A possible role of cytokinin in mediating long-distance nitrogen signaling in the promotion of sylleptic branching in hybrid poplar

Nitrogen fertilization of roots enhances shoot growth in plants and cytokinins are known to initiate bud outgrowth in shoots. Is it possible that root-derived cytokinins may play a role in long-distance signaling for nitrogen availability in the promotion of sylleptic branching in hybrid poplar? Nitrogen fertilization in the form of 5 mM NH4NO3, KNO3 or NH4Cl was applied to roots of three hybrid poplar clones exhibiting contrasting degrees of sylleptic branching. Cytokinin (0.1-1 mM benzyladenine, BA) was applied directly to lateral buds of shoots. Glutamate, asparagine and glutamine were also applied as drops to buds or as foliar sprays. NH4NO3, KNO3 and NH4Cl all usually enhanced sylleptic branching within a week in the high sylleptic clone (11-11) but in four out of five trials there was no effect in the low sylleptic clone (47-174). NH4NO3 added directly to buds had no effect. Also, glutamate, asparagine and glutamine had no effect. However, 1 mM BA promoted lateral bud outgrowth in all three clones. These results are consistent with the long-distance nitrogen signaling hypothesis of Forde and Sakakibara wherein nitrogen is transduced to cytokinin via enhanced ipt activity in the roots and is translocated up the shoot with the subsequent promotion of leaf/bud outgrowth.