins. In rice (Oryza sativa), the GF14e gene, which encodes a 14-3-3 protein, is induced during effector-triggered immunity (ETI) associated with pathogens such as Xanthomonas oryzae pv. oryzae (Xoo). To determine whether the GF14e gene plays a direct role in resistance to disease in rice, we suppressed its expression by RNAi silencing. GF14e suppression was correlated with the appearance of a lesion-mimic (LM) phenotype in the transgenic plants at 3 weeks after sowing. This indicates inappropriate regulation of cell death, a phenotype that is frequently associated with enhanced resistance to pathogens. GF14e-silenced rice plants showed high levels of resistance to a virulent strain of Xoo compared with plants that were not silenced. Enhanced resistance was correlated with GF14e silencing prior to and after development of the LM phenotype, higher basal expression of a defense response peroxidase gene (POX22.3), and accumulation of reactive oxygen species (ROS). In addition, GF14e-silenced plants also exhibit enhanced resistance to the necrotrophic fungal pathogen Rhizoctonia solani. Together, our findings suggest that GF14e negatively affects the induction of plant defense response genes, cell death and broad-spectrum resistance in rice.

AtPUB19, a U-box E3 ubiquitin ligase, negatively regulates abscisic acid and drought responses in Arabidopsis thaliana

Ubiquitination is an important protein post-translational modification, which is involved in various cellular processes in higher plants, and U-box E3 ligases play important roles in diverse functions in eukaryotes. Here, we describe the functions of Arabidopsis thaliana PUB19 (AtPUB19), which we demonstrated in an in vitro assay to encode a U-box type E3 ubiquitin ligase. AtPUB19 was up-regulated by drought, salt, cold, and abscisic acid (ABA). Down-regulation of AtPUB19 led to hypersensitivity to ABA, enhanced ABA-induced stomatal closing, and enhanced drought tolerance, while AtPUB19 overexpression resulted in the reverse phenotypes. Molecular analysis showed that the expression levels of a number of ABA and stress marker genes were altered in both AtPUB19 overexpressing and atpub19-1 mutant plants. In summary, our data show that AtPUB19 negatively regulates ABA and drought responses in A. thaliana.

The pepper E3 ubiquitin ligase RING1 gene, CaRING1, is required for cell death and the salicylic acid-dependent defense response

Ubiquitination is essential for ubiquitin/proteasome-mediated protein degradation in plant development and defense. Here, we identified a novel E3 ubiquitin ligase RING1 gene, CaRING1, from pepper (Capsicum annuum). In pepper, CaRING1 expression is induced by avirulent Xanthomonas campestris pv vesicatoria infection. CaRING1 contains an amino-terminal transmembrane domain and a carboxyl-terminal RING domain. In addition, it displays in vitro E3 ubiquitin ligase activity, and the RING domain is essential for E3 ubiquitin ligase activity in CaRING1. CaRING1 also localizes to the plasma membrane. In pepper plants, virus-induced gene silencing of CaRING1 confers enhanced susceptibility to avirulent X. campestris pv vesicatoria infection, which is accompanied by compromised hypersensitive cell death, reduced expression of PATHOGENESIS-RELATED1, and lowered salicylic acid levels in leaves. Transient expression of CaRING1 in pepper leaves induces cell death and the defense response that requires the E3 ubiquitin ligase activity of CaRING1. By contrast, overexpression of CaRING1 in Arabidopsis (Arabidopsis thaliana) confers enhanced resistance to hemibiotrophic Pseudomonas syringae pv tomato and biotrophic Hyaloperonospora arabidopsidis infections. Taken together, these results suggest that CaRING1 is involved in the induction of cell death and the regulation of ubiquitination during the defense response to microbial pathogens.

