A Phosphonate Natural Product Made by Pantoea ananatis is Necessary and Sufficient for the Hallmark Lesions of Onion Center Rot.

Pantoea ananatis is the primary cause of onion center rot. Genetic data suggest that a phosphonic acid natural product is required for pathogenesis; however, the nature of the molecule is unknown. Here, we show that P. ananatis produces at least three phosphonates, two of which were purified and structurally characterized. The first, designated pantaphos, was shown to be 2-(hydroxy[phosphono]methyl)maleate; the second, a probable biosynthetic precursor, was shown to be 2-(phosphonomethyl)maleate. Purified pantaphos is both necessary and sufficient for the hallmark lesions of onion center rot. Moreover, when tested against mustard seedlings, the phytotoxic activity of pantaphos was comparable to the widely used herbicides glyphosate and phosphinothricin. Pantaphos was also active against a variety of human cell lines but was significantly more toxic to glioblastoma cells. Pantaphos showed little activity when tested against a variety of bacteria and fungi.IMPORTANCEPantoea ananatis is a significant plant pathogen that targets a number of important crops, a problem that is compounded by the absence of effective treatments to prevent its spread. Our identification of pantaphos as the key virulence factor in onion center rot suggests a variety of approaches that could be employed to address this significant plant disease. Moreover, the general phytotoxicity of the molecule suggests that it could be developed into an effective herbicide to counter the alarming rise in herbicide-resistant weeds.

Cadmium Stress Reprograms ROS/RNS Homeostasis in Phytophthora infestans (Mont.) de Bary.

Heavy metal pollution causes many soils to become a toxic environment not only for plants, but also microorganisms; however, little is known how heavy metal contaminated environment affects metabolism of phytopathogens and their capability of infecting host plants. In this study the oomycete Phytophthora infestans (Mont.) de Bary, the most harmful pathogen of potato, growing under moderate cadmium stress (Cd, 5 mg/L) showed nitro-oxidative imbalance associated with an enhanced antioxidant response. Cadmium notably elevated the level of nitric oxide, superoxide and peroxynitrite that stimulated nitrative modifications within the RNA and DNA pools in the phytopathogen structures. In contrast, the protein pool undergoing nitration was diminished confirming that protein tyrosine nitration is a flexible element of the oomycete adaptive strategy to heavy metal stress. Finally, to verify whether Cd is able to modify P. infestans pathogenicity, a disease index and molecular assessment of disease progress were analysed indicating that Cd stress enhanced aggressiveness of vr P. infestans towards various potato cultivars. Taken together, Cd not only affected hyphal growth rate and caused biochemical changes in P. infestans structures, but accelerated the pathogenicity as well. The nitro-oxidative homeostasis imbalance underlies the phytopathogen adaptive strategy and survival in the heavy metal contaminated environment.

Effect of the level of "Candidatus Liberibacter solanacearum" infection on the development of zebra chip disease in different potato genotypes at harvest and post storage.

Potato psyllid (Bactericera cockerelli Sulc)-transmitted "Candidatus Liberibacter solanacearum" (Lso) has been negatively impacting the potato industry in the United States as well as other potato-producing countries. Lso has been linked to a condition known as zebra chip (ZC) that affects yield and quality of potato tubers. Efforts to find sources of resistance to ZC have primarily focused on greenhouse evaluations based on a single inoculation time prior to harvest. Plant response to infection, however, could be influenced by the developmental stage of the host plant, and ZC may continue to develop after harvest. The objectives of this study were to quantify Lso inoculation success, Lso titer, ZC severity and Lso development during storage in eight potato genotypes. These evaluations were conducted on plants infested with Lso-positive psyllids at 77, 12, and 4 days before vine removal (DBVR). The evaluated genotypes were categorized according to their relative resistance to Lso and tolerance to ZC symptoms. Lso inoculation success in the genotype family A07781, derived from Solanum chacoense, was lower than that of the susceptible control ('Russet Burbank'). A07781-4LB and A07781-3LB genotypes were characterized relatively resistant to the pathogen and highly tolerant to ZC symptoms, while A07781-10LB was categorized as susceptible to Lso but relatively tolerant to symptom expression. In stored potatoes, increase in Lso concentrations was observed for all infestation times. However, significantly higher Lso titer was detected in tubers infested 12 DBVR and the effect was similar across genotypes. Overall, the A07781 family can be considered as a promising source of resistance or tolerance to ZC.

