Rice OsACDR1 (Oryza sativa accelerated cell death and resistance 1) is a potential positive regulator of fungal disease resistance

A ubiquitin-specific protease functions in regulating cell death and immune responses in rice

Ubiquitin-specific proteases (UBPs) process deubiquitination in eukaryotic organisms and are widely involved in plant development and responses to environmental stress. However, their role in cell death and plant immunity remains largely unknown. Here, we identified a rice lesion mimic mutant (LMM) and cloned its causative gene, LMM22. Both dysfunction and overexpression of LMM22 gave rise to the hypersensitive response-like cell death, reactive oxygen species bursts, and activated defence responses. LMM22 encodes an active UBP that is localised to the endoplasmic reticulum (ER) and displays a constitutive expression pattern in rice. LMM22 interacts with SPOTTED LEAF 35 (SPL35), a coupling of ubiquitin conjugation to ER degradation domain-containing protein that is known to participate in ubiquitination and the regulation of cell death and disease response in rice. Additional analyses suggest that LMM22 can positively regulate and stabilise the abundance of SPL35 protein likely through its deubiquitination activity. These data therefore improve our understanding of the function of UBP in rice innate immune responses by demonstrating that LMM22 functions as a critical regulator of SPL35 in cell death and disease resistance.

Defensive Molecules Momilactones A and B: Function, Biosynthesis, Induction and Occurrence

Labdane-related diterpenoids, momilactones A and B were isolated and identified in rice husks in 1973 and later found in rice leaves, straws, roots, root exudate, other several Poaceae species and the moss species Calohypnum plumiforme. The functions of momilactones in rice are well documented. Momilactones in rice plants suppressed the growth of fungal pathogens, indicating the defense function against pathogen attacks. Rice plants also inhibited the growth of adjacent competitive plants through the root secretion of momilactones into their rhizosphere due to the potent growth-inhibitory activity of momilactones, indicating a function in allelopathy. Momilactone-deficient mutants of rice lost their tolerance to pathogens and allelopathic activity, which verifies the involvement of momilactones in both functions. Momilactones also showed pharmacological functions such as anti-leukemia and anti-diabetic activities. Momilactones are synthesized from geranylgeranyl diphosphate through cyclization steps, and the biosynthetic gene cluster is located on chromosome 4 of the rice genome. Pathogen attacks, biotic elicitors such as chitosan and cantharidin, and abiotic elicitors such as UV irradiation and CuCl2 elevated momilactone production through jasmonic acid-dependent and independent signaling pathways. Rice allelopathy was also elevated by jasmonic acid, UV irradiation and nutrient deficiency due to nutrient competition with neighboring plants with the increased production and secretion of momilactones. Rice allelopathic activity and the secretion of momilactones into the rice rhizosphere were also induced by either nearby Echinochloa crus-galli plants or their root exudates. Certain compounds from Echinochloa crus-galli may stimulate the production and secretion of momilactones. This article focuses on the functions, biosynthesis and induction of momilactones and their occurrence in plant species.

A Single Amino Acid Substitution in MIL1 Leads to Activation of Programmed Cell Death and Defense Responses in Rice

Lesion mimic mutants are an ideal model system for elucidating the molecular mechanisms of programmed cell death and defense responses in rice. In this study, we identified a lesion mimic mutant termed miner infection like 1-1 (mil1-1). The mil1-1 exhibited lesions on the leaves during development, and the chloroplasts of mil1-1 leaves were disrupted. Reactive oxygen species were found to accumulate in mil1-1 leaves. Cell death and DNA fragmentation were observed in mil1-1 leaves, indicating that the cells in the spots of mil1-1 leaves experienced programmed cell death. Most agronomic traits decreased in mil1-1, suggesting that the growth retardation in mil1-1 caused reduced per-plant grain yield. However, the mutation of MIL1 activated the expression of pathogen response genes and enhanced resistance to bacterial blight. The MIL1 gene was cloned using the positional cloning approach. A missense mutation 751 bp downstream of ATG was found in mil1-1. The defects of mil1-1 were able to be rescued by delivering a wild-type MIL1 gene into mil1-1. MIL1 encoded hydroperoxide lyase 3 (OsHPL3), and the expression of OsHPL3 was induced via hormone and abiotic stresses. Our findings provide insights into the roles of MIL1 in regulating programmed cell death, development, yield, and defense responses in rice.

OsSPL88 Encodes a Cullin Protein that Regulates Rice Growth and Development

Plant lesion mimics refer to necrotic spots spontaneously produced by the plant without mechanical damage, pathogen invasion, and adversity stress. Here, we isolated and characterized two rice (Oryza sativa L) mutants, namely, spl88-1 (spotted leaf88-1) and spl88-2 (spotted leaf88-2), which were identified from an ethyl methanesulfonate-mutagenized japonica cultivar Xiushui 11 population. Physiological and biochemical experiments indicated that more ROS accumulated in spl88-1 and spl88-2 than in wild type. spl88-1 and spl88-2 displayed spontaneous cell death and enhanced their resistance to bacterial blight by affecting the expression of defense-related genes. We isolated SPL88 by map-based cloning, which encoded a highly conserved Cullin protein. A single base deletion was detected in spl88-1 and spl88-2, in which the 132nd base C of SPL88-1 and the 381th base T of SPL88-2 were deleted, causing premature termination of protein translation. SPL88 was expressed in root, stem, leaf, leaf sheath, and panicle. The Cullin protein was localized in the cytoplasm and nucleus. The aforementioned results indicate that SPL88 regulates the growth and development of rice by affecting the expression of defense-related genes.

Rice Lesion Mimic Gene Cloning and Association Analysis for Disease Resistance

Lesion mimic mutants refer to a class of mutants that naturally form necrotic lesions similar to allergic reactions on leaves in the absence of significant stress or damage and without being harmed by pathogens. Mutations in most lesion mimic genes, such as OsACL-A2 and OsSCYL2, can enhance mutants’ resistance to pathogens. Lesion mimic mutants are ideal materials for studying programmed cell death (PCD) and plant defense mechanisms. Studying the genes responsible for the rice disease-like phenotype is of great significance for understanding the disease resistance mechanism of rice. In this paper, the nomenclature, occurrence mechanism, genetic characteristics, regulatory pathways, and the research progress on the cloning and disease resistance of rice lesion mimic mutant genes were reviewed, in order to further analyze the various lesion mimic mutants of rice. The mechanism lays a theoretical foundation and provides a reference for rice breeding.

Chlorosis seedling lethality 1 encoding a MAP3K protein is essential for chloroplast development in rice

BACKGROUND

Mitogen-activated protein kinase (MAPK) cascades are conserved signaling modules in eukaryotic organisms and play essential roles in immunity and stress responses. However, the role of MAPKs in chloroplast development remains to be evidently established.

