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Beneficial rhizobacteria promote plant growth and protect plants against phytopathogens. Effective colonization on plant roots is critical for the rhizobacteria to exert beneficial activities. How bacteria migrate swiftly in the soil of semisolid or solid nature remains unclear. Here we report that sucrose, a disaccharide ubiquitously deployed by photosynthetic plants for fixed carbon transport and storage, and abundantly secreted from plant roots, promotes solid surface motility (SSM) and root colonization by Bacillus subtilis through a previously uncharacterized mechanism. Sucrose induces robust SSM by triggering a signaling cascade, first through extracellular synthesis of polymeric levan, which in turn stimulates strong production of surfactin and hyper-flagellation of the cells. B. subtilis poorly colonizes the roots of Arabidopsis thaliana mutants deficient in root-exudation of sucrose, while exogenously added sucrose selectively shapes the rhizomicrobiome associated with the tomato plant roots, promoting specifically bacilli and pseudomonad. We propose that sucrose activates a signaling cascade to trigger SSM and promote rhizosphere colonization by B. subtilis . Our findings also suggest a practicable approach to boost prevalence of beneficial Bacillus species in plant protection.

Magnesium (Mg) is an essential nutrient for a wide array of fundamental physiological and biochemical processes in plants. It largely involves chlorophyll synthesis, production, transportation, and utilization of photoassimilates, enzyme activation, and protein synthesis. As a multifaceted result of the introduction of high-yielding fertilizer-responsive cultivars, intensive cropping without replenishment of Mg, soil acidification, and exchangeable Mg (Ex-Mg) leaching, Mg has become a limiting nutrient for optimum crop production. However, little literature is available to better understand distinct responses of plants to Mg deficiency, the geographical distribution of soil Ex-Mg, and the degree of Mg deficiency. Here, we summarize the current state of knowledge of key plant responses to Mg availability and, as far as possible, highlight spatial Mg distribution and the magnitude of Mg deficiency in different cultivated regions of the world with a special focus on China. In particular, ~55% of arable lands in China are revealed Mg-deficient (< 120 mg kg −1 soil Ex-Mg), and Mg deficiency literally becomes increasingly severe from northern (227–488 mg kg −1 ) to southern (32–89 mg kg −1 ) China. Mg deficiency primarily traced back to higher depletion of soil Ex-Mg by fruits, vegetables, sugarcane, tubers, tea, and tobacco cultivated in tropical and subtropical climate zones. Further, each unit decline in soil pH from neutral reduced ~2-fold soil Ex-Mg. This article underscores the physiological importance of Mg, potential risks associated with Mg deficiency, and accordingly, to optimize fertilization strategies for higher crop productivity and better quality.

Magnesium deficiency is a frequently occurring limiting factor for crop production due to low levels of exchangeable Mg (ex-Mg) in acidic soil, which negatively affects sustainability of agriculture development. How Mg fertilization affects crop yield and subsequent physiological outcomes in different crop species, as well as agronomic efficiencies of Mg fertilizers, under varying soil conditions remain particular interesting questions to be addressed. A meta-analysis was performed with 570 paired observations retrieved from 99 field research articles to compare effects of Mg fertilization on crop production and corresponding agronomic efficiencies in different production systems under varying soil conditions. The mean value of yield increase and agronomic efficiency derived from Mg application was 8.5% and 34.4 kg kg respectively, when combining all yield measurements together, regardless of the crop type, soil condition, and other factors. Under severe Mg deficiency (ex-Mg < 60 mg kg ), yield increased up to 9.4%, nearly two folds of yield gain (4.9%) in the soil containing more than 120 mg kg ex-Mg. The effects of Mg fertilization on yield was 11.3% when soil pH was lower than 6.5. The agronomic efficiency of Mg fertilizers was negatively correlated with application levels of Mg, with 38.3 kg kg at lower MgO levels (0-50 kg ha ) and 32.6 kg kg at higher MgO levels (50-100 kg ha ). Clear interactions existed between soil ex-Mg, pH, and types and amount of Mg fertilizers in terms of crop yield increase. With Mg supplementation, Mg accumulation in the leaf tissues increased by 34.3% on average; and concentrations of sugar in edible organs were 5.5% higher compared to non-Mg supplemented treatments. Our analysis corroborated that Mg fertilization enhances crop performance by improving yield or resulting in favorable physiological outcomes, providing great potentials for integrated Mg management for higher crop yield and quality.

