Systemic acquired resistance, NPR1, and pathogenesis-related genes in wheat and barley

In Arabidopsis, systemic acquired resistance （SAR） is established beyond the initial infection by a pathogen or is directly induced by treatment with salicylic acid （SA） or its functional analogs, 2,6-dichloroisonicotinic acid （INA） and benzothiadiazole （BTH）. NPR1 protein is considered the master regulator of SAR in both SA signal sensing and transduction. In wheat （Triticum aesfivum） and barley （Hordeum vulgare）, both pathogen infection and BTH treatment can induce broad-spectrum resistance to various diseases, including powdery mildew, leaf rust, Fusarium head blight, etc. However, three different types of SAR-like responses including acquired resistance （AR）, systemic immunity （SI）, and BTH-induced resistance （BIR） seem to be achieved by activating different gene pathways. Recent research on wheat and barley NPR1 homologs in AR and SI has provided the initial clue for understanding the mechanism of SAR in these two plant species. In this review, the specific features ofAR, Si, and BIR in wheat and barley were summarized and compared with that of SAR in model plants of Arabidopsis and rice. Research updates on downstream genes of SAR, including pathogenesis-related （PR） and BTH-induced genes, were highlighted.

Systemic Acquired Resistance and Signal Transduction in Plant

Systemic acquired resistance (SAR), known as the broad-spectrum, inducible plant immunity, is a defense response triggered by pathogen infection. The response starts from the recognition of plant resistance (R) with the corresponding avirulence (avr) gene from the pathogen. There are some genes for convergence of signals downstream of different R/avr interacting partners into a single signaling pathway. Salicylic acid (SA) is required for the induction of SAR and involved in transducing the signal in target tissues. The SA signal is transduced through NPR1, a nuclear-localized protein that interacts with transcription factors that are involved in regulating SA-mediated gene expression. Some chemicals that mimic natural signaling compounds can also activate SAR. The application of biochemical activators to agriculture for plant protection is a novel idea for developing green chemical pesticide.

Glycosylation of N-hydroxy-pipecolic acid equilibrates between systemic acquired resistance response and plant growth

N-hydroxy-pipecolic acid(NHP)activates plant systemic acquired resistance(SAR).Enhanced defense responses are typically accompanied by deficiency in plant development and reproduction.Despite of extensive studies on SAR induction,the effects of NHPmetabolismon plant growth remain largely unclear.In this study,we discovered that NHP glycosylation is a critical factor that fine-tunes the tradeoff between SAR defense and plant growth.We demonstrated that a UDP-glycosyltransferase(UGT76B1)forming NHP glycoside(NHPG)controls the NHP to NHPG ratio.Consistently,the ugt76b1 mutant exhibits enhanced SAR response and an inhibitory effect on plant growth,while UGT76B1 overexpression attenuates SAR response,promotes growth,and delays senescence,indicating that NHP levels are dependent on UGT76B1 function in the course of SAR.Furthermore,our results suggested that,upon pathogen attack,UGT76B1-mediated NHP glycosylation forms a‘‘hand brake’’on NHP accumulation by attenuating the positive regulation of NHP biosynthetic pathway genes,highlighting the complexity of SAR-associated networks.In addition,we showed that UGT76B1-mediated NHP glycosylation in the local site is important for fine-tuning SAR response.Our results implicate that engineering plant immunity through manipulating the NHP/NHPG ratio is a promising method to balance growth and defense response in crops.

Regulatory and Functional Aspects of Indolic Metabolism in Plant Systemic Acquired Resistance

Tryptophan-derived, indolic metabolites possess diverse functions in Arabidopsis innate immunity to microbial pathogen infection. Hers, we investigate the functional role and regulatory characteristics of indolic metabolism in Arabidopsis systemic acquired resistance （SAR） triggered by the bacterial pathogen Pseudomonas syringae. Indolic metabolism is broadly activated in both P. syringae-inoculated and distant, non-inoculated leaves. At inoculation sites, camalexin, indol-3-ylmethylamine （13A）, and indole-3-carboxylic acid （ICA） are the major accumulating compounds. Camalexin accumulation is positively affected by MYB122, and the cytochrome P450 genes CYP81F1 and CYP81F2. Local 13A production, by contrast, occurs via indole glucosinolate breakdown by PEN2- dependent and independent pathways. Moreover, exogenous application of the defense hormone salicylic acid stimulates 13A generation at the expense of its precursor indol-3-ylmethylglucosinolate （13M）, and the SAR regulator pipecolic acid primes plants for enhanced P. syringae-induced activation of distinct branches of indolic metabolism. In uninfected systemic tissue, the metabolic response is more specific and associated with enhanced levels of the indolics 13A, ICA, and indole-3-carbaldehyde （ICC）. Systemic indole accumulation fully depends on functional CYP79B2/3, PEN2, and MYB34/51/122, and requires functional SAR signaling. Genetic analyses suggest that systemi- cally elevated indoles are dispensable for SAR and associated systemic increases of salicylic acid. However, soil-grown but not hydroponically -cultivated cyp79b2/3 and pen2 plants, both defective in indolic secondary metabolism, exhibit pre-induced immunity, which abrogates their intrinsic ability to induce SAR.

