Coronatine, syringomycin, syringopeptin, tabtoxin, and phaseolotoxin are the most intensively studied phytotoxins of Pseudomonas syringae, and each contributes significantly to bacterial virulence in plants. Coronatine functions partly as a mimic of methyl jasmonate, a hormone synthesized by plants undergoing biological stress. Syringomycin and syringopeptin form pores in plasma membranes, a process that leads to electrolyte leakage. Tabtoxin and phaseolotoxin are strongly antimicrobial and function by inhibiting glutamine synthetase and ornithine carbamoyltransferase, respectively. Genetic analysis has revealed the mechanisms responsible for toxin biosynthesis. Coronatine biosynthesis requires the cooperation of polyketide and peptide synthetases for the assembly of the coronafacic and coronamic acid moieties, respectively. Tabtoxin is derived from the lysine biosynthetic pathway, whereas syringomycin, syringopeptin, and phaseolotoxin biosynthesis requires peptide synthetases. Activation of phytotoxin synthesis is controlled by diverse environmental factors including plant signal molecules and temperature. Genes involved in the regulation of phytotoxin synthesis have been located within the coronatine and syringomycin gene clusters; however, additional regulatory genes are required for the synthesis of these and other phytotoxins. Global regulatory genes such as gacS modulate phytotoxin production in certain pathovars, indicating the complexity of the regulatory circuits controlling phytotoxin synthesis. The coronatine and syringomycin gene clusters have been intensively characterized and show potential for constructing modified polyketides and peptides. Genetic reprogramming of peptide and polyketide synthetases has been successful, and portions of the coronatine and syringomycin gene clusters could be valuable resources in developing new antimicrobial agents.

https://mmbr.asm.org/content/mmbr/63/2/266.full.pdf

Pseudomonas syringae Phytotoxins: Mode of Action, Regulation, and Biosynthesis by Peptide and Polyketide Synthetases

Phenolic-storing cells : keys to programmed cell death and periderm formation in wilt disease resistance and in general defence responses in plants?

Preface.- List of contributors.- 1. Introduction to allelopathy Chou,C-H.- 2. Basic pathways for the origin of allelopathic compounds Seigler, D.S.- Methodological aspects. 3. Clues in the search of new herbicides Dayan, F.E., Duke, S.O.- 4. Distinguishing allelopathy from resource competition: the role of density Weidenhamer, J.D.- 5. Toxicity in allelopathy: in silico approach Lo Piparo, E. et al.- Physiological aspects of allelopathy. 6. Allelochemicals and photosynthesis Zhou, Y.H., Yu, J.Q.- 7. Cell cycle analyses for understanding growth inhibition Sanchez-Moreiras, A.M. et al. 8. Detoxification of allelochemicals. The case of bezoxazolin-2(3H)-one (BOA) Schulz, M. et al.- 9. Allelopathy and abiotic stress Pedrol, M.N. et al.- 10. Allelopathy and biotic stresses Gawronska, H., Golisz, A.- 11. Plant secondary metabolites. Targets and mechanisms of allelopathy Lotina-Hennsen, B. et al.- 12. Mitochondria as a site of allelochemical action Ishii-Iwamoto, E.L. et al.- Ecophysiology and allelopathy. 13. Weed germination, seedling growth and their lesson for allelopathy in agriculture Aliotta, A. et al.- 14. Allelopathy: A soil system perspective Blum, U.- 15. Microorganisms and allelopathy: A one-sided approach Vokou, D. et al.- Ecological aspects of allelopathy. 16. Ecological relationships and allelopathy Sinkkonen, A.- 17. Resistance and susceptibility of plant communities to invasion:revisiting rabotnov's ideas about community homeostasis Callaway, R.M., Hierro, J.L.- Allelopathy in different environments. 18. Allelopathy in marine ecosystems Graneli, E., Pavia, H.- 19. Allelopathy in aquatic environments Erhard, D.- 20. Forest ecosystems and allelopathy Reigosa, M.J., Gonzalez, L.- 21. Allelopathic interactions in agroecosystems Kohli, R.K. et al.- Applied aspects of allelopathy. 22. Playing with chemistry: studies on Orobanche spp. germination stimulants Macias, F.A. et al.- 23. Modes of action of phytotoxins from plants Duke, S.O., Dayan, F.E.- 24. Allelopathy in ecological sustainable agriculture Narwal, S.S.- 25. Parasitic weeds and allelopathy: from the hypothesis to the proof Qasem, J.R.-