Characterization and genetic analysis of a light- and temperature-sensitive spotted-leaf mutant in rice

A rice spotted-leaf mutant was isolated from an ethane methyl sulfonate (EMS) -induced IR64 mutant bank. The mutant, designated as spl30 (spotted-leaf30), displayed normal green leaf color under shade but exhibited red-brown lesions under natural summer field conditions. Initiation of the lesions was induced by light and the symptom was enhanced at 33 (°) C relative to 26 (°) C. Histochemical staining did not show cell death around the red-brown lesions. Chlorophyll contents in the mutant were significantly lower than those of the wild type while the ratio of chlorophyll a/b remained the same, indicating that spl30 was impaired in biosynthesis or degradation of chlorophyll. Disease reaction patterns of the mutant to Xanthomonas oryzae pv. oryzae were largely unchanged to most races tested except for a few strains. Genetic analysis showed that the mutation was controlled by a single recessive gene, tentatively named spl30(t), which co-segregated with RM15380 on chromosome 3, and was delimited to a 94 kb region between RM15380 and RM15383. Spl30(t) is likely a novel rice spotted-leaf gene since no other similar genes have been identified near the chromosomal region. The genetic data and recombination populations provided in this study will enable further fine-mapping and cloning of the gene.

Altered germination and subcellular localization patterns for PUB44/SAUL1 in response to stress and phytohormone treatments

Background

In plants, the ubiquitin-proteasome system is emerging as a significant regulatory system throughout the plant lifecycle. The ubiquitination of a target protein requires the sequential actions of the E1, E2 and E3 enzymes, with the latter E3 enzyme conferring target selection in this process. There are a large number of predicted E3 enzymes in plant genomes, and very little is known about the functions of many of these predicted genes. Here we report here an analysis of two closely-related members of the Arabidopsis Plant U-box (PUB) family of E3 ubiquitin ligases, PUB43 and PUB44.

Principal Findings

Homozygous pub44/pub44 mutant seedlings were found displayed a seedling lethal phenotype and this corresponded with widespread cell death lesions throughout the cotyledons and roots. Interestingly, heterozygous PUB44/pub44 seedlings were wild-type in appearance yet displayed intermediate levels of cell death lesions in comparison to pub44/pub44 seedlings. In contrast, homozygous pub43/pub43 mutants were viable and did not show any signs of cell death despite the PUB43 gene being more highly expressed than PUB44. The PUB44 mutants are not classical lesion mimic mutants as they did not have increased resistance to plant pathogens. We also observed increased germination rates in mutant seeds for both PUB44 and PUB43 under inhibitory concentrations of abscisic acid. Finally, the subcellular localization of PUB44 was investigated with transient expression assays in BY-2 cells. Under varying conditions, PUB44 was observed to be localized to the cytoplasm, plasma membrane, or nucleus.

Conclusions

Based on mutant plant analyses, the Arabidopsis PUB43 and PUB44 genes are proposed to function during seed germination and early seedling growth. Given PUB44's ability to shuttle from the nucleus to the plasma membrane, PUB44 may be active in different subcellular compartments as part of these biological functions.

Direct ubiquitination of pattern recognition receptor FLS2 attenuates plant innate immunity

Innate immune responses are triggered by the activation of pattern-recognition receptors (PRRs). The Arabidopsis PRR FLAGELLIN-SENSING 2 (FLS2) senses bacterial flagellin and initiates immune signaling through association with BAK1. The molecular mechanisms underlying the attenuation of FLS2 activation are largely unknown. We report that flagellin induces recruitment of two closely related U-box E3 ubiquitin ligases, PUB12 and PUB13, to FLS2 receptor complex in Arabidopsis. BAK1 phosphorylates PUB12 and PUB13 and is required for FLS2-PUB12/13 association. PUB12 and PUB13 polyubiquitinate FLS2 and promote flagellin-induced FLS2 degradation, and the pub12 and pub13 mutants displayed elevated immune responses to flagellin treatment. Our study has revealed a unique regulatory circuit of direct ubiquitination and turnover of FLS2 by BAK1-mediated phosphorylation and recruitment of specific E3 ligases for attenuation of immune signaling.

Direct ubiquitination of pattern recognition receptor FLS2 attenuates plant innate immunity

Innate immune responses are triggered by the activation of pattern-recognition receptors (PRRs). The Arabidopsis PRR FLAGELLIN-SENSING 2 (FLS2) senses bacterial flagellin and initiates immune signaling through association with BAK1. The molecular mechanisms underlying the attenuation of FLS2 activation are largely unknown. We report that flagellin induces recruitment of two closely related U-box E3 ubiquitin ligases, PUB12 and PUB13, to FLS2 receptor complex in Arabidopsis. BAK1 phosphorylates PUB12 and PUB13 and is required for FLS2-PUB12/13 association. PUB12 and PUB13 polyubiquitinate FLS2 and promote flagellin-induced FLS2 degradation, and the pub12 and pub13 mutants displayed elevated immune responses to flagellin treatment. Our study has revealed a unique regulatory circuit of direct ubiquitination and turnover of FLS2 by BAK1-mediated phosphorylation and recruitment of specific E3 ligases for attenuation of immune signaling.

