Stem-nodule symbiosis and its unusual properties

Abstract

Although the phenomenon of stem nodulation was first reported in 1928 in Aeschynomene spp. by Hagerup (8), actual interest arose only after the recent discovery of Sesbania rostrata (4). S. rostrata is a remarkably fast-growing and high N₂-fixing legume with a great potential as a source of N for rice (11). This interest has led to the identification of yet another stem-nodulating species, Aeschynomene afraspera, with comparable or higher potential than S. rostrata (3). Stem nodulation, a phenomenon that evolved in nature as a result of adaptation to flooding, is characterized by several unique features: 1) increasing number of infection sites with plant growth, 2) cyclic nature of symbiosis, 3) tolerance for combined N, 4) presence of photosynthetic cells around the nodules, 5) dinitrogen-dependent growth and N₂ fixation by stem-nodule rhizobia, and 6) photosynthetic nature of stem-nodule rhizobia. Due to these unusual features, there has been extensive research on stem nodulation recently. This paper reviews the most recent findings on stem nodulation.