Diclofop-methyl affects microbial rhizosphere community and induces systemic acquired resistance in rice

Diclofop-methyl（DM）,a widely used herbicide in food crops,may partly contaminate the soil surface of natural ecosystems in agricultural area and exert toxic effects at low dose to nontarget plants.Even though rhizosphere microorganisms strongly interact with root cells,little is known regarding their potential modulating effect on herbicide toxicity in plants.Here we exposed rice seedlings（Xiushui 63） to 100 μg/L DM for 2 to 8 days and studied the effects of DM on rice rhizosphere microorganisms,rice systemic acquired resistance（SAR） and rice-microorganisms interactions.The results of metagenomic 16 S rDNA Illumina tags show that DM increases bacterial biomass and affects their community structure in the rice rhizosphere.After DM treatment,the relative abundance of the bacterium genera Massilia and Anderseniella increased the most relative to the control.In parallel,malate and oxalate exudation by rice roots increased,potentially acting as a carbon source for several rhizosphere bacteria.Transcriptomic analyses suggest that DM induced SAR in rice seedlings through the salicylic acid（but not the jasmonic acid） signal pathway.This response to DM stress conferred resistance to infection by a pathogenic bacterium,but was not influenced by the presence of bacteria in the rhizosphere since SAR transcripts did not change significantly in xenic and axenic plant roots exposed to DM.The present study provides new insights on the response of rice and its associated microorganisms to DM stress.

Articulating beneficial rhizobacteria-mediated plant defenses through induced systemic resistance:A review

Induced systemic resistance(ISR)is a mechanism by which certain plant beneficial rhizobacteria and fungi produce immunity,which can stimulate crop growth and resilience against various phytopathogens,insects,and parasites.These beneficial rhizobacteria and fungi improve plant performance by regulating hormone signaling,including salicylic acid(SA),jasmonic acid(JA),prosystemin,pathogenesis-related gene 1,and ethylene(ET)pathways,which activate the gene expression of ISR,the synthesis of secondary metabolites,various enzymes,and volatile compounds that ultimately induce defense mechanisms in plant.To protect themselves from disease,plants have various advanced defense mechanisms in which local acquired resistance,systemic gene silencing,systemic wound response,systemic acquired resistance(SAR),and ISR are involved.Several rhizobacteria activate the SA-dependent SAR pathway by producing SA at the root’s surface.In contrast,other rhizobacteria can activate different signaling pathways independent of SA(SA-independent ISR pathways)such as those dependent on JA and ET signaling.The main objective of this review is to provide insight into the types of induced resistance utilized for plant defense.Further to this,the genetic approaches used to suppress disease-causing genes,i.e.,RNA interference and antisense RNA,which are still underutilized in sustainable agriculture,along with the current vision for virus-induced gene silencing are also discussed.

Salicylic Acid and its Function in Plant Immunity

The small phenolic compound salicylic acid （SA） plays an important regulatory role in multiple physiological processes including plant im- mune response. Significant progress has been made during the past two decades in understanding the SA-mediated defense signaling network. Characterization of a number of genes functioning in SA biosynthesis, conjugation, accumulation, signaling, and crosstalk with other hormones such as jasmonic acid, ethylene, abscisic acid, auxin, gibberellic acid, cytokinin, brassinosteroid, and peptide hormones has sketched the finely tuned immune response network. Full understanding of the mech- anism of plant immunity will need to take advantage of fast developing genomics tools and bioinformatics techniques. However, elucidating genetic components involved in these pathways by conventional ge- netics, biochemistry, and molecular biology approaches will continue to be a major task of the community. High-throughput method for SA quantification holds the potential for isolating additional mutants related to SA-mediated defense signaling.

Synthesis and bioactivity of N-tert-butyl-N′-acyl-5-methy 1-1,2,3-thiadiazole-4-carbohydrazides

A series of novel N-tert-butyl-N＇-acyl-5-methyl-1,2,3-thiadiazole-4-carbohydrazides were designed and synthesized. Their structures were characterized by melting points, ^1H NMR, IR, ESI-MS, and elemental analysis. The bioassay tests indicated that compound 7o exhibited excellent direct anti-TMV activity and induction activity in vivo at 50μg/mL, which was better than that of Ninamycin and tiadinial. Our studies indicated that 1,2,3-thiadiazole was an active substructure for novel pesticide development.