OsSDIR1 overexpression greatly improves drought tolerance in transgenic rice

Recent genomic and genetic analyses based on Arabidopsis suggest that ubiquitination plays crucial roles in the plant response to abiotic stress and the phytohormone abscisic acid (ABA). However, few such studies have been reported in rice as a monocotyledonous model plant. Taking advantage of strategies in biochemistry, molecular cell biology and genetics, the RING-finger containing E3 ligase OsSDIR1 (Oryza sativa SALT-AND DROUGHT-INDUCED RING FINGER 1) was found to be a candidate drought tolerance gene for engineering of crop plants. The expression of OsSDIR1 was detected in all tissues of rice and up-regulated by drought and NaCl, but not by ABA. In vitro ubiquitination assays demonstrated that OsSDIR1 is a functional E3 ubiquitin ligase and that the RING finger region is required for its activity. OsSDIR1 could complement the drought sensitive phenotype of the sdir1 mutant and overexpressing transgenic Arabidopsis were more sensitive to ABA, indicating that the OsSDIR1 gene is a functional ortholog of SDIR1. Upon drought treatment, the OsSDIR1-transgenic rice showed strong drought tolerance compared to control plants. Analysis of the stomata aperture revealed that there were more closed stomatal pores in transgenic plants than those of control plants. This result was also confirmed by the water loss assay and leaf related water content (RWC) measurements during drought treatment. Thus, we demonstrated that monocot- and dicot- SDIR1s are conserved yet have diverse functions.

Isolation, fine mapping and expression profiling of a lesion mimic genotype, spl(NF4050-8) that confers blast resistance in rice

We evaluated a large collection of Tos17 mutant panel lines for their reaction to three different races of Magnaporthe oryzae and identified a lesion mimic mutant, NF4050-8, that showed lesions similar to naturally occurring spl5 mutant and enhanced resistance to all the three blast races tested. Nested modified-AFLP using Tos17-specific primers and southern hybridization experiments of segregating individuals indicated that the lesion mimic phenotype in NF4050-8 is most likely due to a nucleotide change acquired during the culturing process and not due to Tos17 insertion per se. Inheritance and genetic analyses in two japonica × indica populations identified an overlapping genomic region of 13 cM on short arm of chromosome 7 that was linked with the lesion mimic phenotype. High-resolution genetic mapping using 950 F(3) and 3,821 F(4) plants of NF4050-8 × CO39 delimited a 35 kb region flanked by NBARC1 (5.262 Mb) and RM8262 (5.297 Mb), which contained 6 ORFs; 3 of them were 'resistance gene related' with typical NBS-LRR signatures. One of them harbored a NB-ARC domain, which had been previously demonstrated to be associated with cell death in animals. Microarray analysis of NF4050-8 revealed significant up-regulation of numerous defense/pathogenesis-related genes and down-regulation of heme peroxidase genes. Real-time PCR analysis of WRKY45 and PR1b genes suggested possible constitutive activation of a defense signaling pathway downstream of salicylic acid but independent of NH1 in these mutant lines of rice.

Identification and transcriptional profiling of differentially expressed genes associated with resistance to Pseudoperonospora cubensis in cucumber

To identify genes induced during Pseudoperonospora cubensis (Berk. and Curk.) Rostov. infection in cucumber (Cucumis sativus L.), the suppression subtractive hybridization (SSH) was performed using mixed cDNAs prepared from cucumber seedlings inoculated with the pathogen as a tester and cDNA from uninfected cucumber seedlings as a driver. A forward subtractive cDNA library (FSL) and a reverse subtractive cDNA library (RSL) were constructed, from which 1,416 and 1,128 recombinant clones were isolated, respectively. Differential screening of the preferentially expressed recombinant clones identified 58 unique expressed sequence tags (ESTs) from FSL and 29 from RSL. The ESTs with significant protein homology were sorted into 13 functional categories involved in nearly the whole process of plant defense such as signal transduction and cell defense, transcription, cell cycle and DNA processing, protein synthesis, protein fate, proteins with binding functions, transport, metabolism and energy. The expressions of twenty-five ESTs by real-time quantitative RT-PCR confirmed that differential gene regulation occurred during P. cubensis infection and inferred that higher and earlier expression of transcription factors and signal transduction associated genes together with ubiquitin/proteasome and polyamine biosynthesis pathways may contribute to the defense response of cucumber to P. cubensis infection. The transcription profiling of selected down-regulated genes revealed that suppression of the genes in reactive oxygen species scavenging system and photosynthesis pathway may inhibit disease development in the host tissue.

