Bathellier Camille, Tcherkez Guillaume, Mauve Caroline, Bligny Richard, Gout Elizabeth, Ghashghaie Jaleh
Laboratoire d'Ecologie, Systématique et Evolution (ESE), CNRS-UMR 8079 - IFR 87, Bâtiment 362, Université Paris-Sud, 91405-Orsay cedex, France.
Rapid Commun Mass Spectrom. 2009 Sep;23(18):2847-56. doi: 10.1002/rcm.4198.
The response of root metabolism to variations in carbon source availability is critical for whole-plant nitrogen (N) assimilation and growth. However, the effect of changes in the carbohydrate input to intact roots is currently not well understood and, for example, both smaller and larger values of root:shoot ratios or root N uptake have been observed so far under elevated CO(2). In addition, previous studies on sugar starvation mainly focused on senescent or excised organs while an increasing body of data suggests that intact roots may behave differently with, for example, little protein remobilization. Here, we investigated the carbon and nitrogen primary metabolism in intact roots of French bean (Phaseolus vulgaris L.) plants maintained under continuous darkness for 4 days. We combined natural isotopic (15)N/(14)N measurements, metabolomic and (13)C-labeling data and show that intact roots continued nitrate assimilation to glutamate for at least 3 days while the respiration rate decreased. The activity of the tricarboxylic acid cycle diminished so that glutamate synthesis was sustained by the anaplerotic phosphoenolpyruvate carboxylase fixation. Presumably, the pentose phosphate pathway contributed to provide reducing power for nitrate reduction. All the biosynthetic metabolic fluxes were nevertheless down-regulated and, consequently, the concentration of all amino acids decreased. This is the case of asparagine, strongly suggesting that, as opposed to excised root tips, protein remobilization in intact roots remained very low for 3 days in spite of the restriction of respiratory substrates.
根系代谢对碳源可利用性变化的响应对于整株植物的氮同化和生长至关重要。然而，目前对于完整根系碳水化合物输入变化的影响尚不清楚，例如，到目前为止，在二氧化碳浓度升高的情况下，观察到根冠比或根系氮吸收的值既有变小的情况，也有变大的情况。此外，先前关于糖饥饿的研究主要集中在衰老或切除的器官上，而越来越多的数据表明，完整根系的行为可能有所不同，例如，蛋白质再动员很少。在这里，我们研究了在连续黑暗条件下维持4天的菜豆（Phaseolus vulgaris L.）完整根系中的碳和氮初级代谢。我们结合了天然同位素（15）N/（14）N测量、代谢组学和（13）C标记数据，结果表明，完整根系在呼吸速率下降的情况下，至少3天内持续将硝酸盐同化为谷氨酸。三羧酸循环的活性降低，使得谷氨酸的合成通过回补磷酸烯醇式丙酮酸羧化酶固定得以维持。据推测，磷酸戊糖途径有助于为硝酸盐还原提供还原力。然而，所有生物合成代谢通量均下调，因此所有氨基酸的浓度均降低。天冬酰胺就是这种情况，这强烈表明，与切除的根尖不同，尽管呼吸底物受到限制，但完整根系中的蛋白质再动员在3天内仍然非常低。
