Soil Nutrient Bioavailability—A Mechanistic Approach

Role of organic amendments on enhanced bioremediation of heavy metal(loid) contaminated soils

Iron and phosphorus availability is low in many soils; hence, microorganisms and plants have evolved mechanisms to acquire these nutrients by altering the chemical conditions that affect their solubility. In plants, this includes exudation of organic acid anions and acidification of the rhizosphere by release of protons in response to iron and phosphorus deficiency. Grasses (family Poaceae) and microorganisms further respond to Fe deficiency by production and release of specific chelators (phytosiderophores and siderophores, respectively) that complex Fe to enhance its diffusion to the cell surface. In the rhizosphere, the mutual demand for Fe and P results in competition between plants and microorganisms with the latter being more competitive due to their ability to decompose plant-derived chelators and their proximity to the root surface; however microbial competitiveness is strongly affected by carbon availability. On the other hand, plants are able to avoid direct competition with microorganisms due to the spatial and temporal variability in the amount and composition of exudates they release into the rhizosphere. In this review, we present a model of the interactions that occur between microorganisms and roots along the root axis, and discuss advantages and limitations of methods that can be used to study these interactions at nanometre to centimetre scales. Our analysis suggests mechanisms such as increasing turnover of microbial biomass or enhanced nutrient uptake capacity of mature root zones that may enhance plant competitiveness could be used to develop plant genotypes with enhanced efficiency in nutrient acquisition. Our model of interactions between plants and microorganisms in the rhizosphere will be useful for understanding the biogeochemistry of P and Fe and for enhancing the effectiveness of fertilization.

Rhizosphere interactions between microorganisms and plants govern iron and phosphorus acquisition along the root axis – model and research methods

As an essential plant macronutrient, the low availability of phosphorus (P) in most soils imposes serious limitation on crop production. Plants have evolved complex responsive and adaptive mechanisms for acquisition, remobilization and recycling of phosphate (Pi) to maintain P homeostasis. Spatio-temporal molecular, physiological, and biochemical Pi deficiency responses developed by plants are the consequence of local and systemic sensing and signaling pathways. Pi deficiency is sensed locally by the root system where hormones serve as important signaling components in terms of developmental reprogramming, leading to changes in root system architecture. Root-to-shoot and shoot-to-root signals, delivered through the xylem and phloem, respectively, involving Pi itself, hormones, miRNAs, mRNAs, and sucrose, serve to coordinate Pi deficiency responses at the whole-plant level. A combination of chromatin remodeling, transcriptional and posttranslational events contribute to globally regulating a wide range of Pi deficiency responses. In this review, recent advances are evaluated in terms of progress toward developing a comprehensive understanding of the molecular events underlying control over P homeostasis. Application of this knowledge, in terms of developing crop plants having enhanced attributes for P use efficiency, is discussed from the perspective of agricultural sustainability in the face of diminishing global P supplies.

http://www.rbc-scau.cn/sites/files/2014-Bill-jipb.pdf

Molecular mechanisms underlying phosphate sensing, signaling, and adaptation in plants.

http://www.rbc-scau.cn/sites/files/Wang_et_al_Plant_Science_2010.pdf

Acquisition or utilization, which is more critical for enhancing phosphorus efficiency in modern crops

Genotypic differences in acquiring immobile P exist among species or cultivars within one species. We investigated the P-efficiency mechanisms of rapeseed (Brassica napus L.) in low P soil by measuring plant growth, P acquisition and rhizosphere properties. Two genotypes with different P efficiencies were grown in a root-compartment experiment under low P (P15: 15 mg P kg−1) and high P (P100: 100 mg P kg−1) treatments. The P-efficient genotype produced more biomass, and had a high seed yield and high P acquisition efficiency under low P treatment. Under both P treatments, both genotypes decreased inorganic P (Pi) and organic P (Po) fractions in the rhizosphere soil. However there was no decrease in NaHCO3-Po at P100. For the P15 treatment, the concentrations of NaHCO3-Po and NaOH-Po were negatively correlated with soil acid phosphatase activity. The P-efficient genotype 102 differed from the P-inefficient genotype 105 in the following ways. In the rhizosphere the soil pH was lower, acid phosphatase activity was higher, and depletion of P was greater. Further the depletion zones were wider. These results suggested that improving P efficiency based on the character of P efficiency acquisition in P-efficient genotype would be a potential approach for maintaining rapeseed yield potential in soils with low P bioavailability.

