Activation of the jasmonic acid pathway by depletion of the hydroperoxide lyase OsHPL3 reveals crosstalk between the HPL and AOS branches of the oxylipin pathway in rice

Comparative Metabolomic Analysis Reveals the Role of OsHPL1 in the Cold-Induced Metabolic Changes in Rice

Cytochrome P450 (CYP74) family members participate in the generation of oxylipins and play essential roles in plant adaptation. However, the metabolic reprogramming mediated by CYP74s under cold stress remains largely unexplored. Herein, we report how cold-triggered OsHPL1, a member of the CYP74 family, modulates rice metabolism. Cold stress significantly induced the expression of OsHPL1 and the accumulation of OPDA (12-oxo-phytodienoic acid) and jasmonates in the wild-type (WT) plants. The absence of OsHPL1 attenuates OPDA accumulation to a low temperature. Then, we performed a widely targeted metabolomics study covering 597 structurally annotated compounds. In the WT and hpl1 plants, cold stress remodeled the metabolism of lipids and amino acids. Although the WT and hpl1 mutants shared over one hundred cold-affected differentially accumulated metabolites (DAMs), some displayed distinct cold-responding patterns. Furthermore, we identified 114 and 56 cold-responding DAMs, specifically in the WT and hpl1 mutants. In conclusion, our work characterized cold-triggered metabolic rewiring and the metabolic role of OsHPL1 in rice.

Advances in the Genetic Basis and Molecular Mechanism of Lesion Mimic Formation in Rice

Plant lesion mutation usually refers to the phenomenon of cell death in green tissues before senescence in the absence of external stress, and such mutants also show enhanced resistance to some plant pathogens. The occurrence of lesion mimic mutants in rice is affected by gene mutation, reactive oxygen species accumulation, an uncontrolled programmed cell death system, and abiotic stress. At present, many lesion mimic mutants have been identified in rice, and some genes have been functionally analyzed. This study reviews the occurrence mechanism of lesion mimic mutants in rice. It analyzes the function of rice lesion mimic mutant genes to elucidate the molecular regulation pathways of rice lesion mimic mutants in regulating plant disease resistance.

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.

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.

A conserved clathrin-coated vesicle component, OsSCYL2, regulates plant innate immunity in rice

Clathrin-mediated vesicle trafficking (CMVT) is a fundamental process in all eukaryotic species, and indispensable to organism's growth and development. Recently, it has been suggested that CMVT also plays important roles in the regulation of plant immunity. However, the molecular link between CMVT and plant immunity is largely unknown. SCY1-LIKE2 (SCYL2) is evolutionally conserved among the eukaryote species. Loss-of-function of SCYL2 in Arabidopsis led to severe growth defects. Here, we show that mutation of OsSCYL2 in rice gave rise to a novel phenotype-hypersensitive response-like (HR) cell death in a light-dependent manner. Although mutants of OsSCYL2 showed additional defects in the photosynthetic system, they exhibited enhanced resistance to bacterial pathogens. Subcellular localisation showed that OsSCYL2 localized at Golgi, trans-Golgi network and prevacuolar compartment. OsSCYL2 interacted with OsSPL28, subunit of a clathrin-associated adaptor protein that is known to regulate HR-like cell death in rice. We further showed that OsSCYL2-OsSPL28 interaction is mediated by OsCHC1. Collectively, we characterized a novel component of the CMVT pathway in the regulation of plant immunity. Our work also revealed unidentified new functions of the very conserved SCYL2. It thus may provide new breeding targets to achieve both high yield and enhanced resistance in crops.

Salicylic Acid Is Required for Broad-Spectrum Disease Resistance in Rice

Rice plants contain high basal levels of salicylic acid (SA), but some of their functions remain elusive. To elucidate the importance of SA homeostasis in rice immunity, we characterized four rice SA hydroxylase genes (OsSAHs) and verified their roles in SA metabolism and disease resistance. Recombinant OsSAH proteins catalyzed SA in vitro, while OsSAH3 protein showed only SA 5-hydroxylase (SA5H) activity, which was remarkably higher than that of other OsSAHs that presented both SA3H and SA5H activities. Amino acid substitutions revealed that three amino acids in the binding pocket affected SAH enzyme activity and/or specificity. Knockout OsSAH2 and OsSAH3 (sahKO) genes conferred enhanced resistance to both hemibiotrophic and necrotrophic pathogens, whereas overexpression of each OsSAH gene increased susceptibility to the pathogens. sahKO mutants showed increased SA and jasmonate levels compared to those of the wild type and OsSAH-overexpressing plants. Analysis of the OsSAH3 promoter indicated that its induction was mainly restricted around Magnaporthe oryzae infection sites. Taken together, our findings indicate that SA plays a vital role in immune signaling. Moreover, fine-tuning SA homeostasis through suppression of SA metabolism is an effective approach in studying broad-spectrum disease resistance in rice.

Disruption of OsPHD1, Encoding a UDP-Glucose Epimerase, Causes JA Accumulation and Enhanced Bacterial Blight Resistance in Rice

Lesion mimic mutants (LMMs) have been widely used in experiments in recent years for studying plant physiological mechanisms underlying programmed cell death (PCD) and defense responses. Here, we identified a lesion mimic mutant, lm212-1, which cloned the causal gene by a map-based cloning strategy, and verified this by complementation. The causal gene, OsPHD1, encodes a UDP-glucose epimerase (UGE), and the OsPHD1 was located in the chloroplast. OsPHD1 was constitutively expressed in all organs, with higher expression in leaves and other green tissues. lm212-1 exhibited decreased chlorophyll content, and the chloroplast structure was destroyed. Histochemistry results indicated that H₂O₂ is highly accumulated and cell death is occurred around the lesions in lm212-1. Compared to the wild type, expression levels of defense-related genes were up-regulated, and resistance to bacterial pathogens Xanthomonas oryzae pv. oryzae (Xoo) was enhanced, indicating that the defense response was activated in lm212-1, ROS production was induced by flg22, and chitin treatment also showed the same result. Jasmonic acid (JA) and methyl jasmonate (MeJA) increased, and the JA signaling pathways appeared to be disordered in lm212-1. Additionally, the overexpression lines showed the same phenotype as the wild type. Overall, our findings demonstrate that OsPHD1 is involved in the regulation of PCD and defense response in rice.

Identification and fine mapping of lesion mimic mutant spl36 in rice (Oryza sativa L.)

In the absence of pathogen attack, lesion mimic mutants (LMMs) in plants undergo spontaneous cell death and develop necrosis or apoptosis-like lesions on the leaves or sheath, resembling symptoms of hypersensitive response. In-depth research has been conducted on LMMs, especially regarding the molecular mechanisms underlying programmed cell death and disease resistance. In this study, the spotted leaf 36 (spl36) mutant was identified as a typical LMM, showing lesions on both the leaf blade and leaf sheath. The formation of lesions was found to be caused by cell death accompanied by accumulation of hydrogen peroxide and degradation of chloroplasts. Compared with wild-type, the main agronomic traits such as plant height, effective panicle number, panicle length, grain per panicle, seed setting rate, and 1000-grain weight of spl36 were significantly reduced. The defence and pathogenesis-related genes PR1a, PR1b, PR10, and NPR1, were transcriptionally activated in mutant spl36 without pathogen attack. Genetic analysis showed that the mutant phenotype was controlled by the gene SPL36, which was mapped to an interval of 260 kb at the end of the long arm on chromosome 11. Pathogen inoculation analysis showed that spl36 has enhanced resistance to sheath blight, rice blast, and bacterial blight.

