Nodulation potential and phenotypic diversity of rhizobia nodulating cowpea isolated from major growing areas of Ethiopia

Abstract

Rhizobia are legume specific, and they need to be characterized by different methods. This study assessed the nodulation potential and phenotypic diversity of rhizobia nodulating cowpea [Vigna unguiculata (L.) Walp.] isolated from major growing areas of Ethiopia. From 30 soil samples, 28 (93.3%) samples induced nodulation. Twenty-eight isolates obtained from nodulated plants were evaluated using presumptive tests and different phenotypic characteristics. On yeast extract mannitol agar (YEMA) containing Congo red (CR), the isolates produced colony colors that were whitish to pale pink. All isolates showed no growth on peptone glucose agar and the isolates were capable of growing on ketolactose medium but did not turn the media color to yellow. All the isolates were also confirmed to be gram-negative, rodshaped, and nonspore-forming. The colony texture study revealed that 78.57% of the isolates had a large mucoid colony, whereas 21.43% exhibited a large watery colony texture with diameters ranging between 1 and 5 mm. Depending on the colony color on YEMA containing bromothymol blue, 22 (78.57%) isolates were slow growing, whereas six (21.43%) isolates were fast-growing rhizobia. The cluster analysis based on phenotypic characteristics revealed a grouping of the isolates into two clusters at a similarity level of 75% and six further subclusters at a similarity level of ∼91%, indicating variations and diversity among the isolates. Overall, this study revealed significant variations among the cowpea nodulating rhizobial isolates and suggested that the soils of Ethiopia harbor populations of phenotypically diverse isolates. A further test to confirm these findings using genetic characterization and inclusion of reference strain is highly recommended. INTRODUCTIONCowpea [Vigna unguiculata (L.) Walp.] is a multifunctional legume crop that provides food for humans, feed for livestock, and serves as a valuable and dependable revenue-generating commodity for farmers and grain traders. Cowpea can fix Abbreviations: BTB, bromothymol blue; CR, Congo red; YEMA, yeast extract mannitol agar.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.