Use of hairy root system to study signaling pathways during nodule formation

Nodule formation by a specialized group of plants is one of the most beneficial plant-environment interactions, where atmospheric nitrogen is biologically fixed into ammonia, which is subsequently converted to nitrates and amino acids. The molecular basis of nodule formation has been studied in detail, and work done in the past few years has led to seminal discoveries, connecting the initial signal perception by the root hairs to the downstream signaling events and finally to cellular and developmental changes that result in organogenesis and nodule formation. Although the physiology of biological nitrogen fixation has been well known for many years, the exquisite molecular details of nodule formation have been made possible, mostly, by using the transgenic hairy roots on composite plants. Development of hairy roots by Agrobacterium rhizogenes (A. rhizogenes) infection provides an excellent experimental system to rapidly and efficiently evaluate the effect of changes in the expression of specific genes or gene families on a range of root phenotypes. By using this system, the Nod factor receptor-mediated signaling has been linked to the infection thread formation and nodule organogenesis, two critical events of nodulation. The use of hairy root system has made it possible to uncover the details of signaling and developmental events using molecular genetics, genomics, proteomics, and cell biological approaches, making the nodulation signaling pathway one of the best understood in leguminous plants. This article provides an overview of multiple rhizobium-legume interaction studies that utilized the hairy root system to uncover the signaling pathways and offers perspectives on its future uses in the context of the development of novel gene-editing capabilities in plants.