Arabidopsis SAP5 functions as a positive regulator of stress responses and exhibits E3 ubiquitin ligase activity

AtSAP5, one of approximately 14 members of the Stress Associated Protein gene family in Arabidopsis, was identified by its expression in response to salinity, osmotic, drought and cold stress. AtSAP5 shows strong homology to OSISAP1, an A20/AN1-type zinc finger protein implicated in stress tolerance in rice. To evaluate the function of AtSAP5 in the regulation of abiotic stress responses, transgenic Arabidopsis plants that over-express AtSAP5 (35S::AtSAP5) were characterized, along with wild-type and T-DNA knock-down plants. Plants that over-express AtSAP5 showed increased tolerance to environmental challenges including salt stress, osmotic stress and water deficit. Comparison of gene expression patterns between 35S::AtSAP5 transgenic plants and wild-type plants under normal conditions and water deficit stress indicated that over-expression of AtSAP5 correlates with up-regulation of drought stress responsive gene expression. Analysis of transgenic plants that express GFP-AtSAP5 showed that it is localized primarily in nuclei of root cells and recombinant AtSAP5 has E3 ubiquitin ligase activity in vitro. These results indicate that AtSAP5 has E3 ligase activity and acts as a positive regulator of stress responses in Arabidopsis.

Rice RING protein OsBBI1 with E3 ligase activity confers broad-spectrum resistance against Magnaporthe oryzae by modifying the cell wall defence

Emerging evidence suggests that E3 ligases play critical roles in diverse biological processes, including innate immune responses in plants. However, the mechanism of the E3 ligase involvement in plant innate immunity is unclear. We report that a rice gene, OsBBI1, encoding a RING finger protein with E3 ligase activity, mediates broad-spectrum disease resistance. The expression of OsBBI1 was induced by rice blast fungus Magnaporthe oryzae, as well as chemical inducers, benzothiadiazole and salicylic acid. Biochemical analysis revealed that OsBBI1 protein possesses E3 ubiquitin ligase activity in vitro. Genetic analysis revealed that the loss of OsBBI1 function in a Tos17-insertion line increased susceptibility, while the overexpression of OsBBI1 in transgenic plants conferred enhanced resistance to multiple races of M. oryzae. This indicates that OsBBI1 modulates broad-spectrum resistance against the blast fungus. The OsBBI1-overexpressing plants showed higher levels of H(2)O(2) accumulation in cells and higher levels of phenolic compounds and cross-linking of proteins in cell walls at infection sites by M. oryzae compared with wild-type (WT) plants. The cell walls were thicker in the OsBBI1-overexpressing plants and thinner in the mutant plants than in the WT plants. Our results suggest that OsBBI1 modulates broad-spectrum resistance to blast fungus by modifying cell wall defence responses. The functional characterization of OsBBI1 provides insight into the E3 ligase-mediated innate immunity, and a practical tool for constructing broad-spectrum resistance against the most destructive disease in rice.

Genome-wide association study of quantitative resistance to southern leaf blight in the maize nested association mapping population

Nested association mapping (NAM) offers power to resolve complex, quantitative traits to their causal loci. The maize NAM population, consisting of 5,000 recombinant inbred lines (RILs) from 25 families representing the global diversity of maize, was evaluated for resistance to southern leaf blight (SLB) disease. Joint-linkage analysis identified 32 quantitative trait loci (QTLs) with predominantly small, additive effects on SLB resistance. Genome-wide association tests of maize HapMap SNPs were conducted by imputing founder SNP genotypes onto the NAM RILs. SNPs both within and outside of QTL intervals were associated with variation for SLB resistance. Many of these SNPs were within or near sequences homologous to genes previously shown to be involved in plant disease resistance. Limited linkage disequilibrium was observed around some SNPs associated with SLB resistance, indicating that the maize NAM population enables high-resolution mapping of some genome regions.