RESULTS

In this study, a rice chlorosis seedling lethality 1 (csl1) mutant with a Zhonghua11 (ZH11, japonica) background was isolated. Seedlings of the mutant were characterized by chlorotic leaves and death after the trefoil stage, and chloroplasts were observed to contain accumulated starch granules. Molecular cloning revealed that OsCSL1 encoded a MAPK kinase kinase22 (MKKK22) targeted to the endoplasmic reticulum (ER), and functional complementation of OsCSL1 was found to restore the normal phenotype in csl1 plants. The CRISPR/Cas9 technology was used for targeted disruption of OsCSL1, and the OsCSL1-Cas9 lines obtained therein exhibited yellow seedlings which phenocopied the csl1 mutant. CSL1/MKKK22 was observed to establish direct interaction with MKK4, and altered expression of MKK1 and MKK4 was detected in the csl1 mutant. Additionally, disruption of OsCSL1 led to reduced expression of chloroplast-associated genes, including chlorophyll biosynthetic genes, plastid-encoded RNA polymerases, nuclear-encoded RNA polymerase, and nuclear-encoded chloroplast genes.

CONCLUSIONS

The findings of this study revealed that OsCSL1 played roles in regulating the expression of multiple chloroplast synthesis-related genes, thereby affecting their functions, and leading to wide-ranging defects, including chlorotic seedlings and severely disrupted chloroplasts containing accumulated starch granules.

Enhanced SA and Ca2+ signaling results in PCD-mediated spontaneous leaf necrosis in wheat mutant wsl

Leaf is the major photosynthesis organ and the key source of wheat (Triticum aestivum L.) grain. Spotted leaf (spl) mutant is a kind of leaf lesion mimic mutants (LMMs) in plants, which is an ideal material for studying the mechanisms of leaf development. In this study, we report the leaf abnormal development molecular mechanism of a spl mutant named white stripe leaf (wsl) derived from wheat cultivar Guomai 301 (WT). Histochemical observation indicated that the leaf mesophyll cells of the wsl were destroyed in the necrosis regions. To explore the molecular regulatory network of the leaf development in mutant wsl, we employed transcriptome analysis, histochemistry, quantitative real-time PCR (qRT-PCR), and observations of the key metabolites and photosynthesis parameters. Compared to WT, the expressions of the chlorophyll synthesis and photosynthesis-related homeotic genes were repressed; many genes in the WRKY transcription factor (TF) families were highly expressed; the salicylic acid (SA) and Ca²⁺ signal transductions were enhanced in wsl. Both the chlorophyll contents and the photosynthesis rate were lower in wsl. The contents of SA and reactive oxygen species (ROS) were significantly higher, and the leaf rust resistance was enhanced in wsl. Based on the experimental data, a primary molecular regulatory model for leaf development in wsl was established. The results indicated that the SA accumulation and enhanced Ca²⁺ signaling led to programmed cell death (PCD), and ultimately resulted in spontaneous leaf necrosis of wsl. These results laid a solid foundation for further research on the molecular mechanism of leaf development in wheat.

SPL36 Encodes a Receptor-like Protein Kinase that Regulates Programmed Cell Death and Defense Responses in Rice

Lesion mimic mutants spontaneously produce disease spots in the absence of biotic or abiotic stresses. Analyzing lesion mimic mutants' sheds light on the mechanisms underlying programmed cell death and defense-related responses in plants. Here, we isolated and characterized the rice (Oryza sativa) spotted leaf 36 (spl36) mutant, which was identified from an ethyl methanesulfonate-mutagenized japonica cultivar Yundao population. spl36 displayed spontaneous cell death and enhanced resistance to rice bacterial pathogens. Gene expression analysis suggested that spl36 functions in the disease response by upregulating the expression of defense-related genes. Physiological and biochemical experiments indicated that more cell death occurred in spl36 than the wild type and that plant growth and development were affected in this mutant. We isolated SPL36 by map-based cloning. A single base substitution was detected in spl36, which results in a cysteine-to-arginine substitution in SPL36. SPL36 is predicted to encode a receptor-like protein kinase containing leucine-rich domains that may be involved in stress responses in rice. spl36 was more sensitive to salt stress than the wild type, suggesting that SPL36 also negatively regulates the salt-stress response. These findings suggest that SPL36 regulates the disease resistance response in rice by affecting the expression of defense- and stress-related genes.

Pathogen-inducible OsMPKK10.2-OsMPK6 cascade phosphorylates the Raf-like kinase OsEDR1 and inhibits its scaffold function to promote rice disease resistance

Mitogen-activated protein kinase (MAPK) cascades regulate a myriad of plant biological processes, including disease resistance. Plant genomes encode a large number of MAPK kinase kinases (MAPKKKs) that can be divided into two subfamilies, namely MEKK-like kinases and Raf-like kinases. Thus far, the functions of MEKK-like MAPKKKs have been relatively well characterized, but the roles of Raf-like MAPKKKs in plant MAPK cascades remain less understood. Here, we report the role of OsEDR1, a Raf-like MAPKKK, in the regulation of the MAPK cascade in rice response to the bacterial pathogen Xanthomonas oryzae pv. oryzicola (Xoc). We found that OsEDR1 inhibits OsMPKK10.2 (a MAPK kinase) activity through physical interaction. Upon Xoc infection, OsMPKK10.2 is phosphorylated at S304 to activate OsMPK6 (a MAPK). Interestingly, activated OsMPK6 phosphorylates OsEDR1 at S861, which destabilizes OsEDR1 and thus releases the inhibition of OsMPKK10.2, leading to increased OsMPKK10.2 activity and enhanced resistance of rice plants to Xoc. Taken together, these results provide new insights into the functions of Raf-like kinases in the regulation of the MAPK cascade in plant immunity.

Update on the Roles of Rice MAPK Cascades

The mitogen-activated protein kinase (MAPK) cascades have been validated playing critical roles in diverse aspects of plant biology, from growth and developmental regulation, biotic and abiotic stress responses, to phytohormone signal transduction or responses. A classical MAPK cascade consists of a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK), and a MAPK. From the 75 MAPKKKs, eight MAPKKs, and 15 MAPKs of rice, a number of them have been functionally deciphered. Here, we update recent advances in knowledge of the roles of rice MAPK cascades, including their components and complicated action modes, their diversified functions controlling rice growth and developmental responses, coordinating resistance to biotic and abiotic stress, and conducting phytohormone signal transduction. Moreover, we summarize several complete MAPK cascades that harbor OsMAPKKK-OsMAPKK-OsMAPK, their interaction with different upstream components and their phosphorylation of diverse downstream substrates to fulfill their multiple roles. Furthermore, we state a comparison of networks of rice MAPK cascades from signal transduction crosstalk to the precise selection of downstream substrates. Additionally, we discuss putative concerns for elucidating the underlying molecular mechanisms and molecular functions of rice MAPK cascades in the future.

