Phenotypic and Genotypic Diversity and Symbiotic Effectiveness of Rhizobia Isolated from Acacia sp. Grown in Morocco

Phenotypic and genotypic diversity as well as symbiotic effectiveness of twenty one symbionts isolated from nodules of four Acacia species (Acacia melanoxylon, A. mearnsii, A. saligna and Acacia tortillis) grown in Morocco were evaluated. Growth in various media (YMA, YMA-BBT, YMA-RC), colony morphology, tolerance to stress factors (temperature, pH, salt concentrations, various heavy metals) and different carbon sources were used as phenotypic markers. The genetic characterization was studied by Random Amplification of Polymorphic DNA (RAPD) and 16S rRNA gene sequencing. The symbiotic effectiveness of rhizobial isolates on acacia was evaluated using plant nodulation assay in controlled conditions. Results showed that twelve isolates were fast growing, all of the twenty one isolates tolerated pH between 4.8 and 8.8 and temperatures of 14, 22, 28, and 37°C, while only five could grow at 44°C and three at 55°C. Highest resistance to heavy metals was recorded for arsenate, mercury, chromium; in contrast, low resistance was noted for copper (15 isolates resisted to10 mg mL-1). Most of the isolates were able to grow in a wide range of carbohydrates. The genotypic analysis of the tested isolates showed that they could be regrouped into three principal clusters based on their RAPD profiles. Acacia saligna was principally nodulated by Bradyrhizobium sp. while the other species were nodulated by different rhizobial species. Both phenotypic and genotypic studies showed great rhizobial isolate’s diversity. The symbiotic effectiveness tests showed that two isolates (AsF and AsAg1) had high N2 fixing potential on Acacia saligna.