Downstream divergence of the ethylene signaling pathway for harpin-stimulated Arabidopsis growth and insect defense

Bacillus subtilis 26D Triggers Induced Systemic Resistance against Rhopalosiphum padi L. by Regulating the Expression of Genes AGO, DCL and microRNA in Bread Spring Wheat

Bacillus subtilis 26D is a plant growth-promoting endophytic bacteria capable of inducing systemic resistance through the priming mechanism, which includes plant genome reprogramming and the phenomenon of RNA interference (RNAi) and microRNA (miRNAs). The phloem-feeding insect bird cherry-oat aphid Rhopalosiphum padi L. is a serious pest that causes significant damage to crops throughout the world. However, the function of plant miRNAs in the response to aphid infestation remains unclear. The results of this work showed that B. subtilis 26D stimulated aphid resistance in wheat plants, inducing the expression of genes of hormonal signaling pathways ICS, WRKY13, PR1, ACS, EIN3, PR3, and ABI5. In addition, B. subtilis 26D activated the RNAi mechanism and regulated the expression of nine conserved miRNAs through activation of the ethylene, salicylic acid (SA), and abscisic acid (ABA) signaling pathways, which was demonstrated by using treatments with phytohormones. Treatment of plants with SA, ethylene, and ABA acted in a similar manner to B. subtilis 26D on induction of the expression of the AGO4, AGO5 and DCL2, DCL4 genes, as well as the expression of nine conserved miRNAs. Different patterns of miRNA expression were found in aphid-infested plants and in plants treated with B. subtilis 26D or SA, ethylene, and ABA and infested by aphids, suggesting that miRNAs play multiple roles in the plant response to phloem-feeding insects, associated with effects on hormonal signaling pathways, redox metabolism, and the synthesis of secondary metabolites. Our study provides new data to further elucidate the fine mechanisms of bacterial-induced priming. However, further extensive work is needed to fully unravel these mechanisms.

Importin β1 Mediates Nuclear Entry of EIN2C to Confer the Phloem-Based Defense against Aphids

Ethylene Insensitive 2 (EIN2) is an integral membrane protein that regulates ethylene signaling towards plant development and immunity by release of its carboxy-terminal functional portion (EIN2C) into the nucleus. The present study elucidates that the nuclear trafficking of EIN2C is induced by importin β1, which triggers the phloem-based defense (PBD) against aphid infestations in Arabidopsis. In plants, IMPβ1 interacts with EIN2C to facilitate EIN2C trafficking into the nucleus, either by ethylene treatment or by green peach aphid infestation, to confer EIN2-dependent PBD responses, which, in turn, impede the phloem-feeding activity and massive infestation by the aphid. In Arabidopsis, moreover, constitutively expressed EIN2C can complement the impβ1 mutant regarding EIN2C localization to the plant nucleus and the subsequent PBD development in the concomitant presence of IMPβ1 and ethylene. As a result, the phloem-feeding activity and massive infestation by green peach aphid were highly inhibited, indicating the potential value of EIN2C in protecting plants from insect attacks.

Effects of harpin and carbendazim on antioxidant accumulation in young jujube leaves

Jujube leaf tea is a functional beverage that soothes the nerves. In this study, we evaluated the effects of carbendazim and harpin on disease index, biomass accumulation, H2O2, antioxidant contents, and phenyl alanine ammonia lyase (PAL) activity in young jujube leaves. Compared to harpin, carbendazim decreased the disease index and induced higher H2O2 content. Additionally, the pesticide reduced young leaf biomass accumulation. In contrast, harpin increased vitamin C, glutathione, total phenolics, and total antioxidant capacity in young leaves compared to carbendazim. Compared with the control, harpin enhanced the PAL activity. Carbendazim residues were present in treated leaves for 14 days. Our study findings provide a method for improving jujube leaf tea quality from a pesticide utilization perspective.

Characteristics, Roles and Applications of Proteinaceous Elicitors from Pathogens in Plant Immunity

In interactions between pathogens and plants, pathogens secrete many molecules that facilitate plant infection, and some of these compounds are recognized by plant pattern recognition receptors (PRRs), which induce immune responses. Molecules in both pathogens and plants that trigger immune responses in plants are termed elicitors. On the basis of their chemical content, elicitors can be classified into carbohydrates, lipopeptides, proteinaceous compounds and other types. Although many studies have focused on the involvement of elicitors in plants, especially on pathophysiological changes induced by elicitors in plants and the mechanisms mediating these changes, there is a lack of up-to-date reviews on the characteristics and functions of proteinaceous elicitors. In this mini-review, we provide an overview of the up-to-date knowledge on several important families of pathogenic proteinaceous elicitors (i.e., harpins, necrosis- and ethylene-inducing peptide 1 (nep1)-like proteins (NLPs) and elicitins), focusing mainly on their structures, characteristics and effects on plants, specifically on their roles in plant immune responses. A solid understanding of elicitors may be helpful to decrease the use of agrochemicals in agriculture and gardening, generate more resistant germplasms and increase crop yields.

Exogenously applied melatonin alleviates the damage in cucumber plants caused by Aphis goosypii through altering the insect behavior and inducing host plant resistance

BACKGROUND

Aphis gossypii Glover is the main pest found in most cucumber-producing areas. Melatonin (MT) has been widely studied in protecting plants from environmental stresses and pathogens. However, little knowledge is available on the impact of MT on insect resistance.

RESULTS

The fecundity of aphids on MT-treated cucumber leaves was inhibited. Interestingly, MT-treated plants were more attractive to aphids, which would prevent the large-scale transmission of viruses caused by the random movement of aphids. Meanwhile, MT caused varying degrees of change in enzyme activities related to methylesterified HG degradation, antioxidants, defense systems and membrane lipid peroxidation. Furthermore, transcriptomic analysis showed that MT induced 2360 differentially expressed genes (DEGs) compared with the control before aphid infection. These DEGs mainly were enriched in hormone signal transduction, MAPK signaling pathway, and plant-pathogen interaction, revealing that MT can help plants acquire inducible resistance and enhance plant immunity. Subsequently, 2397 DEGs were identified after aphid infection. Further analysis showed that MT-treated plants possessed stronger JA signal, reactive oxygen species stability, and the ability of flavonoid synthesis under aphid infection, while mediating plant growth and sucrose metabolism.

CONCLUSION

In summary, MT as an environmentally friendly substance mitigated aphid damage to cucumbers by affecting the aphids themselves and enhancing plant resistance. This will facilitate exploring sustainable MT-based strategies for cucumber aphid control. © 2022 Society of Chemical Industry.

Identifying Soybean Pod Borer (Leguminivora glycinivorella) Resistance QTLs and the Mechanism of Induced Defense Using Linkage Mapping and RNA-Seq Analysis

The soybean pod borer (Leguminivora glycinivorella) (SPB) is a major cause of soybean (Glycine max L.) yield losses in northeast Asia, thus it is desirable to elucidate the resistance mechanisms involved in soybean response to the SPB. However, few studies have mapped SPB-resistant quantitative trait loci (QTLs) and deciphered the response mechanism in soybean. Here, we selected two soybean varieties, JY93 (SPB-resistant) and K6 (SPB-sensitive), to construct F₂ and F_2:3 populations for QTL mapping and collected pod shells before and after SPB larvae chewed on the two parents to perform RNA-Seq, which can identify stable QTLs and explore the response mechanism of soybean to the SPB. The results show that four QTLs underlying SPB damage to seeds were detected on chromosomes 4, 9, 13, and 15. Among them, qESP-9-1 was scanned in all environments, hence it can be considered a stable QTL. All QTLs explained 0.79 to 6.09% of the phenotypic variation. Meanwhile, 2298 and 3509 DEGs were identified for JY93 and K6, respectively, after the SPB attack, and most of these genes were upregulated. Gene Ontology enrichment results indicated that the SPB-induced and differently expressed genes in both parents are involved in biological processes such as wound response, signal transduction, immune response, and phytohormone pathways. Interestingly, secondary metabolic processes such as flavonoid synthesis were only significantly enriched in the upregulated genes of JY93 after SPB chewing compared with K6. Finally, we identified 18 candidate genes related to soybean pod borer resistance through the integration of QTL mapping and RNA-Seq analysis. Seven of these genes had similar expression patterns to the mapping parents in four additional soybean germplasm after feeding by the SPB. These results provide additional knowledge of the early response and induced defense mechanisms against the SPB in soybean, which could help in breeding SPB-resistant soybean accessions.

Effects of Harpin and Flg22 on Growth Enhancement and Pathogen Defense in Cannabis sativa Seedlings

Pathogen-associated molecular patterns, PAMPs, are a diverse group of molecules associated with pathogenic microbes and are known to activate immune response and in some cases enhance growth in plants. Two PAMPs, harpin and flg22, have shown these affects in various plant species. PAMPs are known to activate basal immunity, the ethylene signaling pathway, alter gene expression and change plant composition. Pretreatment with harpin enhanced hemp seedling resistance to Pythium aphanidermatum, while flg22 failed to induce the defense mechanism towards P. aphanidermatum. In the absence of the pathogen, both harpin and flg22 enhanced seedling growth when compared to the water control. Ethylene is a hormone involved in both plant defense signaling and growth. Both harpin and flg22 pretreatment induced certain ethylene responsive genes but not all the genes examined, indicating that harpin and flg22 act differently in ethylene and potentially defense signaling. In addition, both harpin and flg22 induced CsFRK1 and CsPR1, two marker genes for plant innate immunity. Both PAMPs can enhance growth but likely induce different defense signaling pathways.

