A local accumulation of the Ralstonia solanacearum PopA protein in transgenic tobacco renders a compatible plant-pathogen interaction incompatible

Making Use of Plant uORFs to Control Transgene Translation in Response to Pathogen Attack

Reducing crop loss to diseases is urgently needed to meet increasing food production challenges caused by the expanding world population and the negative impact of climate change on crop productivity. Disease-resistant crops can be created by expressing endogenous or exogenous genes of interest through transgenic technology. Nevertheless, enhanced resistance by overexpressing resistance-produced genes often results in adverse developmental affects. Upstream open reading frames (uORFs) are translational control elements located in the 5' untranslated region (UTR) of eukaryotic mRNAs and may repress the translation of downstream genes. To investigate the function of three uORFs from the 5' -UTR of ACCELERATED CELL 11 (uORFsACD11), we develop a fluorescent reporter system and find uORFsACD11 function in repressing downstream gene translation. Individual or simultaneous mutations of the three uORFsACD11 lead to repression of downstream translation efficiency at different levels. Importantly, uORFsACD11-mediated translational inhibition is impaired upon recognition of pathogen attack of plant leaves. When coupled with the PATHOGENESIS-RELATED GENE 1 (PR1) promoter, the uORFsACD11 cassettes can upregulate accumulation of Arabidopsis thaliana LECTIN RECEPTOR KINASE-VI.2 (AtLecRK-VI.2) during pathogen attack and enhance plant resistance to Phytophthora capsici. These findings indicate that the uORFsACD11 cassettes can be a useful toolkit that enables a high level of protein expression during pathogen attack, while for ensuring lower levels of protein expression at normal conditions.

Preliminary Studies on the Antibacterial Mechanism of a New Plant-Derived Compound, 7-Methoxycoumarin, Against Ralstonia solanacearum

Ralstonia solanacearum (R. solanacearum) is one of the most devastating plant bacterial pathogens and leads to serious economic losses in crops worldwide. In this study, the antibacterial mechanism of 7-methoxycoumarin, a new coumarin antibiotic, was preliminarily investigated by the observation of symptoms and physical and biochemical analyses. The results showed that 7-methoxycoumarin significantly suppressed bacterial growth of R. solanacearum, with the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) values of 75 and 175 mg/L, respectively. Electron microscopy observations showed that the bacterial cell membrane was destroyed after 7-methoxycoumarin treatment. Biofilm formation of R. solanacearum was significantly suppressed by 7-methoxycoumarin at concentrations ranging from 25 to 100 mg/L. Furthermore, virulence-associated genes epsE, hrpG, and popA of R. solanacearum were significantly inhibited by 7-methoxycoumarin. The application of 7-methoxycoumarin effectively suppressed tobacco bacterial wilt progress in pot experiments, with relative control efficiencies of 83.61, 68.78, and 58.11% at 6, 8, and 10 days post inoculation, respectively.

Identification of RipAZ1 as an avirulence determinant of Ralstonia solanacearum in Solanum americanum

Ralstonia solanacearum causes bacterial wilt disease in many plant species. Type III-secreted effectors (T3Es) play crucial roles in bacterial pathogenesis. However, some T3Es are recognized by corresponding disease resistance proteins and activate plant immunity. In this study, we identified the R. solanacearum T3E protein RipAZ1 (Ralstonia injected protein AZ1) as an avirulence determinant in the black nightshade species Solanum americanum. Based on the S. americanum accession-specific avirulence phenotype of R. solanacearum strain Pe_26, 12 candidate avirulence T3Es were selected for further analysis. Among these candidates, only RipAZ1 induced a cell death response when transiently expressed in a bacterial wilt-resistant S. americanum accession. Furthermore, loss of ripAZ1 in the avirulent R. solanacearum strain Pe_26 resulted in acquired virulence. Our analysis of the natural sequence and functional variation of RipAZ1 demonstrated that the naturally occurring C-terminal truncation results in loss of RipAZ1-triggered cell death. We also show that the 213 amino acid central region of RipAZ1 is sufficient to induce cell death in S. americanum. Finally, we show that RipAZ1 may activate defence in host cell cytoplasm. Taken together, our data indicate that the nucleocytoplasmic T3E RipAZ1 confers R. solanacearum avirulence in S. americanum. Few avirulence genes are known in vascular bacterial phytopathogens and ripAZ1 is the first one in R. solanacearum that is recognized in black nightshades. This work thus opens the way for the identification of disease resistance genes responsible for the specific recognition of RipAZ1, which can be a source of resistance against the devastating bacterial wilt disease.

