Sequential expression of two late nodulin genes in the infected cells of alfalfa root nodules.

The Nms-22 and leghemoglobin (Lb) genes are expressed exclusively in the infected cells of alfalfa root nodules. Expression of these two late nodulin genes originated at distinct cellular boundaries within the symbiotic region of the nodule. The Nms-22 gene was expressed in all infected cells, including those just adjacent to the meristematic region. Lb gene expression was induced in older infected cells and was most prominent in the mature region of the nodule. Despite this temporal separation of gene expression, both the Nms-22 and Lb genes were expressed in nodules elicited by bacA mutants in which bacteroid development has been blocked just after release from the infection thread.