Magnesium (Mg) deficiency is becoming a widespread limiting factor for crop production. How crops adapt to Mg limitation remains largely unclear at the molecular level. Using hydroponic-cultured tomato seedlings, we found that total Mg 2+ content significantly decreased by ∼80% under Mg limitation while K + and Ca 2+ concentrations increased. Phylogenetic analysis suggested that Mg transporters (MRS2/MGTs) constitute a previously uncharacterized 3-clade tree in planta with two rounds of asymmetric duplications, providing evolutionary evidence for further molecular investigation. In adaptation to internal Mg deficiency, the expression of six representative MGT s (two in the shoot and four in the root) was up-regulated in Mg-deficient plants. Contradictory to the transcriptional elevation of most of MGT s, Mg limitation resulted in the ∼50% smaller root system. Auxin concentrations particularly decreased by ∼23% in the Mg-deficient root, despite the enhanced accumulation of gibberellin, cytokinin, and ABA. In accordance with such auxin reduction was overall transcriptional down-regulation of thirteen genes controlling auxin biosynthesis ( TAR / YUCs ), transport ( LAXs, PINs ), and signaling ( IAAs, ARFs ). Together, systemic down-tuning of gene expression in the auxin signaling pathway under Mg limitation preconditions a smaller tomato root system, expectedly stimulating MGT transcription for Mg uptake or translocation.

Topography can significantly influence the atmospheric deposition of heavy metals on the surface soil and further shape their spatial distribution. This study was carried out in Northwestern Guizhou Province, a key agricultural region contaminated with cadmium (Cd), lead (Pb), and zinc (Zn) in agricultural soil. In this study, geographical detectors were employed to quantitatively assess the impact of topography on the spatial distribution of Cd, Pb, and Zn in surface soils. Moreover, the influence of topography on the spatial distribution of Cd, Pb, and Zn in the surface soil was further ascertained using geographical detectors. The results demonstrated that the bulk atmospheric deposition fluxes of Cd in Dawan (DW), Dongfeng (DF), Jinzhong (JZ), Shuanglong (SL), and Heishitou (HST) were 1.538, 0.766, 0.802, 0.365, and 0.186 mg m − 2  a − 1 , respectively; For Pb, the corresponding values were 47.13, 34.88, 38.25, 16.95 and 35.76 mg m − 2  a − 1 , respectively; in the case of Zn, the fluxes were 579.26, 82.38, 83.20, 45.98, and 29.13 mg m − 2  a − 1 , respectively. The atmospheric deposition fluxes of Cd, Pb, and Zn demonstrated a general downward trend from DW to SL because of rainfall reduction with increasing elevation. Industrial activities can significantly increase atmospheric deposition fluxes of Cd, Pb, and Zn. However, the atmospheric deposition fluxes of Cd, Pb, and Zn in the control area HST were significantly lower due to the interception effect of mountain. The results of the Pb isotopic analysis further supported the above conclusions. It also indicated that the atmospheric Pb in DW, DF, JZ, and SL were contaminated by multiple anthropogenic sources, whereas the atmospheric Pb in HST was primarily polluted by burning coal from residents. The geographical detector analysis results demonstrated that the spatial distribution of Cd, Pb, and Zn in the surface soil at JZ was affected by the interaction of topography with organic matter and particle size. In SL, the influence of the interactions of all topographies with physical and chemical characters on the spatial distribution of soil Cd and Pb was significant. The aspect was the only topographical factor influencing Zn. Furthermore, topography and pH were the leading factors affecting the spatial distribution of soil Cd, Pb, and Zn in HST. Therefore, this research provides a scientific basis for the identification of pollution sources and the development of effective soil heavy metal control strategies.

