The rhizosphere of the halophytic grass Sporobolus robustus Kunth hosts rhizobium genospecies that are efficient on Prosopis juliflora (Sw.) DC and Vachellia seyal (Del.) P.J.H. Hurter seedlings

Abstract

The aim of this study was to survey the abundance and genetic diversity of legume-nodulating rhizobia (LNR) in the rhizosphere of a salt-tolerant grass, Sporobolus robustus Kunth, in the dry and rainy seasons along a salinity gradient, and to test their effectiveness on Prosopis juliflora (SW.) DC and Vachellia seyal (Del.) P.J.H. Hurter seedlings. The results showed a significant decrease in LNR population density and diversity in response to salinity, particularly during the dry season. A phylogenetic analysis of the 16S-23S rRNA ITS region clustered the 232 rhizobium isolates into three genera and 12 distinct representative genotypes: Mesorhizobium (8 genotypes), Ensifer (2 genotypes) and Rhizobium (2 genotypes). Of these genotypes, 2 were only found in the dry season, 4 exclusively in the rainy season and 6 were found in both seasons. Isolates of the Mesorhizobium and Ensifer genera were more abundant than those of Rhizobium, with 55%, 44% and 1% of the total strains, respectively. The abundance of the Mesorhizobium isolates appeared to increase in the dry season, suggesting that they were more adapted to environmental aridity than Ensifer genospecies. Conversely, Ensifer genospecies were more tolerant of high salinity levels than the other genospecies. However, Ensifer genospeciesproved to be the most efficient strains on P. juliflora and V. seyal seedlings. We concluded that S. robustus hosts efficient rhizobium strains in its rhizosphere, suggesting its ability to act as a nurse plant to facilitate seedling recruitment of P. juliflora and V. seyal in saline soils.

Keywords: Bioremediation; Nitrogen fixation; Nurse plant; Phylogeny; Salt tolerance.