The large, diverse, and robust arsenal of Ralstonia solanacearum type III effectors and their in planta functions

The type III secretion system with its delivered type III effectors (T3Es) is one of the main virulence determinants of Ralstonia solanacearum, a worldwide devastating plant pathogenic bacterium affecting many crop species. The pan-effectome of the R. solanacearum species complex has been exhaustively identified and is composed of more than 100 different T3Es. Among the reported strains, their content ranges from 45 to 76 T3Es. This considerably large and varied effectome could be considered one of the factors contributing to the wide host range of R. solanacearum. In order to understand how R. solanacearum uses its T3Es to subvert the host cellular processes, many functional studies have been conducted over the last three decades. It has been shown that R. solanacearum effectors, as those from other plant pathogens, can suppress plant defence mechanisms, modulate the host metabolism, or avoid bacterial recognition through a wide variety of molecular mechanisms. R. solanacearum T3Es can also be perceived by the plant and trigger immune responses. To date, the molecular mechanisms employed by R. solanacearum T3Es to modulate these host processes have been described for a growing number of T3Es, although they remain unknown for the majority of them. In this microreview, we summarize and discuss the current knowledge on the characterized R. solanacearum species complex T3Es.

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.

Molecular breeding approaches for production of disease-resilient commercially important tobacco

Tobacco is one of the most widely cultivated nonfood cash crops, a source of income, model organism for plant molecular research, a natural pesticide and of pharmaceutical importance. First domesticated in South Americas, the modern-day tobacco (Nicotiana tabacum) is now cultivated in more than 125 countries to generate revenues worth billions of dollars each year. However, the production of this crop is highly threatened by the global presence of devastating infectious agents, which cause huge fiscal loss. These threats have been battled through breeding for acquiring disease resilience in tobacco plants, first, via conventional and now with the use of modern molecular breeding approaches. For efficacy and precision, the characterization of the genetic components underlying disease resistance is the key tool in tobacco for resistance breeding programs. The past few decades have witnessed significant progress in resilience breeding through advanced molecular techniques. The current review discusses history of tobacco breeding since its time of origin till date, highlighting the most widely used techniques and recent advances in molecular research and strategies for resistance breeding. In addition, we narrate the budding possibilities for the future. This review will provide a comprehensive and valuable information for the tobacco growers and researchers to deal with the destructive infectious diseases.

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.

Identification and functional characterization of the soybean GmaPPO12 promoter conferring Phytophthora sojae induced expression

Identification of pathogen-inducible promoters largely lags behind cloning of the genes for disease resistance. Here, we cloned the soybean GmaPPO12 gene and found that it was rapidly and strongly induced by Phytophthorasojae infection. Computational analysis revealed that its promoter contained many known cis-elements, including several defense related transcriptional factor-binding boxes. We showed that the promoter could mediate induction of GUS expression upon infection in both transient expression assays in Nicotianabenthamiana and stable transgenic soybean hairy roots. Importantly, we demonstrated that pathogen-induced expression of the GmaPPO12 promoter was higher than that of the soybean GmaPR1a promoter. A progressive 5' and 3' deletion analysis revealed two fragments that were essential for promoter activity. Thus, the cloned promoter could be used in transgenic plants to enhance resistance to phytophthora pathogens, and the identified fragment could serve as a candidate to produce synthetic pathogen-induced promoters.

Hpa2 required by HrpF to translocate Xanthomonas oryzae transcriptional activator-like effectors into rice for pathogenicity

Xanthomonas oryzae pv. oryzicola, the causative agent of bacterial leaf streak, injects a plethora of effectors through the type III secretion system (T3SS) into rice cells to cause disease. The T3SS, encoded by the hrp genes, is essential for the pathogen to elicit the hypersensitive response (HR) in nonhost tobacco and for pathogenicity in host rice. Whether or not a putative lytic transglycosylase, Hpa2, interacts with a translocon protein, HrpF, to facilitate bacterial pathogenicity remains unknown. Here we demonstrated that both the hpa2 and hrpF genes are required for the pathogenicity of X. oryzae pv. oryzicola strain RS105 in rice but not for HR induction in tobacco. The expression of hpa2 was positively regulated by HrpG and HrpD6 but not by HrpX. In vivo secretion and subcellular localization analyses confirmed that Hpa2 secretion is dependent on HpaB (a T3SS exit protein) and that Hpa2 binds to the host cell membrane. Protein-protein assays demonstrated that Hpa2 interacts with HrpF. In planta translocation of AvrXa10 indicated that the mutation in hpa2 and hrpF inhibits the injection of the HpaB-dependent transcriptional activator-like (TAL) effector into rice. These findings suggest that Hpa2 and HrpF form a complex to translocate T3S effectors into plant cells for pathogenesis in host rice.