Amongst various environmental constraints, abiotic stresses are increasing the risk of food insecurity worldwide by limiting crop production and disturbing the geographical distribution of food crops. Millets are known to possess unique features of resilience to adverse environments, especially infertile soil conditions, although the underlying mechanisms are yet to be determined. The small diploid genome, short stature, excellent seed production, C photosynthesis, and short life cycle of foxtail millet make it a very promising model crop for studying nutrient stress responses. Known to be a drought-tolerant crop, it responds to low nitrogen and low phosphate by respective reduction and enhancement of its root system. This special response is quite different from that shown by maize and some other cereals. In contrast to having a smaller root system under low nitrogen, foxtail millet enhances biomass accumulation, facilitating root thickening, presumably for nutrient translocation. The low phosphate response of foxtail millet links to the internal nitrogen status, which tends to act as a signal regulating the expression of nitrogen transporters and hence indicates its inherent connection with nitrogen nutrition. Altogether, the low nitrogen and low phosphate responses of foxtail millet can act as a basis to further determine the underlying molecular mechanisms. Here, we will highlight the abiotic stress responses of foxtail millet with a key note on its low nitrogen and low phosphate adaptive responses in comparison to other crops.

Excessive N fertilization results in low N-use efficiency (NUE) without any yield benefits and can have profound, long-term environmental consequences including soil acidification, N leaching and increased production of greenhouse gases. Improving NUE in crop production has been a longstanding, worldwide challenge. A crucial strategy to improve NUE is to enhance N uptake by roots. Taking maize as a model crop, we have compared root dry weight (RDW), root/shoot biomass ratio (R/S), and NUE of maize grown in the field in China and in western countries using data from 106 studies published since 1959. Detailed analysis revealed that the differences in the RDW and R/S of maize at silking in China and the western countries were not derived from variations in climate, geography, and stress factors. Instead, NUE was positively correlated with R/S and RDW; R/S and NUE of maize varieties grown in western countries were significantly greater than those grown in China. We then testified this conclusion by conducting field trials with representative maize hybrids in China (ZD958 and XY335) and the US (P32D79). We found that US P32D79 had a better root architecture for increased N uptake and removed more mineral N than Chinese cultivars from the 0-60 cm soil profile. Reported data and our field results demonstrate that a large and deep root, with an appropriate architecture and higher stress tolerance (higher plant density, drought and N deficiency), underlies high NUE in maize production. We recommend breeding for these traits to reduce the N-fertilizer use and thus N-leaching in maize production and paying more attention to increase tolerance to stresses in China.

