Two functional soybean genes encoding p34cdc2 protein kinases are regulated by different plant developmental pathways.

We have isolated two cDNA clones (cdc2-S5 and cdc2-S6) encoding p34cdc2 protein kinases, homologs of yeast cdc2/CDC28 genes, from a soybean nodule cDNA library. The two sequences share 90% sequence homology in the coding regions. The 5' and 3' noncoding regions are distinct from each other, however, indicating that at least two genes encode p34cdc2 protein kinases in soybean. Both sequences can rescue the cdc28 mutation in Saccharomyces cerevisiae but rescue it with different efficiency. Genomic Southern analysis showed the existence of two copies for each of these genes, which are not closely linked and are nonallelic. The relative expression level of the two soybean p34cdc2 genes varies in different tissues. Expression of cdc2-S5 is higher in roots and root nodules, whereas cdc2-S6 is more actively expressed in aerial tissues, indicating that regulation of these two p34cdc2 genes is coupled with plant developmental pathways. Expression of cdc2-S5 is, furthermore, enhanced after Rhizobium infection, whereas cdc2-S6 fails to respond, suggesting that cdc2-S5 plays a role in nodule initiation and organogenesis. This latter gene preferentially responds to auxin (alpha-naphthaleneacetic acid) treatment, indicating that phytohormones may be involved in the control of cell division mediated by Rhizobium infection. Thus, different p34cdc2 protein kinases may control cell division in different tissues in a multicellular organism and respond to different signals--e.g., phytohormones.