Coordinated regulation of plant immunity by poly(ADP-ribosyl)ation and K63-linked ubiquitination

Poly(ADP-ribosyl)ation(PARylation)is a posttranslational modification reversibly catalyzed by poly(ADP-ribose)polymerases(PARPs)and poly(ADP-ribose)glycohydrolases(PARGs)and plays a key role in multi-ple cellular processes.The molecular mechanisms by which PARylation regulates innate immunity remain largely unknown in eukaryotes.Here we show that Arabidopsis UBC13A and UBC13B,the major drivers of lysine 63(K63)-linked polyubiquitination,directly interact with PARPs/PARGs.Activation of pathogen-associated molecular pattern(PAMP)-triggered immunity promotes these interactions and enhances PARylation of UBC13.Both parp1 parp2 and ubc13a ubc13b mutants are compromised in immune responses with increased accumulation of total pathogenesis-related(PR)proteins but decreased accu-mulation of secreted PR proteins.Protein disulfide-isomerases(PDIs),essential components of endo-plasmic reticulum quality control(ERQC)that ensure proper folding and maturation of proteins destined for secretion,complex with PARPs/PARGs and are PARylated upon PAMP perception.Significantly,PARylation of UBC13 regulates K63-linked ubiquitination of PDIs,which may further promote their disulfide isomerase activities for correct protein folding and subsequent secretion.Taken together,these results indicate that plant immunity is coordinately regulated by PARylation and K63-linked ubiquitination.

Osdof28: A New Member of the DOF Transcription Factor Family from Rice

DOF is a novel family of plant-specific proteins that share a unique and highly conserved DNA binding domain with one CX2CX21CX2C zinc finger motif. In this study, the Osdof28 gene, which codes a characteristic amino acid sequence of the DOF transcription factor family, was screened from rice （Oryza sativa japonica） using a yeast one-hybrid assay. Great amounts of the Osdof28 transcripts were found to accumulate in stems and leaves, with less in the roots, and no detectable transcription found in the germs. Osdof28 can be induced by salicylic acid and INA, which suggests that it may be related to the plant systemic acquired resistance （SAR）. The relationship was confirmed through biological induction of SAR using Xanthomonascampestrispv. oryzae, with more expression of Osdof28 observed in the systemic tissues after infection.

Novel plant activators with thieno[2,3-d]-1,2,3-thiadiazole-6-carboxylate scaffold: Synthesis and bioactivity

The 1,2,3-thiadiazole-carboxylate moiety was reported to be an important pharmacophore of plant activators.In this study,a series of novel plant activators based on thieno[2,3-d]-1,2,3-thiadiazole-6-carboxylate were designed and synthesized and their biological activity as plant activators was studied.The structures of the novel compounds were identifed by1H NMR,19F NMR and HRMS.The in vivo bioassay showed that these novel compounds had good effcacy against seven plant diseases.Especially,compounds 1a and 1c were more potent than the commercialized plant activator BTH.Almost no fungicidal activity was observed for the active compounds in the in vitro assay,which matched the requirements as plant activators.

Analysis of Salicylic Acid Induced Proteins in Rice

An analysis using SDS\|PAGE of acidic and basic protein fractions extracted from rice seedling treated with salicylic acid (SA) yielded several new proteins, some of which are similar in relative molecular mass to PR\|1a,c, PR\|2, 2e and PR\|3d, 3e of tobacco. Direct assays for peroxidases and β\|1,3\|glucanases demonstrated that the activities of the two enzymes in the rice seedlings increased rapidly with time after SA treatment, reaching a maximum 6 days after treatment.Disease resistance tests showed that SA treated rice seedlings stunted the development of blight lesions and displayed higher resistance to rice blight pathogen ( Xanthomonas oryzea pv. oryzea ). The data suggest that the treatment with SA, even for plants with high endogenous SA levels such as rice, may induce the appearance of new proteins and the formation of disease resistance. The results contribute to the analysis of the SA role in rice systemic acquired resistance.

The discovery of new scaffold of plant activators：From salicylic acid to benzotriazole

Started from salicylic acid（SA） and related commercialized plant activators,based on molecular threedimensional shape and pharmacophore similarity comparison（SHAFTS）,a new lead compound benzotriazole was predicted and a series of benzotriazole derivatives were designed and synthesized.The bioassay showed that benzotriazole had high activity against a broad spectrum of diseases including fungi and oomycetes in vivo,but no activity in vitro.And the introduction of proper groups at the1＇-position and 5＇-position was beneficial to the activity.So,they had the potential to be exploited as novel plant activators.