ABSTRACT
The interaction between the Brazilian pioneer legume Sesbania virgata and its microsymbiont Azorhizobium doebereinerae leads to the formation of nitrogen-fixing nodules on roots that grow either in well-aerated soils or in wetlands. We studied the initiation and development of nodules under these alternative conditions. To this end, light and fluorescence microscopy were used to follow the bacterial colonisation and invasion into the host and, by means of transmission electron microscopy, we could observe the intracellular entry. Under hydroponic conditions, intercellular invasion took place at lateral root bases and mature nodules were round and determinate. However, on roots grown in vermiculite that allows aerated growth, bacteria also entered via root hair invasion and nodules were both of the determinate and indeterminate type. Such versatility in entry and developmental plasticity, as previously described in Sesbania rostrata, enables efficient nodulation in both dry and wet environments and are an important adaptive feature of this group of semi-tropical plants that grow in temporarily flooded habitats.
Subject(s)
Azorhizobium/physiology , Plant Root Nodulation/physiology , Sesbania/physiology , Aluminum Silicates , Brazil , Floods , Green Fluorescent Proteins , Hydroponics , Microscopy, Electron, Transmission , Nitrogen Fixation , Plant Roots/microbiology , Plant Roots/physiology , Plant Roots/ultrastructure , Root Nodules, Plant/microbiology , Root Nodules, Plant/physiology , Root Nodules, Plant/ultrastructure , Sesbania/microbiology , Sesbania/ultrastructure , Symbiosis , WetlandsABSTRACT
Thirty-four rhizobium strains were isolated from root nodules of the fast-growing woody native species Sesbania virgata in different regions of southeast Brazil (Minas Gerais and Rio de Janeiro States). These isolates had cultural characteristics on YMA quite similar to Azorhizobium caulinodans (alkalinization, scant extracellular polysaccharide production, fast or intermediate growth rate). They exhibited a high similarity of phenotypic and genotypic characteristics among themselves and to a lesser extent with A. caulinodans. DNA:DNA hybridization and 16SrRNA sequences support their inclusion in the genus Azorhizobium, but not in the species A. caulinodans. The name A. doebereinerae is proposed, with isolate UFLA1-100 (=BR5401, =LMG9993=SEMIA 6401) as the type strain.