Efficient expression and purification of soluble HarpinEa protein by translation initiation region codon optimization

HarpinEa protein can stimulate plants to produce defense responses to resist the attack of pathogens, improve plant immune resistance, and promote plant growth. This has extremely high application value in agriculture. To efficiently express soluble HarpinEa protein, in this study, we expressed HarpinEa protein with a 6× His-tag in Escherichia coli BL21 (DE3). Because of the low level of expression of HarpinEa protein in E. coli, three rounds of synonymous codon optimization were performed on the +53 bp of the translation initiation region (TIR) of HarpinEa. Soluble HarpinEa protein after optimization accounted for 50.3% of the total soluble cellular protein expressed. After purification using a Ni Bestarose Fast Flow column, the purity of HarpinEa protein exceeded 95%, and the yield reached 227.5 mg/L of culture medium. The purified HarpinEa protein was sensitive to proteases and exhibited thermal stability. It triggered visible hypersensitive responses after being injected into tobacco leaves for 48 h. Plants treated with HarpinEa showed obvious growth-promoting and resistance-improving performance. Thus, the use of TIR synonymous codon optimization successfully achieved the economical, efficient, and soluble production of HarpinEa protein.

A salicylic acid inducible mulberry WRKY transcription factor, MiWRKY53 is involved in plant defence response

MiWRKY53 is expressed in response to various stresses and hormones. Although it is localized in the nucleus, it shows no transcriptional activation. Role of SA-mediated plant defence response is demonstrated. WRKY transcription factors are one the largest gene families in plants involved in almost every process in plants including development, physiological processes, and stress response. Salicylic acid (SA) is key regulator of biotic stress against various pathogens in plants acting via its multiple mechanisms to induce defence response. Herein, we have identified and functionally validated WRKY53 from mulberry (Morus indica var. K2). MiWRKY53 expressed differentially in response to different stress and hormonal treatments. MiWRKY53 belongs to group III of WKRY gene family, localized in nucleus, and lacks transcriptional activation activity in yeast. Hormone responsive behaviour of MiWRKY53 Arabidopsis overexpression (OE) transgenics preferentially was noted in root growth assay in response to Salicylic acid (SA). Arabidopsis overexpression plants also displayed alteration in leaf phenotype having wider leaves than the wild-type plants. PR-1 transcripts were higher in MiWRKY53 Arabidopsis OE plants and they displayed resistance towards biotrophic pathogen Pseudomonas syringae PstDC3000. MiWRKY53 Mulberry OE transgenics also depicted SA-responsive behaviour. Several hormones and stress-related cis-acting elements were also identified in the 1.2-Kb upstream regulatory region (URR) of MiWRKY53. Functional characterization of full-length promoter region revealed that it is induced by SA and further analysis of deletion constructs helped in the identification of minimal promoter responsible for its inducibility by SA. Altogether, the findings from this study point towards the SA preferential behaviour of MiWRKY53 and its function as regulator of plant defence response through SA-mediated mechanisms.

Rice aquaporin OsPIP2;2 is a water-transporting facilitator in relevance to drought-tolerant responses

In rice (Oryza sativa), the PLASMA MEMBRANE INTRINSIC PROTEIN (PIP) family of aquaporin has 11 members, OsPIP1;1 to OsPIP1;3, and OsPIP2;1 to OsPIP2;8, which are hypothesized to facilitate the transport of H2O and other small compounds across cell membranes. To date, however, only OsPIP1;2, OsPIP2;1, and OsPIP2;4 have been demonstrated for substrate selectivity in their source plant (rice). In this study, OsPIP2;2 was characterized as the most efficient facilitator of H2O transport across cell membranes in comparison with the other 10 OsPIPs. In concomitant tests of all OsPIPs, four genes (OsPIP1;3, OsPIP2;1, OsPIP2;2, and OsPIP2;4) were induced to express in leaves of rice plants following a physiological drought stress, while OsPIP2;2 was expressed to the highest level. After de novo expression in frog oocytes and yeast cells, the four OsPIP proteins were localized to the plasma membranes in trimer and tetramer and displayed the activity to increase the membrane permeability to H2O. In comparison, OsPIP2;2 was most supportive to H2O import to oocytes and yeast cells. After de novo expression in tobacco protoplasts, OsPIP2;2 exceeded OsPIP1;3, OsPIP2;1, and OsPIP2;4 to support H2O transport across the plasma membranes. OsPIP2;2-mediated H2O transport was accompanied by drought-tolerant responses, including increases in concentrations of proline and polyamines, both of which are physiological markers of drought tolerance. In rice protoplasts, H2O transport and drought-tolerant responses, which included expression of marker genes of drought tolerance pathway, were considerably enhanced by OsPIP2;2 overexpression but strongly inhibited by the gene silencing. Furthermore, OsPIP2;2 played a role in maintenance of the cell membrane integrity and effectively protected rice cells from electrolyte leakage caused by the physiological drought stress. These results suggest that OsPIP2;2 is a predominant facilitator of H2O transport in relevance to drought tolerance in the plant.

Response of hrpZPsph-transgenic N. benthamiana plants under cadmium stress

The hrpZPsph gene from Pseudomonas syringae pv. phaseolicola, in its secretable form (SP/hrpZPsph), has previously proven capable of conferring resistance against rhizomania disease as well as abiotic stresses in Nicotiana benthamiana plants, while enhancing plant growth. This study aimed at investigating the response of SP/hrpZPsph-expressing plants under cadmium stress. Transgenic N. benthamiana lines, homozygous for the SP/hrpZPsph gene, and wild-type plants were exposed to Cd at different stress levels (0, 50, 100, 150 μΜ CdCl2). Plants' response to stress was assessed at germination and at the whole plant level on the basis of physiological and growth parameters, including seed germination percentage, shoot and root length, total chlorophyll content, fresh and dry root weight, as well as overall symptomatology, and Cd content in leaves and roots. At germination phase, significant differences were noted in germination rates and post-germination growth among stress levels, with Cd effects being in most cases analogous to the level applied but also among plant categories. Although seedling growth was adversely affected in all plant categories, especially at high stress level, lines #6 and #9 showed the lowest decrease in root and shoot length over control. The superiority of these lines was further manifested at the whole plant level by the absence of stress-attributed symptoms and the low or zero reduction in chlorophyll content. Interestingly, a differential tissue-specific Cd accumulation pattern was observed in wt- and hrpZPsph-plants, with the former showing an increased Cd content in leaves and the latter retaining Cd in the roots. These data are discussed in the context of possible mechanisms underlying the hrpZPsph-based Cd stress resistance.

Coiled-Coil N21 of Hpa1 in Xanthomonas oryzae pv. oryzae Promotes Plant Growth, Disease Resistance and Drought Tolerance in Non-Hosts via Eliciting HR and Regulation of Multiple Defense Response Genes

Acting as a typical harpin protein, Hpa1 of Xanthomonas oryzae pv. oryzae is one of the pathogenic factors in hosts and can elicit hypersensitive responses (HR) in non-hosts. To further explain the underlying mechanisms of its induced resistance, we studied the function of the most stable and shortest three heptads in the N-terminal coiled-coil domain of Hpa1, named N21Hpa1. Proteins isolated from N21-transgenic tobacco elicited HR in Xanthi tobacco, which was consistent with the results using N21 and full-length Hpa1 proteins expressed in Escherichia coli. N21-expressing tobacco plants showed enhanced resistance to tobacco mosaic virus (TMV) and Pectobacterium carotovora subsp. carotovora (Pcc). Spraying of a synthesized N21 peptide solution delayed the disease symptoms caused by Botrytis cinerea and Monilinia fructicola and promoted the growth and drought tolerance of plants. Further analysis indicated that N21 upregulated the expression of multiple plant defense-related genes, such as genes mediated by salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) signaling, and genes related to reactive oxygen species (ROS) biosynthesis. Further, the bioavailability of N21 peptide was better than that of full-length Hpa1Xoo. Our studies support the broad application prospects of N21 peptide as a promising succedaneum to biopesticide Messenger or Illite or other biological pharmaceutical products, and provide a basis for further development of biopesticides using proteins with similar structures.

The stability of the coiled-coil structure near to N-terminus influence the heat resistance of harpin proteins from Xanthomonas

BACKGROUND

Heat resistance is a common characteristic of harpins, a class of proteins found in Gram-negative bacteria, which may be related to the stability of coiled-coil (CC) structure. The CC structure is a ubiquitous protein folding and assembly motif made of α-helices wrapping around each other forming a supercoil. Specifically, whether the stability of the CC structure near to N-terminus of four selected harpin proteins from Xanthomonas (hereafter referred to as Hpa1) would influence their characteristics of heat resistance was investigated. We used bioinformatics approach to predict the structure of Hpa1, used the performance of hypersensitive response (HR)-induction activity of Hpa1 and circular dichroism (CD) spectral analyses to detect the relationship between the stability of the CC structure of Hpa1 and heat resistance.

