Ammonium uptake and metabolism alleviate PEG-induced water stress in rice seedlings

Ammonium (NH4+) can enhance the water stress induced drought tolerance of rice seedlings in comparison to nitrate (NO3−) nutrition. To investigate the mechanism involved in nitrogen (N) uptake, N metabolism and transcript abundance of associated genes, a hydroponic experiment was conducted in which different N sources were supplied to seedlings growing under water stress. Compared to nitrate, ammonium prevented water stress-induced biomass, leaf SPAD and photosynthesis reduction to a significantly larger extent. Water stress significantly increased root nitrate reductase (NR) and nitrite reductase (NiR) activities, but decreased leaf NiR and glutamate synthetase (GS) activities under NO3− supply, causing lower nitrate content in roots and higher in leaves. In contrast, under NH4+ supply root GS and glutamine oxoglutarate aminotransferase (GOGAT) activities were significantly decreased under water stress, but remained higher in leaves, compared to NO3− treatment, which was beneficial for the transport and assimilation of ammonium in leaves. 15N tracing assays demonstrated that rice 15N uptake rate and accumulation were significant reduced under water stress, but were higher in plants supplied with NH4+ than with NO3−. Therefore, the formers showed higher leaf soluble sugar, proline and amino acids contents, and in turn, associated with a higher photosynthesis rate and biomass accumulation. Most genes related to NO3− uptake and reduction in roots and leaves were down-regulated; however, two ammonium transporter genes closely related to NH4+ uptake (AMT1;2 and AMT1;3) were up-regulated in response to water stress. Overall, our findings suggest that ammonium supply alleviated waters tress in rice seedlings, mainly by increasing root NH4+ uptake and leaf N metabolism.