Genotypic differences in phosphorus acquisition and the rhizosphere properties of Brassica napus in response to low phosphorus stress

Application of phosphorus (P) fertilizer is important in crop production because of the low bioavailability of phosphorus to plants in both acidic and calcareous soils. Although rapeseed (Brassica napus) is generally sensitive to P deficiency, different cultivars differ widely in this respect. Differences in P uptake and utilization between two rapeseed cultivars, one P-efficient (‘97081’) and one P-inefficient (‘97009’), were evaluated in solution culture by studying the changes in root morphology and parameters of P uptake kinetics in response to low-P stress. The P-efficient cultivar had lower Km and Cmin values and higher Vmax and developed longer and denser lateral root hair with greater number of root tips and branches under low-P stress, which resulted in a better developed root system and more efficient uptake of P. That, in turn, led to higher concentration and accumulation of P in the plants, culminating in higher biomass production. However, P utilization efficiency (biomass production per unit...

Genotypic differences in root morphology and phosphorus uptake kinetics in brassica napus under low phosphorus supply

Identification and characterization of improved nitrogen efficiency in interspecific hybridized new-type Brassica napus.

Boron (B) deficiency is one of the major causes of growth inhibition and yield reduction in Brassica napus (B. napus). However, the molecular mechanisms of low B adaptation in B. napus are largely unknown. Here, fifty-one BnaWRKY transcription factors were identified as responsive to B deficiency in B. napus, in which BnaAn.WRKY26, BnaA9.WRKY47, BnaA1.WKRY53 and BnaCn.WRKY57 were tested in yeast one-hybrid assays and showed strong binding activity with conserved sequences containing a W box in the promoters of the B transport-related genes BnaNIP5;1s and BnaBOR1s. Green fluorescent protein fused to the target protein demonstrated the nuclear localization of BnaA9.WRKY47. CRISPR/Cas9-mediated knockout lines of BnaA9.WRKY47 in B. napus had increased sensitivity to low B and lower contents of B than wild-type plants. In contrast, overexpression of BnaA9.WRKY47 enhanced the adaptation to low B with higher B contents in tissues than in wild-type plants. Consistent with the phenotypic response and B accumulation in these transgenic lines, the transcription activity of BnaA3.NIP5;1, a B efficiency candidate gene, was decreased in the knockout lines but was significantly increased in the overexpressing lines under low B conditions. Electrophoretic mobility shift assays, transient expression experiments in tobacco and in situ hybridizations showed that BnaA9.WRKY47 directly activated BnaA3.NIP5;1 expression through binding to the specific cis-element. Taken together, our findings support BnaWRKYs as new participants in response to low B, and BnaA9.WRKY47 contributes to the adaptation of B. napus to B deficiency through up-regulating BnaA3.NIP5;1 expression to facilitate efficient B uptake.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/pbi.13288

Transcription factor BnaA9.WRKY47 contributes to the adaptation of Brassica napus to low boron stress by up-regulating the boric acid channel gene BnaA3.NIP5;1.

Considerable genetic variation in agronomic nitrogen (N) use efficiency (NUE) has been reported among genotypes of Brassica napus. However, the physiological and molecular mechanisms underpinning these differences remain poorly understood. In this study, physiological and genetic factors impacting NUE were identified in field trials and hydroponic experiments using two B. napus genotypes with contrasting NUE. The results showed that the N-efficient genotype (D4-15) had greater N uptake and utilization efficiencies, more root tips, larger root surface and root volume, and higher N assimilation and photosynthesis capacity than the N-inefficient genotype (D2-1). Genomic analysis revealed that D4-15 had a greater genome diversity related to NUE than D2-1. By combining genomic and transcriptomic analysis, genes involved in photosynthesis and C/N metabolism were implicated in conferring NUE. Co-expression network analysis of genes that differed between the two genotypes suggested gene clusters impacting NUE. A nitrate transporter gene BnaA06g04560D (NRT2.1) and two vacuole nitrate transporter CLC genes (BnaA02g11800D and BnaA02g28670D) were up-regulated by N starvation in D4-15 but not in D2-1. The study revealed that high N uptake and utilization efficiencies, maintained photosynthesis and coordinated C/N metabolism confer high NUE in B. napus, and identified candidate genes that could facilitate breeding for enhanced NUE in B. napus.

Xiangsheng Ye

Papers

Genotypic differences in phosphorus acquisition and the rhizosphere properties of Brassica napus in response to low phosphorus stress

Genotypic differences in root morphology and phosphorus uptake kinetics in brassica napus under low phosphorus supply

Identification and characterization of improved nitrogen efficiency in interspecific hybridized new-type Brassica napus.

Transcription factor BnaA9.WRKY47 contributes to the adaptation of Brassica napus to low boron stress by up-regulating the boric acid channel gene BnaA3.NIP5;1.

Comparative genome and transcriptome analysis unravels key factors of nitrogen use efficiency in Brassica napus L