Evidence that synergism between potassium and nitrate enhances the alleviation of ammonium toxicity in rice seedling roots

Ammonium toxicity in plants is considered a global phenomenon, but the primary mechanisms remain poorly characterized. Here, we show that although the addition of potassium or nitrate partially alleviated the inhibition of rice seedling root growth caused by ammonium toxicity, the combination of potassium and nitrate clearly improved the alleviation, probably via some synergistic mechanisms. The combined treatment with potassium and nitrate led to significantly improved alleviation effects on root biomass, root length, and embryonic crown root number. The aberrant cell morphology and the rhizosphere acidification level caused by ammonium toxicity, recovered only by the combined treatment. RNA sequencing analysis and weighted gene correlation network analysis (WGCNA) revealed that the transcriptional response generated from the combined treatment involved cellulose synthesis, auxin, and gibberellin metabolism. Our results point out that potassium and nitrate combined treatment effectively promotes cell wall formation in rice, and thus, effectively alleviates ammonium toxicity.

Influence of Aluminum at Low pH on the Rhizosphere Processes of Masson Pine (Pinus Massoniana Lamb)

Trees in general are very tolerant of aluminum (Al, mainly Al3+ at pH ≦ 5.0), and the small effects seen in the contaminated soils may mislead people that the contamination is unimportant. We believe that the assessments with Al-sensitive Masson pine could have revealed a bigger difference. The key point of this study was to characterize the Al toxicity for Masson Pine. The objectives were to discover the specific eco-physiological relationship between pine roots and rhizosphere Al, and to investigate the Al effects on several parameters, measured in the rhizosphere of Masson pine. Masson pine seedlings were cultivated on a hydroponic setup. Through comprehensive laboratory dose-gradient experiments, Al-triggered composition of the root-released compounds and several rhizospheric parameters were determined by chromatography or spectroscopy. This study gives an important evidence of the Al-toxicity effects on the composition of root-released compounds and the root growth of Masson pine. Results showed that higher rhizospheric Al at pH 4.5 might contribute to increased release of sugars, and also could stimulate the release of oxalic acid and malic acid. The total of secreted amino acids were correlated with the rhizosphere Al. Zero additional Al induced no rhizosphere pH elevation, but Al-induced rhizosphere acidification (pH from 4.50 to 4.22) was observed at Al 100 µM. Greater additions of Al (>300 µM) suppressed the rhizosphere acidification at pH 3.92. Added Al had a negative effect on the dry weight of pine roots, but an opposite effect on Al accumulated in the roots was observed. The four endogenous hormones were also determined in the pine roots. Gibberellic acid (GA3) decreased, whereas abscisic acid (ABA) increased simultaneously with the addition of Al. Their inflexional concentrations were most frequently observed at 100 µM, which might be the threshold of Al toxicity for Masson pine. The secondary metabolites assayed have been studied in relation to the rhizospheric Al. The rhizosphere Al species at low pH can trigger pine roots to release the sugars (glucose, fructose + aldose), organic acids (oxalic acid, and malic acid), amino acids, secondary metabolites, and endogenous hormones during their growth. Meanwhile it also affected the growth of pine roots. This is an extensive study, which can help understanding the toxicity of Al to this important pioneer species of acid forest soils in south China.

Differential response of pea (Pisum sativum L.) genotypes to iron deficiency in relation to the growth, rhizosphere acidification and ferric chelate reductase activities

Calcareous soils are known problematic lands for agricultural systems because of the low availability of nutrients, particularly iron (Fe). The so-called strategy I plant (e. g. Pea, Pisum sativum L.) which groups dicotyledons and monocots other than grasses, developed root membrane activities that contribute to the improvement of Fe availability. Among the functions considered to be a critical phase in iron absorption is rhizosphere acidification by H-ATPase and Fe(III) reduced by Fe(III) chelate reducctase (FeCR). In order to experimentally investigate the importance of root FeCR in Fe nutrition, its relationship with rhizosphere acidification and the genotypic differences in response to iron deficiency in pea (Pisum sativum L.), a glasshouse experiment was conducted hydroponically on four genotypes Merveille de Kelvedon (MK); Lincoln (Lin); Douce de Provence (DP) and Alexandra (Alex). Plants of each genotype were distributed into two plots, the first one received full nutrient solution (+ Fe), the second one received nutrient solution devoid of iron (- Fe). Plant growth, Fe distribution, SPAD index and root acidification and ferric chelate reductase activities were evaluated. Fe deficiency decreased plant growth and SPAD index along with the significant increase of H-ATPase and FeCR activities. Some genotypic differences were observed as follows; Alex showed high tolerance to Fe deprivation as compared to other genotypes. Important H-ATPase and FeCR activities, high Fe use efficiency and adequate membrane efficiency are the main reasons for this tolerance. These physiological parameters could be used as tools of tolerance for further breeding programs

