Signaling Pathway Research Unit, RIKEN Center for Sustainable Resource Science, 1-7-22, Suehiro-cho, Tsurumi, Yokohama 230-0045, Japan.
J Exp Bot. 2013 Jul;64(10):2701-12. doi: 10.1093/jxb/ert122. Epub 2013 May 16.
Medicago truncatula is an important model plant for characterization of P deficiency on leguminous plants at the physiological and molecular levels. Growth optimization of this plant with regard to P supply is the first essential step for elucidation of the role of P in regulation of nodulation. Hence, a study was carried out to address the growth pattern of M. truncatula hydroponically grown at different gradual increases in P levels. The findings revealed that M. truncatula had a narrow P regime, with an optimum P level (12 μM P) which is relatively close to the concentration that induces P toxicity. The accumulated P concentration (2.7 mg g(-1) dry matter), which is normal for other crops and legumes, adversely affected the growth of M. truncatula plants. Under P deficiency, M. truncatula showed a higher symbiotic efficiency with Sinorhizobium meliloti 2011 in comparison with S. meliloti 102F51, partially as a result of higher electron allocation to N2 versus H(+). The total composition of free amino acids in the phloem was significantly affected by P deprivation. This pattern was found to be almost exclusively the result of the increase in the asparagine level, suggesting that asparagine might be the shoot-derived signal that translocates to the nodules and exerts the down-regulation of nitrogenase activity. Additionally, P deprivation was found to have a strong influence on the contents of the nodule carbon metabolites. While levels of sucrose and succinate tended to decrease, a higher accumulation of malate was observed. These findings have provided evidence that N2 fixation of M. truncatula is mediated through an N feedback mechanism which is closely related to nodule carbon metabolism.
蒺藜苜蓿是研究豆科植物磷缺乏生理和分子机制的重要模式植物。优化其磷素供应的生长条件是阐明磷素在调控结瘤作用中的地位的首要前提。因此，我们进行了一项研究，探讨了在不同逐渐增加的磷素水平下，蒺藜苜蓿水培生长的生长模式。结果表明，蒺藜苜蓿的磷素需求范围较窄，最适磷素水平（12 μM P）与诱导磷素毒性的浓度较为接近。而对于其他作物和豆科植物来说，正常积累的磷素浓度（2.7 mg g(-1)干重）会对蒺藜苜蓿的生长产生不利影响。在磷素缺乏条件下，蒺藜苜蓿与 Sinorhizobium meliloti 2011 的共生效率高于 S. meliloti 102F51，这部分是由于电子分配给 N2 与 H(+)的比例更高所致。韧皮部游离氨基酸的总组成受磷素剥夺的显著影响。这种模式几乎完全是由于天冬酰胺水平的增加所致，表明天冬酰胺可能是从地上部转运到根瘤并下调固氮酶活性的信号物质。此外，磷素缺乏对根瘤碳代谢物的含量有强烈影响。虽然蔗糖和琥珀酸的水平趋于降低，但观察到苹果酸的积累更高。这些发现为蒺藜苜蓿的 N2 固定是通过与根瘤碳代谢密切相关的 N 反馈机制介导提供了证据。