WRKY10 transcriptional regulatory cascades in rice are involved in basal defense and Xa1-mediated resistance

WRKY proteins play essential roles as negative or positive regulators of pathogen defense. This study explored the roles of different OsWRKY proteins in basal defense and Xa1-mediated resistance to Xanthomonas oryzae pv. oryzae (Xoo) infection in rice. Assays of disease in OsWRKY10KD and OsWRKY88KD lines following infection with an incompatible Xoo race, which induced Xa1-mediated resistance in wild-type plants, showed that OsWRKY10 and OsWRKY88 were positive regulators of Xa1-mediated resistance. OsWRKY10 also acted as a positive regulator in basal defense by directly or indirectly activating transcription of defense-related genes. OsWRKY10 activated the OsPR1a promoter by binding to specific WRKY binding sites. Two transcriptional regulatory cascades of OsWRKY10 were identified in basal defense and Xa1-mediated resistance. In the first transcriptional regulatory cascade, OsWRKY47 acted downstream of OsWRKY10 whereas OsWRKY51 acted upstream. OsWRKY10 activated OsPR1a in two distinct ways: by binding to its promoter and, at the same time, by indirect activation through OsWRKY47. In the second transcriptional regulatory cascade, OsWRKY47 acted downstream of OsWRKY10, and OsWRKY88 acted upstream. These OsWRKY10 transcriptional regulatory cascades played important roles in basal defense and Xa1-mediated resistance to enable the mounting of a rapid immune response against pathogens.

OsMAPKKK63 is involved in salt stress response and seed dormancy control

Approximately 75 MAP kinase kinase kinases (MAPKKKs) have been identified in the rice genome. However, only a few of them have been functionally characterized. In this paper, we report the function of a rice MAPKKK, OsMAPKKK63. OsMAPKKK63 was found to be induced by several abiotic stresses, including high salinity, chilling and drought. Our data indicate that OsMAPKKK63 possesses in vitro kinase activity and that it interacts with rice MAP kinase kinase OsMKK1 and OsMKK6. The two rice MKKs are known mediator of the salt stress response, implying that OsMAPKKK63 may be involved in the high salinity response. Our analysis of an OsMAPKKK63 knockout mutant indeed demonstrated that it is necessary for normal response to high salt. On the other hand, OsMAPKKK63 OX lines exhibited viviparous phenotype in both rice and Arabidopsis. The result suggests that OsMAPKKK63 may also be involved in seed dormancy control.

Identification of a Novel Semi-Dominant Spotted-Leaf Mutant with Enhanced Resistance to Xanthomonas oryzae pv. oryzae in Rice

Many spotted-leaf mutants show enhanced disease resistance to multiple pathogen attacks; however, the mechanisms are largely unknown. Here, we reported a novel semi-dominant spotted-leaf mutant 24 (spl24) obtained from an ethyl methane sulfonate (EMS)-induced IR64 mutant bank. spl24 developed tiny brown lesions on the leaf tip and spread down gradually to the leaf base as well as the sheath at the early heading stage. The performances of major agronomic traits such as the plant height, panicle length, number of panicles/plant, and 1000-grain weight were significantly altered in spl24 when compared to the wild-type IR64. Furthermore, spl24 exhibited a premature senescing phenotype with degeneration of nuclear acids, significantly reduced soluble protein content, increased level of malonaldehyde (MDA), and lowered activities of reactive oxygen species (ROS) scavenging enzymes. Disease evaluation indicated that spl24 showed enhanced resistance to multiple races of Xanthomonas oryzae pv. oryzae, the causal pathogen of bacterial leaf blight in rice, with elevated expression of pathogenesis-related genes, salicylic acid (SA) signaling pathway-associated genes revealed by real-time quantitative PCR and high-throughput RNA sequencing analysis. Genetic analysis and gene mapping indicated that the lesion mimic phenotype was controlled by a novel semi-dominant nuclear gene. The mutation, tentatively termed as OsSPL24, was in a 110 kb region flanked by markers Indel-33 and Indel-12 in chromosome 11. Together, our data suggest that spl24 is a novel lesion mimic mutant with enhanced innate immunity and would facilitate the isolation and functional characterization of the target gene.

Functional inactivation of OsGCNT induces enhanced disease resistance to Xanthomonas oryzae pv. oryzae in rice

BACKGROUND

Spotted-leaf mutants are important to reveal programmed cell death and defense-related pathways in rice. We previously characterized the phenotype performance of a rice spotted-leaf mutant spl21 and narrowed down the causal gene locus spl21(t) to an 87-kb region in chromosome 12 by map-based cloning.

RESULT

We showed that a single base substitution from A to G at position 836 in the coding sequence of Oryza sativa beta-1,6-N-acetylglucosaminyl transferase (OsGCNT), effectively mutating Tyr to Cys at position 279 in the translated protein sequence, was responsible for the spotted-leaf phenotype as it could be rescued by functional complementation. Compared to the wild type IR64, the spotted-leaf mutant spl21 exhibited loss of chlorophyll, breakdown of chloroplasts, down-regulation of photosynthesis-related genes, and up-regulation of senescence associated genes, which indicated that OsGCNT regulates premature leaf senescence. Moreover, the enhanced resistance to the bacterial leaf blight pathogen Xanthomonas oryzae pv. oryzae, up-regulation of pathogenesis-related genes and increased level of jasmonate which suggested that OsGCNT is a negative regulator of defense response in rice. OsGCNT was expressed constitutively in the leaves, sheaths, stems, roots, and panicles, and OsGCNT-GFP was localized to the Golgi apparatus. High throughput RNA sequencing analysis provided further evidence for the biological effects of loss of OsGCNT function on cell death, premature leaf senescence and enhanced disease resistance in rice. Thus, we demonstrated that the novel OsGCNT regulated rice innate immunity and immunity-associated leaf senescence probably by changing the jasmonate metabolic pathway.

CONCLUSIONS

These results reveal that a novel gene Oryza sativa beta-1,6-N-acetylglucosaminyl transferase (OsGCNT) is responsible for the spotted-leaf mutant spl21, and OsGCNT acts as a negative-regulator mediating defense response and immunity-associated premature leaf senescence probably by activating jasmonate signaling pathway.

Transcriptomic Analysis of Oryza sativa Leaves Reveals Key Changes in Response to Magnaporthe oryzae MSP1

Rice blast disease, caused by Magnaporthe oryzae, results in an extensive loss of rice productivity. Previously, we identified a novel M. oryzae secreted protein, termed MSP1 which causes cell death and pathogen-associated molecular pattern (PAMP)-triggered immune (PTI) responses in rice. Here, we report the transcriptome profile of MSP1-induced response in rice, which led to the identification of 21,619 genes, among which 4,386 showed significant changes (P < 0.05 and fold change > 2 or < 1/2) in response to exogenous MSP1 treatment. Functional annotation of differentially regulated genes showed that the suppressed genes were deeply associated with photosynthesis, secondary metabolism, lipid synthesis, and protein synthesis, while the induced genes were involved in lipid degradation, protein degradation, and signaling. Moreover, expression of genes encoding receptor-like kinases, MAPKs, WRKYs, hormone signaling proteins and pathogenesis-related (PR) proteins were also induced by MSP1. Mapping these differentially expressed genes onto various pathways revealed critical information about the MSP1-triggered responses, providing new insights into the molecular mechanism and components of MSP1-triggered PTI responses in rice.

