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Three strains of Streptomyces griseus (CAI-24, CAI-121 and CAI-127) and one strain each of Streptomyces africanus (KAI-32) and Streptomyces coelicolor (KAI-90) were reported by us as biocontrol agents against Fusarium wilt, caused by Fusarium oxysporum f. sp. ciceri (FOC), and as plant growth-promoters (PGP) in chickpea. In the present study, the combined effect of these Streptomyces strains as a consortium were assessed for their biocontrol potential against Fusarium wilt and PGP in chickpea. Based on their compatibility, biocontrol ability and PGP performance, two consortia were assembled, consortium-1 having all the five strains of Streptomyces sp. and consortium-2 having the two promising strains (CAI-127 and KAI-32). Under greenhouse conditions, consortium-1 and consortium-2 were found to reduce the Fusarium wilt disease incidence by 55% and 74%, while under field conditions, these were by 86% and 96% in year-1 and by 54% and 69% in year-2, respectively, when compared to the positive control (only FOC treated). Shoot samples treated with consortia + FOC contained significantly enhanced antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, glutathione reductase and phenylalanine ammonia-lyase, when compared to the positive control (only FOC treated) or the negative control samples (neither FOC nor consortia treated). When the consortia were evaluated for their PGP traits under field conditions in two chickpea cultivars, JG11 and ICCV2, and in two consecutive years, nodule number was found to enhance up to 25%, nodule weight up to 49%, leaf area up to 37%, leaf weight up to 43%, root weight up to 23%, shoot weight up to 35%, seed weight up to 30%, seed number up to 29%, total dry matter up to 22% and grain yield up to 22% over the un-inoculated control plants. This study had demonstrated that the selected consortium of Streptomyces spp. has a greater potential for biological control of Fusarium wilt disease and PGP in chickpea.

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Streptomycesalbus strain CAI-21 has been previously reported to have plant growth-promotion abilities in chickpea, pigeonpea, rice, and sorghum. The strain CAI-21 and its secondary metabolite were evaluated for their biocontrol potential against charcoal rot disease in sorghum caused by Macrophomina phaseolina. Results exhibited that CAI-21 significantly inhibited the growth of the pathogen, M. phaseolina, in dual-culture (15 mm; zone of inhibition), metabolite production (74% inhibition), and blotter paper (90% inhibition) assays. When CAI-21 was tested for its biocontrol potential under greenhouse and field conditions following inoculation of M. phaseolina by toothpick method, it significantly reduced the number of internodes infected (75% and 45% less, respectively) and length of infection (75% and 51% less, respectively) over the positive control (only M. phaseolina inoculated) plants. Under greenhouse conditions, scanning electron microscopic analysis showed that the phloem and xylem tissues of the CAI-21-treated shoot samples were intact compared to those of the diseased stem samples. The culture filtrate of the CAI-21 was purified by various chromatographic techniques, and the active compound was identified as “organophosphate” by NMR and MS. The efficacy of organophosphate was found to inhibit the growth of M. phaseolina in the poisoned food technique. This study indicates that S.albus CAI-21 and its active metabolite organophosphate have the potential to control charcoal rot in sorghum.

The Stenotrophomonas maltophilia k279a (Stm) Hex gene encodes a polypeptide of 785 amino acid residues, with an N-terminal signal peptide. StmHex was cloned without signal peptide and expressed as an 83.6 kDa soluble protein in Escherichia coli BL21 (DE3). Purified StmHex was optimally active at pH 5.0 and 40 °C. The Vmax, Km and kcat/Km for StmHex towards chitin hexamer were 10.55 nkat (mg protein)(-1), 271 μM and 0.246 s(-1) mM(-1), while the kinetic values with chitobiose were 30.65 nkat (mg protein)(-1), 2365 μM and 0.082 s(-1) mM(-1), respectively. Hydrolytic activity on chitooligosaccharides indicated that StmHex was an exo-acting enzyme and yielded N-acetyl-D-glucosamine (GlcNAc) as the final product. StmHex hydrolysed chitooligosaccharides (up to hexamer) into GlcNAc within 60 min, suggesting that this enzyme has potential for use in large-scale production of GlcNAc from chitooligosaccharides.

Main conclusion The findings of this study suggest that the selected five strains of Streptomyces spp. could be used for biological control of charcoal rot disease in sorghum.

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