Appropriate NH4+/NO3– Ratio Triggers Plant Growth and Nutrient Uptake of Flowering Chinese Cabbage by Optimizing the pH Value of Nutrient Solution

Compared with sole nitrogen (N), the nutrition mixture of ammonium (NH4+) and nitrate (NO3–) is known to better improve crop yield and quality. However, the mechanism underlying this improvement remains unclear. In the present study, we analyzed the changes in nutrient solution composition, content of different N forms in plant tissues and exudates, and expression of plasma membrane (PM) H+-ATPase genes (HAs) under different NH4+/NO3– ratios (0/100, 10/90, 25/75, 50/50 as control, T1, T2, and T3) in flowering Chinese cabbage. We observed that compared with the control, T1 and T2 increased the economical yield of flowering Chinese cabbage by 1.26- and 1.54-fold, respectively, whereas T3 significantly reduced plant yield. Compared with the control, T1–T3 significantly reduced the NO3– content and increased the NH4+, amino acid, and soluble protein contents of flowering Chinese cabbage to varying extents. T2 significantly increased the N use efficiency (NUE), whereas T3 significantly decreased it to only being 70.25% of that of the control. Owing to the difference in N absorption and utilization among seedlings, the pH value of the nutrient solution differed under different NH4+/NO3– ratios. At harvest, the pH value of T2 was 5.8; in the control and T1, it was approximately 8.0, and in T3 it was only 3.6. We speculated that appropriate NH4+/NO3– ratios may improve N absorption and assimilation and thus promote the growth of flowering Chinese cabbage, owing to the suitable pH value. On the contrary, addition of excessive NH4+ may induce rhizosphere acidification and ammonia toxicity, causing plant growth inhibition. We further analyzed the transcription of PM H+-ATPase genes (HAs). HA1 and HA7 transcription in roots was significantly down-regulated by the addition of the mixture of NH4+ and NO3–, whereas the transcription of HA2, HA9 in roots and HA7, HA8, and HA10 in leaves was sharply up-regulated by the addition of the mixture; the transcription of HA3 was mainly enhanced by the highest ratio of NH4+/NO3–. Our results provide valuable information about the effects of treatments with different NH4+/NO3– ratios on plant growth and N uptake and utilization.

Will Phosphate Bio-Solubilization Stimulate Biological Nitrogen Fixation in Grain Legumes?

Biological nitrogen fixation (BNF) refers to a bacterially mediated process by which atmospheric N2 is reduced, either symbiotically or non-symbiotically, into ammonia (NH3) in the presence of the enzyme complex nitrogenase. In N2-fixing grain legumes, BNF is often hampered under low phosphorus (P) availability. The P status of legumes, particularly nodules, as well as P availability in the rhizosphere, play a vital role in regulating BNF. Aside from increasing P availability via fertilization, other plant traits (i.e., extensive rooting system and their spatial distribution, hyper-nodulation, root exudates, rhizosphere acidification, and heterogeneity) contribute to greater P uptake and hence more effective BNF. The positive interaction between P availability and BNF can be exploited through beneficial soil P solubilizing microorganisms (PSM). These microorganisms can increase plant-available P by modifying either rhizosphere soil processes or promoting plant traits, which lead to increased P uptake by the production of plant growth-promoting substances, both of which could indirectly influence the efficiency of BNF in legumes. In this review, we report on the importance of microbial P bio-solubilization as a pathway for improving BNF in grain legumes via PSM and P solubilizing bacteria (PSB). Because BNF in legumes is a P-requiring agro-ecological process, the ability of soil PSB to synergize with the rhizobial strains is likely a key belowground process worth investigating for advanced research aiming to improve rhizosphere biological functions necessary for sustainable legume-based cropping systems.