Identification on mitogen-activated protein kinase signaling cascades by integrating protein interaction with transcriptional profiling analysis in cotton

Plant mitogen-activated protein kinase (MAPK) cascades play important roles in development and stress responses. In previous studies, we have systematically investigated the mitogen-activated protein kinase kinase (MKK) and MAPK gene families in cotton. However, the complete interactions between MAPK gene family members in MAPK signaling cascade is poorly characterized. Herein, we investigated the mitogen-activated protein kinase kinase kinase (MAPKKK) family members and identified a total of 89 MAPKKK genes in the Gossypium raimondii genome. We cloned 51 MAPKKKs in G. hirsutum and investigated the interactions between MKK and MAPKKK proteins through yeast-two hybrid assays. A total of 18 interactive protein pairs involved in 14 MAPKKKs and six MKKs were found. Among these, 13 interactive pairs had not been reported previously. Gene expression patterns revealed that 12 MAPKKKs were involved in diverse signaling pathways triggered by hormone treatments or abiotic stresses. By combining the MKK-MAPK and MKK-MAPKKK protein interactions with gene expression patterns, 38 potential MAPK signaling modules involved in the complicated cross-talks were identified, which provide a basis on elucidating biological function of the MAPK cascade in response to hormonal and/or stress responses. The systematic investigation in MAPK signaling cascades will lay a foundation for understanding the functional roles of different MAPK cascades in signal transduction pathways, and for the improvement of various defense responses in cotton.

Rice WRKY11 Plays a Role in Pathogen Defense and Drought Tolerance

BACKGROUND

Plants are frequently subjected to abiotic and biotic stresses, and WRKY proteins play a pivotal role in the response to such stress. OsWRKY11 is induced by pathogens, drought, and heat, suggesting a function in biotic and abiotic stress responses.

RESULTS

This study identified OsWRKY11, a member of WRKY group IIc. It is a transcriptional activator that localized to the nucleus. Ectopic expression of OsWRKY11 resulted in enhanced resistance to a bacterial pathogen, Xanthomonas oryzae pv. oryzae; resistance was compromised in transgenic lines under-expressing OsWRKY11. Ectopic expression of OsWRKY11 resulted in constitutive expression of defense-associated genes, whereas knock-down (kd) of OsWRKY11 reduced expression of defense-associated genes during pathogen attack, suggesting that OsWRKY11 activates defense responses. OsWRKY11 bound directly to the promoter of CHITINASE 2, a gene associated with defense, and activated its transcription. In addition, ectopic expression of OsWRKY11 enhanced tolerance to drought stress and induced constitutive expression of drought-responsive genes. Induction of drought-responsive genes was compromised in OsWRKY11-kd plants. OsWRKY11 also bound directly to the promoter of a drought-responsive gene, RAB21, activating its transcription. In addition, OsWRKY11 protein levels were controlled by the ubiquitin-proteasome system.

CONCLUSION

OsWRKY11 integrates plant responses to pathogens and abiotic stresses by positively modulating the expression of biotic and abiotic stress-related genes.

The Rice SPOTTED LEAF4 (SPL4) Encodes a Plant Spastin That Inhibits ROS Accumulation in Leaf Development and Functions in Leaf Senescence

Lesion mimic mutants (LMMs) are usually controlled by single recessive mutations that cause the formation of necrotic lesions without pathogen invasion. These genetic defects are useful to reveal the regulatory mechanisms of defense-related programmed cell death in plants. Molecular evidence has been suggested that some of LMMs are closely associated with the regulation of leaf senescence in rice (Oryza sativa). Here, we characterized the mutation underlying spotted leaf4 (spl4), which results in lesion formation and also affects leaf senescence in rice. Map-based cloning revealed that the γ ray-induced spl4-1 mutant has a single base substitution in the splicing site of the SPL4 locus, resulting in a 13-bp deletion within the encoded microtubule-interacting-and-transport (MIT) spastin protein containing an AAA-type ATPase domain. The T-DNA insertion spl4-2 mutant exhibited spontaneous lesions similar to those of the spl4-1 mutant, confirming that SPL4 is responsible for the LMM phenotype. In addition, both spl4 mutants exhibited delayed leaf yellowing during dark-induced or natural senescence. Western blot analysis of spl4 mutant leaves suggested possible roles for SPL4 in the degradation of photosynthetic proteins. Punctate signals of SPL4-fused fluorescent proteins were detected in the cytoplasm, similar to the cellular localization of animal spastin. Based on these findings, we propose that SPL4 is a plant spastin that is involved in multiple aspects of leaf development, including senescence.

LMM5.1 and LMM5.4, two eukaryotic translation elongation factor 1A-like gene family members, negatively affect cell death and disease resistance in rice

Lesion mimic mutant (LMM) genes, stimulating lesion formation in the absence of pathogens, play significant roles in immune response. In this study, we characterized a rice lesion mimic mutant, lmm5, which displayed light-dependent spontaneous lesions. Additionally, lmm5 plants exhibited enhanced resistance to all of the tested races of Magnaporthe oryzae and Xanthomonas oryzae pv. oryzae (Xoo) by increasing the expression of defense-related genes and the accumulation of hydrogen peroxide. Genetic analysis showed that the lesion mimic phenotype of lmm5 was controlled by two genes, lmm5.1 and lmm5.4, which were isolated with a map-based cloning strategy. Remarkably, LMM5.1 and LMM5.4 share a 97.4% amino acid sequence identity, and they each encode a eukaryotic translation elongation factor 1A (eEF1A)-like protein. Besides, LMM5.1 and LMM5.4 were expressed in a tissue-specific and an indica-specific manner, respectively. In addition, high-throughput mRNA sequencing analysis confirmed that the basal immunity was constitutively activated in the lmm5 mutant. Taken together, these results suggest that the homologous eEF1A-like genes, LMM5.1 and LMM5.4, negatively affect cell death and disease resistance in rice.

Updated Rice Kinase Database RKD 2.0: enabling transcriptome and functional analysis of rice kinase genes

BACKGROUND

Protein kinases catalyze the transfer of a phosphate moiety from a phosphate donor to the substrate molecule, thus playing critical roles in cell signaling and metabolism. Although plant genomes contain more than 1000 genes that encode kinases, knowledge is limited about the function of each of these kinases. A major obstacle that hinders progress towards kinase characterization is functional redundancy. To address this challenge, we previously developed the rice kinase database (RKD) that integrated omics-scale data within a phylogenetics context.

RESULTS

An updated version of rice kinase database (RKD) that contains metadata derived from NCBI GEO expression datasets has been developed. RKD 2.0 facilitates in-depth transcriptomic analyses of kinase-encoding genes in diverse rice tissues and in response to biotic and abiotic stresses and hormone treatments. We identified 261 kinases specifically expressed in particular tissues, 130 that are significantly up- regulated in response to biotic stress, 296 in response to abiotic stress, and 260 in response to hormones. Based on this update and Pearson correlation coefficient (PCC) analysis, we estimated that 19 out of 26 genes characterized through loss-of-function studies confer dominant functions. These were selected because they either had paralogous members with PCC values of <0.5 or had no paralog.

