Alteration of nitrogen metabolism in rice variety 'Nipponbare' induced by alkali stress

Aims Alkali stress (AS) is an important agricultural contaminant and has complex effects on plant metabolism, specifically root physiology. The aim of this study was to test the role of nitrogen metabolism regulation in alkali tolerance of rice variety 'Nipponbare'. Methods In this study, the rice seedlings were subjected to salinity stress (SS) or AS. Growth, the contents of inorganic ions, NH4+-nitrogen (free amino acids), and NO3−-nitrogen in the stressed seedlings were then measured. The expression of some critical genes involved in nitrogen metabolism were also assayed to test their roles in the regulation of nitrogen metabolism during adaptation of rice variety 'Nipponbare' to AS. Results AS showed a stronger inhibiting effect on rice variety 'Nipponbare' growth than SS. AS may have more complex effects on nitrogen metabolism than SS. Conclusions Effects of AS on the nitrogen metabolism of rice variety 'Nipponbare' mainly comprised two mechanisms. Firstly, in roots, AS caused the reduction of NO3− content, which caused two harmful consequences, the large downregulation of OsNR1 expression and the subsequent reduction of NH4+ production in roots. On the other hand, under AS (pH, 9.11), almost all the NH4+ was changed to NH3, which caused a severe deficiency of NH4+ surrounding the roots. Both events might cause a severe deficiency of NH4+ in roots. Under AS, the increased expression of several OsAMT family members in roots might be an adaptative response to the reduction of NH4+ content in roots or the NH4+ deficiency in rhizosphere. Also, the down-regulation of OsNADH-GOGAT and OsGS1;2 in roots might be due to NH4+ deficiency in roots. Secondly, in shoots, AS caused a larger acuumulatiuon of Na+, which possibly affected photorespiration and led to a continuous decrease of NH4+ production in shoots, and inhibited the expression of OsFd-GOGAT and OsGS2 in chloroplasts.