RESULTS

Each of four-selected Hpa1 has two α-helical regions with one in their N-terminus that could form CC structure, and the other in their C-terminus that could not. And the important amino acid residues involved in the CC motifs are located on helices present on the surface of these proteins, indicating they may engage in the formation of oligo mericaggregates, which may be responsible for HR elicitation by harpins and their high thermal stability. Increased or decreased the probability of forming a CC could either induce a stronger HR response or eliminate the ability to induce HR in tobacco after high temperature treatment. In addition, although the four Hpa1 mutants had little effect on the induction of HR by Hpa1, its thermal stability was significantly decreased. The α-helical content increased with increasing temperature, and the secondary structures of Hpa1 became almost entirely α-helices when the temperature reached 200 °C. Moreover, the stability of the CC structure near to N-terminus was found to be positively correlated with the heat resistance of Hpa1.

CONCLUSIONS

The stability of the CC structure might sever as an inner drive for mediating the heat resistance of harpin proteins. Our results offer a new insight into the interpretation of the mechanism involved in the heat resistance of harpin protein and provide a theoretical basis for further harpin function investigations and structure modifications.

WY7 is a newly identified promoter from the rubber powdery mildew pathogen that regulates exogenous gene expression in both monocots and dicots

Promoters are very important for transcriptional regulation and gene expression, and have become invaluable tools for genetic engineering. Owing to the characteristics of obligate biotrophs, molecular research into obligate biotrophic fungi is seriously lagging behind, and very few of their endogenous promoters have been developed. In this study, a WY7 fragment was predicted in the genome of Oidium heveae Steinmann using PromoterScan. Its promoter function was verified with transient transformations (Agrobacterium tumefaciens-mediated transformation, ATMT) in Nicotiana tabacum cv. Xanthi nc. The analysis of the transcription range showed that WY7 could regulate GUS expression in both monocots (Zea mays Linn and Oryza sativa L. spp. Japonica cv. Nipponbare) and dicots (N. tabacum and Hylocereus undulates Britt). The results of the quantitative detection showed that the GUS transient expression levels when regulated by WY7 was more than 11.7 times that of the CaMV 35S promoter in dicots (N. tabacum) and 5.13 times that of the ACT1 promoter in monocots (O. sativa). GUS staining was not detected in the T1 generation of the WY7-GUS transgenic N. tabacum. This showed that WY7 is an inducible promoter. The cis elements of WY7 were predicted using PlantCARE, and further experiments indicated that WY7 was a low temperature- and salt-inducible promoter. Soluble proteins produced by WY7-hpa1Xoo transgenic tobacco elicited hypersensitive responses (HR) in N. tabacum leaves. N. tabacum transformed with pBI121-WY7-hpa1Xoo exhibited enhanced resistance to the tobacco mosaic virus (TMV). The WY7 promoter has a lot of potential as a tool for plant genetic engineering. Further in-depth studies will help to better understand the transcriptional regulation mechanisms of O. heveae.

Identification of potential candidate genes controlling pea aphid tolerance in a Pisum fulvum high-density integrated DArTseq SNP-based genetic map

BACKGROUND

Pea (Pisum sativum) is one of the most important temperate grain legumes in the world, and its production is severely constrained by the pea aphid (Acyrthosiphon pisum). Wild relatives, such as P. fulvum, are valuable sources of allelic diversity to improve the genetic resistance of cultivated pea species against A. pisum attack. To unravel the genetic control underlying resistance to the pea aphid attack, a quantitative trait loci (QTL) analysis was performed using the previously developed high density integrated genetic linkage map originated from an intraspecific recombinant inbred line (RIL) population (P. fulvum: IFPI3260 × IFPI3251).

RESULTS

We accurately evaluated specific resistance responses to pea aphid that allowed the identification, for the first time, of genomic regions that control plant damage and aphid reproduction. Eight QTLs associated with tolerance to pea aphid were identified in LGs I, II, III, IV and V, which individually explained from 17.0% to 51.2% of the phenotypic variation depending on the trait scored, and as a whole from 17.0% to 88.6%. The high density integrated genetic linkage map also allowed the identification of potential candidate genes co-located with the QTLs identified.

CONCLUSIONS

Our work shows how the survival of P. fulvum after the pea aphid attack depends on the triggering of a multi-component protection strategy that implies a quantitative tolerance. The genomic regions associated with the tolerance responses of P. fulvum during A. pisum infestation have provided six potential candidate genes that could be useful in marker-assisted selection (MAS) and genomic assisted breeding (GAB) after functional validation in the future. © 2019 Society of Chemical Industry.

Ralstonia solanacearum elicitor RipX Induces Defense Reaction by Suppressing the Mitochondrial atpA Gene in Host Plant

RipX of Ralstonia solanacearum is translocated into host cells by a type III secretion system and acts as a harpin-like protein to induce a hypersensitive response in tobacco plants. The molecular events in association with RipX-induced signaling transduction have not been fully elucidated. This work reports that transient expression of RipX induced a yellowing phenotype in Nicotiana benthamiana, coupled with activation of the defense reaction. Using yeast two-hybrid and split-luciferase complementation assays, mitochondrial ATP synthase F1 subunit α (ATPA) was identified as an interaction partner of RipX from N. benthamiana. Although a certain proportion was found in mitochondria, the YFP-ATPA fusion was able to localize to the cell membrane, cytoplasm, and nucleus. RFP-RipX fusion was found from the cell membrane and cytoplasm. Moreover, ATPA interacted with RipX at both the cell membrane and cytoplasm in vivo. Silencing of the atpA gene had no effect on the appearance of yellowing phenotype induced by RipX. However, the silenced plants improved the resistance to R. solanacearum. Moreover, qRT-PCR and promoter GUS fusion experiments revealed that the transcript levels of atpA were evidently reduced in response to expression of RipX. These data demonstrated that RipX exerts a suppressive effect on the transcription of atpA gene, to induce defense reaction in N. benthamiana.

HpaXpm, a novel harpin of Xanthomonas phaseoli pv. manihotis, acts as an elicitor with high thermal stability, reduces disease, and promotes plant growth

BACKGROUND

Harpins are proteins secreted by the type III secretion system of Gram-negative bacteria during pathogen-plant interactions that can act as elicitors, stimulating defense and plant growth in many types of non-host plants. Harpin-treated plants have higher resistance, quality and yields and, therefore, harpin proteins may potentially have many valuable agricultural applications. Harpins are characterized by high thermal stability at 100 °C. However, it is unknown whether harpins are still active at temperatures above 100 °C or whether different temperatures affect the activity of the harpin protein in different ways. The mechanism responsible for the heat stability of harpins is also unknown.

RESULTS

We identified a novel harpin, HpaXpm, from the cassava blight bacteria Xanthomonas phaseoli pv. manihotis HNHK. The predicted secondary structure and 3-D structure indicated that the HpaXpm protein has two β-strand domains and two major α-helical domains located at the N- and C-terminal regions, respectively. A phylogenetic tree generated using the maximum likelihood method grouped HpaXpm in clade I of the Hpa1 group along with harpins produced by other Xanthomonas spp. (i.e., HpaG-Xag, HpaG-Xcm, Hpa1-Xac, and Hpa1Xm). Phenotypic assays showed that HpaXpm induced the hypersensitive response (HR), defense responses, and growth promotion in non-host plants more effectively than Hp1Xoo (X. oryzae pv. oryzae). Quantitative real-time PCR analysis indicated that HpaXpm proteins subjected to heat treatments at 100 °C, 150 °C, or 200 °C were still able to stimulate the expression of function-related genes (i.e., the HR marker genes Hin1 and Hsr203J, the defense-related gene NPR1, and the plant growth enhancement-related gene NtEXP6); however, the ability of heat-treated HpaXpm to induce HR was different at different temperatures.

CONCLUSIONS

These findings add a new member to the harpin family. HpaXpm is heat-stable up to 200 °C and is able to stimulate powerful beneficial biological functions that could potentially be more valuable for agricultural applications than those stimulated by Hpa1Xoo. We hypothesize that the extreme heat resistance of HpaXpm is because the structure of harpin is very stable and, therefore, the HpaXpm structure is less affected by temperature.

Trichoderma harzianum- and Methyl Jasmonate-Induced Resistance to Bipolaris sorokiniana Through Enhanced Phenylpropanoid Activities in Bread Wheat (Triticum aestivum L.)