CONCLUSION

Compared with the previous version of RKD, RKD 2.0 enables more effective estimations of functional redundancy or dominance because it uses comprehensive expression profiles rather than individual profiles. The integrated analysis of RKD with PCC establishes a single platform for researchers to select rice kinases for functional analyses.

A Raf-like MAPKKK gene, GhRaf19, negatively regulates tolerance to drought and salt and positively regulates resistance to cold stress by modulating reactive oxygen species in cotton

Mitogen-activated protein kinase kinase kinases (MAPKKKs) function at the top level of MAPK cascades and play important roles in plant development and stress responses. Although MAPKKKs comprise the largest family in the MAPK cascades, very few Raf-like MAPKKKs have been functionally identified, especially in the economically important crop cotton. In this study, a Raf-like MAPKKK gene, GhRaf19, was characterized for the first time in cotton. Our data show that the expression of GhRaf19 was inhibited by PEG and NaCl and induced by cold (4°C) and H₂O₂. Furthermore, when GhRaf19 was silenced in cotton using virus-induced gene silencing (VIGS), tolerance to drought and salt stress were enhanced, the accumulation of reactive oxygen species (ROS) was reduced, and ROS-related gene expression was increased. Consistent with these results, in N. benthamiana, overexpressing-GhRaf19 reduced tolerance to drought and salt. However, GhRaf19-silenced plants showed lowered resistance to cold in cotton, and this effect was correlated with the accumulation of ROS. In contrast, overexpressing GhRaf19 in N. benthamiana increased resistance to cold by inducing higher levels of expression and activity of ROS-related antioxidant genes/enzymes. These results indicate that GhRaf19 negatively regulates tolerance to drought and salt and positively regulates resistance to cold stress by modulating cellular ROS in cotton.

Magnaporthe oryzae Effector AVR-Pii Helps to Establish Compatibility by Inhibition of the Rice NADP-Malic Enzyme Resulting in Disruption of Oxidative Burst and Host Innate Immunity

Plant disease resistance occurs as a hypersensitive response (HR) at the site of attempted pathogen invasion. This specific event is initiated in response to recognition of pathogen-associated molecular pattern (PAMP) and subsequent PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI). Both PTI and ETI mechanisms are tightly connected with reactive oxygen species (ROS) production and disease resistance that involves distinct biphasic ROS production as one of its pivotal plant immune responses. This unique oxidative burst is strongly dependent on the resistant cultivars because a monophasic ROS burst is a hallmark of the susceptible cultivars. However, the cause of the differential ROS burst remains unknown. In the study here, we revealed the plausible underlying mechanism of the differential ROS burst through functional understanding of the Magnaporthe oryzae (M. oryzae) AVR effector, AVR-Pii. We performed yeast two-hybrid (Y2H) screening using AVR-Pii as bait and isolated rice NADP-malic enzyme2 (Os-NADP-ME2) as the rice target protein. To our surprise, deletion of the rice Os-NADP-ME2 gene in a resistant rice cultivar disrupted innate immunity against the rice blast fungus. Malic enzyme activity and inhibition studies demonstrated that AVR-Pii proteins specifically inhibit in vitro NADP-ME activity. Overall, we demonstrate that rice blast fungus, M. oryzae attenuates the host ROS burst via AVR-Pii-mediated inhibition of Os-NADP-ME2, which is indispensable in ROS metabolism for the innate immunity of rice. This characterization of the regulation of the host oxidative burst will help to elucidate how the products of AVR genes function associated with virulence of the pathogen.

Mutation of SPOTTED LEAF3 (SPL3) impairs abscisic acid-responsive signalling and delays leaf senescence in rice

Lesion mimic mutants commonly display spontaneous cell death in pre-senescent green leaves under normal conditions, without pathogen attack. Despite molecular and phenotypic characterization of several lesion mimic mutants, the mechanisms of the spontaneous formation of cell death lesions remain largely unknown. Here, the rice lesion mimic mutant spotted leaf3 (spl3) was examined. When grown under a light/dark cycle, the spl3 mutant appeared similar to wild-type at early developmental stages, but lesions gradually appeared in the mature leaves close to heading stage. By contrast, in spl3 mutants grown under continuous light, severe cell death lesions formed in developing leaves, even at the seedling stage. Histochemical analysis showed that hydrogen peroxide accumulated in the mutant, likely causing the cell death phenotype. By map-based cloning and complementation, it was shown that a 1-bp deletion in the first exon of Oryza sativa Mitogen-Activated Protein Kinase Kinase Kinase1 (OsMAPKKK1)/OsEDR1/OsACDR1 causes the spl3 mutant phenotype. The spl3 mutant was found to be insensitive to abscisic acid (ABA), showing normal root growth in ABA-containing media and delayed leaf yellowing during dark-induced and natural senescence. Expression of ABA signalling-associated genes was also less responsive to ABA treatment in the mutant. Furthermore, the spl3 mutant had lower transcript levels and activities of catalases, which scavenge hydrogen peroxide, probably due to impairment of ABA-responsive signalling. Finally, a possible molecular mechanism of lesion formation in the mature leaves of spl3 mutant is discussed.

Secondary metabolites in fungus-plant interactions

Fungi and plants are rich sources of thousands of secondary metabolites. The genetically coded possibilities for secondary metabolite production, the stimuli of the production, and the special phytotoxins basically determine the microscopic fungi-host plant interactions and the pathogenic lifestyle of fungi. The review introduces plant secondary metabolites usually with antifungal effect as well as the importance of signaling molecules in induced systemic resistance and systemic acquired resistance processes. The review also concerns the mimicking of plant effector molecules like auxins, gibberellins and abscisic acid by fungal secondary metabolites that modulate plant growth or even can subvert the plant defense responses such as programmed cell death to gain nutrients for fungal growth and colonization. It also looks through the special secondary metabolite production and host selective toxins of some significant fungal pathogens and the plant response in form of phytoalexin production. New results coming from genome and transcriptional analyses in context of selected fungal pathogens and their hosts are also discussed.

Molecular identification and interaction assay of the gene (OsUbc13) encoding a ubiquitin-conjugating enzyme in rice

The ubiquitin (Ub)-conjugating enzyme, Ubc13, has been known to be involved in error-free DNA damage tolerance (or post-replication repair) via catalyzing Lys63-linked polyubiquitin chains formation together with a Ubc variant. However, its functions remain largely unknown in plant species, especially in monocotyledons. In this study, we cloned a Ub-conjugating enzyme, OsUbc13, that shares the conserved domain of Ubc with AtUBC13B in Oryza sativa L., which encodes a protein of 153 amino acids; the deduced sequence shares high similarities with other homologs. Real-time quantitative polymerase chain reaction (PCR) indicated that OsUbc13 transcripts could be detected in all tissues examined, and the expression level was higher in palea, pistil, stamen, and leaf, and lower in root, stem, and lemma; the expression of OsUbc13 was induced by low temperature, methylmethane sulfate (MMS), and H(2)O(2), but repressed by mannitol, abscisic acid (ABA), and NaCl. OsUbc13 was probably localized in the plasma and nuclear membranes. About 20 proteins, which are responsible for the positive yeast two-hybrid interaction of OsUbc13, were identified. These include the confirmed OsVDAC (correlated with apoptosis), OsMADS1 (important for development of floral organs), OsB22EL8 (related to reactive oxygen species (ROS) scavenging and DNA protection), and OsCROC-1 (required for formation of Lys63 polyubiquitylation and error-free DNA damage tolerance). The molecular characterization provides a foundation for the functional study of OsUbc13.