Comparison between tumors in plants and human beings: Mechanisms of tumor development and therapy with secondary plant metabolites.

BACKGROUND: Human tumors are still a major threat to human health and plant tumors negatively affect agricultural yields. Both areas of research are developing largely independent of each other. Treatment of both plant and human tumors remains unsatisfactory and novel therapy options are urgently needed. HYPOTHESIS: The concept of this paper is to compare cellular and molecular mechanisms of tumor development in plants and human beings and to explore possibilities to develop novel treatment strategies based on bioactive secondary plant metabolites. The interdisciplinary discourse may unravel commonalities and differences in the biology of plant and human tumors as basis for rational drug development. RESULTS: Plant tumors and galls develop upon infection by bacteria (e.g. Agrobacterium tumefaciens and A. vitis, which harbor oncogenic T-DNA) and by insects (e.g. gall wasps, aphids). Plant tumors are benign, i.e. they usually do not ultimately kill their host, but they can lead to considerable economic damage due to reduced crop yields of cultivated plants. Human tumors develop by biological carcinogenesis (i.e. viruses and other infectious agents), chemical carcinogenesis (anthropogenic and non-anthropogenic environmental toxic xenobiotics) and physical carcinogenesis (radioactivity, UV-radiation). The majority of human tumors are malignant with lethal outcome. Although treatments for both plant and human tumors are available (antibiotics and apathogenic bacterial strains for plant tumors, cytostatic drugs for human tumors), treatment successes are non-satisfactory, because of drug resistance and the severe adverse side effects. In human beings, attacks by microbes are repelled by cellular immunity (i.e. innate and acquired immune systems). Plants instead display chemical defense mechanisms, whereby constitutively expressed phytoanticipin compounds compare to the innate human immune system, the acquired human immune system compares to phytoalexins, which are induced by appropriate biotic or abiotic stressors. Some chemical weapons of this armory of secondary metabolites are also active against plant galls. There is a mutual co-evolution between plant defense and animals/human beings, which was sometimes referred to as animal plant warfare. As a consequence, hepatic phase I-III metabolization and excretion developed in animals and human beings to detoxify harmful phytochemicals. On the other hand, plants invented "pro-drugs" during evolution, which are activated and toxified in animals by this hepatic biotransformation system. Recent efforts focus on phytochemicals that specifically target tumor-related mechanisms and proteins, e.g. angiogenic or metastatic inhibitors, stimulators of the immune system to improve anti-tumor immunity, specific cell death or cancer stem cell inhibitors, inhibitors of DNA damage and epigenomic deregulation, specific inhibitors of driver genes of carcinogenesis (e.g. oncogenes), inhibitors of multidrug resistance (i.e. ABC transporter efflux inhibitors), secondary metabolites against plant tumors. CONCLUSION: The exploitation of bioactive secondary metabolites to treat plant or human tumors bears a tremendous therapeutic potential. Although there are fundamental differences between human and plant tumors, either isolated phytochemicals and their (semi)synthetic derivatives or chemically defined and standardized plant extracts may offer new therapy options to decrease human tumor incidence and mortality as well as to increase agricultural yields by fighting crown galls.

Isolation and structural characterization of a non-diketopiperazine phytotoxin from a potato pathogenic Streptomyces strain.

Two Streptomyces spp. strains responsible for potato common scab infections in Uruguay which do not produce diketopiperazines were identified through whole-genome sequencing, and the virulence factor produced by one of them was isolated and characterized. Phylogenetic analysis showed that both pathogenic strains can be identified as S. niveiscabiei, and the structure of the phytotoxin was elucidated as that of the polyketide desmethylmensacarcin using MS and NMR methods. The metabolite is produced in yields of ∼200 mg/L of culture media, induces deep necrotic lesions on potato tubers, stuns root and shoot growth in radish seedlings, and is comparatively more aggressive than thaxtomin A. This is the first time that desmethylmensacarcin, a member of a class of compounds known for their antitumor and antibiotic activity, is associated with phytotoxicity. More importantly, it represents the discovery of a new virulence factor related to potato common scab, an economically-important disease affecting potato production worldwide.