The aim of the present study was to evaluate the impact of Trichoderma harzianum UBSTH-501- and methyl jasmonate-induced systemic resistance and their integration on the spot blotch pathogen, Bipolaris sorokiniana through enhanced phenylpropanoid activities in bread wheat (Triticum aestivum L.). It was found that the application of MeJA (>100 mg L^-1) inhibits the germination of B. sorokiniana spores under controlled laboratory conditions. To assess the effect of MeJA (150 mg L^-1) in combination with the biocontrol agent T. harzianum UBSTH-501 in vivo, a green house experiment was conducted. For this, biocontrol agent T. harzianum UBSTH-501 was applied as seed treatment, whereas MeJA (150 mg L^-1) was applied 5 days prior to pathogen inoculation. Results indicated that application of MeJA (150 mg L^-1) did not affect the root colonization of wheat by T. harzianum UBSTH-501 in the rhizosphere. The combined application of T. harzianum UBSTH-501 and MeJA also enhanced indole acetic acid production in the rhizosphere (4.92 μg g^-1 of soil) which in turn helps in plant growth and development. Further, the combined application found to enhance the activities of defense related enzymes viz. catalase (5.92 EU min^-1 g^-1 fresh wt.), ascorbate peroxidase [μmol ascorbate oxidized (mg prot)^-1 min^-1], phenylalanine ammonia lyase (102.25 μmol cinnamic acid h^-1 mg^-1 fresh wt.) and peroxidase (6.95 Unit mg^-1 min^-1 fresh wt.) significantly in the plants under treatment which was further confirmed by assessing the transcript level of PAL and peroxidase genes using semi-quantitative PCR approach. The results showed manifold increase in salicylic acid (SA) along with enhanced accumulation of total free phenolics, ferulic acid, caffeic acid, coumaric acid, and chlorogenic acid in the leaves of the plants treated with the biocontrol agent alone or in combination with MeJA. A significant decrease in the disease severity (17.46%) and area under disease progress curve (630.32) were also observed in the treatments with biocontrol agent and MeJA in combination as compared to B. sorokiniana alone treated plant (56.95% and 945.50, respectively). Up-regulation of phenylpropanoid cascades in response to exogenous application of MeJA and the biocontrol agent was observed. It was depicted from the results that PAL is the primary route for lignin production in wheat which reduces cell wall disruption and tissue disintegration and increases suberization and lignification of the plant cell as seen by Scanning Electron microphotographs. These results clearly indicated that exogenous application of MeJA with T. harzianum inducing JA- and/or SA-dependent defense signaling after pathogen challenge may increase the resistance to spot blotch by stimulating enzymatic activities and the accumulation of phenolic compounds in a cooperative manner. This study apparently provides the evidence of biochemical cross-talk and physiological responses in wheat following MeJA and biocontrol agent treatment during the bio-trophic infection.

The protein elicitor Hrip1 enhances resistance to insects and early bolting and flowering in Arabidopsis thaliana

The elicitor Hrip1 isolated from necrotrophic fungus Alternaria tenuissima, could induce systemic acquired resistance in tobacco to enhance resistance to tobacco mosaic virus. In the present study, we found that the transgenic lines of Hrip1-overexpression in wild type (WT) Arabidopsis thaliana were more resistant to Spodoptera exigua and were early bolting and flowering than the WT. A profiling of transcription assay using digital gene expression profiling was used for transgenic and WT Arabidopsis thaliana. Differentially expressed genes including 40 upregulated and three downregulated genes were identified. In transgenic lines of Hrip1-overexpression, three genes related to jasmonate (JA) biosynthesis were significantly upregulated, and the JA level was found to be higher than WT. Two GDSL family members (GLIP1 and GLIP4) and pathogen-related gene, which participated in pathogen defense action, were upregulated in the transgenic line of Hrip1-overexpression. Thus, Hrip1 is involved in affecting the flower bolting time and regulating endogenous JA biosynthesis and regulatory network to enhance resistance to insect.

Introduction of the harpinXooc-encoding gene hrf2 in soybean enhances resistance against the oomycete pathogen Phytophthora sojae

Phytophthora root and stem rot (PRR) caused by an oomycete pathogen Phytophthora sojae is one of the most devastating and widespread diseases throughout soybean-producing regions worldwide. The diversity and variability of P. sojae races make effective control of the pathogen challenging. Here, we introduced an elicitor of plant defense response, the harpinXooc-encoding hrf2 gene from the rice bacterial pathogen Xanthomonas oryzae pv. oryzicola into soybean and evaluated resistance to P. sojae infection. Molecular analysis confirmed the integration and expression of hrf2 in the transgenic soybean. After inoculation with P. sojae, non-transformed control (NC) plants exhibited typical PRR symptoms, including necrotic and wilting leaves, and plant death, whereas most of the transgenic plants showed slightly chlorotic leaves and developed normally. Through T3 to T5 generations, the transgenic events displayed milder disease symptoms and had higher survival rates compared to NC plants, indicating enhanced and stable resistance to P. sojae infection, whereas without P. sojae inoculation, no significant differences in agronomic traits were observed between the transgenic and non-transformed plants. Moreover, after inoculation with P. sojae, significant upregulation of a set of plant defense-related genes, including salicylic acid- and jasmonic acid-dependent and hypersensitive response-related genes was observed in the transgenic plants. Our results indicate that hrf2 expression in transgenic soybean significantly enhanced resistance to P. sojae by eliciting multiple defense responses mediated by different signaling pathways. The potential functional role of the hrf2 gene in plant defense against P. sojae and other pathogens makes it a promising tool for broadening disease resistance in soybean.

High temperatures affect the hypersensitive reaction, disease resistance and gene expression induced by a novel harpin HpaG-Xcm

Harpin proteins are produced by plant-pathogenic Gram-negative bacteria and regulate bacterial pathogenicity by inducing plant growth and defence responses in non-hosts. HpaG-Xcm, a novel harpin protein, was identified from Xanthomonas citri pv. mangiferaeindicae, which causes bacterial black spot of mango. Here, we describe the predicted structure and functions of HpaG-Xcm and investigate the mechanism of heat resistance. The HpaG-Xcm amino acid sequence contains seven motifs and two α-helices, in the N- and C-terminals, respectively. The N-terminal α-helical region contains two heptads, which form the coiled-coil (CC) structure. The CC region, which is on the surface of HpaG-Xcm, forms oligomeric aggregates by forming hydrophobic interactions between hydrophobic amino acids. Like other harpins, HpaG-Xcm was heat stable, promoted root growth and induced a hypersensitive response (HR) and systemic acquired resistance in non-host plants. Subjecting HpaG-Xcm to high temperatures altered the gene expression induced by HpaG-Xcm in tobacco leaves, probably due to changes in the spatial structure of HpaG-Xcm. Phenotypic tests revealed that the high-temperature treatments reduced the HR and disease resistance induced by HpaG-Xcm but had little effect on growth promotion. These findings indicate that the stability of interactions between CC and plants may be associated with thermal stability of HpaG-Xcm.

Genetic Transformation of Cotton with the Harpin-Encoding Gene hpaXoo of Xanthomonas oryzae pv. oryzae and Evaluation of Resistance Against Verticillium Wilt

The soilborne fungal pathogen Verticillium dahliae Kleb causes Verticillium wilt in a wide range of crops including cotton (Gossypium hirsutum). To date, most upland cotton varieties are susceptible to V. dahliae, and the breeding for cotton varieties with the resistance to Verticillium wilt has not been successful. Hpa1Xoo is a harpin protein from Xanthomonas oryzae pv. oryzae which induces the hypersensitive cell death in plants. When hpa1Xoo was transformed into the susceptible cotton line Z35 through Agrobacterium-mediated transformation, the transgenic cotton line (T-34) with an improved resistance to Verticillium dahliae was obtained. Here, we describe the related research approach, such as Western blot, Southern blot, immuno-gold labeling, evaluation of resistance to Verticillium dahliae, and how to detect the micro-hypersensitive response and oxidative burst elicited by harpin_Xoo in plant tissue.

Quantitative Proteomics Analysis of Lettuce (Lactuca sativa L.) Reveals Molecular Basis-Associated Auxin and Photosynthesis with Bolting Induced by High Temperature

Bolting is a key process in the growth and development of lettuce (Lactuca sativa L.). A high temperature can induce early bolting, which decreases both the quality and production of lettuce. However, knowledge of underlying lettuce bolting is still lacking. To better understand the molecular basis of bolting, a comparative proteomics analysis was conducted on lettuce stems, during the bolting period induced by a high temperature (33 °C) and a control temperature (20 °C) using iTRAQ-based proteomics, phenotypic measures, and biological verifications using qRT-PCR and Western blot. The high temperature induced lettuce bolting, while the control temperature did not. Of the 5454 identified proteins, 619 proteins presented differential abundance induced by high-temperature relative to the control group, of which 345 had an increased abundance and 274 had a decreased abundance. Proteins with an abundance level change were mainly enriched in pathways associated with photosynthesis and tryptophan metabolism involved in auxin (IAA) biosynthesis. Moreover, among the proteins with differential abundance, proteins associated with photosynthesis and tryptophan metabolism were increased. These findings indicate that a high temperature enhances the function of photosynthesis and IAA biosynthesis to promote the process of bolting, which is in line with the physiology and transcription level of IAA metabolism. Our data provide a first comprehensive dataset for gaining novel understanding of the molecular basis underlying lettuce bolting induced by high temperature. It is potentially important for further functional analysis and genetic manipulation for molecular breeding to breed new cultivars of lettuce to restrain early bolting, which is vital for improving vegetable quality.