Transcriptome profiling of the spl5 mutant reveals that SPL5 has a negative role in the biosynthesis of serotonin for rice disease resistance

BACKGROUND

Rice mutant, spl5 (spotted leaf 5), has spontaneous hypersensitive-like lesions on its leaves and shows enhanced resistance to pathogens, indicating that SPL5 plays a role in programmed cell death (PCD) and disease resistance. To understand the molecular mechanism of SPL5 gene, we investigated the transcriptome profiles of the spl5 mutant leaves with few lesions (FL) and leaves with many lesions (ML) compared to the wild-type (WT) leaves respectively by microarray.

RESULTS

The data from microarray revealed that 243 and 896 candidate genes (Fold change ≥ 3.0) were up- or down-regulated in the spl5-FL and spl5-ML, respectively, and a large number of these genes involved in biotic defense responses or reactive oxygen species (ROS) metabolism. Interestingly, according to our microarray and real-time PCR assays, the expressions of a transcription factor OsWRKY14 and genes responsible for the biosynthesis of serotonin, anthranilate synthase (AS), indole-3-glycerolphosphate synthase (IGPS), tryptophan synthase (TS) and tryptophan decarboxylase (TDC) were significantly up-regulated in the spl5 mutant. It has been reported previously that TS and TDC expressions are regulated by OsWRKY14 in rice, which raises the possibility that OsWRKY14 regulates serotonin production through the up-regulation of TS and TDC. Our HPLC analysis further confirmed that serotonin levels were higher in the leaves of spl5 mutant than that in WT.

CONCLUSIONS

Since the serotonin plays a critical role in inducing disease-resistance, the increased serotonin level may contribute, at least partly, to the disease resistance in spl5. The SPL5 gene may act as a negative regulatory factor activating the serotonin metabolic pathway, and these results might provide a new insight into the spl5-induced defense response mechanisms in plants.

Multiple phytohormones and phytoalexins are involved in disease resistance to Magnaporthe oryzae invaded from roots in rice

Blast, caused by the fungus Magnaporthe oryzae, is one of the most devastating diseases of rice worldwide. Phenylalanine ammonia lyase (PAL) is a key enzyme in the phenylpropanoid pathway, which leads to the biosynthesis of defense-related phytohormone salicylic acid (SA) and flavonoid-type phytoalexins sakuranetin and naringenin. However, the roles and biochemical features of individual rice PALs in defense responses to pathogens remain unclear. Here, we report that rice OsPAL06, which can catalyze the formation of trans-cinnamate using l-phenylalanine, is involved in rice root-M. oryzae interaction. OsPAL06-knockout mutant showed increased susceptibility to M. oryzae invaded from roots and developed typical leaf blast symptoms, accompanied by nearly complete disappearance of sakuranetin and naringenin and a two-third reduction of the SA level in roots. This mutant also showed compensatively induced expression of chalcone synthase, which is involved in flavonoid biosynthesis, isochorismate synthase 1, which is putatively involved in SA synthesis via another pathway, reduced jasmonate content and increased ethylene content. These results suggest that OsPAL06 is a positive regulator in preventing M. oryzae infection from roots. It may regulate defense by promoting both phytoalexin accumulation and SA signaling that synergistically and antagonistically interacts with jasmonate- and ethylene-dependent signaling, respectively.

Protein interactome analysis of 12 mitogen-activated protein kinase kinase kinase in rice using a yeast two-hybrid system

The mitogen-activated protein kinase (MAPK) cascade is composed at least of MAP3K (for MAPK kinase kinase), MAP2K, and MAPK family modules. These components together play a central role in mediating extracellular signals to the cell and vice versa by interacting with their partner proteins. However, the MAP3K-interacting proteins remain poorly investigated in plants. Here, we utilized a yeast two-hybrid system and bimolecular fluorescence complementation in the model crop rice (Oryza sativa) to map MAP3K-interacting proteins. We identified 12 novel nonredundant interacting protein pairs (IPPs) representing 11 nonredundant interactors using 12 rice MAP3Ks (available as full-length cDNA in the rice KOME (http://cdna01.dna.affrc.go.jp/cDNA/) at the time of experimental design and execution) as bait and a rice seedling cDNA library as prey. Of the 12 MAP3Ks, only six had interacting protein partners. The established MAP3K interactome consisted of two kinases, three proteases, two forkhead-associated domain-containing proteins, two expressed proteins, one E3 ligase, one regulatory protein, and one retrotransposon protein. Notably, no MAP3K showed physical interaction with either MAP2K or MAPK. Seven IPPs (58.3%) were confirmed in vivo by bimolecular fluorescence complementation. Subcellular localization of 14 interactors, together involved in nine IPPs (75%) further provide prerequisite for biological significance of the IPPs. Furthermore, GO of identified interactors predicted their involvement in diverse physiological responses, which were supported by a literature survey. These findings increase our knowledge of the MAP3K-interacting proteins, help in proposing a model of MAPK modules, provide a valuable resource for developing a complete map of the rice MAPK interactome, and allow discussion for translating the interactome knowledge to rice crop improvement against environmental factors.

Biosynthesis, elicitation and roles of monocot terpenoid phytoalexins

A long-standing goal in plant research is to optimize the protective function of biochemical agents that impede pest and pathogen attack. Nearly 40 years ago, pathogen-inducible diterpenoid production was described in rice, and these compounds were shown to function as antimicrobial phytoalexins. Using rice and maize as examples, we discuss recent advances in the discovery, biosynthesis, elicitation and functional characterization of monocot terpenoid phytoalexins. The recent expansion of known terpenoid phytoalexins now includes not only the labdane-related diterpenoid superfamily but also casbane-type diterpenoids and β-macrocarpene-derived sequiterpenoids. Biochemical approaches have been used to pair pathway precursors and end products with cognate biosynthetic genes. The number of predicted terpenoid phytoalexins is expanding through advances in cereal genome annotation and terpene synthase characterization that likewise enable discoveries outside the Poaceae. At the cellular level, conclusive evidence now exists for multiple plant receptors of fungal-derived chitin elicitors, phosphorylation of membrane-associated signaling complexes, activation of mitogen-activated protein kinase, involvement of phytohormone signals, and the existence of transcription factors that mediate the expression of phytoalexin biosynthetic genes and subsequent accumulation of pathway end products. Elicited production of terpenoid phytoalexins exhibit additional biological functions, including root exudate-mediated allelopathy and insect antifeedant activity. Such findings have encouraged consideration of additional interactions that blur traditionally discrete phytoalexin classifications. The establishment of mutant collections and increasing ease of genetic transformation assists critical examination of further biological roles. Future research directions include examination of terpenoid phytoalexin precursors and end products as potential signals mediating plant physiological processes.

