Bailey Karen J, Leegood Richard C
Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
J Exp Bot. 2016 Apr;67(9):2901-11. doi: 10.1093/jxb/erw132. Epub 2016 Apr 6.
Measurements of amino acids in the guttation fluid and in the xylem exudates of cut leaves from intact plants provide evidence of the remarkable efficiency with which these nitrogenous compounds are reabsorbed from the xylem sap. This could be achieved by mechanisms involving intercellular transport and/or metabolism. Developmental changes in transcripts and protein showed that transcripts for phosphoenolpyruvate carboxykinase (PEPCK) increased from the base to the leaf tip, and were markedly increased by supplying asparagine. Supplying amino acids also increased the amounts of protein of PEPCK and, to a lesser extent, of pyruvate, Pi dikinase. PEPCK is present in the hydathodes, stomata and vascular parenchyma of rice leaves. Evidence for the role of PEPCK was obtained by using 3-mercaptopicolinic acid (MPA), a specific inhibitor of PEPCK, and by using an activation-tagged rice line that had an increase in PEPCK activity, to show that activation of PEPCK resulted in a decrease in N in the guttation fluid and that treatment by MPA resulted in an increase in amino acids in the guttation fluid and xylem sap towards the leaf tip. Furthermore, increasing PEPCK activity decreased the amount of guttation fluid, whereas decreasing PEPCK activity increased the amount of xylem sap or guttation fluid towards the leaf tip. The findings suggest the following hypotheses: (i) both metabolism and transport are involved in xylem recycling and (ii) excess N is the signal involved in modulating xylem hydraulics, perhaps via nutrient regulation of water-transporting aquaporins. Water relations and vascular metabolism and transport are thus intimately linked.
对完整植株伤流液和离体叶片木质部渗出液中氨基酸的测量,为这些含氮化合物从木质部汁液中被高效重吸收提供了证据。这可以通过涉及细胞间运输和/或代谢的机制来实现。转录本和蛋白质的发育变化表明,磷酸烯醇式丙酮酸羧激酶(PEPCK)的转录本从叶基部到叶尖增加,并且通过供应天冬酰胺显著增加。供应氨基酸也增加了PEPCK的蛋白质含量,以及在较小程度上增加了丙酮酸磷酸双激酶的含量。PEPCK存在于水稻叶片的排水器、气孔和维管束薄壁组织中。通过使用PEPCK的特异性抑制剂3-巯基吡啶甲酸(MPA),以及使用PEPCK活性增加的激活标签水稻品系,获得了PEPCK作用的证据,以表明PEPCK的激活导致伤流液中氮含量降低,而MPA处理导致伤流液和向叶尖的木质部汁液中氨基酸含量增加。此外,增加PEPCK活性减少了伤流液的量,而降低PEPCK活性增加了向叶尖的木质部汁液或伤流液的量。这些发现提出了以下假设:(i)代谢和运输都参与木质部循环,(ii)过量的氮是参与调节木质部水力的信号,可能是通过对水通道蛋白的营养调节。因此,水分关系与维管束代谢和运输密切相关。