OsPUB15, an E3 ubiquitin ligase, functions to reduce cellular oxidative stress during seedling establishment

The plant U-box (PUB) protein functions as an E3 ligase to poly-ubiquitinate a target protein for its degradation or post-translational modification. Here, we report functional roles for OsPUB15, which encodes a cytosolic U-box protein in the class-II PUB family. Self-ubiquitination assays showed that bacterially expressed MBP-OsPUB15 protein has E3 ubiquitin ligase activity. A T-DNA insertional mutation in OsPUB15 caused severe growth retardation and a seedling-lethal phenotype. Mutant seeds did not produce primary roots, and their shoot development was significantly delayed. Transgenic plants expressing the OsPUB15 antisense transcript phenocopied these mutant characters. The abnormal phenotypes were partially rescued by two antioxidants, catechin and ascorbic acid. Germinating seeds in the dark also recovered the rootless defect. Levels of H2O2 and oxidized proteins were higher in the knock-out mutant compared with the wild type. OsPUB15 transcript levels were increased upon H2O2, salt and drought stresses; plants overexpressing the gene grew better than the wild type under high salinity. These results indicate that PUB15 is a regulator that reduces reactive oxygen species (ROS) stress and cell death.

Altered germination and subcellular localization patterns for PUB44/SAUL1 in response to stress and phytohormone treatments

BACKGROUND

In plants, the ubiquitin-proteasome system is emerging as a significant regulatory system throughout the plant lifecycle. The ubiquitination of a target protein requires the sequential actions of the E1, E2 and E3 enzymes, with the latter E3 enzyme conferring target selection in this process. There are a large number of predicted E3 enzymes in plant genomes, and very little is known about the functions of many of these predicted genes. Here we report here an analysis of two closely-related members of the Arabidopsis Plant U-box (PUB) family of E3 ubiquitin ligases, PUB43 and PUB44.

PRINCIPAL FINDINGS

Homozygous pub44/pub44 mutant seedlings were found displayed a seedling lethal phenotype and this corresponded with widespread cell death lesions throughout the cotyledons and roots. Interestingly, heterozygous PUB44/pub44 seedlings were wild-type in appearance yet displayed intermediate levels of cell death lesions in comparison to pub44/pub44 seedlings. In contrast, homozygous pub43/pub43 mutants were viable and did not show any signs of cell death despite the PUB43 gene being more highly expressed than PUB44. The PUB44 mutants are not classical lesion mimic mutants as they did not have increased resistance to plant pathogens. We also observed increased germination rates in mutant seeds for both PUB44 and PUB43 under inhibitory concentrations of abscisic acid. Finally, the subcellular localization of PUB44 was investigated with transient expression assays in BY-2 cells. Under varying conditions, PUB44 was observed to be localized to the cytoplasm, plasma membrane, or nucleus.

CONCLUSIONS

Based on mutant plant analyses, the Arabidopsis PUB43 and PUB44 genes are proposed to function during seed germination and early seedling growth. Given PUB44's ability to shuttle from the nucleus to the plasma membrane, PUB44 may be active in different subcellular compartments as part of these biological functions.

C-terminal armadillo repeats are essential and sufficient for association of the plant U-box armadillo E3 ubiquitin ligase SAUL1 with the plasma membrane

Ubiquitination plays important roles in plant growth and development. Whereas ubiquitin-dependent protein degradation and modulation in the cytoplasm and nucleus are well established in plants, ubiquitination events mediated by E3 ubiquitin ligases at the plasma membrane are largely unknown. Here, it is demonstrated that the suppressor of premature senescence and cell death SENESCENCE-ASSOCIATED UBIQUITIN LIGASE 1 (SAUL1), a plant U-box armadillo repeat (PUB-ARM) E3 ubiquitin ligase, localizes at the plasma membrane. Among the members of the PUB-ARM protein family, this localization is unique to SAUL1 and its two closest homologues. A novel armadillo repeat domain was identified at the SAUL1 C-terminus that directs specific association with the plasma membrane and is crucial for SAUL1 function in vivo. The data suggest that a small subgroup of PUB-ARM proteins including SAUL1 have functions at the plasma membrane probably by modifying target proteins by ubiquitination.