Harpin-inducible defense signaling components impair infection by the ascomycete Macrophomina phaseolina

Soybean (Glycine max) infection by the charcoal rot (CR) ascomycete Macrophomina phaseolina is enhanced by the soybean cyst nematode (SCN) Heterodera glycines. We hypothesized that G. max genetic lines impairing infection by M. phaseolina would also limit H. glycines parasitism, leading to resistance. As a part of this M. phaseolina resistance process, the genetic line would express defense genes already proven to impair nematode parasitism. Using G. max[DT97-4290/PI 642055], exhibiting partial resistance to M. phaseolina, experiments show the genetic line also impairs H. glycines parasitism. Furthermore, comparative studies show G. max[DT97-4290/PI 642055] exhibits induced expression of the effector triggered immunity (ETI) gene NON-RACE SPECIFIC DISEASE RESISTANCE 1/HARPIN INDUCED1 (NDR1/HIN1) that functions in defense to H. glycines as compared to the H. glycines and M. phaseolina susceptible line G. max[Williams 82/PI 518671]. Other defense genes that are induced in G. max[DT97-4290/PI 642055] include the pathogen associated molecular pattern (PAMP) triggered immunity (PTI) genes ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), NONEXPRESSOR OF PR1 (NPR1) and TGA2. These observations link G. max defense processes that impede H. glycines parasitism to also potentially function toward impairing M. phaseolina pathogenicity. Testing this hypothesis, G. max[Williams 82/PI 518671] genetically engineered to experimentally induce GmNDR1-1, EDS1-2, NPR1-2 and TGA2-1 expression leads to impaired M. phaseolina pathogenicity. In contrast, G. max[DT97-4290/PI 642055] engineered to experimentally suppress the expression of GmNDR1-1, EDS1-2, NPR1-2 and TGA2-1 by RNA interference (RNAi) enhances M. phaseolina pathogenicity. The results show components of PTI and ETI impair both nematode and M. phaseolina pathogenicity.

Overexpression of SSBXoc, a Single-Stranded DNA-Binding Protein From Xanthomonas oryzae pv. oryzicola, Enhances Plant Growth and Disease and Salt Stress Tolerance in Transgenic Nicotiana benthamiana

We previously reported that SSBXoc, a highly conserved single-stranded DNA-binding protein from Xanthomonas spp., was secreted through the type III secretion system (T3SS) and functioned as a harpin-like protein to elicit the hypersensitive response (HR) in the non-host plant, tobacco. In this study, we cloned SsbXoc gene from X. oryzae pv. oryzicola (Xoc), the causal agent of bacterial leaf streak in rice, and transferred it into Nicotiana benthamiana via Agrobacterium-mediated transformation. The expression of SsbXoc in transgenic N. benthamiana enhanced growth of both seedling and adult plants. When inoculated with the harpin Hpa1 or the pathogen Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), the accumulation of reactive oxygen species (ROS) was increased more in SsbXoc transgenic lines than that in wild-type (WT) plants. The expression of pathogenesis-related protein genes (PR1a and SGT1), HR marker genes (HIN1 and HSR203J) and the mitogen-activated protein kinase pathway gene, MPK3, was significantly higher in transgenic lines than in WT after inoculation with Pst DC3000. In addition, SsbXoc transgenic lines showed the enhanced resistance to the pathogenic bacteria P. s. tabaci and the improved tolerance to salt stress, accompanied by the elevated transcription levels of the defense- and stress-related genes. Taken together, these results indicate that overexpression of the SsbXoc gene in N. benthamiana significantly enhanced plant growth and increased tolerance to disease and salt stress via modulating the expression of the related genes, thus providing an alternative approach for development of plants with improved tolerance against biotic and abiotic stresses.

Over-expression of the Pseudomonas syringae harpin-encoding gene hrpZm confers enhanced tolerance to Phytophthora root and stem rot in transgenic soybean

Phytophthora root and stem rot (PRR) caused by Phytophthora sojae is one of the most devastating diseases reducing soybean (Glycine max) production all over the world. Harpin proteins in many plant pathogenic bacteria were confirmed to enhance disease and insect resistance in crop plants. Here, a harpin protein-encoding gene hrpZpsta from the P. syringae pv. tabaci strain Psta218 was codon-optimized (renamed hrpZm) and introduced into soybean cultivars Williams 82 and Shennong 9 by Agrobacterium-mediated transformation. Three independent transgenic lines over-expressing hrpZm were obtained and exhibited stable and enhanced tolerance to P. sojae infection in T2-T4 generations compared to the non-transformed (NT) and empty vector (EV)-transformed plants. Quantitative real-time PCR (qRT-PCR) analysis revealed that the expression of salicylic acid-dependent genes PR1, PR12, and PAL, jasmonic acid-dependent gene PPO, and hypersensitive response (HR)-related genes GmNPR1 and RAR was significantly up-regulated after P. sojae inoculation. Moreover, the activities of defense-related enzymes such as phenylalanine ammonia lyase (PAL), polyphenoloxidase (PPO), peroxidase, and superoxide dismutase also increased significantly in the transgenic lines compared to the NT and EV-transformed plants after inoculation. Our results suggest that over-expression of the hrpZm gene significantly enhances PRR tolerance in soybean by eliciting resistance responses mediated by multiple defense signaling pathways, thus providing an alternative approach for development of soybean varieties with improved tolerance against the soil-borne pathogen PRR.

Transgenic expression of plant-specific insert of potato aspartic proteases (StAP-PSI) confers enhanced resistance to Botrytis cinerea in Arabidopsis thaliana

The plant-specific insert of Solanum tuberosum aspartic proteases (StAP-PSI) has high structural similarity with NK-lysin and granulysin, two saposin-like proteins (SAPLIPs) with antimicrobial activity. Recombinant StAP-PSI and some SAPLIPs show antimicrobial activity against pathogens that affect human and plants. In this work, we transformed Arabidopsis thaliana plants with StAP-PSI encoding sequence with its corresponding signal peptide under the control of the cauliflower mosaic virus (CaMV) 35S promoter. Results obtained show that StAP-PSI significantly enhances Arabidopsis resistance against Botrytis cinerea infection. StAP-PSI is secreted into the leaf apoplast and acts directly against pathogens; thereby complementing plant innate immune responses. Data obtained from real-time PCR assays show that the constitutive expression of StAP-PSI induces the expression of genes that regulate jasmonic acid signalling pathway, such as PDF1.2, in response to infection due to necrotrophic pathogens. On the other hand, according to the data described for other antimicrobial peptides, the presence of the StAP-PSI protein in the apoplast of A. thaliana leaves is responsible for the expression of salicylic acid-associated genes, such as PR-1, irrespective of infection with B. cinerea. These results indicate that the increased resistance demonstrated by A. thaliana plants that constitutively express StAP-PSI owing to B. cinerea infection compared to the wild-type plants is a consequence of two factors, i.e., the antifungal activity of StAP-PSI and the overexpression of A. thaliana defense genes induced by the constitutive expression of StAP-PSI. We suggest that the use of this protein would help in minimizing the ecological and health risks that arise from the use of pesticides. We suggest that the use of this protein would help in minimizing the ecological and health risks that arise from the spreading of resistance of agriculturally important pathogens.

Functional regions of HpaXm as elicitors with specific heat tolerance induce the hypersensitive response or plant growth promotion in nonhost plants

HpaXm produced by the cotton leaf blight bacterium Xanthomonas citri subsp. malvacearum is a novel harpin elicitor of the induced hypersensitive response (HR) in tobacco. We investigated whether fragments of HpaXm, compared with fragments of Hpa1Xoo, are sufficient for HR or plant growth promotion (PGP) elicitation using four synthetic peptides (HpaXm_35-51, HpaXm_10-39, Hpa1Xoo_36-52 and Hpa1Xoo_10-40). We also heated the fragments to determine the heat tolerance of the functional fragments. HpaXm_35-51 and Hpa1Xoo_36-52 induced hypersensitive response (HR). Bursts of reactive oxygen intermediates (ROI) induced by HpaXm_35-51 and Hpa1Xoo_36-52 were earlier and stronger than those induced by HpaXm and Hpa1Xoo. In plants treated with HpaXm_35-51 or Hpa1Xoo_36-52, the expression of the HR marker genes Hin1 and Hsr203J and the active oxygen metabolism related gene AOX were significantly upregulated. These findings suggest that the predicted α-helical structures of the HpaXm_35-51 and Hpa1Xoo_36-52 fragments are crucial for HR. PGP result by soaking seeds in unheated/heated HpaXm_10-39 or Hpa1Xoo_10-40 solution prior to transfer, which obviously enhances root growth and the aerial parts of plants. The PGP related gene NtEXP6 was greatly enhanced when plants were sprayed with a solution of HpaXm_10-39 or Hpa1Xoo_10-40; heated fragment treatments induced higher levels of NtEXP6 expression than unheated HpaXm fragments. In addition, HR marker genes induced by the heated fragments had lower expression levels than when induced with unheated HpaXm fragments. Moreover, the expression levels of HR marker genes and PGP related genes induced by treatment with Hpa1Xoo fragments before or after heating were the opposite of those induced by HpaXm fragments. Different functional fragments of harpin and different harpins with the same functional region have different degrees of heat tolerance. Therefore, the heat resistance of harpin is conservative, but the degree of heat tolerance of the functional fragments is specific.