Playing the "Harp": evolution of our understanding of hrp/hrc genes

With the advent of recombinant DNA techniques, the field of molecular plant pathology witnessed dramatic shifts in the 1970s and 1980s. The new and conventional methodologies of bacterial molecular genetics put bacteria center stage. The discovery in the mid-1980s of the hrp/hrc gene cluster and the subsequent demonstration that it encodes a type III secretion system (T3SS) common to Gram negative bacterial phytopathogens, animal pathogens, and plant symbionts was a landmark in molecular plant pathology. Today, T3SS has earned a central role in our understanding of many fundamental aspects of bacterium-plant interactions and has contributed the important concept of interkingdom transfer of effector proteins determining race-cultivar specificity in plant-bacterium pathosystems. Recent developments in genomics, proteomics, and structural biology enable detailed and comprehensive insights into the functional architecture, evolutionary origin, and distribution of T3SS among bacterial pathogens and support current research efforts to discover novel antivirulence drugs.

Strategies of attack and defense in plant-oomycete interactions, accentuated for Phytophthora parasitica Dastur (syn. P. Nicotianae Breda de Haan)

Oomycetes from the genus Phytophthora are fungus-like plant pathogens that are devastating for agriculture and natural ecosystems. Due to their particular physiological characteristics, no efficient treatments against diseases caused by these microorganisms are presently available. To develop such treatments, it appears essential to dissect the molecular mechanisms that determine the interaction between Phytophthora species and host plants. Available data are scarce, and genomic approaches were mainly developed for the two species, Phytophthora infestans and Phytophthora sojae. However, these two species are exceptions from, rather than representative species for, the genus. P. infestans is a foliar pathogen, and P. sojae infects a narrow range of host plants, while the majority of Phytophthora species are quite unselective, root-infecting pathogens. To represent this majority, Phytophthora parasitica emerges as a model for the genus, and genomic resources for analyzing its interaction with plants are developing. The aim of this review is to assemble current knowledge on cytological and molecular processes that are underlying plant-pathogen interactions involving Phytophthora species and in particular P. parasitica, and to place them into the context of a hypothetical scheme of co-evolution between the pathogen and the host.

Cladosporium fulvum CfHNNI1 induces hypersensitive necrosis, defence gene expression and disease resistance in both host and nonhost plants

Nonhost resistance as a durable and broad-spectrum defence strategy is of great potential for agricultural applications. We have previously isolated a cDNA showing homology with genes encoding bZIP transcription factors from tomato leaf mould pathogen Cladosporium fulvum. Upon expression, the cDNA results in necrosis in C. fulvum host tomato and nonhost tobacco plants and is thus named CfHNNI1 (for C . f ulvum host and nonhost plant necrosis inducer 1). In the present study we report the induction of necrosis in a variety of nonhost plant species belonging to three families by the transient in planta expression of CfHNNI1 using virus-based vectors. Additionally, transient expression of CfHNNI1 also induced expression of the HR marker gene LeHSR203 and greatly reduced the accumulation of recombinant Potato virus X. Stable CfHNNI1 transgenic tobacco plants were generated in which the expression of CfHNNI1 is under the control of the pathogen-inducible hsr203J promoter. When infected with the oomycetes pathogen Phytophthora parasitica var. nicotianae, these transgenic plants manifested enhanced expression of CfHNNI1 and subsequent accumulation of CfHNNI1 protein, resulting in high expression of the HSR203J and PR genes, and strong resistance to the pathogen. The CfHNNI1 transgenic plants also exhibited induced resistance to Pseudomonas syringae pv. tabaci and Tobacco mosaic virus. Furthermore, CfHNNI1 was highly expressed and the protein was translocated into plant cells during the incompatible interactions between C. fulvum and host and nonhost plants. Our results demonstrate that CfHNNI1 is a potential general elicitor of hypersensitive response and nonhost resistance.

Ca2+-dependent lipid binding and membrane integration of PopA, a harpin-like elicitor of the hypersensitive response in tobacco

PopA is released by type III secretion from the bacterial plant pathogen Ralstonia solanacearum and triggers the hypersensitive response (HR) in tobacco. The function of PopA remains obscure, mainly because mutants lacking this protein are not altered in their ability to interact with plants. In an attempt to identify the site of PopA activity in plant cells, we generated transgenic tobacco plants expressing the popA gene under the control of an inducible promoter. Immunocytologic analysis revealed that the HR phenotype of these plants correlated with the presence of PopA at the plant plasma membrane. Membrane localization was observed irrespective of whether the protein was designed to accumulate in the cytoplasm or to be secreted by the plant cell, suggesting a general lipid-binding ability. We found that the protein had a high affinity for sterols and sphingolipids in vitro and that it required Ca2+ for both lipid binding and oligomerization. In addition, the protein was integrated into liposomes and membranes from Xenopus laevis oocytes where it formed ion-conducting pores. These characteristics suggest that PopA is part of a system that aims to attach the host cell plasma membrane and to allow molecules cross this barrier.