Rice Lesion Mimic Mutants (LMM): The Current Understanding of Genetic Mutations in the Failure of ROS Scavenging during Lesion Formation

Rice lesion mimic mutants (LMMs) form spontaneous lesions on the leaves during vegetative growth without pathogenic infections. The rice LMM group includes various mutants, including spotted leaf mutants, brown leaf mutants, white-stripe leaf mutants, and other lesion-phenotypic mutants. These LMM mutants exhibit a common phenotype of lesions on the leaves linked to chloroplast destruction caused by the eruption of reactive oxygen species (ROS) in the photosynthesis process. This process instigates the hypersensitive response (HR) and programmed cell death (PCD), resulting in lesion formation. The reasons for lesion formation have been studied extensively in terms of genetics and molecular biology to understand the pathogen and stress responses. In rice, the lesion phenotypes of most rice LMMs are inherited according to the Mendelian principles of inheritance, which remain in the subsequent generations. These rice LMM genetic traits have highly developed innate self-defense mechanisms. Thus, although rice LMM plants have undesirable agronomic traits, the genetic principles of LMM phenotypes can be used to obtain high grain yields by deciphering the efficiency of photosynthesis, disease resistance, and environmental stress responses. From these ailing rice LMM plants, rice geneticists have discovered novel proteins and physiological causes of ROS in photosynthesis and defense mechanisms. This review discusses recent studies on rice LMMs for the Mendelian inheritances, molecular genetic mapping, and the genetic definition of each mutant gene.

Repressed OsMESL expression triggers reactive oxygen species-mediated broad-spectrum disease resistance in rice

A few reports have indicated that a single gene confers resistance to bacterial blight, sheath blight and rice blast. In this study, we identified a novel disease resistance mutant gene, methyl esterase-like (osmesl) in rice. Mutant rice with T-DNA insertion displayed significant resistance to bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo), sheath blight caused by Rhizoctonia solani and rice blast caused by Magnaporthe oryzae. Additionally, CRISPR-Cas9 knockout mutants and RNAi lines displayed resistance to these pathogens. Complementary T-DNA mutants demonstrated a phenotype similar to the wild type (WT), thereby indicating that osmesl confers resistance to pathogens. Protein interaction experiments revealed that OsMESL affects reactive oxygen species (ROS) accumulation by interacting with thioredoxin OsTrxm in rice. Moreover, qRT-PCR results showed significantly reduced mRNA levels of multiple ROS scavenging-related genes in osmesl mutants. Nitroblue tetrazolium staining showed that the pathogens cause ROS accumulation, and quantitative detection revealed significantly increased levels of H₂ O₂ in the leaves of osmesl mutants and RNAi lines after infection. The abundance of JA, a hormone associated with disease resistance, was significantly more in osmesl mutants than in WT plants. Overall, these results suggested that osmesl enhances disease resistance to Xoo, R. solani and M. oryzae by modulating the ROS balance.

Application of jasmonic acid at the stage of visible brown necrotic spots in Magnaporthe oryzae infection as a novel and environment-friendly control strategy for rice blast disease

Rice blast disease is one of the most common rice diseases worldwide. It is essential to improve disease resistance through environment-friendly methods, while maintaining yield and quality parameters. In this study, jasmonic acid (JA), a plant hormone with anti-fungal activity, was obtained, at both low (100 μmol/L) and high (400 μmol/L) concentrations in rice leaves, before, during, and after infection, respectively. JA could inhibit germination and appressorium formation of rice blast spores in a dose-dependent manner. A total of 400-μmol/L JA treatment significantly enhanced cell viability and endogenous JA level in rice leaves. Furthermore, rice leaves inoculated with Magnaporthe oryzae and sprayed with JA 72 h post-inoculation showed the maximum symptom relief and the highest endogenous JA production among all treatment approaches. The expressions of defense-related genes, OsPR10a and OsAOS2, were highly up-regulated in response to JA, whereas OsEDS1 was down-regulated. Hence, we revealed that exogenous JA could activate JA signaling to effectively control the symptoms of rice blast.

Characterization of full-length transcriptome in Saccharum officinarum and molecular insights into tiller development

BACKGROUND

Although extensive breeding efforts are ongoing in sugarcane (Saccharum officinarum L.), the average yield is far below the theoretical potential. Tillering is an important component of sugarcane yield, however, the molecular mechanism underlying tiller development is still elusive. The limited genomic data in sugarcane, particularly due to its complex and large genome, has hindered in-depth molecular studies.

RESULTS

Herein, we generated full-length (FL) transcriptome from developing leaf and tiller bud samples based on PacBio Iso-Seq. In addition, we performed RNA-seq from tiller bud samples at three developmental stages (T0, T1 and T2) to uncover key genes and biological pathways involved in sugarcane tiller development. In total, 30,360 and 20,088 high-quality non-redundant isoforms were identified in leaf and tiller bud samples, respectively, representing 41,109 unique isoforms in sugarcane. Likewise, we identified 1063 and 1037 alternative splicing events identified in leaf and tiller bud samples, respectively. We predicted the presence of coding sequence for 40,343 isoforms, 98% of which was successfully annotated. Comparison with previous FL transcriptomes in sugarcane revealed 2963 unreported isoforms. In addition, we characterized 14,946 SSRs from 11,700 transcripts and 310 lncRNAs. By integrating RNA-seq with the FL transcriptome, 468 and 57 differentially expressed genes (DEG) were identified in T1vsT0 and T2vsT0, respectively. Strong up-regulation of several pyruvate phosphate dikinase and phosphoenolpyruvate carboxylase genes suggests enhanced carbon fixation and protein synthesis to facilitate tiller growth. Similarly, up-regulation of linoleate 9S-lipoxygenase and lipoxygenase genes in the linoleic acid metabolism pathway suggests high synthesis of key oxylipins involved in tiller growth and development.

CONCLUSIONS

Collectively, we have enriched the genomic data available in sugarcane and provided candidate genes for manipulating tiller formation and development, towards productivity enhancement in sugarcane.

Olive (Olea europaea L.) Genetic Transformation: Current Status and Future Prospects

Olive (Olea europaea L.) is the most characteristic and important oil crop of the Mediterranean region. Traditional olive cultivation is based on few tens cultivars of ancient origin. To improve this crop, novel selections with higher tolerance to biotic and abiotic stress, adaptable to high-density planting systems and resilient to climate change are needed; however, breeding programs are hindered by the long juvenile period of this species and few improved genotypes have been released so far. Genetic transformation could be of great value, in the near future, to develop new varieties or rootstocks in a shorter time; in addition, it has currently become an essential tool for functional genomic studies. The recalcitrance of olive tissues to their in vitro manipulation has been the main bottleneck in the development of genetic transformation procedures in this species; however, some important traits such as fungal resistance, flowering or lipid composition have successfully been manipulated through the genetic transformation of somatic embryos of juvenile or adult origin, providing a proof of the potential role that this technology could have in olive improvement. However, the optimization of these protocols for explants of adult origin is a prerequisite to obtain useful materials for the olive industry. In this review, initially, factors affecting plant regeneration via somatic embryogenesis are discussed. Subsequently, the different transformation approaches explored in olive are reviewed. Finally, transgenic experiments with genes of interest undertaken to manipulate selected traits are discussed.

Planthopper-Secreted Salivary Disulfide Isomerase Activates Immune Responses in Plants

The small brown planthopper (Laodelphax striatellus; SBPH) is a piercing-sucking insect that secretes salivary proteins into its plant host during feeding. However, the mechanisms by which these salivary proteins regulate plant defense responses remain poorly understood. Here, we identified the disulfide isomerase (LsPDI1) in the SBPH salivary proteome. LsPDI1 was highly expressed in the SBPH salivary glands and secreted into rice plants during feeding. Transient in planta LsPDI1 expression in the absence of signal peptide induced reactive oxygen species (ROS) burst, cell death, callose deposition, and jasmonic acid (JA) signaling pathway. Deletion mutant analysis revealed that either the a-b-b' or the b-b'-a' domains in LsPDI1 are required to induce cell death in plants. LsPDI1 and its orthologs were highly conserved among various planthopper species and strongly induced ROS burst and cell death in plants. Transient in Nicotiana benthamiana LsPDI1 expression impaired the performance of Spodoptera frugiperda and Myzus persicae on host plants. Hence, LsPDI1 is an important salivary elicitor that enhances plant resistance to insects by inducing the calcium, ROS, and JA signaling pathways. The findings of this study provide novel insights into the molecular mechanisms underlying plant-insect interactions.

