Limited P availability in several agricultural areas is one of the key challenges facing current agriculture. Exploiting P-solubilizing bacteria (PSB) has been an emerging bio-solution for a higher ...rhizosphere P-availability, meanwhile the above- and below-ground interactions that PSB would trigger remain unclear over plant growing stages. We hypothesized that PSB effects on plant growth may be greater on root traits that positively links with aboveground physiology more than the commonly believed rhizosphere P bio-solubilization. In this study, five contrasting PSB (Pseudomonas spp.) isolates (low "PSB1", moderate "PSB2 and PSB4" and high "PSB3 and PSB5" P-solubilizing capacity "PSC") were used to investigate above- and below-ground responses in wheat fertilized with rock P (RP) under controlled conditions. Our findings show that all PSB isolates increased wheat root traits, particularly PSB5 which increased root biomass and PSB3 that had greater effect on root diameter in 7-, 15- and 42-day old plants. The length, surface and volume of roots significantly increased along with higher rhizosphere available P in 15- and 42-day old plants inoculated with PSB4 and PSB2. Shoot biomass significantly increased with both PSB2 and PSB5. Root and shoot physiology significantly improved with PSB1 (lowest PSC) and PSB4 (moderate PSC), notably shoot total P (78.38%) and root phosphatase activity (390%). Moreover, nutrients acquisition and chlorophyll content increased in inoculated plants and was stimulated (PSB2, PSB4) more than rhizosphere P-solubilization, which was also revealed by the significant above- and below-ground inter-correlations, mainly chlorophyll and both total (R = 0.75, p = 0.001**) and intracellular (R = 0.7, p = 0.000114*) P contents. These findings demonstrate the necessity to timely monitor the plant-rhizosphere continuum responses, which may be a relevant approach to accurately evaluate PSB through considering below- and above-ground relationships; thus enabling unbiased interpretations prior to field applications.
The rhizosphere is a hot spot and a source of beneficial microorganisms known as plant growth-promoting rhizobacteria (PGPR). From the alfalfa (
Medicago sativa
) rhizosphere, 115 bacteria were ...isolated, and from the screening for PGP traits, 26 interesting isolates were selected as PGP rhizobacteria for the next tests. The objective of this study was to use a consortium of PGPR to enhance the growth of faba-bean under phosphate (P) deficiency by taking advantage of their ability to release phosphorus from rock phosphate (RP). Several examined strains were found to have a relatively high activity on P solubilization, auxin, siderophore, ammoniac production, antifungal activity, and the ability to tolerate hypersalinity and water stress. 16S rRNA gene sequencing of the collection revealed six different genera, including
Bacillus
(46.15%),
Siccibacter
(23.07%), and
Acinetobacter
(15.38%) which were identified as the most abundant. Three of the interesting strains (
Siccibacter colletis, Enterobacter huaxiensis
, and
Pantoea
sp.) showed high plant growth promotion traits and no antagonism with
Rhizobium laguerreae
. These three bacteria were retained to establish a rhizobia-including consortium. The inoculation of faba-bean plants with the consortium improved growth parameters as root and shoot dried biomasses and some physiological criteria (chlorophyll content and P uptake under low P availability conditions), and the increase reached 40%. Our study could be the first report of faba-bean growth promotion by a multi-strain PGPR-rhizobia consortium involving
S. colletis, E. huaxiensis
, and
Pantoea
sp. Thus, this consortium could be recommended for faba-bean inoculation, particularly under P-limiting conditions.
Although phosphate solubilizing bacteria (PSB) have been globally reported to improve soil phosphorus (P) availability and plant growth, technical gaps such as the lack of an ideal screening ...approach, is yet to be addressed. The potential of non-halo-forming PSB remains underestimated because of the currently adopted screening protocols that exclusively consider halo-forming and PSB with high phosphorus solubilization (PS) capacities. Yet, caution should be taken to properly assess PSB with contrasting PS rates regardless of the presence or absence of the solubilization halo.
This study sought to examine the PS rate and plant growth promotion ability of 12 PSB categorized as high PSB (H-PSB), medium PSB (M-PSB), and low PSB (L-PSB) based on their PS rates of rock phosphate (RP). The non-halo-forming PSB Arthrobacter pascens was categorized as H-PSB, which might have been eliminated during the classical screening process. In addition, induction of organic acids and phosphatase activity in rhizosphere soils by H-, M-, and L-PSB was proportional to increased wheat P content by 143.22, 154.21, and 77.76 mg P g-1 compared to uninoculated plants (18.1 mg P g-1).
Isolates considered as M- and L-PSB could positively influence wheat above-ground physiology and root traits as high as H-PSB. In addition, non-halo-forming PSB revealed significant PS rates along with positive effects on plant growth as high as halo-forming PSB.
