Effects of cytokinin on ethylene production and nodulation in pea (Pisum sativum) cv. Sparkle.

In this study, we were interested in learning if cytokinins play a role in the developmental process that leads to nodulation in the pea cv. Sparkle. We demonstrate that the application of the synthetic cytokinin BAP (6-benzyl-amino-purine) results in a number of nodulation-related changes. BAP stimulates the production of ethylene, a known inhibitor of nodulation. At low levels (up to 1 &mgr;M), BAP also stimulates nodulation but as its concentration is increased (up to 25 &mgr;M), nodule number decreases. In BAP-treated roots, the infection threads are abnormal; they are twisted, very knotty, and generally grow in a direction parallel to the root surface. In addition, the centers of cell division in the inner cortex are very few. Thus, BAP-treated Sparkle appears to phenocopy the low-nodulating pea mutant R50 [Guinel FC, Sloetjes LL (2000) Ethylene is involved in the nodulation phenotype of Pisum sativum R50 (sym 16), a pleiotropic mutant that nodulates poorly and has pale green leaves. J Exp Bot 51: 885-894]. However, it appears doubtful that there is a direct correlation between the actions of cytokinin and ethylene in causing a reduction in nodule organogenesis because nodulation is not restored by treating BAP-treated Sparkle with ethylene inhibitors.