Robert Chênevert

Flavonoid levels in roots ofMedicago sativa are modulated by the developmental stage of the symbiosis and the root colonizing arbuscular mycorrhizal fungus

Abundant data on the effect of flavonoids on spore germination, hyphal growth and root colonization by AMF are available. Moreover, the flavonoid pattern in mycorrhizal roots changes, thus flavonoids have been suggested as arbuscular mycorrhizal signalling compounds. In our work we studied the accumulation of flavonoids in roots of Medicago sativa i) after the exposure of uncolonized roots to sterile solutions containing Glomus intraradices tissue, ii) at three different stages of colonization by G. mosseae, iii) colonized by G. mosseae, G. intraradices or Gigaspora rosea. We could show that flavonoid accumulation in M. sativa roots i) is induced before root colonization, pointing towards the presence of a fungal-derived signal, ii) depends on the developmental stage of the symbiosis and iii) depends on the root-colonizing arbuscular mycorrhizal fungus. The data presented indicate not only a time-specificity of the flavonoid accumulation during the mycorrhizal association, but also an arbuscular mycorrhizal fungal-specificity. The possible functions of the flavonoid pattern changes are discussed.

ATP binding by glutamyl‐tRNA synthetase is switched to the productive mode by tRNA binding

Aminoacyl‐tRNA synthetases catalyze the formation of an aminoacyl‐AMP from an amino acid and ATP, prior to the aminoacyl transfer to tRNA. A subset of aminoacyl‐tRNA synthetases, including glutamyl‐tRNA synthetase (GluRS), have a regulation mechanism to avoid aminoacyl‐AMP formation in the absence of tRNA. In this study, we determined the crystal structure of the ‘non‐productive’ complex of Thermus thermophilus GluRS, ATP and L‐glutamate, together with those of the GluRS·ATP, GluRS·tRNA·ATP and GluRS·tRNA·GoA (a glutamyl‐AMP analog) complexes. In the absence of tRNAGlu, ATP is accommodated in a ‘non‐productive’ subsite within the ATP‐binding site, so that the ATP α‐phosphate and the glutamate α‐carboxyl groups in GluRS· ATP·Glu are too far from each other (6.2 Å) to react. In contrast, the ATP‐binding mode in GluRS·tRNA· ATP is dramatically different from those in GluRS·ATP·Glu and GluRS·ATP, but corresponds to the AMP moiety binding mode in GluRS·tRNA·GoA (the ‘productive’ subsite). Therefore, tRNA binding to GluRS switches the ATP‐binding mode. The interactions of the three tRNAGlu regions with GluRS cause conformational changes around the ATP‐binding site, and allow ATP to bind to the ‘productive’ subsite.

Flavonoids released by carrot (Daucus carota) seedlings stimulate hyphal development of vesicular-arbuscular mycorrhizal fungi in the presence of optimal CO2 enrichment

Carbon dioxide has been previously identified as a critical volatile factor that stimulates hyphal growth ofGigaspora margarita, a vesiculararbuscular mycorrhizal fungus, and we determined the optimal concentration at 2.0%. The beneficial effect of CO2 on fungal development is also visible in the presence of stimulatory (quercetin, myricetin) or inhibitory (naringenin) flavonoids. Sterile root exudates from carrot seedlings stimulate the hyphal development ofG. margarita in the presence of optimal CO2 enrichment. Three flavonols (quercetin, kaempferol, rutin or quercetin 3-rutinoside) and two flavones (apigenin, luteolin) were identified in carrot root exudates by means of HPLC retention time. Flavonols like quercetin and kaempferol are known to have stimulatory effects on hyphal growth ofG. margarita.

Degradation of 2,4,6-trinitrotoluene by Serratia marcescens

A strain of Serratia marcescens, isolated from the soil of a contaminated site, degraded 2,4,6-trinitrotoluene (TNT) as the sole source of carbon and energy. At an initial concentration of 50mg , TNT was totally degraded in 48h under aerobic conditions in a minimal salt medium. Reduction intermediates (4-amino-2,6-dinitrotoluene and 2-amino-4,6-dinitrotoluene) were observed. The presence of a surfactant (Tween 80) is essential to facilitate rapid degradation.

Chemoenzymatic synthesis of both enantiomers of baclofen

Abstract Both enantiomers of baclofen have been synthesized in five steps from 4-chlorocinnamic acid. The key step is the highly stereoselective enzymatic hydrolysis of dimethyl 3-(4-chlorophenyl)glutarate by chymotrypsin.

Isolation and identification of flavonoids from Ri T-DNA-transformed roots (Daucus carota) and their significance in vesicular-arbuscular mycorrhiza

Abstract The functional symbiotic system using Ri T-DNA-transformed roots and the vesicular-arbuscular mycorrhizal fungus Gigaspora margarita has been used in several studies of mycorrhizal formation. From the extracts of these transformed roots six flavonoids were isolated and identified: three flavonols (quercetin, kaempferol, rutin or quercetin-3-rutinoside) and three flavones (apigenin, chrysin and luteolin). The exudates contained the same compounds with the exception of chrysin which was present in trace amounts. The significance of their presence in relation to their role as molecular signals in early events of symbiosis establishment is discussed.

Enzymatic desymmetrization of meso cis-2,6- and cis,cis-2,4,6-substituted piperidines. Chemoenzymatic synthesis of (5S,9S)-(+)-indolizidine 209D

Abstract The stereoselective acylation of meso piperidines 3a , b by vinyl acetate (solvent and acyl donor) in the presence of Candida antarctica lipase gave the corresponding (2 S ,6 R ) and (2 S ,4 R ,6 R ) monoesters 2a , b in high enantiomeric purity. (5 S ,9 S )-(+)-Indolizidine 209D was prepared in eight steps from (2 S ,6 R )- 2a .

Glutamylsulfamoyladenosine and pyroglutamylsulfamoyladenosine are competitive inhibitors of E. coli glutamyl-tRNA synthetase

5′-O-[N-(l-glutamyl)-sulfamoyl]adenosine is a potent competitive inhibitor of E. coli glutamyl-tRNA synthetase with respect to glutamic acid (Ki=2.8 nM) and is the best inhibitor of this enzyme. It is a weaker inhibitor of mammalian glutamyl-tRNA synthetase (Ki=70 nM). The corresponding 5′-O-[N-(l-pyroglutamyl)-sulfamoyl]adenosine is a weak inhibitor (Ki=15 μM) of the E. coli enzyme.

Chemoenzymatic synthesis of chiral β-azidoalcohols. Application to the preparation of chiral aziridines and aminoalcohols

Abstract From the microbiological reduction of 3-azido-2-octanone, 3-azido-4-phenyl-2-butanone and 1-azido-1-phenyl-2-propanone, homochiral isomers of the corresponding β-azidoalcohols were prepared. These “σ-bichiral” synthons were used to prepare all the stereoisomers of 2-methyl-3-n-pentylaziridine and 2-methyl-3-benzylaziridine and some homochiral aminoalcohols.

Photochemical rearrangement of phenyl benzoate in the presence of cyclodextrins and amylose

Abstract The photorearrangement of phenyl benzoate in aqueous medium is changed in the presence of cyclodextrins or amylose; the ortho/para migration ratio is altered in favor of the para position.