A harpin elicitor induces the expression of a coiled-coil nucleotide binding leucine rich repeat (CC-NB-LRR) defense signaling gene and others functioning during defense to parasitic nematodes

The bacterial effector harpin induces the transcription of the Arabidopsis thaliana NON-RACE SPECIFIC DISEASE RESISTANCE 1/HARPIN INDUCED1 (NDR1/HIN1) coiled-coil nucleotide binding leucine rich repeat (CC-NB-LRR) defense signaling gene. In Glycine max, Gm-NDR1-1 transcripts have been detected within root cells undergoing a natural resistant reaction to parasitism by the syncytium-forming nematode Heterodera glycines, functioning in the defense response. Expressing Gm-NDR1-1 in Gossypium hirsutum leads to resistance to Meloidogyne incognita parasitism. In experiments presented here, the heterologous expression of Gm-NDR1-1 in G. hirsutum impairs Rotylenchulus reniformis parasitism. These results are consistent with the hypothesis that Gm-NDR1-1 expression functions broadly in generating a defense response. To examine a possible relationship with harpin, G. max plants topically treated with harpin result in induction of the transcription of Gm-NDR1-1. The result indicates the topical treatment of plants with harpin, itself, may lead to impaired nematode parasitism. Topical harpin treatments are shown to impair G. max parasitism by H. glycines, M. incognita and R. reniformis and G. hirsutum parasitism by M. incognita and R. reniformis. How harpin could function in defense has been examined in experiments showing it also induces transcription of G. max homologs of the proven defense genes ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), TGA2, galactinol synthase, reticuline oxidase, xyloglucan endotransglycosylase/hydrolase, alpha soluble N-ethylmaleimide-sensitive fusion protein (α-SNAP) and serine hydroxymethyltransferase (SHMT). In contrast, other defense genes are not directly transcriptionally activated by harpin. The results indicate harpin induces pathogen associated molecular pattern (PAMP) triggered immunity (PTI) and effector-triggered immunity (ETI) defense processes in the root, activating defense to parasitic nematodes.

SG2-Type R2R3-MYB Transcription Factor MYB15 Controls Defense-Induced Lignification and Basal Immunity in Arabidopsis

Lignification of cell wall appositions is a conserved basal defense mechanism in the plant innate immune response. However, the genetic pathway controlling defense-induced lignification remains unknown. Here, we demonstrate the Arabidopsis thaliana SG2-type R2R3-MYB transcription factor MYB15 as a regulator of defense-induced lignification and basal immunity. Loss of MYB15 reduces the content but not the composition of defense-induced lignin, whereas constitutive expression of MYB15 increases lignin content independently of immune activation. Comparative transcriptional and metabolomics analyses implicate MYB15 as necessary for the defense-induced synthesis of guaiacyl lignin and the basal synthesis of the coumarin metabolite scopoletin. MYB15 directly binds to the secondary wall MYB-responsive element consensus sequence, which encompasses the AC elements, to drive lignification. The myb15 and lignin biosynthetic mutants show increased susceptibility to the bacterial pathogen Pseudomonas syringae, consistent with defense-induced lignin having a major role in basal immunity. A scopoletin biosynthetic mutant also shows increased susceptibility independently of immune activation, consistent with a role in preformed defense. Our results support a role for phenylalanine-derived small molecules in preformed and inducible Arabidopsis defense, a role previously dominated by tryptophan-derived small molecules. Understanding the regulatory network linking lignin biosynthesis to plant growth and defense will help lignin engineering efforts to improve the production of biofuels and aromatic industrial products as well as increase disease resistance in energy and agricultural crops.

Three MYB genes co-regulate the phloem-based defence against English grain aphid in wheat

Plant phloem-based defence (PBD) against phloem-feeding insects is characteristic of the sieve occlusion by phloem lectins and β-1,3-glucan callose, both of which are produced under regulation by ethylene and MYB transcription factors. Wheat PBD requires β-1,3-glucan synthase-like proteins GSL2, GSL10, and GSL12, and may also require insect-resistant mannose-binding lectins Hfr-1 and Wci-1, which can accumulate in the phloem upon aphid feeding. This study elucidates whether any of the 73 MYB genes identified previously in the common wheat Triticum aestivum genome plays a role in wheat PBD activation with regard to the GSLs and lectins. Wheat MYB genes TaMYB19, TaMYB29, and TaMYB44 are highly activated in response to infestation of English grain aphid, and their silencing facilitates aphid feeding on wheat phloem and represses wheat PBD responses. Repressed PBD is shown to decrease aphid-induced callose deposition in wheat leaf epidermis and decrease aphid-induced expression of genes GSL2, GSL10, GSL12, Hfr-1, and Wci-1 in wheat leaf tissues. Based on single gene silencing effects, TaMYB19, TaMYB29, and TaMYB44 contribute 55-82% of PBD responses. However, the contributions of TaMYB genes to PBD are eliminated by ethylene signalling inhibitors, while simultaneous silencing of the three TaMYB genes cancels the tested PBD responses. Therefore, TaMYB19, TaMYB29, and TaMYB44 are co-regulators of wheat PBD and execute this function through crosstalk with the ethylene signalling pathway.

Zoospore exudates from Phytophthora nicotianae affect immune responses in Arabidopsis

Zoospore exudates play important roles in promoting zoospore communication, homing and germination during plant infection by Phytophthora. However, it is not clear whether exudates affect plant immunity. Zoospore-free fluid (ZFF) and zoospores of P. nicotianae were investigated comparatively for effects on resistance of Arabidopsis thaliana Col-0 and mutants that affect signaling mediated by salicylic acid (SA) and jasmonic acid (JA): eds16 (enhanced disease susceptibility16), pad4 (phytoalexin deficient4), and npr1 (nonexpressor of pathogenesis-related genes1). Col-0 attracted more zoospores and had severe tissue damage when flooded with a zoospore suspension in ZFF. Mutants treated with ZFF alone developed disease symptoms similar to those inoculated with zoospores and requirements of EDS16 and PAD4 for plant responses to zoospores and the exudates was apparent. Zoospore and ZFFs also induced expression of the PR1 and PDF1.2 marker genes for defense regulated by SA and JA, respectively. However, ZFF affected more JA defense signaling, down regulating PR1 when SA signaling or synthesis is deficient, which may be responsible for Arabidopsis mutant plants more susceptible to infection by high concentration of P. nicotianae zoospores. These results suggest that zoospore exudates can function as virulence factors and inducers of plant immune responses during plant infection by Phytophthora.

The signal peptide-like segment of hpaXm is required for its association to the cell wall in transgenic tobacco plants

Harpins, encoded by hrp (hypersensitive response and pathogenicity) genes of Gram-negative plant pathogens, are elicitors of hypersensitive response (HR). HpaXm is a novel harpin-like protein described from cotton leaf blight bacteria, Xanthomonas citri subsp. malvacearum-a synonym of X. campestris pv. malvacearum (Smith 1901-1978). A putative signal peptide (1-MNSLNTQIGANSSFL-15) of hpaXm was predicted in the nitroxyl-terminal (N-terminal)by SignalP (SignalP 3.0 server). Here, we explored the function of the N-terminal leader peptide like segment of hpaXm using transgenic tobacco (Nicotiana tabacum cv. Xanthi nc.). Transgenic tobacco lines expressing the full-length hpaXm and the signal peptide-like segment-deleted mutant hpaXmΔLP were developed using transformation mediated by Agrobacterium tumefaciens. The target genes were confirmed integrated into the tobacco genomes and expressed normally. Using immune colloidal-gold detection technique, hpaXm protein was found to be transferred to the cytoplasm, the cell membrane, and organelles such as chloroplasts, mitochondria, and nucleus, as well as the cell wall. However, the deletion mutant hpaXmΔLP expressed in transgenic tobacco was found unable to cross the membrane to reach the cell wall. Additionally, soluble proteins extracted from plants transformed with hpaXm and hpaXmΔLP were bio-active. Defensive micro-HR induced by the transgene expression of hpaXm and hpaXmΔLP were observed on transgenic tobacco leaves. Disease resistance bioassays to tobacco mosaic virus (TMV) showed that tobacco plants transformed with hpaXm and with hpaXmΔLP exhibited enhanced resistance to TMV. In summary, the N-terminal signal peptide-like segment (1-45 bp) in hpaXm sequence is not necessary for transgene expression, bioactivity of hpaXm and resistance to TMV in transgenic tobacco, but is required for the protein to be translocated to the cell wall.

