The roots of nodulins.

Nodulin gene expression is an integral and highly specific part of the formation of nitrogenfixing nodules on the roots of leguminous plants. Dependent on the time of expression during root nodule development, nodulin genes can be divided into early and late nodulin genes. A brief overview of the functions assigned to early and late nodulins is presented. We hypothesize that nodulin genes originate from regular plant genes that evolved to fit the regulatory and/or physiological constraints of symbiotic nitrogen fixation. Data on nodulins and nodulin genes, nodulation taxonomy and nodule development are evaluated in the light of this hypothesis.