Threat of establishment of non-indigenous potato blackleg and tuber soft rot pathogens in Great Britain under climate change.

Potato blackleg and soft rot caused by Pectobacterium and Dickeya species are among the most significant bacterial diseases affecting potato production globally. In this study we estimate the impact of future temperatures on establishment of non-indigenous but confirmed Pectobacterium and Dickeya species in Great Britain (GB). The calculations are based on probabilistic climate change data and a model fitted to disease severity data from a controlled environment tuber assay with the dominant potato blackleg and soft rot-causing species in GB (P. atrosepticum), and three of the main causative agents in Europe (P. carotovorum subsp. brasiliense, P. parmentieri, Dickeya solani). Our aim was to investigate if the European strains could become stronger competitors in the GB potato ecosystem as the climate warms, on the basis of their aggressiveness in tubers at different temperatures. Principally, we found that the tissue macerating capacity of all four pathogens will increase in GB under all emissions scenarios. The predominant Pectobacterium and Dickeya species in Europe are able to cause disease in tubers under field conditions currently seen in GB but are not expected to become widely established in the future, at least on the basis of their aggressiveness in tubers relative to P. atrosepticum under GB conditions. Our key take-home messages are that the GB potato industry is well positioned to continue to thrive via current best management practices and continued reinforcement of existing legislation.

Nutritional status of the cauliflower cultivar 'verona' grown with omission of out added macronutrients.

Knowledge of plant nutritional status allows an understanding of the physiological responses of plants to crop fertilization. A hydroponic experiment evaluated the symptoms of macronutrient deficiency in cauliflower 'Verona' and determined: a) the macronutrient contents of foliar tissues when visual symptoms were observed, b) macronutrients content of foliar and inflorescence tissues at harvest. The effect of nutrient deficiency on inflorescence mass was also evaluated. Nitrogen deficiency caused chlorosis followed by purple color in the old leaves, while P deficiency caused only chlorosis in old leaves. Chlorosis at the edge of old leaves progressing to the center of the leaves was observed with the omission of K, and after was observed necrosis in the chlorotic areas. Ca deficiency caused tip burn in new leaves, while Mg deficiency caused internerval chlorosis in old leaves. The omission of each macronutrient reduced inflorescence dry matter. This deleterious effect was larger for N, P, and K deficiencies, reducing inflorescence dry matter by 87, 49, and 42%, respectively. When the nutrient solutions without N, P, K, Ca, or Mg were supplied to cauliflower plants, the macronutrient contents at harvest were 8.8, 0.6, 3.5, 13.0, and 0.8 g kg-1 in the foliar tissues and 27.3, 2.2, 21.6, 1.1, and 0.7 g kg-1 in the inflorescence tissues, respectively.

Transcriptome-wide identification of the genes responding to replanting disease in Rehmannia glutinosa L. roots.

The development of the medicinal plant Rehmannia glutinosa L. are severely declined when are replanted on the soil of the preceding crops being themselves. The biological basis of this so called "replanting disease" is unknown. Here, we have exploited the parallel sequencing capacity of both RNA-seq and DGE technology to ascertain what genes are responsive to the replanting disease in roots of R. glutinosa. RNA-seq analysis generated 99,708 non-redundant consensus sequences from the roots of the first year (R1) and the second year (R2) replanted R. glutinosa plants. From this set, a total of 48,616 transcripts containing a complete or partial encoding region was identified. Based on this resource, two DGE tag libraries were established to capture the transcriptome differences between the R1 and R2 libraries. Finally, a set of 2,817 (1,676 up- and 1,141 down-regulated) differentially transcribed genes was screened, and 114 most strongly differentially transcribed genes were identified by DGE analysis between first year and replanted plants. Furthermore, a more detailed examination of 16 selected candidates was carried out by qRT-PCR. The indication was that replanting could promote Ca(2+) signal transduction and ethylene synthesis, resulting in forming of the replanting disease. We analyzed the biomass indexes of replanted R. glutinosa roots by irrigating Ca(2+) signal blockers. The results suggested that the alleviation of the disease impairment could be the decrease of Ca(2+) signal transduction. This study provided a global survey of the root transcriptome in replanted R. glutinosa roots at the tuberous root expansion stage. As a result, a number of candidate genes underlying the replanting disease have been identified.

