The regulation of ammonium translocation in plants.
The regulation of ammonium translocation in plants.
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DOI：
10.1093/jexbot/53.370.883
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发表时间：
2002-04
影响因子：
6.9
通讯作者：
J. Schjoerring;Søren Husted;Gisela Mäck;M. Mattsson
中科院分区：
文献类型：
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作者：
J. Schjoerring;Søren Husted;Gisela Mäck;M. Mattsson
关于NH（+）（4）是否在木质部中从根转移到地上部存在很大争议。在这篇论文中，它表明，这样的易位确实可以发生，但其他代谢物，如氨基酸和胺的干扰可能会引起很大的不确定性木质部NH（+）（4）浓度的大小。消除干扰需要通过例如甲酸或甲醇来稳定样品。NH（+）（4）的后续定量应在中性pH下用2-巯基乙醇作为还原剂通过OPA-荧光法进行，因为该方法灵敏可靠。绝不能使用基于Berthelot反应的比色法，因为它们容易给出错误的结果。在以NO（-）（3）作为唯一氮源生长的油菜和番茄植株的木质部汁液和叶质外体溶液中，测量到显著浓度的NH（+）（4），超过1 mM。当NO（-）（3）被NH（+）（4）取代时，木质部汁液NH（+）（4）浓度随着外部浓度的增加和暴露于NH（+）（4）的时间而增加。高达11%的易位N由NH（+）构成（4）。谷氨酰胺合成酶（GS）将NH（+）（4）结合到谷氨酰胺中，但当供应高水平的NH（+）（4）时，根GS活性和表达受到抑制。在茎基部上方木质部汁液中测得的铵浓度与叶片质外体溶液中的NH（+）（4）浓度高度相关。幼叶往往有较高的质外体NH（+）（4）浓度比老非衰老叶。尽管木质部NH（+）（4）浓度下降，但NH（+）（4）通量（浓度乘以蒸腾水流量）随温度的升高而增加。叶质外体NH（+）（4）的恢复涉及叶肉细胞质膜中的高亲和力和低亲和力转运蛋白。目前的知识，这些转运和他们的监管进行了讨论。
Much controversy exists about whether or not NH(+)(4) is translocated in the xylem from roots to shoots. In this paper it is shown that such translocation can indeed take place, but that interference from other metabolites such as amino acids and amines may give rise to large uncertainties about the magnitude of xylem NH(+)(4) concentrations. Elimination of interference requires sample stabilization by, for instance, formic acid or methanol. Subsequent quantification of NH(+)(4) should be done by the OPA-fluorometric method at neutral pH with 2-mercaptoethanol as the reducing agent since this method is sensitive and reliable. Colorimetric methods based on the Berthelot reaction should never be used, as they are prone to give erroneous results. Significant concentrations of NH(+)(4), exceeding 1 mM, were measured in both xylem sap and leaf apoplastic solution of oilseed rape and tomato plants growing with NO(-)(3) as the sole N source. When NO(-)(3) was replaced by NH(+)(4), xylem sap NH(+)(4) concentrations increased with increasing external concentrations and with time of exposure to NH(+)(4). Up to 11% of the translocated N was constituted by NH(+)(4). Glutamine synthetase (GS) incorporates NH(+)(4) into glutamine, but root GS activity and expression were repressed when high levels of NH(+)(4) were supplied. Ammonium concentrations measured in xylem sap sampled just above the stem base were highly correlated with NH(+)(4) concentrations in apoplastic solution from the leaves. Young leaves tended to have higher apoplastic NH(+)(4) concentrations than older non-senescing leaves. The flux of NH(+)(4) (concentration multiplied by transpirational water flow) increased with temperature despite a decline in xylem NH(+)(4) concentration. Retrieval of leaf apoplastic NH(+)(4) involves both high and low affinity transporters in the plasma membrane of mesophyll cells. Current knowledge about these transporters and their regulation is discussed.