A new class of plant homeobox genes is expressed in specific regions of determinate symbiotic root nodules

A cDNA containing a homeobox sequence was isolated from a soybean nodule-sp ecific expression library. This homeobox cDNA, Ndx (nodulin homeobox), repr esents a small gene family with at least two members in soybean (Glycine ma x) and three in Lotus japonicus. One complete 3304 bp Ndx cDNA from L. japo nicus encodes a protein, NDX, of 958 amino acids. An unusual type of homeod omain that differs in two of the most conserved amino acid positions in the consensus sequence is located close to the C-terminal and appears to be th e only DNA-binding domain. Weak Ndx gene expression in the root increases v ery shortly after infection with Rhizobium and remains throughout nodule de velopment. In situ hybridizations show cell-specific expression patterns th at suggest developmentally separate regions in maturing determinate nodules . Thus in the maturing nodule Ndx and leghemoglobin genes are expressed in a mutually exclusive fashion. The Ndx transcript is also detectable in the young nodule primordium. Ndx expression is not confined to the root nodule since Ndx is also expressed in shoot and root meristems, indicating that th e Ndx gene products might also be involved in developmental processes in ot her plant tissues.