Turning lateral roots into nodules

Abstract

The evolutionary origin of legume root nodules that help them grow is revealed Nitrogen is critical for plant growth but must be acquired from the soil in reduced forms such as nitrate. Often this occurs through biological fixation whereby nitrogen-fixing bacteria, such as rhizobia, live symbiotically in root nodules of legumes. Theories concerning the evolutionary origins of legume root nodules range from them being highly modified stems or lateral roots (1, 2). The latter origin is supported by rhizobia inducing lateral root-like nodules in non-leguminous Parasponia by triggering pericycle cell divisions, which normally produce lateral roots (3). However, in legume roots, rhizobia also induce cortical cell divisions, triggering many morphological differences to lateral root organogenesis (2). Two studies—by Soyano et al. (4) on page 1021 of this issue and by Schiessl et al. (5)—report key regulatory components of lateral root organogenesis to have been hijacked in legumes to coordinate nodule formation. This could open avenues to engineer nitrogen fixation in nonlegume crops.