Hydroperoxide lyase modulates defense response and confers lesion-mimic leaf phenotype in soybean (Glycine max (L.) Merr.)

Allene oxide synthase (AOS) and hydroperoxide lyase (HPL) are two important members of P450 enzymes metabolizing hydroperoxy fatty acid to produce jasmonates and aldehydes respectively, which function in response to diverse environmental and developmental stimuli. However, their exact roles in soybean have not been clarified. In present study, we identified a lesion-mimic mutant in soybean named NT302, which exhibits etiolated phenotype together with chlorotic and spontaneous lesions on leaves at R3 podding stage. The underlying gene was identified as GmHPL encoding hydroperoxide lyase by map-based cloning strategy. Sequence analysis demonstrated that a single nucleotide mutation created a premature termination codon (Gln20-Ter), which resulted in a truncated GmHPL protein in NT302. GmHPL RNA was significantly reduced in NT302 mutant, while genes in AOS branch of the 13-LOX pathway were up-regulated in NT302. The mutant exhibited higher susceptibility to bacterial leaf pustule (BLP) disease, but increased resistance against common cutworm (CCW) pest. GmHPL was significantly induced in response to MeJA, wounding, and CCW in wild type soybean. Virus induced gene silencing (VIGS) of GhHPL genes gave rise to similar lesion-mimic leaf phenotypes in upland cotton, coupled with upregulation of the expression of JA biosynthesis and JA-induced genes. Our study provides evidence that competition exist between HPL and AOS branches in 13-LOX pathway of the oxylipin metabolism in soybean, thereby plays essential roles in modulation of plant development and defense.

Transcriptome and metabolome profiling unveiled mechanisms of tea (Camellia sinensis) quality improvement by moderate drought on pre-harvest shoots

Conventional wisdom holds that tea (Camellia sinensis) quality can be improved by drought. To clarify the underlying mechanism, a conjoint analysis of transcriptome and metabolome profiling was carried out in tea shoots harvested under different soil water contents (SWCs). Drought had little impact on theanine, catechins and caffeine in field conditions. Besides the flavor contributions of amino acid and their derivatives, organic acids, and nucleotides and their derivatives, the obviously increased isoflavonoids and glycosylflavonoids and the sharply decreased lipids are suggested to play key roles, which is mainly due to substantial increases of type III polyketide synthase B (PKSB), flavonol synthase/flavanone 3-hydroxylase (FLS), and UDP-glycosyltransferases (UGTs), as well as the significant repression of anthocyanidin synthase (ANS) and R2R3MYBs, and downregulated lipid metabolisms. Genes of GDSL esterase/lipase (GDSL), abscisic acid (ABA) and jasmonate (JA) signaling were found to play important roles in both flavonoid accumulation and lipid reduction. These findings increased our understanding of how moderate drought improves taste and aroma of tea by interfering in the metabolism of fresh leaves, which provides new insight into balancing compounds in pre-harvest tea shoots.

The modulation of stomatal conductance and photosynthetic parameters is involved in Fusarium head blight resistance in wheat

Fusarium head blight (FHB) is one of the most devastating fungal diseases affecting grain crops and Fusarium graminearum is the most aggressive causal species. Several evidences shown that stomatal closure is involved in the first line of defence against plant pathogens. However, there is very little evidence to show that photosynthetic parameters change in inoculated plants. The aim of the present study was to study the role of stomatal regulation in wheat after F. graminearum inoculation and explore its possible involvement in FHB resistance. RT-qPCR revealed that genes involved in stomatal regulation are induced in the resistant Sumai3 cultivar but not in the susceptible Rebelde cultivar. Seven genes involved in the positive regulation of stomatal closure were up-regulated in Sumai3, but it is most likely, that two genes, TaBG and TaCYP450, involved in the negative regulation of stomatal closure, were strongly induced, suggesting that FHB response is linked to cross-talk between the genes promoting and inhibiting stomatal closure. Increasing temperature of spikes in the wheat genotypes and a decrease in photosynthetic efficiency in Rebelde but not in Sumai3, were observed, confirming the hypothesis that photosynthetic parameters are related to FHB resistance.

Transcriptional landscape of pathogen-responsive lncRNAs in rice unveils the role of ALEX1 in jasmonate pathway and disease resistance

Plant defence is multilayered and is essential for surviving in a changing environment. The discovery of long noncoding RNAs (lncRNAs) has dramatically extended our understanding of post-transcriptional gene regulation in diverse biological processes. However, the expression profile and function of lncRNAs in disease resistance are still largely unknown, especially in monocots. Here, we performed strand-specific RNA sequencing of rice leaves infected by Xanthomonas oryzae pv. Oryzae (Xoo) in different time courses and systematically identified 567 disease-responsive rice lncRNAs. Target analyses of these lncRNAs showed that jasmonate (JA) pathway was significantly enriched. To reveal the interaction between lncRNAs and JA-related genes, we studied the coexpression of them and found 39 JA-related protein-coding genes to be interplayed with 73 lncRNAs, highlighting the potential modulation of lncRNAs in JA pathway. We subsequently identified an lncRNA, ALEX1, whose expression is highly induced by Xoo infection. A T-DNA insertion line constructed using enhancer trap system showed a higher expression of ALEX1 and exerted a significant resistance to rice bacterial blight. Functional study revealed that JA signalling is activated and the endogenous content of JA and JA-Ile is increased. Overexpressing ALEX1 in rice further confirmed the activation of JA pathway and resistance to bacterial blight. Our findings reveal the expression of pathogen-responsive lncRNAs in rice and provide novel insights into the connection between lncRNAs and JA pathway in the regulation of plant disease resistance.

The CYP74 Gene Family in Watermelon: Genome-Wide Identification and Expression Profiling Under Hormonal Stress and Root-Knot Nematode Infection

Allene oxide synthase (AOS) and hydroperoxide lyase (HPL), members of the CYP74 gene family, are branches of the oxylipin pathway and play vital roles in plant responses to a number of stresses. In this study, four HPL genes and one AOS gene were identified in the watermelon genome, which were clustered into three subfamilies (CYP74A, CYP74B and CYP74C). Sequence analysis revealed that most HPL and AOS proteins from various plants contain representative domains, including Helix-I region, Helix-K region (ExxR) and Heme-binding domain. A number of development-, stress-, and hormone-related cis-elements were found in the promoter regions of the ClAOS and ClHPL genes, and the detected ClAOS and ClHPL genes were differentially expressed in different tissues and fruit development stages, as well as in response to various hormones. In addition, red light could enhance the expression of ClAOS in root-knot nematode-infected leaves and roots of watermelon, implying that ClAOS might play a primary role in red light-induced resistance against root-knot nematodes. These findings lay a foundation for understanding the specific function of CYP74 genes in watermelon.

Metabolic Control within the Jasmonate Biochemical Pathway

Regulation of defense and developmental responses by jasmonates (JAs) has been intensively investigated at genetic and transcriptional levels. Plasticity in the jasmonic acid (JA) metabolic pathway as a means to control signal output has received less attention. Although the amplitude of JA responses generally follows the accumulation dynamics of the active hormone jasmonoyl-isoleucine (JA-Ile), emerging evidence has identified cases where this relationship is distorted and that we discuss in this review. JA-Ile is turned over in Arabidopsis by two inducible, intertwined catabolic pathways; one is oxidative and mediated by cytochrome P450 enzymes of the subfamily 94 (CYP94), and the other proceeds via deconjugation by amidohydrolases. Their genetic inactivation has profound effects on JAs homeostasis, including strong JA-Ile overaccumulation, but this correlates with enhanced defense and tolerance to microbial or insect attacks only in the absence of overinduction of negative signaling regulators. By contrast, the impairment of JA oxidation in the jasmonic acid oxidase 2 (jao2) mutant turns on constitutive defense responses without elevating JA-Ile levels in naive leaves and enhances resistance to subsequent biotic stress. This latter and other recent cases of JA signaling are associated with JA-Ile catabolites accumulation rather than more abundant hormone, reflecting increased metabolic flux through the pathway. Therefore, manipulating upstream and downstream JA-Ile homeostatic steps reveals distinct metabolic nodes controlling defense signaling output.

