Characteristics of Indigenous Azospirillum spp. Associated with Peanut Nodules and Compatibility with Bradyrhizobium in Thailand

Abstract

Azospirillum is a free living N2 fixing rhizobacteria which can promote plant growth and plant productivity. This study was performed to investigate the occurrence and to isolate indigenous Azospirillum in the nodules of peanut (Arachis hypogaea L.) from three cultivated areas in Thailand. It was found that the population of Azospirillum in peanut nodules from three soil types by most probable number method with semisolid N free (Nf) medium were 2.4x104, 2.4x103 and 2.3x102 cfu/g respectively. Six bacterial isolates were obtained and were characterized. Based on their morphological, physiological and biochemical characteristics they were putative Azospirillum spp. All of them formed the blue pellicles below the surface of semisolid Nf medium. Their colonies on Congo red agar were red, opaque with dry consistency. They were all Gram negative rods. Based on the ability for utilization of glucose and trisodium citrate as carbon sources, they could be divided into three groups. From 16SrDNA sequencing analysis, 3 isolates belonged to A. oryzae, 2 isolates belonged to A. formosense and 1 isolate belonged to A. brasilense. A. oryzae isolate was similar as A. brasilense isolates for their high acetylene reduction activity (ARA) but A. brasilense produced higher IAA production while the IAA production were low. Among the six isolates, only A. oryzae isolates could solubilize phosphate and produced siderophore. Meanwhile A. formosense isolates had the lowest ARA while the IAA production were high and they had positive antagonism against Bradyrhizobium strain TAL 1000. A. brasilense was selected for further study for co-inoculation with Bradyrhizobium strain TAL 1000 in peanut in the future.