Exogenous γ-aminobutyric acid (GABA) alleviates nitrogen deficiency by mediating nitrate uptake and assimilation in Andrographis paniculata seedlings.

γ-Aminobutyric acid (GABA) plays significant metabolic and signaling roles in plant stress responses. Recent studies have proposed that GABA alleviates plant nitrogen (N) deficient stress; however, the mechanism by which GABA mediates plant N deficiency adaptation remains not yet well understood. Herein we found in a medicinal plant Andrographis paniculata that 5 mmol L-1 exogenous GABA promoted plant growth under N deficient (1 mmol L-1 NO3-) condition, with remarkably increments in total N and NO3- concentrations in plants. GABA increased N assimilation and protein synthesis by up-regulating the activities and expression of N metabolic enzymes. GABA also increased the accumulation of α-ketoglutarate and malate, which could facilitate the assimilation of NO3-. Inhibition of NR by Na2WO4 counteracted the promoting effects of GABA on plant growth, and the effects of GABA were not affected by L-DABA and 3-MP, the inhibitors of GABA transaminase (GABA-T) and glutamate decarboxylase (GAD), respectively. These results suggested that the nutritional role of GABA was excluded in promoting plant growth under low N condition. The results of 15N isotopic tracing and NRTs transcription indicated that exogenous GABA could up-regulate NRT2.4 and NRT3.2 to increase plant NO3- uptake under N deficient condition. Interestingly, primidone, an inhibitor of GABA receptor, impeded the effects of GABA on plant growth and N accumulation. Thus, our results revealed that exogenous GABA acted as a signal to up-regulate NRTs via its receptor to increase NO3- uptake, and subsequently promoted NO3- assimilation to alleviate N deficiency in A. paniculata.

Concentration-dependent dual effects of exogenous sucrose on nitrogen metabolism in Andrographis paniculata.

The effects of exogenous sucrose (Suc) concentrations (0, 0.5, 1, 5, 10 mmol L-1) on carbon (C) and nitrogen (N) metabolisms were investigated in a medicinal plant Andrographis paniculata (Chuanxinlian). Suc application with the concentration of 0.5-5 mmol L-1 significantly promoted plant growth. In contrast, 10 mmol L-1 Suc retarded plant growth and increased contents of anthocyanin and MDA and activity of SOD in comparison to 0.5-5 mmol L-1 Suc. Suc application increased contents of leaf soluble sugar, reducing sugar and trerhalose, as well as isocitrate dehydrogenase (ICDH) activity, increasing supply of C-skeleton for N assimilation. However, total leaf N was peaked at 1 mmol L-1 Suc, which was consistent with root activity, suggesting that exogenous Suc enhanced root N uptake. At 10 mmol L-1 Suc, total leaf N and activities of glutamine synthase (GS), glutamate synthase (GOGAT), NADH-dependent glutamate dehydrogenase (NADH-GDH) and glutamic-pyruvic transaminase (GPT) were strongly reduced but NH4+ concentration was significantly increased. The results revealed that exogenous Suc is an effective stimulant for A. paniculata plant growth. Low Suc concentration (e.g. 1 mmol L-1) increased supply of C-skeleton and promoted N uptake and assimilation in A. paniculata plant, whereas high Suc concentration (e.g. 10 mmol L-1) uncoupled C and N metabolisms, reduced N metabolism and induced plant senescence.

Sulfur Regulates the Trade-Off Between Growth and Andrographolide Accumulation via Nitrogen Metabolism in Andrographis paniculata.

Nitrogen (N) and sulfur (S) are essential mineral nutrients for plant growth and metabolism. Here, we investigated their interaction in plant growth and andrographolide accumulation in medicinal plant Andrographis paniculata grown at different N (4 and 8 mmol·L-1) and S concentration levels (0.1 and 2.4 mmol L-1). We found that increasing the S application rate enhanced the accumulation of andrographolide compounds (AGCs) in A. paniculata. Simultaneously, salicylic acid (SA) and gibberellic acid 4 (GA4) concentrations were increased but trehalose/trehalose 6-phosphate (Tre/Tre6P) concentrations were decreased by high S, suggesting that they were involved in the S-mediated accumulation of AGCs. However, S affected plant growth differentially at different N levels. Metabolite analysis revealed that high S induced increases in the tricarboxylic acid (TCA) cycle and photorespiration under low N conditions, which promoted N assimilation and S metabolism, and simultaneously increased carbohydrate consumption and inhibited plant growth. In contrast, high S reduced N and S concentrations in plants and promoted plant growth under high N conditions. Taken together, the results indicated that increasing the S application rate is an effective strategy to improve AGC accumulation in A. paniculata. Nevertheless, the interaction of N and S affected the trade-off between plant growth and AGC accumulation, in which N metabolism plays a key role.

Organic nitrogen sources promote andrographolide biosynthesis by reducing nitrogen metabolism and increasing carbon accumulation in Andrographis paniculata.

Nitrogen (N) form affects secondary metabolites of medicinal plants, but the physiological and molecular mechanisms remain largely unknown. To fully understand the response of andrographolide biosynthesis to different N forms in Andrographis paniculata, the plants were fed with nutritional solution containing sole N source of nitrate (NO3-), ammonium (NH4+), urea or glycine (Gly), and the growth, carbon (C) and N metabolisms and andrographolide biosynthesis were analyzed. We found that plants grown in urea and Gly performed greater photosynthetic rate and photosynthetic N use efficiency (PNUE) than those grown in NO3- and NH4+. Organic N sources reduced the activities of enzymes involving in C and N metabolisms such as glutamine synthase (GS), glutamate synthase (GOGAT) and NADH-dependent glutamate dehydrogenase (NADH-GDH), invertase (INV), isocitrate dehydrogenase (ICDH) and glycolate oxidase (GO), resulting in reduced depletion of carbohydrates and increased starch accumulation. However, they enhanced andrographolide content by up-regulating the key genes in its biosynthetic pathway including HMGR, DXS, GGPS and ApCPS. Besides, NH4+ decreased leaf SPAD value, contents of soluble protein and amino acids and GO activity, but increased photosynthetic rate and contents of soluble sugar and starch in comparison to NO3-. Andrographolide biosynthesis was also up-regulated. The results revealed that increasing accumulation of carbohydrates, especially starch, was beneficial to the biosynthesis of andrographolide; organic N sources decreased carbohydrate depletion by reducing N metabolism, and promoted plant growth and andrographolide biosynthesis synergistically.