Growth and nodulation of symbiotic Medicago truncatula at different levels of phosphorus availability

Abstract

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.

Keywords: Asparagine; Medicago truncatula; carbon and nitrogen metabolites; feedback; nitrogen fixation; nitrogenase activity; nodulation; phloem; phosphorus availability; plant growth; symbiosis capacity; translocation..

Fig. 1.

Effect of P supply on (A) dry matter (DM) formation and (B) the shoot/root ratio of M. truncatula plants hydroponically grown for 9 weeks with supplementation with the indicated concentrations of P. Data are the means of four replicates, with error bars representing SE values.

Fig. 2.

Effect of P supply on (A) the number of nodules, (B) total nodule dry matter (DM) accumulation, and (C) individual nodule DM formation in M. truncatula plants hydroponically grown for 9 weeks with supplementation with the indicated concentrations of P. Data are the means of four replicates, with error bars representing SE values.

Fig. 3.

Medicago truncatula leaves with (A) normal and (B) P toxicity symptoms. The plants were grown in nutrient solution at a P concentration of 12 μM or 15 μM, respectively. Toxicity started with a yellowing of the leaf margins, which quickly turned into necrotic tissues.

Fig. 4.

Effect of P supply on (A) phloem free amino acid composition, (B) phloem total free amino acid composition, and (C) relative share of phloem asparagine of the total amino acids in M. truncatula plants hydroponically grown for 9 weeks with supplementation with the indicated concentrations of P. Data are the means of four replicates, with error bars representing SE values. FW, fresh weight; AA, amino acids; ALA, alanine; ARG, arginine; ASN, asparagine; ASP, aspartic acid; GLU, glutamic acid; GLN, glutamine; GLY, glycine; HIS, histidine; ILE, isoleucine; LEU, leucine; LYS, lysine; MET, methionine; PHE, phenylalanine; SER, serine; THR, threonine; TYR, tyrosine; VAL, valine.

Fig. 5.

Effect of P supply on (A) concentrations of organic acids and (B) concentration of sugars in nodules of M. truncatula plants hydroponically grown for 9 weeks with supplementation with the indicated concentrations of P. Data are the means of four replicates, with error bars representing SE values. FW, fresh weight.

Fig. 6.

Simplified representation showing major metabolic routes of C and N metabolism and the key enzymes involved in nodules of an amide-producing legume. AAT, aspartate aminotransferase; ALA, alanine; ASN, asparagine; ASP, aspartic acid; GLN, glutamine; MDH, malate dehydrogenase; N₂ase, nitrogenase; OAA, oxaloacetate; PEP, phosphoenolpyruvate; PEPC, phosphoenolpyruvate carboxylase; SS, sucrose synthase.