The MAPKKK gene family in Gossypium raimondii: genome-wide identification, classification and expression analysis

Mitogen-activated protein kinase (MAPK) cascades are conserved signal transduction pathways in all eukaryotic organisms. MAPKKKs (MAPK kinase kinases) operate at the top levels of these cascades. Recently, this family of genes has been systematically investigated in Arabidopsis, rice and maize, but has not yet been characterized in cotton. In this study, we identified 78 putative MAPKKK genes in the genome of the diploid cotton, Gossypium raimondii. They were classified into three subfamilies, of which 12 were ZIK, 22 were MEKK and 44 were Raf. The ZIK and MEKK genes displayed a scattered genomic distribution across 11 of the 13 chromosomes, whereas Raf genes were distributed across the entire genome. Their conserved patterns observed for introns and additional domains were consistent with the evolutionary relationships inferred from the phylogenetic analysis within subfamily. Transcriptome sequencing data were used to investigate their transcript profiles in mature leaves, 0 day and 3 days post-anthesis (DPA) ovules. Sixty MAPKKK genes were expressed, of which 41 were strongly expressed in mature leaves. Twelve MAPKKK genes were more highly expressed in 3-DPA ovules than in 0-DPA ovules. Our results provide a foundation for future evolutionary and functional characterizations of MAPKKK genes in cotton and probably other Gossypium plants.

Modulation of phytoalexin biosynthesis in engineered plants for disease resistance

Phytoalexins are antimicrobial substances of low molecular weight produced by plants in response to infection or stress, which form part of their active defense mechanisms. Starting in the 1950's, research on phytoalexins has begun with biochemistry and bio-organic chemistry, resulting in the determination of their structure, their biological activity as well as mechanisms of their synthesis and their catabolism by microorganisms. Elucidation of the biosynthesis of numerous phytoalexins has permitted the use of molecular biology tools for the exploration of the genes encoding enzymes of their synthesis pathways and their regulators. Genetic manipulation of phytoalexins has been investigated to increase the disease resistance of plants. The first example of a disease resistance resulting from foreign phytoalexin expression in a novel plant has concerned a phytoalexin from grapevine which was transferred to tobacco. Transformations were then operated to investigate the potential of other phytoalexin biosynthetic genes to confer resistance to pathogens. Unexpectedly, engineering phytoalexins for disease resistance in plants seem to have been limited to exploiting only a few phytoalexin biosynthetic genes, especially those encoding stilbenes and some isoflavonoids. Research has rather focused on indirect approaches which allow modulation of the accumulation of phytoalexin employing transcriptional regulators or components of upstream regulatory pathways. Genetic approaches using gain- or less-of functions in phytoalexin engineering together with modulation of phytoalexin accumulation through molecular engineering of plant hormones and defense-related marker and elicitor genes have been reviewed.

Oscyp71Z2 involves diterpenoid phytoalexin biosynthesis that contributes to bacterial blight resistance in rice

Bacterial blight is one of the most destructive rice diseases, which caused by Xoo, and results in yield losses, endangering worldwide food security. Diterpenoid phytoalexins, a type of antimicrobials produced in rice, are critical for resistance to fungal and bacterial pathogens. This article reports the characterization of the cytochrome P450 gene Oscyp71Z2, which belongs to the CYP71Z subfamily. Overexpression of Oscyp71Z2 in rice enhanced resistance to Xoo at the booting stage. The accumulation of phytoalexins was rapidly and strongly induced in Oscyp71Z2-overexpressing plants, and the transcript levels of genes related to the phytoalexin biosynthesis pathway were elevated. The H₂O₂ concentration in Oscyp71Z2-overexpressing plants was reduced in accordance with the increase in ROS-scavenging ability due to the induction of SOD as well as POD and CAT activation. We also showed that suppression of Oscyp71Z2 had no significantly effect on disease resistance to Xoo in rice. These results demonstrated that Oscyp71Z2 plays an important role in bacterial blight resistance by regulating the diterpenoid phytoalexin biosynthesis and H₂O₂ generation.

Characterization and genetic analysis of a novel rice spotted-leaf mutant HM47 with broad-spectrum resistance to Xanthomonas oryzae pv. oryzae

A stable inherited rice spotted-leaf mutant HM47 derived from an EMS-induced IR64 mutant bank was identified. The mutant expressed hypersensitive response (HR)-like symptoms throughout its whole life from the first leaf to the flag leaf, without pathogen invasion. Initiation of the lesions was induced by light under natural summer field conditions. Expression of pathogenesis-related genes including PAL, PO-C1, POX22.3 and PBZ1 was enhanced significantly in association with cell death and accumulation of H2 O2 at and around the site of lesions in the mutant in contrast to that in the wild-type (WT). Disease reaction to Xanthomonas oryzae pv. oryzae from the Philippines and China showed that HM47 is a broad-spectrum disease-resistant mutant with enhanced resistance to multiple races of bacterial blight pathogens tested. An F2 progeny test showed that bacterial blight resistance to race HB-17 was co-segregated with the expression of lesions. Genetic analysis indicated that the spotted-leaf trait was controlled by a single recessive gene, tentatively named spl(HM47) , flanked by two insertion/deletion markers in a region of approximately 74 kb on the long arm of chromosome 4. Ten open reading frames are predicted, and all of them are expressed proteins. Isolation and validation of the putative genes are currently underway.

Characterization and fine mapping of a necrotic leaf mutant in maize (Zea mays L.)

Maize (Zea mays L.) is a commercially important crop. Its yield can be reduced by mutations in biosynthetic and degradative pathways that cause death. In this paper, we describe the necrotic leaf (nec-t) mutant, which was obtained from an inbred line, 81647. The nec-t mutant plants had yellow leaves with necrotic spots, reduced chlorophyll content, and the etiolated seedlings died under normal growth conditions. Transmission electron microscopy revealed scattered thylakoids, and reduced numbers of grana lamellae and chloroplasts per cell. Histochemical staining suggested that spot formation of nec-t leaves might be due to cell death. Genetic analysis showed that necrosis was caused by the mutation of a recessive locus. Using simple sequence repeat markers, the Nec-t gene was mapped between mmc0111 and bnlg2277 on the short arm of chromosome 2. A total of 1287 individuals with the mutant phenotype from a F2 population were used for physical mapping. The Nec-t gene was located between markers T31 and H8 within a physical region of 131.7 kb.

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.

Phytoalexin transgenics in crop protection--fairy tale with a happy end?