Disruption of gene SPL35, encoding a novel CUE domain-containing protein, leads to cell death and enhanced disease response in rice

Lesion mimic mutants that exhibit spontaneous hypersensitive response (HR)-like necrotic lesions are ideal experimental systems for elucidating molecular mechanisms involved in plant cell death and defence responses. Here we report identification of a rice lesion mimic mutant, spotted leaf 35 (spl35), and cloning of the causal gene by TAIL-PCR strategy. spl35 exhibited decreased chlorophyll content, higher accumulation of H₂ O₂ , up-regulated expression of defence-related marker genes, and enhanced resistance to both fungal and bacterial pathogens of rice. The SPL35 gene encodes a novel CUE (coupling of ubiquitin conjugation to ER degradation) domain-containing protein that is predominantly localized in cytosol, ER and unknown punctate compartment(s). SPL35 is constitutively expressed in all organs, and both overexpression and knockdown of SPL35 cause the lesion mimic phenotype. SPL35 directly interacts with the E2 protein OsUBC5a and the coatomer subunit delta proteins Delta-COP1 and Delta-COP2 through the CUE domain, and down-regulation of these interacting proteins also cause development of HR-like lesions resembling those in spl35 and activation of defence responses, indicating that SPL35 may be involved in the ubiquitination and vesicular trafficking pathways. Our findings provide insight into a role of SPL35 in regulating cell death and defence response in plants.

Downregulation of a CYP74 Rubber Particle Protein Increases Natural Rubber Production in Parthenium argentatum

We report functional genomics studies of a CYP74 rubber particle protein from Parthenium argentatum, commonly called guayule. Previously identified as an allene oxide synthase (AOS), this CYP74 constitutes the most abundant protein found in guayule rubber particles. Transgenic guayule lines with AOS gene expression down-regulated by RNAi (AOSi) exhibited strong phenotypes that included agricultural traits conducive to enhancing rubber yield. AOSi lines had higher leaf and stem biomass, thicker stembark tissues, increased stem branching and improved net photosynthetic rate. Importantly, the rubber content was significantly increased in AOSi lines compared to the wild-type (WT), vector control and AOS overexpressing (AOSoe) lines, when grown in controlled environments both in tissue-culture media and in greenhouse/growth chambers. Rubber particles from AOSi plants consistently had less AOS particle-associated protein, and lower activity (for conversion of 13-HPOT to allene oxide). Yet plants with downregulated AOS showed higher rubber transferase enzyme activity. The increase in biomass in AOSi lines was associated with not only increases in the rate of photosynthesis and non-photochemical quenching (NPQ), in the cold, but also in the content of the phytohormone SA, along with a decrease in JA, GAs, and ABA. The increase in biosynthetic activity and rubber content could further result from the negative regulation of AOS expression by high levels of salicylic acid in AOSi lines and when introduced exogenously. It is apparent that AOS in guayule plays a pivotal role in rubber production and plant growth.

Overexpression of Arabidopsis OPR3 in Hexaploid Wheat (Triticum aestivum L.) Alters Plant Development and Freezing Tolerance

Jasmonates are plant hormones that are involved in the regulation of different aspects of plant life, wherein their functions and molecular mechanisms of action in wheat are still poorly studied. With the aim of gaining more insights into the role of jasmonic acid (JA) in wheat growth, development, and responses to environmental stresses, we have generated transgenic bread wheat plants overexpressing Arabidopsis 12-OXOPHYTODIENOATE REDUCTASE 3 (AtOPR3), one of the key genes of the JA biosynthesis pathway. Analysis of transgenic plants showed that AtOPR3 overexpression affects wheat development, including germination, growth, flowering time, senescence, and alters tolerance to environmental stresses. Transgenic wheat plants with high AtOPR3 expression levels have increased basal levels of JA, and up-regulated expression of ALLENE OXIDE SYNTHASE, a jasmonate biosynthesis pathway gene that is known to be regulated by a positive feedback loop that maintains and boosts JA levels. Transgenic wheat plants with high AtOPR3 expression levels are characterized by delayed germination, slower growth, late flowering and senescence, and improved tolerance to short-term freezing. The work demonstrates that genetic modification of the jasmonate pathway is a suitable tool for the modulation of developmental traits and stress responses in wheat.

Green leaf volatile production by plants: a meta-analysis

666 I. Introduction 667 II. Biosynthesis 667 III. Meta-analysis 669 IV. The type of stress influences the total amount of GLVs released 669 V. Herbivores can modulate the wound-induced release of GLVs 669 VI. Fungal infection greatly induces GLV production 672 VII. Monocots and eudicots respond differentially to different types of stress 673 VIII. The type of stress does not influence the proportion of GLVs per chemical class 673 IX. The type of stress does influence the isomeric ratio within each chemical class 674 X. GLVs: from signal perception to signal transduction 676 XI. GLVs influence the C/N metabolism 677 XII. Interaction with plant hormones 678 XIII. General conclusions and unanswered questions 678 Acknowledgements 679 References 679 SUMMARY: Plants respond to stress by releasing biogenic volatile organic compounds (BVOCs). Green leaf volatiles (GLVs), which are abundantly produced across the plant kingdom, comprise an important group within the BVOCs. They can repel or attract herbivores and their natural enemies; and they can induce plant defences or prime plants for enhanced defence against herbivores and pathogens and can have direct toxic effects on bacteria and fungi. Unlike other volatiles, GLVs are released almost instantly upon mechanical damage and (a)biotic stress and could thus function as an immediate and informative signal for many organisms in the plant's environment. We used a meta-analysis approach in which data from the literature on GLV production during biotic stress responses were compiled and interpreted. We identified that different types of attackers and feeding styles add a degree of complexity to the amount of emitted GLVs, compared with wounding alone. This meta-analysis illustrates that there is less variation in the GLV profile than we presumed, that pathogens induce more GLVs than insects and wounding, and that there are clear differences in GLV emission between monocots and dicots. Besides the meta-analysis, this review provides an update on recent insights into the perception and signalling of GLVs in plants.

Jasmonates: News on Occurrence, Biosynthesis, Metabolism and Action of an Ancient Group of Signaling Compounds

: Jasmonic acid (JA) and its related derivatives are ubiquitously occurring compounds of land plants acting in numerous stress responses and development. Recent studies on evolution of JA and other oxylipins indicated conserved biosynthesis. JA formation is initiated by oxygenation of α-linolenic acid (α-LeA, 18:3) or 16:3 fatty acid of chloroplast membranes leading to 12-oxo-phytodienoic acid (OPDA) as intermediate compound, but in Marchantiapolymorpha and Physcomitrellapatens, OPDA and some of its derivatives are final products active in a conserved signaling pathway. JA formation and its metabolic conversion take place in chloroplasts, peroxisomes and cytosol, respectively. Metabolites of JA are formed in 12 different pathways leading to active, inactive and partially active compounds. The isoleucine conjugate of JA (JA-Ile) is the ligand of the receptor component COI1 in vascular plants, whereas in the bryophyte M. polymorpha COI1 perceives an OPDA derivative indicating its functionally conserved activity. JA-induced gene expressions in the numerous biotic and abiotic stress responses and development are initiated in a well-studied complex regulation by homeostasis of transcription factors functioning as repressors and activators.

