MSU-ERDA Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824.
Plant Physiol. 1978 Aug;62(2):305-12. doi: 10.1104/pp.62.2.305.
Barley (Hordeum vulgare L.) plants at the three-leaf stage were water-stressed by flooding the rooting medium with polyethylene glycol 6000 with an osmotic potential of -19 bars, or by withholding water. While leaf water potential fell and leaf kill progressed, the betaine (trimethylglycine) content of the second leaf blade rose from about 0.4 micromole to about 1.5 micromoles in 4 days. The time course of betaine accumulation resembled that of proline accumulation. Choline levels in unstressed second leaf blades were low (<0.1 micromole per blade) and remained low during water stress. Upon relief of stress, betaine-like proline-remained at a high concentration in drought-killed leaf zones, but betaine did not disappear as rapidly as proline from viable leaf tissue during recovery.When [methyl-(14)C]choline was applied to second leaf blades of intact plants in the growth chamber, water-stressed plants metabolized 5 to 10 times more (14)C label to betaine than control plants during 22 hours. When infiltrated with tracer quantities of [(14)C]formate and incubated for various times in darkness or light, segments cut from water-stressed leaf blades incorporated about 2- to 10-fold more (14)C into betaine than did segments from unstressed leaves. In segments from stressed leaves incubated with [(14)C]formate for about 18 hours in darkness, betaine was always the principal (14)C-labeled soluble metabolite. This (14)C label was located exclusively in the N-methyl groups of betaine, demonstrating that reducing equivalents were available in stressed leaves for the reductive steps of methyl group biosynthesis from formate. Incorporation of (14)C from formate into choline was also increased in stressed leaf tissue, but choline was not a major product formed from [(14)C]formate.These results are consistent with a net de novo synthesis of betaine from 1- and 2-carbon precursors during water stress, and indicate that the betaine so accumulated may be a metabolically inert end product.
在三叶期，大麦（Hordeum vulgare L.）植株通过用聚乙二醇 6000（渗透压势为-19 巴）淹没根系介质或断水来进行水分胁迫。随着叶片水势下降和叶片死亡的发展，第二叶片中的甜菜碱（三甲氨基甘氨酸）含量在 4 天内从约 0.4 微摩尔增加到约 1.5 微摩尔。甜菜碱积累的时间进程与脯氨酸积累相似。在未受胁迫的第二叶片中，胆碱水平较低（<0.1 微摩尔/叶片），在水分胁迫期间仍保持较低水平。胁迫解除后，干旱致死叶片区的类似脯氨酸的甜菜碱仍保持在高浓度，但在恢复过程中，与存活叶片组织中的脯氨酸相比，甜菜碱没有那么快消失。当[甲基-（14）C]胆碱被应用于生长室中完整植株的第二叶片时，水分胁迫下的植物在 22 小时内将 5 到 10 倍更多的（14）C 标记物代谢为甜菜碱，而对照植物则没有。用示踪量的[(14)C]甲酸盐渗透并在黑暗或光照下孵育不同时间后，从胁迫叶片中切下的片段比未胁迫叶片中的片段掺入约 2 到 10 倍更多的（14）C 到甜菜碱中。在黑暗中用[(14)C]甲酸盐孵育约 18 小时的胁迫叶片片段中，甜菜碱始终是主要的（14）C 标记可溶性代谢物。该（14）C 标记仅位于甜菜碱的 N-甲基基团中，表明在胁迫叶片中存在还原当量，可用于甲酸盐的还原步骤将甲基基团生物合成。从甲酸盐掺入（14）C 到胆碱的也在胁迫叶片组织中增加，但胆碱不是从[(14)C]甲酸盐形成的主要产物。这些结果与水分胁迫期间从头从 1-和 2-碳前体合成甜菜碱的净合成一致，并表明如此积累的甜菜碱可能是一种代谢惰性的终产物。
