The hrpZ gene of Pseudomonas syringae pv. phaseolicola enhances resistance to rhizomania disease in transgenic Nicotiana benthamiana and sugar beet

Rational domestication of a plant-based recombinant expression system expands its biosynthetic range

Plant molecular farming aims to provide a green, flexible, and rapid alternative to conventional recombinant expression systems, capable of producing complex biologics such as enzymes, vaccines, and antibodies. Historically, the recombinant expression of therapeutic peptides in plants has proven difficult, largely due to their small size and instability. However, some plant species harbour the capacity for peptide backbone cyclization, a feature inherent in stable therapeutic peptides. One obstacle to realizing the potential of plant-based therapeutic peptide production is the proteolysis of the precursor before it is matured into its final stabilized form. Here we demonstrate the rational domestication of Nicotiana benthamiana within two generations to endow this plant molecular farming host with an expanded repertoire of peptide sequence space. The in planta production of molecules including an insecticidal peptide, a prostate cancer therapeutic lead, and an orally active analgesic is demonstrated.

Plant immunity inducers: from discovery to agricultural application

While conventional chemical fungicides directly eliminate pathogens, plant immunity inducers activate or prime plant immunity. In recent years, considerable progress has been made in understanding the mechanisms of immune regulation in plants. The development and application of plant immunity inducers based on the principles of plant immunity represent a new field in plant protection research. In this review, we describe the mechanisms of plant immunity inducers in terms of plant immune system activation, summarize the various classes of reported plant immunity inducers (proteins, oligosaccharides, chemicals, and lipids), and review methods for the identification or synthesis of plant immunity inducers. The current situation, new strategies, and future prospects in the development and application of plant immunity inducers are also discussed.

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.

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.

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.

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.

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.

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.

Plant Aquaporin AtPIP1;4 Links Apoplastic H2O2 Induction to Disease Immunity Pathways

Hydrogen peroxide (H2O2) is a stable component of reactive oxygen species, and its production in plants represents the successful recognition of pathogen infection and pathogen-associated molecular patterns (PAMPs). This production of H2O2 is typically apoplastic but is subsequently associated with intracellular immunity pathways that regulate disease resistance, such as systemic acquired resistance and PAMP-triggered immunity. Here, we elucidate that an Arabidopsis (Arabidopsis thaliana) aquaporin (i.e. the plasma membrane intrinsic protein AtPIP1;4) acts to close the cytological distance between H2O2 production and functional performance. Expression of the AtPIP1;4 gene in plant leaves is inducible by a bacterial pathogen, and the expression accompanies H2O2 accumulation in the cytoplasm. Under de novo expression conditions, AtPIP1;4 is able to mediate the translocation of externally applied H2O2 into the cytoplasm of yeast (Saccharomyces cerevisiae) cells. In plant cells treated with H2O2, AtPIP1;4 functions as an effective facilitator of H2O2 transport across plasma membranes and mediates the translocation of externally applied H2O2 from the apoplast to the cytoplasm. The H2O2-transport role of AtPIP1;4 is essentially required for the cytoplasmic import of apoplastic H2O2 induced by the bacterial pathogen and two typical PAMPs in the absence of induced production of intracellular H2O2 As a consequence, cytoplasmic H2O2 quantities increase substantially while systemic acquired resistance and PAMP-triggered immunity are activated to repress the bacterial pathogenicity. By contrast, loss-of-function mutation at the AtPIP1;4 gene locus not only nullifies the cytoplasmic import of pathogen- and PAMP-induced apoplastic H2O2 but also cancels the subsequent immune responses, suggesting a pivotal role of AtPIP1;4 in apocytoplastic signal transduction in immunity pathways.

Harpins, multifunctional proteins secreted by gram-negative plant-pathogenic bacteria

Harpins are glycine-rich and heat-stable proteins that are secreted through type III secretion system in gram-negative plant-pathogenic bacteria. Many studies show that these proteins are mostly targeted to the extracellular space of plant tissues, unlike bacterial effector proteins that act inside the plant cells. Over the two decades since the first harpin of pathogen origin, HrpN of Erwinia amylovora, was reported in 1992 as a cell-free elicitor of hypersensitive response (HR), diverse functional aspects of harpins have been determined. Some harpins were shown to have virulence activity, probably because of their involvement in the translocation of effector proteins into plant cytoplasm. Based on this function, harpins are now considered to be translocators. Their abilities of pore formation in the artificial membrane, binding to lipid components, and oligomerization are consistent with this idea. When harpins are applied to plants directly or expressed in plant cells, these proteins trigger diverse beneficial responses such as induction of defense responses against diverse pathogens and insects and enhancement of plant growth. Therefore, in this review, we will summarize the functions of harpins as virulence factors (or translocators) of bacterial pathogens, elicitors of HR and immune responses, and plant growth enhancers.