Comparative phenotypic and physiological characteristics of spotted leaf 6 (spl6) and brown leaf spot2 (bl2) lesion mimic mutants (LMM) in rice

Spontaneous necrotic lesions were found in a lesion mimic mutant brown leaf spot 2 (bl2) without pathogenic infection. Small spots in the seedlings appeared at the four leaves stage and gradually grew into a large round and black area with a gray center on the leaf surfaces. Lower growth habit and lower agronomic trait values with reduced stature, tiller, and panicle number, as well as lower yield potential were noted in the mutants relative to the trait values of the wild-type plants. Microscopic analysis revealed that mesophyll chloroplast was severely damaged or absent in the spotted area of the mutant leaves. Total chlorophyll content, hydrogen peroxide level, and catalase activity were increased at up to 45 days after germination and were dropped at 60 d in the mutant leaves. However, the total protein contents were reduced slightly with a growth period of up to 45 days and were increased at 60 days after germination. A gradual increment of the total ascorbic acid contents in the mutants were observed with advanced plant age, but increased until 45 days and dropped comparatively at 60 days in the wild-type leaves. Increased gene transcriptions of OsPDI and OsGPX1 were noted in the spotted leaves as compared to the non-spotted leaves of the mutant and wild-type leaves, whereas transcripts of OsTPX were transcribed at lower levels in the spotted leaves as compared to the non-spotted leaves. The genetic nature of the bl2 mutant indicated that the F(1) plants evidenced the wild-type phenotype and that bl2 was governed by a single recessive gene.

The Arabidopsis Botrytis Susceptible1 Interactor defines a subclass of RING E3 ligases that regulate pathogen and stress responses

We studied the function of Arabidopsis (Arabidopsis thaliana) Botrytis Susceptible1 Interactor (BOI) in plant responses to pathogen infection and abiotic stress. BOI physically interacts with and ubiquitinates Arabidopsis BOS1, an R2R3MYB transcription factor previously implicated in stress and pathogen responses. In transgenic plants expressing the BOS1-β-glucuronidase transgene, β-glucuronidase activity could be detected only after inhibition of the proteosome, suggesting that BOS1 is a target of ubiquitin-mediated degradation by the proteosome. Plants with reduced BOI transcript levels generated through RNA interference (BOI RNAi) were more susceptible to the necrotrophic fungus Botrytis cinerea and less tolerant to salt stress. In addition, BOI RNAi plants exhibited increased cell death induced by the phytotoxin α-picolinic acid and by a virulent strain of the bacterial pathogen Pseudomonas syringae, coincident with peak disease symptoms. However, the hypersensitive cell death associated with different race-specific resistance genes was unaffected by changes in the level of BOI transcript. BOI expression was enhanced by B. cinerea and salt stress but repressed by the plant hormone gibberellin, indicating a complex regulation of BOI gene expression. Interestingly, BOI RNAi plants exhibit reduced growth responsiveness to gibberellin. We also present data revealing the function of three Arabidopsis BOI-RELATED GENES (BRGs), which contribute to B. cinerea resistance and the suppression of disease-associated cell death. In sum, BOI and BRGs represent a subclass of RING E3 ligases that contribute to plant disease resistance and abiotic stress tolerance through the suppression of pathogen-induced as well as stress-induced cell death.

Identification of genes contributing to quantitative disease resistance in rice

Despite the importance of quantitative disease resistance during a plant's life, little is known about the molecular basis of this type of host-pathogen interaction, because most of the genes underlying resistance quantitative trait loci (QTLs) are unknown. To identify genes contributing to resistance QTLs in rice, we analyzed the colocalization of a set of characterized rice defense-responsive genes and resistance QTLs against different pathogens. We also examined the expression patterns of these genes in response to pathogen infection in the parents of the mapping populations, based on the strategy of validation and functional analysis of the QTLs. The results suggest that defense-responsive genes are important resources of resistance QTLs in rice. OsWRKY45-1 is the gene contributing to a major resistance QTL. NRR, OsGH3-1, and OsGLP members on chromosome 8 contribute alone or collectively to different minor resistance QTLs. These genes function in a basal resistance pathway or in major disease resistance gene-mediated race-specific pathways.