Elicitors of Host Plant Defenses Partially Suppress Cacopsylla pyricola (Hemiptera: Psyllidae) Populations Under Field Conditions

Defense elicitors are products that activate acquired defense responses in plants, thus rendering the plants less susceptible to attack by a broad range of pests. We demonstrated previously under laboratory conditions that foliar applications of the defense elicitors Actigard (acibenzolar-S-methyl), Employ (harpin protein), or ODC (chitosan) to potted pear trees (Pyrus communis L.) each caused an increase in mortality of Cacopsylla pyricola (Förster) (Hemiptera: Psyllidae) nymphs and altered the settling and oviposition behavior of the adults. In this study, we monitored C. pyricola populations over a 3-yr period on orchard-grown trees treated with water (untreated control), Actigard, Employ, or ODC. Fewer nymphs were observed on trees treated with elicitors compared with untreated trees in both 2014 and 2016. A similar but statistically nonsignificant pattern was observed in 2015 when nearly 30% fewer nymphs were observed on trees treated with elicitors versus untreated controls. Observed reductions in psyllid numbers by defense elicitors were modest and do not warrant the use of these products alone for managing C. pyricola. However, these products are often used for management of fire blight, and our observations that elicitors also reduce C. pyricola populations may be useful for system-wide integrated pest management approaches.

Harpin Hpa1 promotes flower development in Impatiens and Parochetus plants

BACKGROUND

The harpin protein Hpa1 has multiple beneficial effects in plants, promoting plant growth and development, increasing crop yield, and inducing plant resistance to pathogens and insect pests. In these effects, the 10-40 residue fragment (Hpa110-42) isolated from the Hpa1 sequence is 1.3 to 7.5-fold more effective than the full length.

RESULTS

This study extends the beneficial effects of Hpa1 and Hpa110-42 to flower development in three species of the garden balsam Impatiens and the garden scoparius Parochetus communis plant. The external application of Hpa1 or Hpa110-42 to the four ornamental plants had three effects, i.e., promoting flower growth, retarding senescence of fully expanded flowers, and increasing anthocyanin concentrations in those flowers and therefore improving their ornamental visages. Based on quantitative comparisons, Hpa110-42 was at least 17 and 42 % more effective than Hpa1 to increase anthocyanin concentrations and to promote the growth of flowers or delay their senescence.

CONCLUSION

Our results suggest that Hpa1 and especially Hpa110-42 have a great potential of horticultural application to increase ornamental merits of the different garden plants.

Evolution of herbivore-induced early defense signaling was shaped by genome-wide duplications in Nicotiana

Herbivore-induced defenses are widespread, rapidly evolving and relevant for plant fitness. Such induced defenses are often mediated by early defense signaling (EDS) rapidly activated by the perception of herbivore associated elicitors (HAE) that includes transient accumulations of jasmonic acid (JA). Analyzing 60 HAE-induced leaf transcriptomes from closely-related Nicotiana species revealed a key gene co-expression network (M4 module) which is co-activated with the HAE-induced JA accumulations but is elicited independently of JA, as revealed in plants silenced in JA signaling. Functional annotations of the M4 module were consistent with roles in EDS and a newly identified hub gene of the M4 module (NaLRRK1) mediates a negative feedback loop with JA signaling. Phylogenomic analysis revealed preferential gene retention after genome-wide duplications shaped the evolution of HAE-induced EDS in Nicotiana. These results highlight the importance of genome-wide duplications in the evolution of adaptive traits in plants.

Virulence of Erwinia amylovora, a prevalent apple pathogen: Outer membrane proteins and type III secreted effectors increase fitness and compromise plant defenses

Until now, no data are available on the outer membrane (OM) proteome of Erwinia amylovora, a Gram-negative plant pathogen, causing fire blight in most of the members of the Rosaceae family. Since the OM forms the interface between the bacterial cell and its environment it is in direct contact with the host. Additionally, the type III secretion system, embedded in the OM, is a pathogenicity factor of E. amylovora. To assess the influence of the OM composition and the secretion behavior on virulence, a 2D-DIGE analysis and gene expression profiling were performed on a high and lower virulent strain, both in vitro and in planta. Proteome data showed an increase in flagellin for the lower virulent strain in vitro, whereas, in planta several interesting proteins were identified as being differently expressed between both the strains. Further, gene expression of nearly all type III secreted effectors was elevated for the higher virulent strain, both in vitro and in planta. As a first, we report that several characteristics of virulence can be assigned to the OM proteome. Moreover, we demonstrate that secreted proteins prove to be the important factors determining differences in virulence between the strains, otherwise regarded as homogeneous on a genome level.

The Protein Elicitor PevD1 Enhances Resistance to Pathogens and Promotes Growth in Arabidopsis

The protein elicitor PevD1, isolated from Verticillium dahlia, could enhance resistance to TMV in tobacco and Verticillium wilt in cotton. Here, the pevd1 gene was over-expressed in wild type (WT) Arabidopsis, and its biological functions were investigated. Our results showed that the transgenic lines were more resistant to Botrytis cinerea and Pseudomonas syringae pv. tomato DC3000 than the WT line was. In transgenic plants, both the germination time and bolting time required were significantly shorter and fresh weights and plant heights were significantly higher than those in the WT line. A transcriptomics study using digital gene expression profiling (DGE) was performed in transgenic and WT Arabidopsis. One hundred and thirty-six differentially expressed genes were identified. In transgenic Arabidopsis, three critical regulators of JA biosynthesis were up-regulated and JA levels were slightly increased. Three important repressors of the ABA-responsive pathway were up-regulated, indicating that ABA signal transduction may be suppressed. One CML and two WRKY TFs involved in Ca²⁺-responsive pathways were up-regulated, indicating that this pathway may have been triggered. In conclusion, we show that PevD1 is involved in regulating several plant endogenous signal transduction pathways and regulatory networks to enhance resistance and promote growth and development in Arabidopsis.

Overexpression of a harpin-encoding gene popW from Ralstonia solanacearum primed antioxidant defenses with enhanced drought tolerance in tobacco plants

KEY MESSAGE

The tobacco plants transformed with popW gene showed enhanced drought tolerance, and the mechanism was found with primed antioxidant defenses and reduced drought stress damages in the transgenic lines. Harpin proteins are elicitors produced by several gram-negative plant pathogenic bacteria, triggering multiple beneficial responses in plants, such as induction of defense response against diverse pathogens and insects, growth promotion, and drought tolerance. In this study, the harpin-encoding gene popW derived from Ralstonia solanacearum ZJ3721 was transferred to tobacco. We examined the tolerance of transgenic tobacco plants toward drought stress under greenhouse conditions and analyzed the molecular mechanisms underlying the enhanced drought tolerance. The results revealed that the transgenic lines primed antioxidant defenses and reduced drought stress damages. In addition, they displayed lower malondialdehyde and relative electrical conductivity, while higher relative water content and recovery intension than the tobacco plants transformed with empty vector pBI121 and the wild-type (WT) plants under drought stress. Furthermore, the transgenic lines displayed a significant increase in peroxidase, superoxide dismutase, catalase activities, and ascorbic acid content compared with control plants under drought stress, and these levels were up to 1.95, 1.68, 1.34, and 1.43 times higher than those of WT plants, respectively. Overexpression of popW in tobacco also significantly enhanced the relative transcript levels of oxidative stress-responsive genes NtAPX, NtCAT1, NtGST, and NtCu/Zn-SOD under drought stress. The relative transcript levels of these genes in the transgenic line PW12 were up to 1.94, 2.36, 5.24, and 3.62 times higher than those of WT plants, respectively. These results confirmed that the popW gene, which was transformed into tobacco primed antioxidant responses, increased tolerance to drought stress in tobacco plants.

Elucidation of defense-related signaling responses to spot blotch infection in bread wheat (Triticum aestivum L.)

Spot blotch disease, caused by Bipolaris sorokiniana, is an important threat to wheat, causing an annual loss of ~17%. Under epidemic conditions, these losses may be 100%, yet the molecular responses of wheat to spot blotch remain almost uncharacterized. Moreover, defense-related phytohormone signaling genes have been poorly characterized in wheat. Here, we have identified 18 central components of salicylic acid (SA), jasmonic acid (JA), ethylene (ET), and enhanced disease susceptibility 1 (EDS1) signaling pathways as well as the genes of the phenylpropanoid pathway in wheat. In time-course experiments, we characterized the reprogramming of expression of these pathways in two contrasting genotypes: Yangmai #6 (resistant to spot blotch) and Sonalika (susceptible to spot blotch). We further evaluated the performance of a population of recombinant inbred lines (RILs) by crossing Yangmai#6 and Sonalika (parents) and subsequent selfing to F10 under field conditions in trials at multiple locations. We characterized the reprogramming of defense-related signaling in these RILs as a consequence of spot blotch attack. During resistance to spot blotch attack, wheat strongly elicits SA signaling (SA biogenesis as well as the NPR1-dependent signaling pathway), along with WRKY33 transcription factor, followed by an enhanced expression of phenylpropanoid pathway genes. These may lead to accumulation of phenolics-based defense metabolites that may render resistance against spot blotch. JA signaling may synergistically contribute to the resistance. Failure to elicit SA (and possibly JA) signaling may lead to susceptibility against spot blotch infection in wheat.

