Research Notes Sequential Expression of Two Late Nodulin Genes in the Infected Cells of Alfalfa Root Nodules. Ted Allen. Department of Biology, Boston College, Chestnut Hill, MA 02167 U.S.A. Sandhya Raja, Geoffrey Ganter, and Kathleen Dunn. Department of Biology, Boston College, Chestnut Hill, MA 02167 U.S.A. MPMI 5:430-434. Accepted 18 May 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992.

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.