Unraveling the Root Nodule Microbiome of Red Gram

Red gram (pigeon pea, ‘arhar’ in Hindi, ‘kandi’ in Telugu) is an important staple crop that significantly contributes to the protein intake in vegetarian diets across many countries, including India. Often referred to as the “poor man’s protein,” red gram thrives as a rainfed crop in the semi-arid regions of the world. Roots of red gram from small and variously shaped nodules that engage in a symbiotic association with nitrogen-fixing bacteria, especially with Bradyrhizobium, one of the commonly known rhizobia. Red gram and other legumes as hosts benefit with the symbiotic association with the rhizobia, because the rhizobia convert atmospheric nitrogen—comprising over 75% of the air—into a form plants can use, thus reducing the need for external nitrogen fertilizers for small-scale farmers.

 

 

Nodules are not just filled with rhizobia alone, but several other microbes are also present in the nodules and around the nodules, which can impact the efficiency of the nodule in fixing atmospheric nitrogen. Prof. Appa Rao Podile and his team at the University of Hyderabad conducted a comprehensive metagenome analysis to explore the factors influencing the microbiome diversity within red gram nodules. They examined different red gram genotypes like Asha, Durga, and Mannem Konda Kandi grown in different types of soils (Alfisol, Vertisol, and Inceptisol), as well as a wild relative of red gram in its native soil.

Supported by the Science and Engineering Research Board (SERB), now known as Anusnadhan National Research Foundation (ANRF), under the Department of Science and Technology (DST), Govt of India, Prof. Appa Rao with his Ph.D. students Dr. Anirban Basu, Dr. Chalasani Danteswari and Dr. PVSRN Sarma, at the Department of Plant Sciences, School of Life Sciences, University of Hyderabad has taken up this systematic study on the microbiome of red gram nodule.

 

 

The presence of non-rhizobial bacterial endophytes in the red gram nodules was evident all through. The position of the nodule on the root, the type of soil in which the red gram is grown, the variety (genotype of the red gram), and other environmental factors that contribute to the composition of the nodule microbiome were revealed through this study. While the diversity of microbiome in the nodule is less compared to bulk soil, it was more specialized, indicting the role of host selection.

Another significant finding was that the microbiome of the wild relative of red gram was dominated by Bradyrhizobium while the cultivated varieties like Asha, Durga, and Mannem Konda Kandi had exhibited diverse bacterial communities. In the process of domestication, the native symbiosis appears to have been compromised.

 

The study also highlighted that soil properties played a more significant role than the host plant genetics in shaping the nodule microbiome, indicating the importance of soil in deciding the microbiome of plants. These findings were published this week in the high-impact journal “Environmental Microbiome”, published by Springer Nature.