Shoot branching in response to nodal roots is mimicked by application of exogenous cytokinin in Trifolium repens

In nodally-rooting prostrate herbs the outgrowth of shoot axillary buds is highly influenced by the supply of a branch-promoting signal exported from nodal roots to the shoot. The aim of this study was to establish whether cytokinin could be a candidate for the positive component within this net root stimulus (NRS). The approach taken was based on the notion that should cytokinin be the activating signal, then the effects on bud outgrowth induced by exogenous supply of cytokinin (6-benzylaminopurine (BAP)) to plants should largely mimic the responses observed when experimental manipulations alter intra-plant supply of NRS. In Trifolium repens experimental results consistently indicated that supply of BAP into the stem vasculature induced responses mimicking those induced by manipulation of NRS supply: it induced the outgrowth of a similar number of distal axillary buds, activated buds to a similar extent, had similar properties of transport along stems, induced a similar dose dependent response in distal buds and also had the ability to induce bud outgrowth in P-deficient plants. These findings indicate a requirement for further detailed hormonal analytical work to confirm this result and identify the nature of the cytokinin(s) involved in the NRS signalling pathway.