Engineering plants with increased disease resistance: how are we going to express it?

Precise control of transgene expression is pivotal to the engineering of plants with increased disease resistance. Many early attempts to boost disease resistance used constitutive overexpression of defence components but frequently this resulted in poor quality plants. It is now clear that the extensive cellular reprogramming associated with defence will reduce yields if uncontrolled defence reactions are activated in uninfected cells. Therefore, for many strategies pathogen-inducible promoters might be the most useful as they limit the cost of resistance by restricting expression to infection sites. Although progress to date has been hindered by a lack of suitable promoters, new research should reveal more potentially useful native promoters. Additionally, the first steps towards 'designer' synthetic promoters have proved encouraging.

Engineering plants with increased disease resistance: what are we going to express?

To engineer plants with increased and durable disease resistance using transgenic technologies we must address two questions. First, what gene or genes do we want to express to improve disease resistance, and second, how are we going to express these genes so that crop yields are actually increased? Emerging technologies are providing us with a plethora of candidate genes that might lead to enhanced crop protection through genetic engineering. These genes can come from plants, from pathogens or from other organisms and several strategies for their manipulation show promise. Here, we discuss recent advances and consider future perspectives for producing plants with durable disease resistance.

Type III protein secretion mechanism in mammalian and plant pathogens

The type III protein secretion system (TTSS) is a complex organelle in the envelope of many Gram-negative bacteria; it delivers potentially hundreds of structurally diverse bacterial virulence proteins into plant and animal cells to modulate host cellular functions. Recent studies have revealed several basic features of this secretion system, including assembly of needle/pilus-like secretion structures, formation of putative translocation pores in the host membrane, recognition of N-terminal/5' mRNA-based secretion signals, and requirement of small chaperone proteins for optimal delivery and/or expression of effector proteins. Although most of our knowledge about the TTSS is derived from studies of mammalian pathogenic bacteria, similar and unique features are learned from studies of plant pathogenic bacteria. Here, we summarize the most salient aspects of the TTSS, with special emphasis on recent findings.

Transgene expression in strawberries driven by a heterologous phloem-specific promoter

Strawberry is susceptible to diseases caused by phytoplasmas, mycoplasma-like prokaryotes restricted to sieve elements in the phloem tissue of infected plants. One strategy to improve strawberry resistance to phytoplasmas involves transgenic expression of anti-microbial peptide genes in phloem. For targeted phloem-specific expression, we constructed a binary vector with an expression cassette bearing the beta-glucuronidase (GUS) reporter gene (uidA) under control of the Arabidopsis sucrose-H+ symporter gene (AtSUC2) promoter. Transgenic strawberry lines were generated with high efficiencies by a modified transformation protocol, which combines the adoption of a 3-day pre-selection period following transformation, and the addition of 10-microM thidiazuron to the regeneration medium. Histological GUS activity indicated that the reporter gene was expressed specifically in phloem of leaves, petioles, and roots of transgenic plants. The results suggest that the transformation protocol and the AtSUC2 promoter may be useful for engineering phytoplasma-resistant transgenic strawberries.

Ectopic expression of Ralstonia solanacearum effector protein PopA early in invasion results in loss of virulence

Ralstonia solanacearum OE1-1 (OE1-1) is pathogenic to tobacco. The type III-secreted effector protein popA of OE1-1 showed 97.6% identity to popA of R. solanacearum GMI1000, which is not pathogenic to tobacco. Reverse transcription-polymerase chain reaction analysis showed that popA in OE1-1 was expressed at 3 h after inoculation (HAI), but not before, in infiltrated-tobacco leaves. Pathogenicity analysis using a popABC operon-deleted mutant of OE1-1 (deltaABC) showed that popABC is not directly involved in the pathogenicity of OE1-1. When Papa, which constitutively expresses popA, was infiltrated into tobacco leaves, popA was expressed by 0.5 HAI. Papa could no longer multiply or spread in tobacco leaves and was no longer virulent. Moreover, the hypersensitive response (HR) and expression of HR-related genes were not induced in Papa-infiltrated leaves. Papa was also avirulent in a tobacco root-dipping inoculation assay. These results suggest that the expression of popA in Papa immediately after invasion triggers the suppression of bacterial proliferation and movement, resulting in loss of virulence. However, Papa retained its virulence when directly inoculated into xylem vessels. This result suggests that tobacco plants can recognize PopA when it is expressed early in disease development, and respond with an effective defense in the intercellular spaces.