Sequential induction of nodulin gene expression in the developing pea nodule.

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Abstract
A set of cDNA clones have been characterized that represent early nodulin mRNAs from pea root nodules. By RNA transfer blot analyses, the different early nodulin mRNAs were found to vary in time course of appearance during the development of the indeterminate pea root nodule. In situ hybridization studies demonstrated that the transcripts were located in different zones, representing subsequent steps in development of the central tissue of the root nodule. ENOD12 transcripts were present in every cell of the invasion zone, whereas ENOD5, ENOD3, and ENOD14 transcripts were restricted to the infected cells in successive but partially overlapping zones of the central tissue. We conclude that the corresponding nodulin genes are expressed at subsequent developmental stages. The amino acid sequence derived from the nucleotide sequences of the cDNAs, in combination with the localization data, showed that ENOD5 is an arabinogalactan-like protein involved in the infection process, whereas ENOD3 and ENOD14 have a cysteine cluster suggesting that these are metal-binding proteins. Furthermore, we showed that there is a clear difference in the way Rhizobium induced the infection-related early nodulin genes ENOD5 and ENOD12. A factor acting over a long distance induced the ENOD12 gene, whereas a factor acting over a short distance activated the ENOD5 gene.