High level resistance against rhizomania disease by simultaneously integrating two distinct defense mechanisms

With the aim of achieving durable resistance against rhizomania disease of sugar beet, the employment of different sources of resistance to Beet necrotic yellow vein virus was pursued. To this purpose, Nicotiana benthamiana transgenic plants that simultaneously produce dsRNA originating from a conserved region of the BNYVV replicase gene and the HrpZ(Psph) protein in a secreted form (SP/HrpZ(Psph)) were produced. The integration and expression of both transgenes as well as proper production of the harpin protein were verified in all primary transformants and selfed progeny (T1, T2). Transgenic resistance was assessed by BNYVV-challenge inoculation on T2 progeny by scoring disease symptoms and DAS-ELISA at 20 and 30 dpi. Transgenic lines possessing single transformation events for both transgenes as well as wild type plants were included in inoculation experiments. Transgenic plants were highly resistant to virus infection, whereas in some cases immunity was achieved. In all cases, the resistant phenotype of transgenic plants carrying both transgenes was superior in comparison with the ones carrying a single transgene. Collectively, our findings demonstrate, for a first time, that the combination of two entirely different resistance mechanisms provide high level resistance or even immunity against the virus. Such a novel approach is anticipated to prevent a rapid virus adaptation that could potentially lead to the emergence of isolates with resistance breaking properties.

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

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

Apoplastic and cytoplasmic location of harpin protein Hpa1Xoo plays different roles in H2O2 generation and pathogen resistance in Arabidopsis

Harpin proteins secreted by phytopathogenic bacteria have been shown to activate the plant defense pathway, which involves transduction of a hydrogen peroxide (H(2)O(2)) signal generated in the apoplast. However, the way in which harpins are recognized in the pathway and what role the apoplastic H(2)O(2) plays in plant defenses are unclear. Here, we examine whether the cellular localization of Hpa1(Xoo), a harpin protein produced by the rice bacterial leaf blight pathogen, impacts H(2)O(2) production and pathogen resistance in Arabidopsis thaliana. Transformation with the hpa1 (Xoo) gene and hpa1 (Xoo) fused to an apoplastic localization signal (shpa1 (Xoo)) generated h pa1 (Xoo)- and sh pa1 (Xoo)-expressing transgenic A . t haliana (HETAt and SHETAt) plants, respectively. Hpa1(Xoo) was associated with the apoplast in SHETAt plants but localized inside the cell in HETAt plants. In addition, Hpa1(Xoo) localization accompanied H(2)O(2) accumulation in both the apoplast and cytoplasm of SHETAt plants but only in the cytoplasm of HETAt plants. Apoplastic H(2)O(2) production via nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) located in the plasma membrane is a common feature of plant defenses. In SHETAt plants, H(2)O(2) was generated in apoplasts in a NOX-dependent manner but accumulated to a greater extent in the cytoplasm than in the apoplast. After being applied to the wild-type plant, Hpa1(Xoo) localized to apoplasts and stimulated H(2)O(2) production as in SHETAt plants. In both plants, inhibiting apoplastic H(2)O(2) generation abrogated both cytoplasmic H(2)O(2) accumulation and plant resistance to bacterial pathogens. These results suggest the possibility that the apoplastic H(2)O(2) is subject to a cytoplasmic translocation for participation in the pathogen defense.

Genomic toolboxes for conservation biologists

Conservation genetics is expanding its research horizon with a genomic approach, by incorporating the modern techniques of next-generation sequencing (NGS). Application of NGS overcomes many limitations of conservation genetics. First, NGS allows for genome-wide screening of markers, which may lead to a more representative estimation of genetic variation within and between populations. Second, NGS allows for distinction between neutral and non-neutral markers. By screening populations on thousands of single nucleotide polymorphism markers, signals of selection can be found for some markers. Variation in these markers will give insight into functional rather than neutral genetic variation. Third, NGS facilitates the study of gene expression. Conservation genomics will increase our insight in how the environment and genes interact to affect phenotype and fitness. In addition, the NGS approach opens a way to study processes such as inbreeding depression and local adaptation mechanistically. Conservation genetics programs are directed to a fundamental understanding of the processes involved in conservation genetics and should preferably be started in species for which large databases on ecology, demography and genetics are available. Here, we describe and illustrate the connection between the application of NGS technologies and the research questions in conservation. The perspectives of conservation genomics programs are also discussed.