A novel approach of preventing Japanese cedar pollen dispersal that is the cause of Japanese cedar pollinosis (JCP) using pollen-specific fungal infection.

In Japan, Japanese cedar pollen dispersal is one of the major causes of pollinosis. Sydowia japonica is an ascomycetous fungus that grows exclusively on the male strobili of Japanese cedar, suggesting a possible mechanism for controlling pollen dispersal. To evaluate this possibility, eleven isolates of S. japonica were collected from around Japan and used as an inoculum to male strobili of Japanese cedar. The treatment demonstrated that the fungus infected only the pollen and prevented pollen dispersal. The fungus did not cause any additional symptoms to other parts of Japanese cedar, such as needles, stems, and buds. All S. japonica isolates collected around Japan could serve to control pollen dispersal. Periodic observation of the fungal pathogenesis with stereomicroscope and scanning electron microscope showed that hyphal fragments and conidia of S. japonica germinated on the surface of male strobili, and the germ tube entered pollen sacs through opening microsporophylls. Within the pollen sacs, the hyphae penetrated pollen gradually, such that all pollen was infected by the fungus by approximately one month before the pollen dispersal season. The infected pollen was destroyed due to the fungal infection and was never released. Our data suggests a novel approach of preventing pollen dispersal using pollen-specific fungal infection.

Role of pectin methylesterases in cellular calcium distribution and blossom-end rot development in tomato fruit.

Blossom-end rot (BER) in tomato fruit (Solanum lycopersicum) is believed to be a calcium (Ca²âº) deficiency disorder, but the mechanisms involved in its development are poorly understood. Our hypothesis is that high expression of pectin methylesterases (PMEs) increases Ca²âº bound to the cell wall, subsequently decreasing Ca²âº available for other cellular functions and thereby increasing fruit susceptibility to BER. The objectives of this study were to evaluate the effect of PME expression, and amount of esterified pectins and Ca²âº bound to the cell wall on BER development in tomato fruit. Wild-type and PME-silenced tomato plants were grown in a greenhouse. At full bloom, flowers were pollinated and Ca²âº was no longer provided to the plants to induce BER. Our results show that suppressing expression of PMEs in tomato fruit reduced the amount of Ca²âº bound to the cell wall, and also reduced fruit susceptibility to BER. Both the wild-type and PME-silenced fruit had similar total tissue, cytosolic and vacuolar Ca²âº concentrations, but wild-type fruit had lower water-soluble apoplastic Ca²âº content and higher membrane leakage, one of the first symptoms of BER. Our results suggest that apoplastic water-soluble Ca²âº concentration influences fruit susceptibility to Ca²âº deficiency disorders.

The possible role of factor C in common scab disease development.

The Gram-positive soil-borne streptomycetes exhibit a complex life cycle that is controlled by extracellular regulatory molecules. One interesting autoregulator is the protein factor C, originally isolated from the culture fluid of S. albidoflavus 45H. Southern hybridizations and database searches revealed that although homologues of factor C are not present in most Streptomyces strains, an exception is the plant pathogenic S. scabies , which causes common scab disease on potato. In S. scabies and related strains pathogenicity involves a large pathogenicity island that carries thaxtomin biosynthetic genes, the nec1 necrogenic factor and other putative virulence genes. Extracellular enzymes, including extracellular esterases, that attack the surface of the tubers and disintegrate the external protective layer are also known to be involved in pathogenicity. In S. albidoflavus 45H, factor C coordinates the expression of many secreted hydrolases. To find out whether esterase is also regulated by factor C, we made a factor C null mutant of strain 45H. The mutant showed a bald phenotype and was impaired in pathogenicity and esterase activity. This is a first indication that extracellular regulatory factors may play a role in the development of potato scab.