Allelopathy : a physiological process with ecological implications

A drop-collapsing test for screening surfactant-producing microorganisms

Herbicides with new modes of action (MOAs) are badly needed due to the rapidly evolving resistance to commercial herbicides, but a new MOA has not been introduced in over 20 years. The greatest pest management challenge for organic agriculture is the lack of effective natural product herbicides. The structural diversity and evolved biological activity of natural phytotoxins offer opportunities for the development of both directly used natural compounds and synthetic herbicides with new target sites based on the structures of natural phytotoxins. Natural phytotoxins are also a source for the discovery of new herbicide target sites that can serve as the focus of traditional herbicide discovery efforts. There are many examples of strong natural phytotoxins with MOAs other than those used by commercial herbicides, which indicates that there are molecular targets of herbicides that can be added to the current repertoire of commercial herbicide MOAs.

http://www.plantphysiol.org/content/plantphysiol/early/2014/04/30/pp.114.239061.full.pdf

Natural Compounds as Next Generation Herbicides

Syringomycin (SR) production by Pseudomonas syringae in potato dextrose broth (PDB) still culture was approximately 20 to 136 times greater than that in potato dextrose agar (PDA) or in PDB shake cultures, respectively. Potato broth supplemented with 1·5% dextrose and 0·4% casamino acids was best for SR production when compared to other formulations of PDB, synthetic medium or biphasic media. Maximum production of SR corresponded with exponential cell growth of P. syringae in vitro . SR activity was completely partitioned into n -butanol from aqueous concentrates of acidified cultures. By means of ion-exchange chromatography (carboxy-methyl cellulose), thin-layer chromatography and Polyacrylamide gel electrophoresis, homogeneous preparations of SR were obtained with specific activities up to 47·9 units/μg. Eleven strains of P. syringae antibiotic toward Geotrichum candidum and representing a broad range of ecotypes were compared for the production of SR or other antibiotics. The only antibiotic isolated from ten of the strains was SR whereas the eleventh strain produced only syringotoxin (ST), a small biocidal polypeptide. Both SR and ST were phytotoxic and mimicked holcus spot on maize leaves at a threshold concentration of 50 units/ml. The threshold toxigenicity of SR to representative isolates of P. syringae ranged from 800 to 6400 units/ml compared with 100 units/ml for G. candidum .

Production and purification of syringomycin, a phytotoxin produced by Pseudomonas syringae

Methods were devised for the large-scale production of syringomycin using stainless steel trays which were poured with agar medium, and then covered with cellophane. The bacteria were grown on the surface of the cellophane. Syringomycin was then extracted from the cellophane and purified by ion exchange column chromatography and partition chromatography; it was a single, low molecular weight, ninhydrin-reactive compound. Nine ninhydrin-reactive spots were detected in the acid hydrolysate of syringomycin by two-dimensional chromatography. Eight of the spots corresponded in RF and color to amino acids tentatively identified as aspartic acid, d -glutamic acid, serine, glycine, α-alanine, valine, d -phenylalanine and d -lysine. Investigation of some of the properties of syringomycin showed that its antibiotic activity was labile in alkaline solutions, especially above pH 8. Syringomycin is soluble in water and the lower alcohols, and insoluble in ether and chloroform. Its isoelectric point is about pH 7. Syringomycin has a wide spectrum of antibiotic activity. A species of each of six genera of bacteria, four different fungal species, a unicellular algal species and a Daphnia species were all inhibited by low levels of syringomycin. The growth of Geotrichum candidum, the most sensitive of the organisms surveyed, was completely inhibited by 24 μg syringomycin per ml. Syringomycin produced phytotoxic symptoms in the leaves of peach shoots when the stems were immersed for 24 h in a water solution of syringomycin containing 600 μg syringomycin per ml. The stems of peach shoots became brown and necrotic after being immersed for one week in a solution containing 6 μg syringomycin per ml. Inoculations of peach shoots with Pseudomonas syringae showed that the symptoms produced by the bacterium were very similar to the phytotoxic symptoms produced by syringomycin. Inactivation of the antifungal activity of syringomycin by mild pH manipulation also decreased its phytotoxicity proportionately. An antibiotic which was indistinguishable from syringomycin was isolated from diseased host tissues. It was concluded that syringomycin is probably the major phytotoxin in the bacterial canker disease of peach trees.