Grapevine fatty acid hydroperoxide lyase generates actin-disrupting volatiles and promotes defence-related cell death

Fatty acid hydroperoxides can generate short-chained volatile aldehydes that may participate in plant defence. A grapevine hydroperoxide lyase (VvHPL1) clustering to the CYP74B class was functionally characterized with respect to a role in defence. In grapevine leaves, transcripts of this gene accumulated rapidly to high abundance in response to wounding. Cellular functions of VvHPL1 were investigated upon heterologous expression in tobacco BY-2 cells. A C-terminal green fluorescent protein (GFP) fusion of VvHPL1 was located in plastids. The overexpression lines were found to respond to salinity stress or the bacterial elicitor harpin by increasing cell death. This signal-dependent mortality response was mitigated either by addition of exogenous jasmonic acid or by treatment with diphenyleneiodonium (DPI), an inhibitor of NADPH oxidases. By feeding different substrates to recombinantly expressed enzyme, VvHPL1 could also be functionally classified as true 13-HPL. The cognate products generated by this 13-HPL were cis-3-hexenal and trans-2-hexenal. Using a GFP-tagged actin marker line, one of these isomeric products, cis-3-hexenal, was found specifically to elicit a rapid disintegration of actin filaments. This response was not only observed in the heterologous system (tobacco BY-2), but also in a grapevine cell strain expressing this marker, as well as in leaf discs from an actin marker grape used as a homologous system. These results are discussed in the context of a role for VvHPL1 in a lipoxygenase-dependent signalling pathway triggering cell death-related defence that bifurcates from jasmonate-dependent basal immunity.

Lack of a Cytoplasmic RLK, Required for ROS Homeostasis, Induces Strong Resistance to Bacterial Leaf Blight in Rice

Many scientific findings have been reported on the beneficial function of reactive oxygen species (ROS) in various cellular processes, showing that they are not just toxic byproducts. The double-edged role of ROS shows the importance of the regulation of ROS level. We report a gene, rrsRLK (required for ROS-scavenging receptor-like kinase), that encodes a cytoplasmic RLK belonging to the non-RD kinase family. The gene was identified by screening rice RLK mutant lines infected with Xanthomonas oryzae pv. oryzae (Xoo), an agent of bacterial leaf blight of rice. The mutant (ΔrrsRLK) lacking the Os01g02290 gene was strongly resistant to many Xoo strains, but not to the fungal pathogen Magnaporthe grisea. ΔrrsRLK showed significantly higher expression of OsPR1a, OsPR1b, OsLOX, RBBTI4, and jasmonic acid-related genes than wild type. We showed that rrsRLK protein interacts with OsVOZ1 (vascular one zinc-finger 1) and OsPEX11 (peroxisomal biogenesis factor 11). In the further experiments, abnormal biogenesis of peroxisomes, hydrogen peroxide (H2O2) accumulation, and reduction of activity of ROS-scavenging enzymes were investigated in ΔrrsRLK. These results suggest that the enhanced resistance in ΔrrsRLK is due to H2O2 accumulation caused by irregular ROS-scavenging mechanism, and rrsRLK is most likely a key regulator required for ROS homeostasis in rice.

The Oxylipin Pathways: Biochemistry and Function

Plant oxylipins form a constantly growing group of signaling molecules that comprise oxygenated fatty acids and metabolites derived therefrom. In the last decade, the understanding of biosynthesis, metabolism, and action of oxylipins, especially jasmonates, has dramatically improved. Additional mechanistic insights into the action of enzymes and insights into signaling pathways have been deepened for jasmonates. For other oxylipins, such as the hydroxy fatty acids, individual signaling properties and cross talk between different oxylipins or even with additional phytohormones have recently been described. This review summarizes recent understanding of the biosynthesis, regulation, and function of oxylipins.

The Brown Midrib Leaf (bml) Mutation in Rice (Oryza sativa L.) Causes Premature Leaf Senescence and the Induction of Defense Responses

Isolating and characterizing mutants with altered senescence phenotypes is one of the ways to understand the molecular basis of leaf aging. Using ethyl methane sulfonate mutagenesis, a new rice (Oryza sativa) mutant, brown midrib leaf (bml), was isolated from the indica cultivar ‘Zhenong34’. The bml mutants had brown midribs in their leaves and initiated senescence prematurely, at the onset of heading. The mutants had abnormal cells with degraded chloroplasts and contained less chlorophyll compared to the wild type (WT). The bml mutant showed excessive accumulation of reactive oxygen species (ROS), increased activities of superoxide dismutase, catalase, and malondialdehyde, upregulation of senescence-induced STAY-GREEN genes and senescence-related transcription factors, and down regulation of photosynthesis-related genes. The levels of abscisic acid (ABA) and jasmonic acid (JA) were increased in bml with the upregulation of some ABA and JA biosynthetic genes. In pathogen response, bml demonstrated higher resistance against Xanthomonas oryzae pv. oryzae and upregulation of four pathogenesis-related genes compared to the WT. A genetic study confirmed that the bml trait was caused by a single recessive nuclear gene (BML). A map-based cloning using insertion/deletion markers confirmed that BML was located in the 57.32kb interval between the L5IS7 and L5IS11 markers on the short arm of chromosome 5. A sequence analysis of the candidate region identified a 1 bp substitution (G to A) in the 5′-UTR (+98) of bml. BML is a candidate gene associated with leaf senescence, ROS regulation, and disease response, also involved in hormone signaling in rice. Therefore, this gene might be useful in marker-assisted backcrossing/gene editing to improve rice cultivars.

A Comparison of the Effects of FATTY ACID DESATURASE 7 and HYDROPEROXIDE LYASE on Plant-Aphid Interactions

The spr2 mutation in tomato (Solanum lycopersicum), which disrupts function of FATTY ACID DESATURASE 7 (FAD7), confers resistance to the potato aphid (Macrosiphum euphorbiae) and modifies the plant’s C6 volatile profiles. To investigate whether C6 volatiles play a role in resistance, HYDROPEROXIDE LYASE (HPL), which encodes a critical enzyme in C6 volatile synthesis, was silenced in wild-type tomato plants and spr2 mutants. Silencing HPL in wild-type tomato increased potato aphid host preference and reproduction on 5-week old plants but had no influence on 3-week old plants. The spr2 mutation, in contrast, conferred strong aphid resistance at both 3 and 5 weeks, and silencing HPL in spr2 did not compromise this aphid resistance. Moreover, a mutation in the FAD7 gene in Arabidopsis thaliana also conferred resistance to the green peach aphid (Myzus persicae) in a genetic background that carries a null mutation in HPL. These results indicate that HPL contributes to certain forms of aphid resistance in tomato, but that the effects of FAD7 on aphids in tomato and Arabidopsis are distinct from and independent of HPL.

Mutation in Rice Abscisic Acid2 Results in Cell Death, Enhanced Disease-Resistance, Altered Seed Dormancy and Development

Lesion mimic mutants display spontaneous cell death, and thus are valuable for understanding the molecular mechanism of cell death and disease resistance. Although a lot of such mutants have been characterized in rice, the relationship between lesion formation and abscisic acid (ABA) synthesis pathway is not reported. In the present study, we identified a rice mutant, lesion mimic mutant 9150 (lmm9150), exhibiting spontaneous cell death, pre-harvest sprouting, enhanced growth, and resistance to rice bacterial and blast diseases. Cell death in the mutant was accompanied with excessive accumulation of H₂O₂. Enhanced disease resistance was associated with cell death and upregulation of defense-related genes. Map-based cloning identified a G-to-A point mutation resulting in a D-to-N substitution at the amino acid position 110 of OsABA2 (LOC_Os03g59610) in lmm9150. Knock-out of OsABA2 through CRISPR/Cas9 led to phenotypes similar to those of lmm9150. Consistent with the function of OsABA2 in ABA biosynthesis, ABA level in the lmm9150 mutant was significantly reduced. Moreover, exogenous application of ABA could rescue all the mutant phenotypes of lmm9150. Taken together, our data linked ABA deficiency to cell death and provided insight into the role of ABA in rice disease resistance.

Metabolomics and Proteomics of Brassica napus Guard Cells in Response to Low CO2

Stomatal guard cell response to various stimuli is an important process that balances plant carbon dioxide (CO2) uptake and water transpiration. Elevated CO2 induces stomatal closure, while low CO2 promotes stomatal opening. The signaling process of elevated CO2 induced stomatal closure has been extensively studied in recent years. However, the mechanism of low CO2 induced stomatal opening is not fully understood. Here we report metabolomic and proteomic responses of Brassica napus guard cells to low CO2 using hyphenated mass spectrometry technologies. A total of 411 metabolites and 1397 proteins were quantified in a time-course study of low CO2 effects. Metabolites and proteins that exhibited significant changes are overrepresented in fatty acid metabolism, starch and sucrose metabolism, glycolysis and redox regulation. Concomitantly, multiple hormones that promote stomatal opening increased in response to low CO2. Interestingly, jasmonic acid precursors were diverted to a branch pathway of traumatic acid biosynthesis. These results indicate that the low CO2 response is mediated by a complex crosstalk between different phytohormones.

