Phenotypic and Genotypic Characterization of Microsymbionts of Acacia Species Plants Grown in South-Western Saudi Arabia

Abstract

Acacias are generally the most dominant tree species in Saudi Arabia and elsewhere in the Arabian Peninsula. In Saudi Arabia, the genetic diversity of rhizobia associated with these legumes has not been examined. The taxonomic diversity of bacteria from root nodules of acacia species growing in south-western regions of Saudi Arabia was investigated. Seventy-five bacterial strains were isolated from root nodules of five acacia species found in the study area. The bacteria were characterized for phenotypic properties and genomic diversity by rep-PCR. Nineteen representative strains from each rep-PCR pattern were further characterized by the sequence analysis of their 16S rRNA gene. The majority of the isolates were fast-growing rhizobia (90%), tolerated 3% NaCl (60%), and grew at temperatures up to 40 °C (68%). On the basis of 16S rRNA gene sequences, the isolates were classified to four known genera of nodule bacteria: Ensifer (28%), Mesorhizobium (28%), Rhizobium (25%), and Bradyrhizobium (10%). The genus Bradyrhizobium bacteria were identified only among Acacia negrii and A. asak isolates. This study is the first report on phenotypic and genomic characterization of acacia nodule symbionts deriving from Saudi Arabia. Acacia species formed symbiosis with diverse rhizobia inhabiting Saudi Arabia soil. Furthermore, non-nodulating endophytic bacteria were also described.