Zheng Xin-Qiang, Hayashibe Etsuko, Ashihara Hiroshi
Department of Advanced Biosciences, Graduate Division of Human Environmental Science, Graduate School of Humanities and Sciences, Ochanomizu University, Bunkyo-ku, Tokyo, 112-8610, Japan.
J Exp Bot. 2005 Jun;56(416):1615-23. doi: 10.1093/jxb/eri156. Epub 2005 Apr 18.
Changes in trigonelline content and in biosynthetic activity were determined in the cotyledons and embryonic axes of etiolated mungbean (Phaseolus aureus) seedlings during germination. Accumulation of trigonelline (c. 240 nmol per pair of cotyledons) was observed in the cotyledons of dry seeds; trigonelline content decreased 2 d after imbibition. Trigonelline content in the embryonic axes increased with seedling growth and reached a peak (c. 380 nmol per embryonic axis) at day 5. Trigonelline content did not change significantly during the differentiation of hypocotyls, and the concentration was greatest in the apical 5 mm. Nicotinic acid and nicotinamide were better precursors for pyridine nucleotide synthesis than quinolinic acid, but no great differences were found in the synthesis of trigonelline from these three precursors. Trigonelline synthesis was always higher in embryonic axes than in cotyledons. Activity of quinolinate phosphoribosyltransferase (EC 2.4.2.19), nicotinate phosphoribosyltransferase (EC 2.4.2.11), and nicotinamidase (EC 3.5.1.19) was found in cotyledons and embryonic axes, but no nicotinamide phosphoribosyltransferase (EC 2.4.2.12) activity was detected. It follows that quinolinic acid and nicotinic acid were directly converted to nicotinic acid mononucleotide by the respective phosphoribosyltransferases, but nicotinamide appeared to be converted to nicotinic acid mononucleotide after conversion to nicotinic acid. Trigonelline synthase (nicotinate N-methyltransferase, EC 2.1.1.7) activity increased in the embryonic axes, but decreased in cotyledons during germination. [14C]Nicotinic acid and trigonelline absorbed by the cotyledons were transported to the embryonic axes during germination. Trigonelline had no effect on the growth of seedlings, but nicotinic acid and nicotinamide significantly inhibited the growth of roots. Based on these findings, the role of trigonelline synthesis in mungbean seedlings is discussed.
在发芽过程中，对黄化绿豆（Phaseolus aureus）幼苗的子叶和胚轴中葫芦巴碱含量及生物合成活性的变化进行了测定。在干种子的子叶中观察到葫芦巴碱的积累（约每对子叶240 nmol）；吸胀2天后葫芦巴碱含量下降。胚轴中的葫芦巴碱含量随着幼苗生长而增加，并在第5天达到峰值（约每胚轴380 nmol）。在 hypocotyls分化过程中，葫芦巴碱含量没有显著变化，且在顶端5 mm处浓度最高。烟酸和烟酰胺是比喹啉酸更好的吡啶核苷酸合成前体，但从这三种前体合成葫芦巴碱时未发现很大差异。胚轴中的葫芦巴碱合成始终高于子叶。在子叶和胚轴中发现了喹啉酸磷酸核糖基转移酶（EC 2.4.2.19）、烟酸磷酸核糖基转移酶（EC 2.4.2.11）和烟酰胺酶（EC 3.5.1.19）的活性，但未检测到烟酰胺磷酸核糖基转移酶（EC 2.4.2.12）的活性。由此可见，喹啉酸和烟酸分别通过各自的磷酸核糖基转移酶直接转化为烟酸单核苷酸，但烟酰胺似乎在转化为烟酸后再转化为烟酸单核苷酸。在发芽过程中，胚轴中的葫芦巴碱合酶（烟酸N-甲基转移酶，EC 2.1.1.7）活性增加，而子叶中的活性降低。发芽期间，子叶吸收的[14C]烟酸和葫芦巴碱被转运到胚轴。葫芦巴碱对幼苗生长没有影响，但烟酸和烟酰胺显著抑制根的生长。基于这些发现，讨论了葫芦巴碱合成在绿豆幼苗中的作用。