Phytoalexins are pathogen induced low molecular weight compounds with antimicrobial activities derived from secondary metabolism. Following their identification, phytoalexins were directly incorporated into the network of plant defense responses. Due to their heterogeneity, the metabolic pathways involved in phytoalexin formation and in particular the regulatory mechanisms remained elusive. Consequently, research focus shifted to the characterization of other components of plant immunity such as defense signaling and resistance mechanisms, including components of systemic acquired and induced systemic resistance, effector and pathogen-associated molecular pattern triggered immunity as well as R-gene resistance. Despite the obtained knowledge on these immunity mechanisms, genetic engineering employing these mechanisms and classical breeding reached too low improvements in crop protection, probably because classical breeding focused on yield performance and taste, rather than pathogen resistance. The increasing demand for disease resistant crop species and the aim to reduce pesticide application therefore requires alternative approaches. Recent advances in the understanding of phytoalexin function, biosynthesis and regulation, in combination with novel methods of molecular engineering and advances in instrumental analysis, returned attention to phytoalexins as a potent target for improving crop protection. Based on this, the advantages as well as potential bottlenecks for molecular approaches of modulating inducible phytoalexins to improve crop protection are discussed.

Ectopic expression of Hrf1 enhances bacterial resistance via regulation of diterpene phytoalexins, silicon and reactive oxygen species burst in rice

Harpin proteins as elicitor derived from plant gram negative bacteria such as Xanthomonas oryzae pv. oryzae (Xoo), Erwinia amylovora induce disease resistance in plants by activating multiple defense responses. However, it is unclear whether phytoalexin production and ROS burst are involved in the disease resistance conferred by the expression of the harpin(Xoo) protein in rice. In this article, ectopic expression of hrf1 in rice enhanced resistance to bacterial blight. Accompanying with the activation of genes related to the phytoalexin biosynthesis pathway in hrf1-transformed rice, phytoalexins quickly and consistently accumulated concurrent with the limitation of bacterial growth rate. Moreover, the hrf1-transformed rice showed an increased ability for ROS scavenging and decreased hydrogen peroxide (H(2)O(2)) concentration. Furthermore, the localization and relative quantification of silicon deposition in rice leaves was detected by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDS). Finally, the transcript levels of defense response genes increased in transformed rice. These results show a correlation between Xoo resistance and phytoalexin production, H(2)O(2), silicon deposition and defense gene expression in hrf1-transformed rice. These data are significant because they provide evidence for a better understanding the role of defense responses in the incompatible interaction between bacterial disease and hrf1-transformed plants. These data also supply an opportunity for generating nonspecific resistance to pathogens.

Rice mitogen-activated protein kinase interactome analysis using the yeast two-hybrid system

Mitogen-activated protein kinase (MAPK) cascades support the flow of extracellular signals to intracellular target molecules and ultimately drive a diverse array of physiological functions in cells, tissues, and organisms by interacting with other proteins. Yet, our knowledge of the global physical MAPK interactome in plants remains largely fragmented. Here, we utilized the yeast two-hybrid system and coimmunoprecipitation, pull-down, bimolecular fluorescence complementation, subcellular localization, and kinase assay experiments in the model crop rice (Oryza sativa) to systematically map what is to our knowledge the first plant MAPK-interacting proteins. We identified 80 nonredundant interacting protein pairs (74 nonredundant interactors) for rice MAPKs and elucidated the novel proteome-wide network of MAPK interactors. The established interactome contains four membrane-associated proteins, seven MAP2Ks (for MAPK kinase), four MAPKs, and 59 putative substrates, including 18 transcription factors. Several interactors were also validated by experimental approaches (in vivo and in vitro) and literature survey. Our results highlight the importance of OsMPK1, an ortholog of tobacco (Nicotiana benthamiana) salicyclic acid-induced protein kinase and Arabidopsis (Arabidopsis thaliana) AtMPK6, among the rice MAPKs, as it alone interacts with 41 unique proteins (51.2% of the mapped MAPK interaction network). Additionally, Gene Ontology classification of interacting proteins into 34 functional categories suggested MAPK participation in diverse physiological functions. Together, the results obtained essentially enhance our knowledge of the MAPK-interacting protein network and provide a valuable research resource for developing a nearly complete map of the rice MAPK interactome.

Increased leaf angle1, a Raf-like MAPKKK that interacts with a nuclear protein family, regulates mechanical tissue formation in the Lamina joint of rice

Mitogen-activated protein kinase kinase kinases (MAPKKKs), which function at the top level of mitogen-activated protein kinase cascades, are clustered into three groups. However, no Group C Raf-like MAPKKKs have yet been functionally identified. We report here the characterization of a rice (Oryza sativa) mutant, increased leaf angle1 (ila1), resulting from a T-DNA insertion in a Group C MAPKKK gene. The increased leaf angle in ila1 is caused by abnormal vascular bundle formation and cell wall composition in the leaf lamina joint, as distinct from the mechanism observed in brassinosteroid-related mutants. Phosphorylation assays revealed that ILA1 is a functional kinase with Ser/Thr kinase activity. ILA1 is predominantly resident in the nucleus and expressed in the vascular bundles of leaf lamina joints. Yeast two-hybrid screening identified six closely related ILA1 interacting proteins (IIPs) of unknown function. Using representative IIPs, the interaction of ILA1 and IIPs was confirmed in vivo. IIPs were localized in the nucleus and showed transactivation activity. Furthermore, ILA1 could phosphorylate IIP4, indicating that IIPs may be the downstream substrates of ILA1. Microarray analyses of leaf lamina joints provided additional evidence for alterations in mechanical strength in ila1. ILA1 is thus a key factor regulating mechanical tissue formation at the leaf lamina joint.

Arabidopsis ENHANCED DISEASE RESISTANCE 1 is required for pathogen-induced expression of plant defensins in nonhost resistance, and acts through interference of MYC2-mediated repressor function

Arabidopsis thaliana exhibits durable resistance, called nonhost resistance, against non-adapted fungal pathogens that typically terminates pathogen entry. The PEN2-dependent indole glucosinolate metabolism pathway is involved in preventing the entry of a range of non-adapted fungi. Here, we report that ENHANCED DISEASE RESISTANCE 1 (EDR1) functions in pre-invasive nonhost resistance. Plants lacking EDR1 exhibit impaired entry resistance to the non-adapted hemibiotrophic Colletotrichum gloeosporioides, in contrast to the enhanced resistance of edr1 against biotrophic infection of a host-adapted powdery mildew fungus. Analysis of the edr1 pen2 double mutant indicates that EDR1 acts in a defense pathway independent from the PEN2 indole glucosinolate pathway. The edr1 mutant also exhibited enhanced susceptibility to host-adapted pathogens, including Colletotrichum higginsianum and necrotrophic Alternaria brassicicola. Comparative transcript profiling revealed that upon C. gloeosporioides inoculation, the expression of four plant defensin genes was severely impaired in edr1, indicating that EDR1 is required for the induced expression of these antifungal proteins. Inactivation of the MYC2-encoded transcription factor fully restored defensin expression in edr1, implying that EDR1 interferes with MYC2 function to abrogate repression of defensin expression. Furthermore, constitutive expression of plant defensin PDF1.2b largely rescued pre-invasive resistance responses in edr1 plants. These results indicate that EDR1 exerts a positive and critical role in resistance responses to hemibiotrophic/necrotrophic fungi, in part by inducing antifungal protein expression through derepression of MYC2 function.

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.

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.