New hosts of Myrothecium spp. in Brazil and a preliminary in vitro assay of fungicides

Myrothecium roridum and M. verrucaria are two plant pathogenic species causing foliar spots in a large number of cultivated plants. This paper aims to study the causal agents of foliar spots in vegetable crops (sweet pepper, tomato and cucumber), ornamental plants (Spathiphyllum wallisii, Solidago canadensis, Anthurium andreanum, Dieffenbachia amoena) and a solanaceous weed plant (Nicandra physaloides). Most of the isolates were identified as M. roridum; only the isolate 'Myr-02' from S. canadensis was identified as M. verrucaria. All the isolates were pathogenic to their original plant hosts and also to some other plants. Some fungicides were tested in vitro against an isolate of M. roridum and the mycelial growth recorded after seven days. Fungicides with quartenary ammonium, tebuconazole and copper were highly effective in inhibiting the mycelial growth of M. roridum. This paper confirms the first record of M. roridum causing leaf spots in sweet pepper, tomato, Spathiphyllum, Anthurium, Dieffenbachia and N. physaloides in Brazil. We also report M. roridum as causal agent of cucumber fruit rot and M. verrucaria as a pathogen of tango plants.

Effects of fly attack (Bactrocera oleae) on the phenolic profile and selected chemical parameters of olive oil.

The phenolic fraction of virgin olive oil influences both its quality and oxidative stability. One of the principal threats of the quality of olive fruit is the olive fly ( Bactrocera oleae) as it alters the chemical composition. The attack of this olive pest has been studied in order to evaluate its influence on the quality of virgin olive oil (free acidity, peroxide value, fatty acid composition, water content, oxidative stability, phenols, and antioxidant power of phenolic fraction). The study was performed using several virgin olive oils obtained from olives with different degrees of fly infestation. They were acquired in different Italian industrial mills from the Abruzzo region. Qualitative and quantitative analyses of phenolic profiles were performed by capillary electrophoresis-diode array detection, and electrochemical evaluation of the antioxidant power of the phenolic fraction was also carried out. These analyses demonstrated that the degree of fly attack was positively correlated with free acidity ( r = 0.77, p < 0.05) and oxidized products ( r = 0.58, p < 0.05), and negatively related to the oxidative stability index ( r = -0.54, p < 0.05) and phenolic content ( r = -0.50, p < 0.05), mainly with secoiridoid compounds. However, it has been confirmed that the phenolic fraction of olive oil depends on several parameters and that a clear correlation does not exist between the percentages of fly attack and phenolic content.

[The virus infection progression in potato plants in conditions of simulated microgravitation].

The influence of simulated microgravitation (clinostating) on the proceeding of mixed virus infection in Zhuravushka variety potato plants, infected with potato viruses X and M (PVX, PVM) was investigated. It was shown, that sensitivity of "virus-plant" system to microgravitation depends both on pathogene species and clinostating mode. Vertical clinostating was found to be more effective than horizontal one. By means of biotest, PCR and ELISA it was detected that potato plants, infected with PVX and PVM are released from PVX under the influence of clinostating during 19-47 days. Assume, that microgravitation is able to induce some protective reactions in plants that influence virus reproduction.

[Free and conjugated forms of salicylic acid: content and role in potato].

Hydrolysis of conjugated forms of salicylic acid and accumulation of its free form was observed after infection of potato tubers (Solanum tuberosum L.) with an incompatible race of phytophthora or treatment with an elicitor (chitosan). Infection of tubers with a compatible race of the pathogen or treatment with a suppressor (laminarin) decreased both the degree of hydrolysis of conjugated forms of salicylic acid and the accumulation of its free form.

The potential use of plant pathogens against crops.

This paper illustrates the characteristics of plant pathogens that have been found to be of most relevance in offensive biological warfare programmes. It shows how states envisaged these pathogens might be used against crops. It assesses whether the Biological and Toxin Weapons Convention can deal adequately with this potential threat.