Metabolic and transcriptional alternations for defense by interfering OsWRKY62 and OsWRKY76 transcriptions in rice

Metabolomic and transcriptomic approaches were used to dissect the enhanced disease resistance in the plants harbouring a RNA interfering construct of OsWRKY62 and OsWRKY76 (dsOW62/76) genes. The primary metabolic pathways were activated in dsOW62/76 compared with wild-type (ZH17) plants, revealed by increased accumulation of amino acids and constituents of citric acid cycle etc. Contents of phenolic acids derived from phenylpropanoid pathway were elevated in dsOW62/76 plants. Importantly, phenolamides, conjugates of the phenolic acids with amines, were detected in large number and mostly at higher levels in dsOW62/76 compared with ZH17 plants; however, the free pools of flavonoids were mostly decreased in dsOW62/76. Salicylic acid (SA) and jasmonic acid (JA)/JA-Ile contents were increased in dsOW62/76 and knockout lines of individual OsWRKY62 and OsWRKY76 genes. Transcription of isochorismate synthase (OsICS1) gene was suppressed in dsOW62/76 and in MeJA-treated rice plants, whereas the transcription level of cinnamoyl-CoA hydratase-dehydrogenase (OsCHD) gene for β-oxidation in peroxisome was increased. The calli with OsCHD mutation showed markedly decreased SA accumulation. These results indicate that OsWRKY62 and OsWRKY76 function as negative regulators of biosynthetic defense-related metabolites and provide evidence for an important role of phenylpropanoid pathway in SA production in rice.

Molecular dissection of the response of the rice Systemic Acquired Resistance Deficient 1 (SARD1) gene to different types of ionizing radiation

PURPOSE

Exposure to ionizing radiation induces plant defenses by regulating the expression of response genes. The systemic acquired resistance deficient 1 (SARD1) is a key gene in plant defense response. In this study, the function of Oryza sativa SARD1 (OsSARD1) was investigated after exposure of seeds/plants to ionizing radiation, jasmonic acid (JA) or salicylic acid (SA).

MATERIALS AND METHODS

Rice seeds exposed to two types of ionizing radiations (gamma ray [GR] and ion beam [IB]) were analyzed by quantitative reverse transcription PCR (qRT-PCR) to identify the genes that are altered in response to ionizing radiation. Then, OsSARD1-overexpressing homozygous Arabidopsis plants were generated to assess the effects of OsSARD1 in the response to irradiation. The phenotypes of these transgenic plants, as well as control plants, were monitored after GR irradiation at doses of 200 and 300 Gray (Gy).

RESULTS

The OsSARD1 transcript was strongly downregulated after exposure to GR and IB irradiation. Previous phylogenetic analysis showed that the Arabidopsis SARD1 (AtSARD1) protein is closely related to Arabidopsis calmodulin-binding protein 60g (AtCBP60g), which is known to be required for activation of SA biosynthesis. In this study, phylogenetic analysis showed that OsSARD1 was grouped with AtSARD1. The OsSARD1 gene was induced after exposure to SA and JA. The biological phenotype of OsSARD1-overexpressing Arabidopsis plants was examined. OsSARD1-overexpressing plants displayed resistance to GR; in comparison with wild-type plants, the height and weight of OsSARD1-overexpressing plants were significantly greater after GR irradiation. In addition, OsSARD1 protein was abundantly accumulated in the nucleus.

CONCLUSIONS

The results indicate that OsSARD1 plays an important role in the regulation of the defense responses to GR and IB irradiation and exhibits phytohormone induced expression.

Jasmonates: biosynthesis, metabolism, and signaling by proteins activating and repressing transcription

The lipid-derived phytohormone jasmonate (JA) regulates plant growth, development, secondary metabolism, defense against insect attack and pathogen infection, and tolerance to abiotic stresses such as wounding, UV light, salt, and drought. JA was first identified in 1962, and since the 1980s many studies have analyzed the physiological functions, biosynthesis, distribution, metabolism, perception, signaling, and crosstalk of JA, greatly expanding our knowledge of the hormone's action. In response to fluctuating environmental cues and transient endogenous signals, the occurrence of multilayered organization of biosynthesis and inactivation of JA, and activation and repression of the COI1-JAZ-based perception and signaling contributes to the fine-tuning of JA responses. This review describes the JA biosynthetic enzymes in terms of gene families, enzymatic activity, location and regulation, substrate specificity and products, the metabolic pathways in converting JA to activate or inactivate compounds, JA signaling in perception, and the co-existence of signaling activators and repressors.

Arabidopsis thaliana SKP1 interacting protein 11 (At2g02870) negatively regulates the release of green leaf volatiles

AtSKIP11, a kelch-repeat containing F-box protein from Arabidopsis thaliana, negatively regulates the HPL pathway and can serve as a potential molecular switch for the biosynthesis of green leaf volatiles.

Separation of enzymatic functions and variation of spin state of rice allene oxide synthase-1 by mutation of Phe-92 and Pro-430

Rice allene oxide synthase-1 mutants carrying F92L, P430A or F92L/P430A amino acid substitution mutations were constructed, recombinant mutant and wild type proteins were purified and their substrate preference, UV-vis spectra and heme iron spin state were characterized. The results show that the hydroperoxide lyase activities of F92L and F92L/P430A mutants prefer 13-hydroperoxy substrate to other hydroperoxydienoic acids or hydroperoxytrienoic acids. The Soret maximum was completely red-shifted in P430A and F92L/P430A mutants, but it was partially shifted in the F92L mutant. ESR spectral data showed that wild type, F92L and P430A mutants occupied high and low spin states, while the F92L/P430A mutant occupied only low spin state. The extent of the red shift of the Soret maximum increased as the population of low spin heme iron increased, suggesting that the spectral shift reflects the high to low transition of heme iron spin state in rice allene oxide synthase-1. Relative to wild type allene oxide synthase-1, the hydroperoxide lyase activities of F92L and F92L/P430A are less sensitive to inhibition by imidazole with (13S or 9S)-hydroperoxydienoic acid as substrate and more sensitive than wild type with (13S)-hydroperoxytrienoic acid as substrate. Our results suggest that hydroperoxydienoic acid is the preferred substrate for the hydroperoxide lyase activity and (13S)-hydroperoxytrienoic acid is the preferred substrate for allene oxide synthase activity of allene oxide synthase-1.

A chloroplast-localized protein LESION AND LAMINA BENDING affects defence and growth responses in rice

Understanding how plants allocate their resources to growth or defence is of long-term importance to the development of new and improved varieties of different crops. Using molecular genetics, plant physiology, hormone analysis and Next-Generation Sequencing (NGS)-based transcript profiling, we have isolated and characterized the rice (Oryza sativa) LESION AND LAMINA BENDING (LLB) gene that encodes a chloroplast-targeted putative leucine carboxyl methyltransferase. Loss of LLB function results in reduced growth and yield, hypersensitive response (HR)-like lesions, accumulation of the antimicrobial compounds momilactones and phytocassanes, and constitutive expression of pathogenesis-related genes. Consistent with these defence-associated responses, llb shows enhanced resistance to rice blast (Magnaporthe oryzae) and bacterial blight (Xanthomonas oryzae pv. oryzae). The lesion and resistance phenotypes are likely to be caused by the over-accumulation of jasmonates (JAs) in the llb mutant including the JA precursor 12-oxo-phytodienoic acid. Additionally, llb shows an increased lamina inclination and enhanced early seedling growth due to elevated brassinosteroid (BR) synthesis and/or signalling. These findings show that LLB functions in the chloroplast to either directly or indirectly repress both JA- and BR-mediated responses, revealing a possible mechanism for controlling how plants allocate resources for defence and growth.