Properties of syringomycin, a wide spectrum antibiotic and phytotoxin produced by Pseudomonas syringae, and its role in the bacterial canker disease of peach trees

Syringomycin, a polypeptide phytotoxin produced by Pseudomonas syringae , is involved in the disease development of bacterial canker of peach. Pathogenicity tests in several plant species indicated that disease development was not always related to the production of syringomycin on agar medium either by isolates of P. syringae , or other isolates such as P. mors-prunorum . At sublethal doses syringomycin inhibited uridylic acid incorporation in Geotrichum candidum but stimulated thymidylic acid incorporation. Syringomycin was found to inhibit tobacco mosaic virus local-lesion formation in tobacco and inhibit synthesis of the RNA phage MS2 in Escherichia coli . The nature of this inhibition was not completely determined, but for MS2 it appeared to involve an inhibition of phage attachment. Droplets of aqueous syringomycin solutions had a lower contact angle with polystyrene plastic than did water; this effect on surface tension indicated a possible membrane action for syringomycin. Further experiments revealed that syringomycin caused rapid hemolysis of rabbit red blood cells, and that leakage of 260 nm absorbing materials occurred from cells of G. candidum treated with syringomycin. Electron micrographs of syringomycin-treated G. candidum cells indicated that massive disruption of membrane integrity occurred with lethal doses, and that membrane abnormalities occurred at sublethal doses. Microautoradiography with 14 C-labeled syringomycin indicated that syringomycin was concentrated mainly over the plasmalemma and nuclear membranes. Inhibition of syringomycin activity by sterols and certain phospholipids in agar media indicated that the binding of syringomycin on the membranes could involve membrane lipids. Calcium and magnesium ions also inhibited syringomycin activity. Rabbit antisera for cell wall material from syringomycin-producing and non-producing isolates of P. syringae reacted strongly with syringomycin; this indicated that syringomycin was structurally related to cell wall materials. It appears that syringomycin is a haptenic group of a larger cell wall antigenic substance and that syringomycin may be a product of the cell wall metabolism of P. syringae . The primary action of syringomycin appears to be a rapid detergent-like lysis of cellular membranes. The action of syringomycin on nucleic acids is probably secondary to its action on membranes.

Studies on the mode of action and biogenesis of the phytotoxin syringomycin

Syringomycin (SR) is a phytotoxin produced by isolates of Pseudomonas syringae which cause holcus spot of maize. When purified to homogeneity and administered to maize leaves or cowpea seedlings. SR induced disease lesions typical of those produced by P. syringae. However, SR was not recovered from either diseased maize leaves or cowpea seedlings infected with P. syringae or in significant amounts from plants treated with SR. Treatment of excised maize leaves with a preparation of [14C]SR (sp. act. 475 ct/min or 20 units/μg) suggested that SR was metabolized to several substances during development of disease lesions. Growth of P. syringae in host tissue corresponded to bacterial populations in vitro which produced concentrations of SR (1600 to 3200 units/ml) many times the threshold concentration necessary to cause disease lesions on maize (approx. 50 units SR/ml). These results indicate that SR may be a primary determinant of disease caused by host-specific strains of P. syringae.

Rôle of syringomycin in holcus spot of maize and systemic necrosis of cowpea caused by Pseudomonas syringae

Bacterial isolates from blast lesions and stem cankers on various species of citrus trees were characterized as Pseudomonas syringae and were pathogenic when inoculated into petioles and axial buds of healthy Washington navel orange trees in both orchard and greenhouse tests. With one exception, B-455, the citrus isolates were host-specific. Inoculations of the citrus isolates into young Lovell peach stems and maize leaves caused no disease symptoms in contrast to the stone fruit ecotype, B-3A, which caused stem cankers on peach and the grass ecotype, 5D430, which caused holcus spot symptoms on maize. The citrus isolates produced variable patterns in their production of bacteriocins and sensitivities to bacteriophages; however, the production of syringtoxin (ST), a wide spectrum biocide and Phytotoxin produced by these isolates, was unique to the citrus ecotype of P. syringae . Healthy petioles of navel orange trees when treated with ST developed symptoms typical of citrus blast lesions. The biocidal spectrum and amino acid composition of toxin preparations from cultures of the citrus isolates, when compared to known ST and syringomycin, established the identity of the Phytotoxin from citrus isolates as ST.

Syringotoxin: a phytotoxin unique to citrus isolates of Pseudomonas syringae

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