Reservoir sediment contamination with heavy metals produced by mining activities has aroused widespread global concern owing to its potential threat to human health. In this study, the total concentrations and speciation of heavy metals (Cd, Pb, Zn) in the Lexi (LX) and Maoshui (MS) reservoirs around the historical artisanal zinc smelting area in Southwest China were determined, and pollution indices were applied to assess the pollution levels and potential ecological risks of the two reservoirs. The results showed that all the detected samples in the two reservoirs presented significant metal accumulation, especially for Cd, as compared with the soil background values in Guizhou Province. Between the two reservoirs, the vertical distribution characteristics of each metal in sediment columns were similar. The heavy metal concentrations of the three columns in the LX reservoir reached their maxima at 35, 15, and 10 cm and showed a trend of first increasing and then decreasing overall. However, the heavy metal contents of the three columns in the MS reservoir all exhibited wave-like characteristics in the vertical direction, and all of them reached a relatively obvious high point at approximately 5 and 30 cm. The geoaccumulation index ( I geo ) and potential ecological risk index ( RI ) indicated that Cd was strongly enriched and represented the main risk factor, and the pollution level of the MS reservoir was significantly higher than that of the LX reservoir. Furthermore, the effect coefficients ( ERMQ ) confirmed that the two reservoirs are likely to have toxic impacts on aquatic organisms and need to be controlled and mitigated. The speciation analysis of heavy metals revealed that Cd was primarily in the acid-extractable fraction (69.57%, 68.28%), Pb was chiefly in the reducible fraction (55.24%, 42.18%) and oxidizable fraction (22.60%, 38.02%), and Zn was mainly in the oxidizable fraction (32.54%, 37.65%) in the LX and MS reservoirs, respectively. The ratios of the secondary phase and primary phase ( RSP ) and risk assessment code ( RAC ) evaluation demonstrated that Cd in the sediments of the two reservoirs presents a very high potential ecological risk, and Pb and Zn were at medium to high ecological risk levels. This study highlighted that the artisanal zinc smelting activities had caused serious heavy metal pollution in reservoir sediments, posing a threat to the local ecological environment.

Main conclusion Glutamine (Gln) is an efficient nitrogen source in promoting aboveground nitrogen and biomass accumulation in ZD958 (an elite maize hybrid with great potential for further genetic improvement) seedlings when conditioning a smaller but adequate root system. Amino acids account for a significant part of nitrogen (N) resources in the soil. However, how amino acid-N affects crop growth remains to be further investigated. Here, glutamine (Gln) application (80% NH 4 NO 3  + 20% Gln; mixed N) enhanced shoot growth of the maize hybrid ZD958. N concentration in the shoot increased, which is associated with favorable increases in SPAD values, GS/GOGAT activities, and accumulation of glutamate, asparagine, total free amino acids and soluble proteins in the shoot under mixed N. On the other hand, root growth was reduced when exposed to Gln as indicated by the significantly lower dry weight, root/shoot ratio, and primary, seminal, crown, and total root lengths, as well as unfavorable physiological alterations. Up-regulation of expression of ZmAMT1.3 , ZmNRT2.1 , and ZmAAP2 in the root and that of ZmAMT1.1 , ZmAMT1.3 , and ZmLHT1 in the shoot preconditioned N over-accumulation in the shoot and facilitated shoot growth, presumably via enhancing N translocation to the shoot, when Gln was supplied. Together, Gln is an efficient N source in promoting aboveground N and biomass accumulation in ZD958 seedlings when conditioning a smaller but adequate root system. Notably, ZD958′s parental lines Z58 and Chang7-2 displayed a wide range of variations in Gln responses, which may be partially attributed to single nucleotide polymorphisms (SNPs) in cis -elements and coding regions revealed in this study and much larger quantities of unidentified genetic variations between Z58 and Chang7-2. Extensive genetic divergence of these two elite inbred lines implied large potentials for further genetic improvement of ZD958 in relation to organic N use efficiency.

Due to the adverse effects of coal-mining activities in Karst areas, it is necessary to monitor the contaminants, analyze their fate in the transport and identify the sources. Therefore, concentrations of contaminants were measured in both groundwater and surface water samples collected from coal-mining area in Karst, located in Zhijin County, Guizhou Province (China). The analysis of spatial–temporal variations of contaminants indicates that (1) the concentrations of metal elements (except Ba) and SO42− tend to decrease along the water flow direction, while the concentrations of Ba and nitrate still show an upward trend and (2) the concentrations of metal elements and SO42− have seasonal variations. Furthermore, the results of multi-statistical analysis which combine Pearson correlation coefficients (PCC) with the principal component analysis (PCA), identify five factors for the data structure of contaminants. What is more, the assessment of the water quality was evaluated by Nemerow index method to better understand the status. The results indicate that water quality in the area was poor, while 52.6% of water samples were identified into Grade IV and 16.7% were identified into Grade V by the Nemerow index method.