The DELLA Protein SLR1 Integrates and Amplifies Salicylic Acid- and Jasmonic Acid-Dependent Innate Immunity in Rice

Gibberellins are a class of tetracyclic plant hormones that are well known to promote plant growth by inducing the degradation of a class of nuclear growth-repressing proteins, called DELLAs. In recent years, GA and DELLAs are also increasingly implicated in plant responses to pathogen attack, although our understanding of the underlying mechanisms is still limited, especially in monocotyledonous crop plants. Aiming to further decipher the molecular underpinnings of GA- and DELLA-modulated plant immunity, we studied the dynamics and impact of GA and DELLA during infection of the model crop rice (Oryza sativa) with four different pathogens exhibiting distinct lifestyles and infection strategies. Opposite to previous findings in Arabidopsis (Arabidopsis thaliana), our findings reveal a prominent role of the DELLA protein Slender Rice1 (SLR1) in the resistance toward (hemi)biotrophic but not necrotrophic rice pathogens. Moreover, contrary to the differential effect of DELLA on the archetypal defense hormones salicylic acid (SA) and jasmonic acid (JA) in Arabidopsis, we demonstrate that the resistance-promoting effect of SLR1 is due at least in part to its ability to boost both SA- and JA-mediated rice defenses. In a reciprocal manner, we found JA and SA treatment to interfere with GA metabolism and stabilize SLR1. Together, these findings favor a model whereby SLR1 acts as a positive regulator of hemibiotroph resistance in rice by integrating and amplifying SA- and JA-dependent defense signaling. Our results highlight the differences in hormone defense networking between rice and Arabidopsis and underscore the importance of GA and DELLA in molding disease outcomes.

Functional diversity of jasmonates in rice

Phytohormone jasmonates (JA) play essential roles in plants, such as regulating development and growth, responding to environmental changes, and resisting abiotic and biotic stresses. During signaling, JA interacts, either synergistically or antagonistically, with other hormones, such as salicylic acid (SA), gibberellin (GA), ethylene (ET), auxin, brassinosteroid (BR), and abscisic acid (ABA), to regulate gene expression in regulatory networks, conferring physiological and metabolic adjustments in plants. As an important staple crop, rice is a major nutritional source for human beings and feeds one third of the world's population. Recent years have seen significant progress in the understanding of the JA pathway in rice. In this review, we summarize the diverse functions of JA, and discuss the JA interplay with other hormones, as well as light, in this economically important crop. We believe that a better understanding of the JA pathway will lead to practical biotechnological applications in rice breeding and cultivation.

Light-modulated seminal wavy roots in rice mediated by nitric oxide-dependent signaling

Rice (Oryza sativa L.) seminal roots from germinated seeds help establish seedlings, but the seminal root growth and morphology are sensitive to environmental factors. Our previous research showed that several indica-type rice varieties such as Taichung native 1 (TCN1) showed light-induced wavy roots. Also, auxin and oxylipins are two signaling factors regulating the wavy root photomorphology. To investigate the signaling pathway, here, we found that nitric oxide (NO) was a second messenger triggering the signal transduction of light stimuli to induce the wavy morphology of seminal roots in rice. Moreover, interactions between oxylipins and phytohormones such as ethylene and auxin participating in the NO-dependent regulatory pathway of light-induced wavy roots were examined. The order of action of signaling components in the pathway was NO, oxylipins, ethylene, and auxin.

Green leaf volatiles: biosynthesis, biological functions and their applications in biotechnology

Plants have evolved numerous constitutive and inducible defence mechanisms to cope with biotic and abiotic stresses. These stresses induce the expression of various genes to activate defence-related pathways that result in the release of defence chemicals. One of these defence mechanisms is the oxylipin pathway, which produces jasmonates, divinylethers and green leaf volatiles (GLVs) through the peroxidation of polyunsaturated fatty acids (PUFAs). GLVs have recently emerged as key players in plant defence, plant-plant interactions and plant-insect interactions. Some GLVs inhibit the growth and propagation of plant pathogens, including bacteria, viruses and fungi. In certain cases, GLVs released from plants under herbivore attack can serve as aerial messengers to neighbouring plants and to attract parasitic or parasitoid enemies of the herbivores. The plants that perceive these volatile signals are primed and can then adapt in preparation for the upcoming challenges. Due to their 'green note' odour, GLVs impart aromas and flavours to many natural foods, such as vegetables and fruits, and therefore, they can be exploited in industrial biotechnology. The aim of this study was to review the progress and recent developments in research on the oxylipin pathway, with a specific focus on the biosynthesis and biological functions of GLVs and their applications in industrial biotechnology.

To die or not to die? Lessons from lesion mimic mutants

Programmed cell death (PCD) is a ubiquitous genetically regulated process consisting in an activation of finely controlled signaling pathways that lead to cellular suicide. Although some aspects of PCD control appear evolutionary conserved between plants, animals and fungi, the extent of conservation remains controversial. Over the last decades, identification and characterization of several lesion mimic mutants (LMM) has been a powerful tool in the quest to unravel PCD pathways in plants. Thanks to progress in molecular genetics, mutations causing the phenotype of a large number of LMM and their related suppressors were mapped, and the identification of the mutated genes shed light on major pathways in the onset of plant PCD such as (i) the involvements of chloroplasts and light energy, (ii) the roles of sphingolipids and fatty acids, (iii) a signal perception at the plasma membrane that requires efficient membrane trafficking, (iv) secondary messengers such as ion fluxes and ROS and (v) the control of gene expression as the last integrator of the signaling pathways.

cDNA-AFLP analysis reveals the adaptive responses of citrus to long-term boron-toxicity

BACKGROUND

Boron (B)-toxicity is an important disorder in agricultural regions across the world. Seedlings of 'Sour pummelo' (Citrus grandis) and 'Xuegan' (Citrus sinensis) were fertigated every other day until drip with 10 μM (control) or 400 μM (B-toxic) H3BO3 in a complete nutrient solution for 15 weeks. The aims of this study were to elucidate the adaptive mechanisms of citrus plants to B-toxicity and to identify B-tolerant genes.

RESULTS

B-toxicity-induced changes in seedlings growth, leaf CO2 assimilation, pigments, total soluble protein, malondialdehyde (MDA) and phosphorus were less pronounced in C. sinensis than in C. grandis. B concentration was higher in B-toxic C. sinensis leaves than in B-toxic C. grandis ones. Here we successfully used cDNA-AFLP to isolate 67 up-regulated and 65 down-regulated transcript-derived fragments (TDFs) from B-toxic C. grandis leaves, whilst only 31 up-regulated and 37 down-regulated TDFs from B-toxic C. sinensis ones, demonstrating that gene expression is less affected in B-toxic C. sinensis leaves than in B-toxic C. grandis ones. These differentially expressed TDFs were related to signal transduction, carbohydrate and energy metabolism, nucleic acid metabolism, protein and amino acid metabolism, lipid metabolism, cell wall and cytoskeleton modification, stress responses and cell transport. The higher B-tolerance of C. sinensis might be related to the findings that B-toxic C. sinensis leaves had higher expression levels of genes involved in photosynthesis, which might contribute to the higher photosyntheis and light utilization and less excess light energy, and in reactive oxygen species (ROS) scavenging compared to B-toxic C. grandis leaves, thus preventing them from photo-oxidative damage. In addition, B-toxicity-induced alteration in the expression levels of genes encoding inorganic pyrophosphatase 1, AT4G01850 and methionine synthase differed between the two species, which might play a role in the B-tolerance of C. sinensis.

CONCLUSIONS

C. sinensis leaves could tolerate higher level of B than C. grandis ones, thus improving the B-tolerance of C. sinensis plants. Our findings reveal some novel mechanisms on the tolerance of plants to B-toxicity at the gene expression level.

