The importance of utilizing nitrate (NO3-) over ammonium (NH4+) as nitrogen source during detoxification of exogenous thiocyanate (SCN-) in Oryza sativa.

Abstract

Thiocyanate (SCN-) is a nitrogen-containing pollutant, which can be involved in the nitrogen (N) cycle and interferes with plant growth. The current study highlights a new insight into the N (nitrate [NO3-] and ammonium [NH4+]) utilization ways in rice seedlings under SCN- exposure to clarify the interactive effect on uptake and assimilation between these N-containing chemicals. Phenotypically, relative growth rates (RGR) of NO3--fed seedlings were significantly higher than NH4+-fed rice seedlings at the same SCN- concentration. Both N fertilizations have no significant influence on SCN- content and its assimilation in rice seedlings. However, significant accumulation of NO3- and NH4+ were detected in shoots prior to roots under SCN- stress. Enzymatic assay and mRNA analysis showed that the carbonyl sulfide (COS) pathway of SCN- degradation occurred in both roots and shoots of NO3--fed seedlings but only evident in roots of NH4+-fed seedlings. Moreover, the effect of SCN- on the activity of nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (GOGAT) was negligible in NO3--fed seedlings, while GOGAT activity was significantly inhibited in shoots of NH4+-fed seedlings. Nitrogen use efficiency (NUE) estimation provided positive evidence in utilizing NO3- over NH4+ as the main N source to support rice seedling growth during detoxification of exogenous SCN-. Overall, SCN- pollution has unexpectedly changed the rice preference for N source which shifted from NH4+ to NO3-, suggesting that the interactions of SCN- with different N sources in terms of uptake and assimilation in rice plants should not be overlooked, especially at the plant N nutritional level.