Ammonium absorption and assimilation in roots of rice mutants knocking out OsACTPK1, an ACT-domain containing protein kinase like protein 1.

Abstract

The Gln-signaling system to regulate expression of certain genes for NH₄⁺ absorption and assimilation in rice roots after a supply of NH₄⁺ has been proposed. OsACTPKs, rice homologs of Arabidopsis Ser/Thr/Tyr protein kinase (AtSTYPK), have putative amino acid-binding ACT domains similar to those of bacterial Gln sensor GlnD. OsACTPK1 mRNA was cumulatively expressed in roots by daily applications of NH₄⁺. As a first step to examine whether OsACTPK1 may be concerned with the Gln-signaling, OsACTPK1-knockout rice mutants (KO lines) by Tos17 insertion were isolated and phenotypes of their seedlings grown under the supply of low to sufficient NH₄⁺ were investigated. Null lines, in which Tos17 had segregated out, were used as controls. Under sufficient NH₄⁺ treatments, inhibition of root elongation, promotion of shoot growth, and increases in total nitrogen and free amino acids contents were observed in KO lines. Analysis of ¹⁵NH₄⁺ uptake into roots revealed Vmax values of high affinity transport systems in KO lines were ca. 2-fold increased. These results suggest that OsACTPK1 may negatively regulate at least NH₄⁺ absorption in seedling roots under the supply of sufficient NH₄⁺.