Complete (1) H and (13) C NMR spectral assignments for the glycoalkaloid dehydrocommersonine.

The normal levels and types of glycoalkaloids found in commercial varieties of potato (Solanum tuberosum) appear to present no hazard to human health. However when wild Solanum species are used in breeding endeavors, new and untested glycoalkaloids may be introduced. Recent studies of domestic crosses with a wild Solanum oplocense accession indicated that the levels of a non-indigenous glycoalkaloid appeared associated with reduced defoliation by the Colorado potato beetle. The non-indigenous glycoalkaloid was isolated from foliage of the wild S. oplocense accession and unambiguously characterized by high-resolution electrospray ionization mass spectrometry and NM analysis as the glycoalkaloid dehydrocommersonine.

A feruloyltyramine trimer isolated from potato common scab lesions.

(1)H NMR analysis established that a potential suberin intermediate isolated from potato common scab lesions contained three O-methyl groups, a phenylcoumaran-type linkage and a conjugated trans double bond. Mass spectral data determined its molecular formula as indicative of a dehydrotrimer structure formed from three feruloyltyramine units. (1)H and (13)C NMR correlation studies supported the structure as that of a grossamide unit (3) linked through its double bond to the feruloyl phenolic of a third feruloyltyramine group. Identification of the feruloyltyramine trimer (4) expands the number of cross-linked intermediates potentially involved in the suberization process and highlights the presence of a second type of inter-unit linkage available for synthesis of the poly-phenolic domains.

The thaxtomin phytotoxins: sources, synthesis, biosynthesis, biotransformation and biological activity.

Thaxtomin phytotoxins, first reported in 1989, are cyclic dipeptides (2,5-diketopiperazines) formed from the condensation of 4-nitrotrytophan and phenylalanine groups. Inclusion of a 4-nitroindole moiety that is an essential requirement for their phytotoxicity makes them unique amongst microbial generated metabolites. Individual thaxtomins differ only in the presence or absence of N-methyl and hydroxyl groups and their respective substitution sites. The name "thaxtomin" was assigned to these compounds in honor of the eminent American phytopathologist Roland Thaxter who first described the nature of the microorganisms responsible for their production. The great interest in the thaxtomins derives mainly from their established roles as virulence factors in the common scab of potato disease and their apparent ability to inhibit cellulose synthesis in developing plant cells. Surveys of comparable scab-inducing organisms from potato producing regions throughout the world have confirmed the generality of the phytotoxin associated process. Descriptions of the isolation, chemical structure determination, total synthesis, biosynthesis, transformation by microorganisms and biological activity of the thaxtomins are comprehensively reviewed.

Characterization of gamma-butyrolactone autoregulatory signaling gene homologs in the angucyclinone polyketide WS5995B producer Streptomyces acidiscabies.

Organisms belonging to the genus Streptomyces produce numerous important secondary metabolites and undergo a sophisticated morphological differentiation program. In many instances these processes are under the control of gamma-butyrolactone (GBL) autoregulatory systems. Streptomyces acidiscabies strain 84.104 produces the secondary metabolite aromatic angucyclinone polyketide WS5995B. In order to explore the role of GBL regulatory circuitry in WS5995B production and morphogenesis in S. acidiscabies, a gene cluster encoding GBL autoregulatory signaling homologs was identified and characterized. Two GBL receptor homologs, sabR and sabS, were found flanking a GBL synthase homolog sabA. Strains carrying mutations in sabS produced elevated levels of WS5995B and displayed conditional morphological defects reminiscent of defects seen in Streptomyces bldA mutants. Notably, sabS possesses a TTA codon predicted to be recognized by tRNA(leu). sabA mutants produced higher levels of WS5995B than the wild-type strain but to a lesser extent than the levels of WS5995B seen in sabS mutants. Purified recombinant SabR and SabS were tested for their abilities to bind predicted AT-rich autoregulatory element (ARE) boxes within the sabRAS region. SabS did not bind any DNA sequences in this region, while SabR bound an ARE box in the region upstream of sabS. Quantitative reverse transcription-PCR analysis revealed higher levels of sabS transcript in sabR mutants than in the wild-type strain, suggesting that sabS expression is repressed by SabR. Based on these data, we propose that the S. acidiscabies sabRAS genes encode components of a signaling pathway which participates in the regulation of WS5995B production and morphogenesis.

