The Three Sisters of Agriculture: An Active Learning Activity on Symbiotic Nitrogen Fixation

Abstract

This active learning exercise introduces students to the plant microbiome and the contributions that bacteria make to plant growth and food production. The Three Sisters are an ancient indigenous practice in which three plant crops and the associated microbiome contribute to each other&rsquo;s growth. In this symbiotic relationship, the associated bacteria (<em>Rhizobium leguminosarum</em> biovar <em>phaseoli</em>) inhabit root nodules in bean plants, converting atmospheric nitrogen to ammonia through the process of nitrogen fixation. The plant takes up the ammonia as its nitrogen source, and provides the bacteria with organic carbon. The Lesson contains a pre-class reading, a 50-minute class session, and an after-class reading. In the first round of small group work during the class, students discuss and report back on the symbiosis between the three plants and between the bacteria and the plants, and the contributions to sustainable agriculture. In the second round of discussion and reporting, students discuss nitrogen fixation, emphasizing the <em>nod</em> genes for polypeptides involved in forming the root nodules and the <em>nif</em> genes that encode the nitrogenase enzyme complex that carries out nitrogen fixation. The after class reading provides students with an example of enhancing plant growth by adding nitrogen-fixing bacteria externally to beans. Altogether, this exercise provides students with a real life scenario relevant to sustainable agriculture. <em>Primary Image:</em>&nbsp;Nitrogen fixation in bean root nodules. Nodule formation starts with the secretion of flavonoids from bean roots, leading to the induction of nodulation factors in the bacteria. After root curling and nodule formation, the enzyme nitrogenase catalyzes the reaction from atmospheric nitrogen to plant accessible ammonia.