Nitrogen deficiency inhibits cell division-determined elongation, but not initiation, of rice tiller buds

Plant branching shoot number is strongly influenced by nitrogen (N) supply status. However, detailed descriptions of this phenomenon and the regulatory mechanism are lacking. In this study, we show that, in rice, in comparison to sufficient supply of ammonium sulfate or ammonium nitrate (2.5 or 5 mM N), low N (0.2 mM) or nitrate as the only N source limited shoot branching, i.e., tillering number. We observed that N deficiency did not affect the initiation, but suppressed the elongation of the tiller buds. We carried out in-situ hybridization of the tiller buds and showed that the expression of histone H4, a marker of S-phase in the cell cycle, could not be detected in the tiller buds that had stopped growing, indicating that cell division was suppressed in the tissues. Consistent with this finding, we further detected that the expression of other cell cycle marker genes was decreased in the N-deficient tiller buds in comparison to N-sufficient tiller buds. In addition, expression of the genes involved in the strigolactone pathway was induced in the tiller buds by N deficiency, in accordance with other reports. These results shed light on the importance of proper N application to control tiller bud outgrowth in grain production.