Synergism between potassium and nitrate enhances the alleviation of ammonium toxicity in rice seedling root

Ammonium toxicity in plants is considered a global phenomenon, but the primary mechanisms remain poorly characterized. Here, we showed that although the addition of potassium (K+) or nitrate (NO3−) partially alleviated the inhibition of rice root growth caused by ammonium toxicity, the coexistence of K+ and NO3− clearly improved the alleviation via a synergistic mechanism. The synergism led to significantly improved alleviation effects on root biomass, length, surface area, number and meristem cell number. The aberrant auxin distribution in root tips, rhizosphere acidification level and abnormal cell morphology in the root cap and elongation zone caused by ammonium toxicity could be recovered by this synergism. RNA sequencing and the weighted gene correlation network analysis (WGCNA) revealed that the mechanism of this synergism involves cellulose synthesis, auxin and gibberellin metabolism regulation at the transcription level.

Fertilizer Potential of Struvite as Affected by Nitrogen Form in the Rhizosphere

Struvite is increasingly considered a promising alternative to mined phosphorus (P) fertilizer. However, its solubility is very low under neutral to alkaline pH while it increases with acidification. Here, we investigated whether supplying ammonium to stimulate rhizosphere acidification might improve struvite solubility at the vicinity of roots and, ultimately, enhance P uptake by plants. Using a RHIZOtest design, we studied changes in soil pH, P availability and P uptake by ryegrass in the rhizosphere and bulk soil supplied with either ammonium or nitrate under three P treatments: no-P, triple super phosphate and struvite. We found that supplying ammonium decreased rhizosphere pH by more than three units, which in turn increased soluble P concentrations by three times compared with nitrate treatments. However, there was no difference between P treatments, which was attributed to the increase of soluble Al concentration in the rhizosphere, which subsequently controlled P availability by precipitating it under the form of variscite-like minerals (predicted using Visual MINTEQ). Moreover, although ammonium supply increased soluble P concentration, it did not improve P uptake by plants, likely due to the absence of P deficiency. Further studies, especially in low-P soils, are thus needed to elucidate the role of nitrogen form on P uptake in the presence of struvite. More generally, our results highlight the complexity of manipulating rhizosphere processes and stress the need to consider all the components of the soil-plant system.

Nitrate transporter 1.1 alleviates lead toxicity in Arabidopsis by preventing rhizosphere acidification

Exposure to Pb2+ increases NRT1.1-mediated uptake of nitrate in Arabidopsis and is associated with decreased uptake of Pb into the plant, which is a consequence of decreased acidification in the rhizosphere.

Partial defoliation of Brachypodium distachyon plants grown in petri dishes under low light increases P and other nutrient levels concomitantly with transcriptional changes in the roots

Background There have been few studies on the partial defoliation response of grass. It has been unclear how partial defoliation may affect roots at the levels of nutrient accumulation and transcriptional regulation. Hereby we report a comprehensive investigation on molecular impacts of partial defoliation by using a model grass species, Brachypodium distachyon. Results Our Inductively Coupled Plasma Mass Spectrometry analyses of B. distachyon revealed shoot- and root-specific accumulation patterns of a group of macronutrients including potassium (K), Phosphorus (P), Calcium (Ca), Magnesium (Mg), and micronutrients including Sodium (Na), iron (Fe), and Manganese (Mn). Meanwhile, our genome-wide profiling of gene expression patterns depicts transcriptional impacts on B. distachyon roots by cutting the aerial portion. The RNAseq analyses identified a total of 1,268 differentially expressed genes in B. distachyon with partial defoliation treatment. Our comprehensive analyses by means of multiple approaches, including Gene Ontology, InterPro and Pfam protein classification, KEGG pathways, and Plant TFDB, jointly highlight the involvement of hormone-mediated wounding response, primary and secondary metabolites, and ion homeostasis, in B. distachyon after the partial defoliation treatment. In addition, evidence is provided that roots respond to partial defoliation by modifying nutrient uptake and rhizosphere acidification rate, indicating that an alteration of the root/soil interaction occurs in response to this practice. Conclusions This study reveals how partial defoliation alters ion accumulation levels in shoots and roots, as well as partial defoliation-induced transcriptional reprogramming on a whole-genome scale, thereby providing insight into the molecular mechanisms underlying the recovery process of grass after partial defoliation.