Preferential assimilation of NH4+ over NO3- in tea plant associated with genes involved in nitrogen transportation, utilization and catechins biosynthesis

Abstract

Physiological effects of ammonium (NH₄⁺) and nitrate (NO₃^-) on tea have confirmed that tea plants prefer NH₄⁺ as the dominant nitrogen (N) source. To investigate the possible explanations for this preference, studies of ¹⁵NH₄⁺ and ¹⁵NO₃- assimilation using hydroponically grown tea plants were conducted. During the time course of ¹⁵NH₄⁺ and ¹⁵NO₃^- assimilation, the absorption of ¹⁵N from ¹⁵NH₄⁺ was more rapid than that from ¹⁵NO₃^-, as there was a more efficient expression pattern of NH₄⁺ transporters compared with that of NO₃^- transporters. ¹⁵NH₄⁺-fed tea plants accumulated more ¹⁵N than ¹⁵NO₃^- fed plants, which was demonstrated by that genes related to primary N assimilation, like CsNR, CsNiR, CsGDH and CsGOGAT, were more affected by ¹⁵NH₄⁺ than ¹⁵NO₃^-. Markedly higher NH₄⁺ concentrations were observed in ¹⁵NH₄⁺-fed tea roots in comparison with NO₃^- treatment, whereas tea plants maintained a balanced concentration of NH₄⁺ in tea leaves under both these two N forms. This maintenance was achieved through the increased expression of genes involved in theanine biosynthesis and the inhibition of genes related to catechins derived from phenylpropanoid pathway. The current results suggest that efficient NH₄⁺ transportation, assimilation, and reutilization enables tea plant as an ammonium preferring plant species.

Keywords: (15)NH(4)(+); (15)NO(3)(−); Assimilation; Catechins; Tea plant; Theanine.