Inhibitory effect on the tobacco mosaic virus infection by a plant RING finger protein

In the yeast two-hybrid screening of plant factors interacting with tobacco mosaic virus (TMV) RNA-dependent RNA polymerase (RdRp), we found a protein containing a RING finger motif in tobacco (Nicotiana tabacum) and designated it as TARF (TMV-associated RING finger protein). TARF is a homologue of a Lotus japonicus RING finger protein (LjnsRING) involved in the symbiotic interaction between L. japonicus and Mesorhizobium loti. When TARF was silenced by virus-induced gene silencing (VIGS) method, TMV RNA accumulation as well as the number of foci formed by GFP-tagged TMV increased drastically. Transient overexpression of TARF reduced the accumulation of TMV. Moreover, TARF transcription was rapidly upregulated by the inoculation of TMV in tobacco plants. These results indicated that TARF is a RING finger protein that inhibits the accumulation of TMV via the interaction of TMV RdRp.

The Medicago truncatula E3 ubiquitin ligase PUB1 interacts with the LYK3 symbiotic receptor and negatively regulates infection and nodulation

LYK3 is a lysin motif receptor-like kinase of Medicago truncatula, which is essential for the establishment of the nitrogen-fixing, root nodule symbiosis with Sinorhizobium meliloti. LYK3 is a putative receptor of S. meliloti Nod factor signals, but little is known of how it is regulated and how it transduces these symbiotic signals. In a screen for LYK3-interacting proteins, we identified M. truncatula Plant U-box protein 1 (PUB1) as an interactor of the kinase domain. In planta, both proteins are localized and interact in the plasma membrane. In M. truncatula, PUB1 is expressed specifically in symbiotic conditions, is induced by Nod factors, and shows an overlapping expression pattern with LYK3 during nodulation. Biochemical studies show that PUB1 has a U-box-dependent E3 ubiquitin ligase activity and is phosphorylated by the LYK3 kinase domain. Overexpression and RNA interference studies in M. truncatula show that PUB1 is a negative regulator of the LYK3 signaling pathway leading to infection and nodulation and is important for the discrimination of rhizobia strains producing variant Nod factors. The potential role of PUB E3 ubiquitin ligases in controlling plant-microbe interactions and development through interacting with receptor-like kinases is discussed.

The ubiquitin/26S proteasome system in plant-pathogen interactions: a never-ending hide-and-seek game

The ubiquitin/26S proteasome system (UPS) plays a central role in plant protein degradation. Over the past few years, the importance of this pathway in plant-pathogen interactions has been increasingly highlighted. UPS is involved in almost every step of the defence mechanisms in plants, regardless of the type of pathogen. In addition to its proteolytic activities, UPS, through its 20S RNase activity, may be part of a still unknown antiviral defence pathway. Strikingly, UPS is not only a weapon used by plants to defend themselves, but also a target for some pathogens that have evolved mechanisms to inhibit and/or use this system for their own purposes. This article attempts to summarize the current knowledge on UPS involvement in plant-microbe interactions, a complex scheme that illustrates the never-ending arms race between hosts and microbes.

SPL28 encodes a clathrin-associated adaptor protein complex 1, medium subunit micro 1 (AP1M1) and is responsible for spotted leaf and early senescence in rice (Oryza sativa)

To expand our understanding of cell death in plant defense responses, we isolated a novel rice (Oryza sativa) spotted leaf mutant (spl28) that displays a lesion mimic phenotype in the absence of pathogen attack through treatment of Hwacheongbyeo (an elite Korean japonica cultivar) with N-methyl-N-nitrosourea (MNU). Early stage development of the spl28 mutant was normal. However, after flowering, spl28 mutants exhibited a significant decrease in chlorophyll content, soluble protein content, and photosystem II efficiency, and high concentrations of reactive oxygen species (ROS), phytoalexin, callose, and autofluorescent phenolic compounds that localized in or around the lesions. The spl28 mutant also exhibited significantly enhanced resistance to rice blast and bacterial blight. Using a map-based cloning approach, we determined that SPL28 encodes a clathrin-associated adaptor protein complex 1, medium subunit micro 1 (AP1M1), which is involved in the post-Golgi trafficking pathway. A green fluorescent protein (GFP) fusion protein of SPL28 (SPL28::GFP) localized to the Golgi apparatus, and expression of SPL28 complemented the membrane trafficking defect of apm1-1 Delta yeast mutants. SPL28 was ubiquitously expressed and contained a highly conserved adaptor complex medium subunit (ACMS) family domain. SPL28 appears to be involved in the regulation of vesicular trafficking, and SPL28 dysfunction causes the formation of hypersensitive response (HR)-like lesions, leading to the initiation of leaf senescence.

