Bhavdish N. Johri

Impact of antifungals producing rhizobacteria on the performance of Vigna radiata in the presence of arbuscular mycorrhizal fungi

Plant growth-promoting rhizobacteria (PGPR) that produce antifungal metabolites are potential threats for the arbuscular mycorrhizal (AM) fungi known for their beneficial symbiosis with plants that is crucially important for low-input sustainable agriculture. To address this issue, we used a compartmented container system where test plants, Vigna radiata, could only reach a separate nutrient-rich compartment indirectly via the hyphae of AM fungi associated with their roots. In this system, where plants depended on nutrient uptake via AM symbiosis, we explored the impact of various PGPR. Plants were inoculated with or without a consortium of four species of AM fungi (Glomus coronatum, Glomus etunicatum, Glomus constrictum, and Glomus intraradices), and one or more of the following PGPR strains: phenazine producing (P+) and phenazine-less mutant (P−), diacetylphloroglucinol (DAPG) producing (G+) and DAPG-less mutant (G−) strains of Pseudomonas fluorescens, and an unknown antifungal metabolite-producing Alcaligenes faecalis strain, SLHRE425 (D). PGPR exerted only a small if any effect on the performance of AM symbiosis. G+ enhanced AM root colonization and had positive effects on shoot growth and nitrogen content when added alone, but not in combination with P+. D negatively influenced AM root colonization, but did not affect nutrient acquisition. Principal component analysis of all treatments indicated correlation between root weight, shoot weight, and nutrient uptake by AM fungus. The results indicate that antifungal metabolites producing PGPR do not necessarily interfere with AM symbiosis and may even promote it thus carefully chosen combinations of such bioinoculants could lead to better plant growth.

Antimycobacterial serratamolides and diacyl peptoglucosamine derivatives from Serratia sp.

The cyclodepsipeptide serratamolide A ( 1) and five closely related compounds together with three new glucosamine derivatives were isolated by bioactivity-guided chromatography from the XAD adsorber resin extract of a Serratia sp. The structures of the compounds were elucidated by 2D NMR and MS analyses. In addition to the known serratamolide A ( 1) with two C 10 alkyl chains, its derivatives always contained one C 10 chain combined with either C 12:1, C 12, C 11, C 9, or C 8 chains. The glucosamine derivatives contained a common core consisting of an N-butyl-alpha-glucopyranosylamide, which was acylated at the C-1 oxygen with valine. The differences between the derivatives arise from the nature of the acyl groups attached to the N-terminus of valine, which were identified as the linear fatty acid moieties C 16:1, C 15, or C 14. Each compound was present in two isomeric forms arising from racemization of the valine moiety. All compounds showed antibiotic activity against Mycobacterium diernhoferi and other rapidly growing mycobacteria.

Xylanolytic activity of thermophilicSporotrichum sp. andMyceliophthora thermophilum

Extracellular xylanase of 5 isolates of thermophilicSporotrichum sp. andMyceliophthora thermophilum was studied in media containing wheat straw, wheat bran, paddy straw, paddy husk, sugar cane bagasse and local grass (Cyanodon dactylon L). Maximum D-xylanase (0·92 IU/ml) was secreted on 6th day bySporotrichum sp. strain 1 in a basal medium that contained 5% (w/v) wheat straw; treatment of straw by 4% NaOH prior to fungal fermentation did not alter xylanase yields appreciably. Temperature and pH optima for enzyme production were 45°C and 5·0 respectively. Enzyme activity showed a temperature optima of 70°C, pH maxima of 5·5 and a substrate level of 3% larch wood xylan. Xylanase ofSporotrichum sp. strain 1 was inducible and highly thermostable; nearly 70% of activity was retained even after 5 min exposure at 80°C. Associated cellulolytic activity (0·12 IU/ml) was detected in the crude enzyme preparation.