The passage of Potato leafroll virus through Myzus persicae gut membrane regulates transmission efficiency.

Potato leafroll virus (PLRV) is transmitted by aphids in a persistent manner. Although virus circulation within the aphid leading to transmission has been well characterized, the mechanisms involved in virus recognition at aphid membranes are still poorly understood. One isolate in our collection (PLRV-14.2) has been shown to be non- or only poorly transmitted by some clones of aphids belonging to the Myzus persicae complex. To determine where the transmission process was blocked within the aphid, three virus transmission procedures were used. PLRV-14.2 could not be transmitted, or was only very poorly transmitted, after acquisition from infected plants or from purified preparations. In contrast, it could be transmitted with more than 70% efficiency when microinjected. Therefore, it is concluded that the gut membrane was a barrier regulating passage of PLRV particles from the gut lumen into the haemocoel of M. persicae. Comparison of coat protein (CP) and readthrough protein (RTP) sequences between poorly and readily transmissible isolates showed that PLRV-14.2 differed from other PLRV isolates by amino acid changes in both of these proteins. It is hypothesized that at least some of the changes found in CP and/or RTP reduced virus recognition by aphid gut receptors, resulting in reduced acquisition and subsequent transmission of PLRV-14.2.

Mutants of Rhizobium tropici strain CIAT899 that do not induce chlorosis in plants.

Type B strains of Rhizobium tropici induce severe foliar chlorosis when applied at planting to seeds of symbiotic host and non-host dicotyledonous plants. A Tn5-induced mutant, designated CT4812, or R. tropici strain CIAT899 that was unable to induce chlorosis was isolated. Cloning and sequencing of the DNA flanking the transposon in CT4812 revealed that the Tn5 insertion is located in a gene similar to glnD, which encodes uridylyltransferase/uridylyl-removing enzyme in enteric bacteria. Two marker-exchange mutants with insertions in glnD also failed to induce chlorosis in bean (Phaseolus vulgaris) plants. The 5'-most insertion in glnD (in mutant strain ME330) abolished the ability of R. tropici to utilize nitrate as a sole carbon source, whereas a mutation in glnD further downstream (in mutant strain ME245) did not have an obvious effect on nitrate utilization. A gene similar to the Salmonella typhimurium virulence gene mviN overlaps the 3' end of the R. tropici glnD homologue. A mutation in mviN had no effect on the ability of CIAT899 to induce chlorosis in bean plants. Therefore the glnD homologue, but not mviN, appears to be required for induction of chlorosis in plants by R. tropici strain CIAT899. A high nitrogen: carbon ratio in the rhizosphere of bean plants also prevented R. tropici from inducing chlorosis in bean plants. Mutations in either the glnD homologue or mviN had no significant effect on root nodule formation or acetylene reduction activity. A mutation in mviN eliminated motility in R. tropici. The sequence data, the inability of the glnD mutant to utilize nitrate, and the role of the R. tropici glnD gene in chlorosis induction in plants, a process that is nitrogen regulated, suggest that glnD plays a role in nitrogen sensing in R. tropici as its homologues do in other organisms.

Rickettsial relative associated with papaya bunchy top disease.

The phylogeny of a previously unidentified, obligate laticifer-inhabiting bacterium associated with the papaya bunchy top disease was investigated. Portions of genes corresponding to those for 16S rRNA, the flavoprotein subunit of succinate dehydrogenase (SdhA), citrate synthase (GltA), and the 17-kDa rickettsial common antigen were isolated and sequenced from the non-cultivable bacterium from diseased plants. Comparative sequence analyses consistently indicated that the bacterium is a member of the alpha-subdivision of the Proteobacteria and of the genus Rickettsia. The rickettsia was detected by polymerase chain reaction in diseased, but not healthy, papaya tissues and in the leafhopper vector, Empoasca papayae, providing further evidence of the possible etiological role of the bacterium in the disease. Although Rickettsia have been found naturally in arthropods and can be pathogenic to humans and other vertebrates, this is the first evidence of its kind implicating a Rickettsia as a plant pathogen.