Synthesis and NMR characteristics of N-acetyl-4-nitro, N-acetyl-5-nitro, N-acetyl-6-nitro and N-acetyl-7-nitrotryptophan methyl esters.

N-acetyl-4-nitrotryptophan methyl ester (2), N-acetyl-5-nitrotryptophan methyl ester (3), N-acetyl-6-nitrotryptophan methyl ester (4) and N-acetyl-7-nitrotryptophan methyl ester (5) were synthesized through a modified malonic ester reaction of the appropriate nitrogramine analogs followed by methylation with BF(3)-methanol. Assignments of the (1)H and (13)C NMR chemical shifts were made using a combination of (1)H-(1)H COSY, (1)H-(13)C HETCOR and (1)H-(13)C selective INEPT experiments.

Characterization of cross-linked hydroxycinnamic acid amides isolated from potato common scab lesions.

Four feruloyl amides, N-trans-feruloyloctopamine (1), N-cis-feruloyloctopamine (2), N-trans-feruloyltyramine (3), N-cis-feruloyltyramine (4), a cross-linked N-trans-feruloyltyramine dimer (5), and a cross-linked N-cis-feruloyltyramine dimer (6) were isolated from potato common scab lesions. The compounds were purified by TLC and characterized by a combination of (1)H and (13)C NMR spectroscopic techniques. The presence of an accompanying minor complex of cross-linked dimers containing both feruloyltyramines and feruloyloctopamines was also demonstrated. This is the first characterization of cross-linked hydroxycinnamic acid amides associated with wound healing in potato (Solanum tuberosum) tubers.

Biotransformation of the Streptomyces scabies phytotoxin thaxtomin A by the fungus Aspergillus niger.

Of several hundred microorganisms randomly selected from the environment, only a fungal isolate identified as Aspergillus niger van Tiegham var. niger was found to transform the phytotoxin thaxtomin A to much less toxic metabolites. The rate and extent of transformation of thaxtomin A was tested under a variety of conditions, including different growth media, biomass concentrations, incubation periods, and shaker speeds. Under optimum conditions the fungus converted thaxtomin A into two major and five minor metabolites. The two major metabolites and three of the five minor metabolites were fully characterized by a combination of mass spectral and nuclear magnetic resonance techniques. When assayed on aseptically produced mini-tubers, the major metabolites proved to be much less phytotoxic than thaxtomin A.

Occurrence of Turnip Scab Caused by Phytotoxin-Producing Streptomyces spp.

Turnips (Brassica rapa L var. rapifera) purchased from a New Brunswick supermarket in the winter of 2003 contained discernable corky scab-like lesions on their surfaces. The turnips were surface-disinfested with a warm-water rinse, and microorganisms from excised lesions were isolated with methodologies developed for the specific isolation of Streptomyces spp. (1). Nine isolates morphologically characteristic of Streptomyces scabies strains (2) were subsequently examined for pathogenicity on potato mini-tubers and production of the scab phytotoxin, thaxtomin A (1). Four of the nine isolates proved pathogenic on potato mini-tubers. When grown on an oatmeal broth medium, only the pathogenic isolates generated thin-layer chromatographic detectable quantities of the requisite phytotoxin, thaxtomin A (1). Previous greenhouse studies have demonstrated the ability of pathogenic Streptomyces spp. isolated from scab lesions on potatoes to infect turnips (2). To our knowledge, this is the first investigation to confirm the presence of thaxtomin A producing isolates on field-grown turnips. References: (1) R. R. King et al. Am. Potato J. 68:675, 1991. (2) R. Loria et al. Plant Dis. 81:836, 1997.

More chemistry of the thaxtomin phytotoxins.

Chemical and biochemical studies indicated the possible involvement of N-acetyltryptophan and 4-nitrotryptophan as intermediates in biosynthesis of the thaxtomin phytotoxins. A search for other potential pathways indirectly resulted in the identification of three unusual thaxtomin analogues derived from the o-thaxtomin A isomer. Investigations to resolve the identity of a previously described thaxtomin A di-glucoside were not supportive of the proposed structure.