Perception, signaling and cross-talk of jasmonates and the seminal contributions of the Daoxin Xie's lab and the Chuanyou Li's lab

Jasmonates (JAs) are lipid-derived signals in plant responses to biotic and abiotic stresses and in development. The most active JA compound is (+)-7-iso-JA-Ile, a JA conjugate with isoleucine. Biosynthesis, metabolism and key components of perception and signal transduction have been identified and numerous JA-induced gene expression data collected. For JA-Ile perception, the SCF(COI1)-JAZ co-receptor complex has been identified and crystalized. Activators such as MYC2 and repressors such as JAZs including their targets were found. Involvement of JA-Ile in response to herbivores and pathogens and in root growth inhibition is among the most studied aspects of JA-Ile signaling. There are an increasing number of examples, where JA-Ile shows cross-talk with other plant hormones. Seminal contributions in JA/JA-Ile research were given by Daoxin Xie's lab and Chuanyou Li's lab, both in Beijing. Here, characterization was done regarding components of the JA-Ile receptor, such as COI1 (JAI1) and SCF, regarding activators (MYCs, MYBs) and repressors (JAV1, bHLH IIId's) of JA-regulated gene expression, as well as regarding components of auxin biosynthesis and action, such as the transcription factor PLETHORA active in the root stem cell niche. This overview reflects the work of both labs in the light of our present knowledge on biosynthesis, perception and signal transduction of JA/JA-Ile and its cross-talk to other hormones.

Functional convergence of oxylipin and abscisic acid pathways controls stomatal closure in response to drought

Membranes are primary sites of perception of environmental stimuli. Polyunsaturated fatty acids are major structural constituents of membranes that also function as modulators of a multitude of signal transduction pathways evoked by environmental stimuli. Different stresses induce production of a distinct blend of oxygenated polyunsaturated fatty acids, "oxylipins." We employed three Arabidopsis (Arabidopsis thaliana) ecotypes to examine the oxylipin signature in response to specific stresses and determined that wounding and drought differentially alter oxylipin profiles, particularly the allene oxide synthase branch of the oxylipin pathway, responsible for production of jasmonic acid (JA) and its precursor 12-oxo-phytodienoic acid (12-OPDA). Specifically, wounding induced both 12-OPDA and JA levels, whereas drought induced only the precursor 12-OPDA. Levels of the classical stress phytohormone abscisic acid (ABA) were also mainly enhanced by drought and little by wounding. To explore the role of 12-OPDA in plant drought responses, we generated a range of transgenic lines and exploited the existing mutant plants that differ in their levels of stress-inducible 12-OPDA but display similar ABA levels. The plants producing higher 12-OPDA levels exhibited enhanced drought tolerance and reduced stomatal aperture. Furthermore, exogenously applied ABA and 12-OPDA, individually or combined, promote stomatal closure of ABA and allene oxide synthase biosynthetic mutants, albeit most effectively when combined. Using tomato (Solanum lycopersicum) and Brassica napus verified the potency of this combination in inducing stomatal closure in plants other than Arabidopsis. These data have identified drought as a stress signal that uncouples the conversion of 12-OPDA to JA and have revealed 12-OPDA as a drought-responsive regulator of stomatal closure functioning most effectively together with ABA.

Making sense of hormone-mediated defense networking: from rice to Arabidopsis

Phytohormones are not only essential for plant growth and development but also play central roles in triggering the plant immune signaling network. Historically, research aimed at elucidating the defense-associated role of hormones has tended to focus on the use of experimentally tractable dicot plants such as Arabidopsis thaliana. Emerging from these studies is a picture whereby complex crosstalk and induced hormonal changes mold plant health and disease, with outcomes largely dependent on the lifestyle and infection strategy of invading pathogens. However, recent studies in monocot plants are starting to provide additional important insights into the immune-regulatory roles of hormones, often revealing unique complexities. In this review, we address the latest discoveries dealing with hormone-mediated immunity in rice, one of the most important food crops and an excellent model for molecular genetic studies in monocots. Moreover, we highlight interactions between hormone signaling, rice defense and pathogen virulence, and discuss the differences and similarities with findings in Arabidopsis. Finally, we present a model for hormone defense networking in rice and describe how detailed knowledge of hormone crosstalk mechanisms can be used for engineering durable rice disease resistance.

Development of disease-resistant rice using regulatory components of induced disease resistance

Infectious diseases cause huge crop losses annually. In response to pathogen attacks, plants activate defense systems that are mediated through various signaling pathways. The salicylic acid (SA) signaling pathway is the most powerful of these pathways. Several regulatory components of the SA signaling pathway have been identified, and are potential targets for genetic manipulation of plants' disease resistance. However, the resistance associated with these regulatory components is often accompanied by fitness costs; that is, negative effects on plant growth and crop yield. Chemical defense inducers, such as benzothiadiazole and probenazole, act on the SA pathway and induce strong resistance to various pathogens without major fitness costs, owing to their 'priming effect.' Studies on how benzothiadiazole induces disease resistance in rice have identified WRKY45, a key transcription factor in the branched SA pathway, and OsNPR1/NH1. Rice plants overexpressing WRKY45 were extremely resistant to rice blast disease caused by the fungus Magnaporthe oryzae and bacterial leaf blight disease caused by Xanthomonas oryzae pv. oryzae (Xoo), the two major rice diseases. Disease resistance is often accompanied by fitness costs; however, WRKY45 overexpression imposed relatively small fitness costs on rice because of its priming effect. This priming effect was similar to that of chemical defense inducers, although the fitness costs were amplified by some environmental factors. WRKY45 is degraded by the ubiquitin-proteasome system, and the dual role of this degradation partly explains the priming effect. The synergistic interaction between SA and cytokinin signaling that activates WRKY45 also likely contributes to the priming effect. With a main focus on these studies, I review the current knowledge of SA-pathway-dependent defense in rice by comparing it with that in Arabidopsis, and discuss potential strategies to develop disease-resistant rice using signaling components.

Isolation, expression, and characterization of a hydroperoxide lyase gene from cucumber

A full-length cDNA coding for hydroperoxide lyase (CsHPL) was isolated from cucumber fruits of No. 26 (Southern China type) and No.14-1 (Northern China type), which differed significantly in fruit flavor. The deduced amino acid sequences of CsHPL from both lines show the same and significant similarity to known plant HPLs and contain typical conserved domains of HPLs. The recombinant CsHPL was confirmed to have 9/13-HPL enzymatic activity. Gene expression levels of CsHPL were measured in different organs, especially in fruits of different development stages of both lines. The HPL activities of fruit were identified basing on the catalytic action of crude enzyme extracts incubating with 13-HPOD (13-hydroperoxy-(9Z,12E)-octadecadienoic acid) and 13-HPOD + 9-HPOD (9-hydroperoxy-(10E,12Z)-octadecadienoic acid), and volatile reaction products were analyzed by GC-MS (gas chromatography-mass spectrometry). CsHPL gene expression in No. 26 fruit occurred earlier than that of total HPL enzyme activity and 13-HPL enzyme activity, and that in No. 14-1 fruit was consistent with total HPL enzyme activity and 9-HPL enzyme activity. 13-HPL enzyme activities decreased significantly and the 9-HPL enzyme activities increased significantly with fruit ripening in both lines, which accounted for the higher content of C6 aldehydes at 0–6 day post-anthesis (dpa) and higher content of C9 aldehydes at 9–12 dpa.

Hormone defense networking in rice: tales from a different world

Recent advances in plant immunity research underpin the pivotal role of small-molecule hormones in regulating the plant defense signaling network. Although most of our understanding comes from studies of dicot plants such as Arabidopsis thaliana, new studies in monocots are providing additional insights into the defense-regulatory role of phytohormones. Here, we review the roles of both classical and more recently identified stress hormones in regulating immunity in the model monocot rice (Oryza sativa) and highlight the importance of hormone crosstalk in shaping the outcome of rice-pathogen interactions. We also propose a defense model for rice that does not support a dichotomy between the pathogen lifestyle and the effectiveness of the archetypal defense hormones salicylic acid (SA) and jasmonic acid (JA).