In situ Localization of Rhizobium mRNAs in Pea Root Nodules: nifA and nifH Localization. Wei-Cai Yang. Department of Molecular Biology, Agricultural University Wageningen, Dreijenlaan 3, 6703 HA Wageningen, Netherlands. Beatrix Horvath, Jan Hontelez, Albert Van Kammen, and Ton Bisseling. Department of Molecular Biology, Agricultural University Wageningen, Dreijenlaan 3, 6703 HA Wageningen, Netherlands.. MPMI 4:464-468. Accepted 7 May 1991. 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, 1991.

Here we demonstrate that Rhizobium mRNAs can efficiently be detected in developing root nodules with the in situ hybridization technique. We have been able to localize the Rhizobium nifH mRNA as well as the transcript of the regulatory nifA gene. Therefore, we expect that the in situ hybridization technique can generally be applied to locate Rhizobium mRNAs in root nodules. In pea nodules, the nifA and nifH mRNAs are first detectable in the third to fourth cell layer of the late symbiotic zone. In these cell layers, these mRNAs are detectable immediately at maximal levels. In older parts of the late symbiotic zone, the level of nifH mRNA remains constant, whereas the level of nifA mRNA decreases. Finally, the pattern of nif mRNA accumulation was compared with that of nodulin mRNAs.