Harpin Hpa1 Interacts with Aquaporin PIP1;4 to Promote the Substrate Transport and Photosynthesis in Arabidopsis

Harpin proteins produced by plant-pathogenic Gram-negative bacteria are the venerable player in regulating bacterial virulence and inducing plant growth and defenses. A major gap in these effects is plant sensing linked to cellular responses, and plant sensor for harpin Hpa1 from rice bacterial blight pathogen points to plasma membrane intrinsic protein (PIP). Here we show that Arabidopsis AtPIP1;4 is a plasma membrane sensor of Hpa1 and plays a dual role in plasma membrane permeability of CO2 and H2O. In particular, AtPIP1;4 mediates CO2 transport with a substantial contribute to photosynthesis and further increases this function upon interacting with Hpa1 at the plasma membrane. As a result, leaf photosynthesis rates are increased and the plant growth is enhanced in contrast to the normal process without Hpa1-AtPIP1;4 interaction. Our findings demonstrate the first case that plant sensing of a bacterial harpin protein is connected with photosynthetic physiology to regulate plant growth.

Plant defence against aphids: the PAD4 signalling nexus

In Arabidopsis thaliana, PHYTOALEXIN DEFICIENT 4 (PAD4) functions as a key player in modulating defence against the phloem sap-feeding aphid Myzus persicae (Sülzer), more commonly known as the green peach aphid (GPA), an important pest of a wide variety of plants. PAD4 controls antibiosis and antixenosis against the GPA. In addition, PAD4 deters aphid feeding from sieve elements on Arabidopsis. In the past few years, substantial progress has been made in dissecting the role of PAD4 and its interaction with other signalling components in limiting aphid infestation. Several key genes/mechanisms involved in providing aphid resistance/susceptibility in Arabidopsis regulate the aphid infestation-stimulated expression of PAD4. Together, PAD4 and its interacting signalling partners provide a critical barrier to curtail GPA colonization of Arabidopsis.

Hpa1 harpin needs nitroxyl terminus to promote vegetative growth and leaf photosynthesis in Arabidopsis

Hpa1 is a harpin protein produced by Xanthomonas oryzae, an important bacterial pathogen of rice, and has the growth-promoting activity in plants. To understand the molecular basis for the function of Hpa1, we generated an inactive variant protein, Hpa1 delta NT, by deleting the nitroxyl-terminal region of the Hpa1 sequence and compared Hpa1 delta NT with the full-length protein in terms of the effects on vegetative growth and related physiological responses in Arabidopsis. When Hpa1 was applied to plants, it acted to enhance the vegetative growth but did not affect the floral development. Enhanced plant growth was accompanied by induced expression of growth-promoting genes in plant leaves. The growth-promoting activity of Hpa1 was further correlated with a physiological consequence shown as promoted leaf photosynthesis as a result of facilitated CO2 conduction through leaf stomata and mesophyll cells. On the contrary, plant growth, growth-promoting gene expression, and the physiological consequence changed little in response to the Hpa1 delta NT treatment. These analyses suggest that Hpa1 requires the nitroxyl-terminus to facilitate CO2 transport inside leaf cells and promote leaf photosynthesis and vegetative growth of the plant.

BOTRYTIS-INDUCED KINASE1 Modulates Arabidopsis Resistance to Green Peach Aphids via PHYTOALEXIN DEFICIENT4

BOTRYTIS-INDUCED KINASE1 (BIK1) plays important roles in induced defense against fungal and bacterial pathogens in Arabidopsis (Arabidopsis thaliana). Its tomato (Solanum lycopersicum) homolog is required for host plant resistance to a chewing insect herbivore. However, it remains unknown whether BIK1 functions in plant defense against aphids, a group of insects with a specialized phloem sap-feeding style. In this study, the potential role of BIK1 was investigated in Arabidopsis infested with the green peach aphid (Myzus persicae). In contrast to the previously reported positive role of intact BIK1 in defense response, loss of BIK1 function adversely impacted aphid settling, feeding, and reproduction. Relative to wild-type plants, bik1 displayed higher aphid-induced hydrogen peroxide accumulation and more severe lesions, resembling a hypersensitive response (HR) against pathogens. These symptoms were limited to the infested leaves. The bik1 mutant showed elevated basal as well as induced salicylic acid and ethylene accumulation. Intriguingly, elevated salicylic acid levels did not contribute to the HR-like symptoms or to the heightened aphid resistance associated with the bik1 mutant. Elevated ethylene levels in bik1 accounted for an initial, short-term repellence. Introducing a loss-of-function mutation in the aphid resistance and senescence-promoting gene PHYTOALEXIN DEFICIENT4 (PAD4) into the bik1 background blocked both aphid resistance and HR-like symptoms, indicating bik1-mediated resistance to aphids is PAD4 dependent. Taken together, Arabidopsis BIK1 confers susceptibility to aphid infestation through its suppression of PAD4 expression. Furthermore, the results underscore the role of reactive oxygen species and cell death in plant defense against phloem sap-feeding insects.

Recessive mutation identifies auxin-repressed protein ARP1, which regulates growth and disease resistance in tobacco

To study the molecular mechanism that underpins crosstalk between plant growth and disease resistance, we performed a mutant screening on tobacco and created a recessive mutation that caused the phenotype of growth enhancement and resistance impairment (geri1). In the geri1 mutant, growth enhancement accompanies promoted expression of growth-promoting genes, whereas repressed expression of defense response genes is consistent with impaired resistance to diseases caused by viral, bacterial, and oomycete pathogens. The geri1 allele identifies a single genetic locus hypothetically containing the tagged GERI1 gene. The isolated GERI1 gene was predicted to encode auxin-repressed protein ARP1, which was determined to be 13.5 kDa in size. The ARP1/GERI1 gene was further characterized as a repressor of plant growth and an activator of disease resistance based on genetic complementation, gene silencing, and overexpression analyses. ARP1/GERI1 resembles pathogen-associated molecular patterns and is required for them to repress plant growth and activate plant immunity responses. ARP1/GERI1 represses growth by inhibiting the expression of AUXIN RESPONSE FACTOR gene ARF8, and ARP1/GERI1 recruits the NPR1 gene, which is essential for the salicylic-acid-mediated defense, to coregulate disease resistance. In conclusion, ARP1/GERI1 is an integral regulator for crosstalk between growth and disease resistance in the plant.

Differential induction of Pisum sativum defense signaling molecules in response to pea aphid infestation

This study demonstrates the sequence of enhanced generation of signal molecules such as phytohormones, i.e. jasmonic acid (JA), ethylene (ET), salicylic acid (SA), and a relatively stable free radical, nitric oxide (NO), in response of Pisum sativum L. cv. Cysterski seedling leaves to the infestation of pea aphid Acyrthosiphon pisum (Harris) at a varied population size. In time from 0 to 96h after A. pisum infestation these signal molecules accumulated transiently. Moreover, the convergence of these signaling pathways occurred. JA and its methyl derivative MeJA reached the first maximum of generation at 24th hour of infestation. An increase in ET and NO generation was observed at 48th hour of infestation. The increase in SA, JA/MeJA and ET concentrations in aphid-infested leaves occurred from the 72nd to 96th hour. In parallel, an increase was demonstrated for the activities of enzymes engaged in the biosynthesis of SA, such as phenylalanine ammonia-lyase (PAL) and benzoic acid 2-hydroxylase (BA2H). Additionally, a considerable post-infestation accumulation of transcripts for PAL was observed. An increase in the activity of lipoxygenase (LOX), an important enzyme in the biosynthesis of JA was noted. This complex signaling network may contribute to the coordinated regulation of gene expression leading to specific defence responses.

Transgenic expression of a functional fragment of harpin protein Hpa1 in wheat induces the phloem-based defence against English grain aphid

The harpin protein Hpa1 has multiple beneficial effects in plants, promoting plant growth and development, increasing crop yield, and inducing resistance to pathogens and insect pests. For these effects, the 10-40 residue fragment (Hpa1₁₀₋₄₂) isolated from the Hpa1 sequence is 1.3- to 7.5-fold more effective than the full-length protein. Here it is reported that the expression of Hpa1₁₀₋₄₂ under the direction of an insect-induced promoter induces the phloem-based defence to English grain aphid, a dominant species of wheat aphids. The expression of Hpa1₁₀₋₄₂ was found to compromise the colonization preference of aphids on the plant and further inhibit aphid reproduction in leaf colonies. In Hpa1₁₀₋₄₂-expressing wheat lines, moreover, aphid feeding from the phloem was repressed in correlation with the phloem-based defence. This defensive mechanism was shown as enhanced expression of wheat genes encoding phloem lectin proteins (PP2-A1 and PP2-A2) and β-1,3-glucan synthase-like enzymes (GSL2, GSL10, and GSL12). Both PP2-A and β-1,3-glucan formed high molecular mass polymers to block phloem sieve plate pores and therefore impede aphid feeding from the phloem. However, the phloem-based defence was impaired by treating plants with ethylene signalling inhibitors, suggesting the requirement for the ethylene signalling pathway. In addition, if Hpa1₁₀₋₄₂-expressing plants were subjected to attack by a small number of aphids, they newly acquired agriculturally beneficial characters, such as enhanced vegetative growth and increased tiller numbers and grain output values. These results suggest that the defensive and developmental roles of Hpa1₁₀₋₄₂ can be integrated into the germplasm of this agriculturally significant crop.