Transcriptome analysis of a wheat near-isogenic line pair carrying Fusarium head blight-resistant and -susceptible alleles

Fusarium head blight (FHB), caused primarily by Fusarium graminearum, decreases grain yield and quality in wheat and barley. Disease severity, deoxynivalenol (DON), fungal biomass, and transcript accumulation were examined in a wheat near-isogenic line pair carrying either the resistant or susceptible allele for the chromosome 3BS FHB-resistance quantitative trait locus (Fhb1). Fhb1 restricts spread of disease symptoms but does not provide resistance to initial infection or initial DON accumulation. Wheat exhibits both induction and repression of large sets of gene transcripts during F. graminearum infection. In addition, a difference in the general timing of transcript accumulation in plants carrying either the resistant or susceptible allele at the Fhb1 locus was detected, and 14 wheat gene transcripts were detected that exhibited accumulation differences between the resistant and susceptible alleles. These results indicate that these may be host responses that differentiate the resistant from the susceptible interaction. Comparative analysis of the wheat-F. graminearum and the barley-F. graminearum interactions revealed a large set of conserved transcript accumulation patterns. However, we also detected gene transcripts that were repressed in wheat but not in barley. Based on the disease symptoms, transcript accumulation data, and comparative analysis of the barley and wheat host response to F. graminearum infection, we developed an integrated model for the interactions of wheat and barley with F. graminearum.

CERBERUS, a novel U-box protein containing WD-40 repeats, is required for formation of the infection thread and nodule development in the legume-Rhizobium symbiosis

Endosymbiotic infection of legume plants by Rhizobium bacteria is initiated through infection threads (ITs) which are initiated within and penetrate from root hairs and deliver the endosymbionts into nodule cells. Despite recent progress in understanding the mutual recognition and early symbiotic signaling cascades in host legumes, the molecular mechanisms underlying bacterial infection processes and successive nodule organogenesis are still poorly understood. We isolated a novel symbiotic mutant of Lotus japonicus, cerberus, which shows defects in IT formation and nodule organogenesis. Map-based cloning of the causal gene allowed us to identify the CERBERUS gene, which encodes a novel protein containing a U-box domain and WD-40 repeats. CERBERUS expression was detected in the roots and nodules, and was enhanced after inoculation of Mesorhizobium loti. Strong expression was detected in developing nodule primordia and the infected zone of mature nodules. In cerberus mutants, Rhizobium colonized curled root hair tips, but hardly penetrated into root hair cells. The occasional ITs that were formed inside the root hair cells were mostly arrested within the epidermal cell layer. Nodule organogenesis was aborted prematurely, resulting in the formation of a large number of small bumps which contained no endosymbiotic bacteria. These phenotypic and genetic analyses, together with comparisons with other legume mutants with defects in IT formation, indicate that CERBERUS plays a critical role in the very early steps of IT formation as well as in growth and differentiation of nodules.

Characterization and mapping of complementary lesion-mimic genes lm1 and lm2 in common wheat

A lesion-mimic phenotype appeared in a segregating population of common wheat cross Yanzhan 1/Zaosui 30. The parents had non-lesion normal phenotypes. Shading treatment and histochemical analyses showed that the lesions were caused by light-dependent cell death and were not associated with pathogens. Studies over two cropping seasons showed that some lines with more highly expressed lesion-mimic phenotypes exhibited significantly lower grain yields than those with the normal phenotype, but there were no significant effects in the lines with weakly expressed lesion-mimic phenotypes. Among yield traits, one-thousand grain weight was the most affected by lesion-mimic phenotypes. Genetic analysis indicated that this was a novel type of lesion mimic, which was caused by interaction of recessive genes derived from each parent. The lm1 (lesion mimic 1) locus from Zaosui 30 was flanked by microsatellite markers Xwmc674 and Xbarc133/Xbarc147 on chromosome 3BS, at genetic distances of 1.2 and 3.8 cM, respectively, whereas lm2 from Yanzhan 1 was mapped between microsatellite markers Xgwm513 and Xksum154 on chromosome 4BL, at genetic distances of 1.5 and 3 cM, respectively. The linked microsatellite makers identified in this study might be useful for evaluating whether potential parents with normal phenotype are carriers of lesion-mimic alleles.