Functional soil microbial communities, including phosphate solubilizing bacteria (PSB), are known for their ability to enhance P availability along with additional properties such as the ...immobilization of trace elements like cadmium (Cd). Bacterial P solubilization and tolerance to Cd are important biological processes that might occur simultaneously with positive influence on plant growth, thus physiological mechanisms controlling both processes need to be deciphered. This interplay was investigated in three PSB isolates (Bacillus siamensis "B. Siam", Rahnella aceris "R. Acer" and Bacillus cereus "B. Cer") and their consortium (PSBCs) with contrasting P solubilizing capacities (high-medium-low), through bacterial culture-based and plant inoculation experiments under increasing Cd concentrations and low P availability conditions (Tri-calcium P "TCP" and Rock P "RP"). Results show that PSB, particularly R. Acer and PSBCs, significantly increased P bioavailability (from TCP) under increasing Cd2+ concentrations (33 ppm “Cd33”, 67 ppm “Cd67”, 134 ppm “Cd134” and 201 ppm “Cd201”). The enhanced bioavailable P strongly correlated (B. Siam and PSBCs) with Cd bioaccumulation capacity, phosphatase enzyme activity, and organic acids (OA) production. Co-application of RP and PSB inoculation under Cd2+ stress (Cd8 and Cd16) improved wheat seedling root growth, P acquisition, and root hairs density in the piliferous zone and root cap. Moreover, inoculation with B. Cer significantly improved root inorganic P (Pi) (384 and 752%) and root acid phosphatases (176 and 131%) under Cd8 and Cd16 compared to uninoculated Cd-stressed seedlings. Such a beneficial root adaptation mechanism could be explained by the significant correlations (mainly under Cd16) between Pi content and morphological traits of wheat roots. Findings from this study demonstrate that P solubilization and Cd tolerance are likely interconnected rhizosphere processes triggered by inoculation that presumably induced the elongation of root hairs and likely the rigidity of the root cell wall to reduce Cd entrance while increasing P acquisition.
•PSB consortium showed high P solubilization and Cd bioaccumulation rates.•Organic acids are involved in both Cd tolerance and P bio-solubilization.•PSB positively impacted wheat P uptake and tolerance to Cd via root hairs elongation and density.•PSB inoculation presumably play crucial roles in crop production under 'low P and Cd' conditions.
Rhizosphere microbes significantly enhance phosphorus (P) availability from a variety of unavailable P pools in agricultural soils. However, little is known about the contribution of root-associated ...microorganisms, notably P solubilizing bacteria (PSB), to enhance the use of polyphosphate (PolyP) fertilizers as well as the key mechanisms involved. This study assesses the ability of four PSB (Bacillus siamensis, Rahnella aceris, Pantoea hericii, Bacillus paramycoides) and their consortium (Cs) to enhance the release rate of available P from two types of PolyP (“PolyB” and “PolyC”) with a focus on the key role of phosphatase enzyme activities and organic acids production. Wheat growth performance and P acquisition efficiency were evaluated in response to co-application of PSB and PolyP. Results showed that inoculation with PSB, notably Cs, significantly enhanced available P from PolyC, PolyB and tri-calcium P. Increased available P in response to inoculation with PSB significantly correlated with medium acidification, organic acids production (notably glycolic acid) and induced activities of acid phosphatase and pyrophosphatase. In planta, the co-application of PSB-PolyP improved wheat plant biomass, root growth and P acquisition, with best results obtained from Cs-PolyP co-application as compared to uninoculated and unfertilized plants. At seedling stage, the co-application of Cs-PolyP (PolyB and PolyC) enhanced root hairs length (125 % and 131 %), root length (26 % and 37 %) and root inorganic P (Pi) content (160 % and 182 %), respectively compared to uninoculated plants. Similarly, at tillering stage, plant biomass (35 % and 47 %), Pi content (43 % and 253 %), P translocation (215 % and 315 %) and soil phosphatases (213 % and 219 %) significantly improved under PolyB and PolyC application, respectively. Findings from this study demonstrate the key role of PSB to enhance the use of PolyP through production of organic acids and phosphatases, exhibiting differential traits patterns between the two PolyP. Improved wheat growth and root P acquisition in response to PSB-PolyP co-application can be attributed to induced rhizosphere processes leading to enhanced available P taken up by roots.
Grain legumes / cereals intercropping systems with microbial inoculants, hold promise for improving crop productivity under stressful conditions. However, the tripartite interaction involving ...intercropping system, associated microbiota and stress combining water deficit and low phosphorus (P) availability remains understudied. This study evaluated the impact of three bacterial consortia (C4, C6, and Cref containing Rhizobium and PSB strains) including single Rhizobium (Rhizobium laguerreae) on the agro-physiological performance of wheat (Triticum durum) and faba bean (Vicia faba) grown as intercrops or sole-crops under P and water deficient conditions. Inoculation, especially with C6, significantly improved shoot and root biomasses of both wheat (up to 66 and 81 %) and faba bean (up to 54 and 266 %) intercrops compared to single Rhizobium inoculation and control treatments. Intercropping generally outperformed sole-cropping in above-ground physiology, root morphological traits, shoot and root P content, with a notable effect in response to C6 exhibiting low microbial biomass P. Changes in bacterial community structure were primarily driven by cropping pattern and water regime rather than bacterial inoculation. Intercropping maintained bacterial diversity but shifted community structure, favoring Proteobacteria. Overall, inoculating intercropped wheat and faba bean with Rhizobium-containing consortia induced beneficial below-ground interspecies interactions under water and P-limiting conditions.
•Rhizobium-containing bacterial consortia enhanced belowground interactions in wheat/faba bean intercrops.•Inoculation enhanced root traits and phosphorus dynamics in rhizosphere soil of both intercropped wheat and faba bean plants.•Water regime and cropping pattern rather than inoculation drove shifts in bacterial community structure.•Bacterial consortia enhanced growth performance of intercropping over sole cropping under low P-availability.
Uncontrolled mine waste disposal globally poses severe environmental and health risks due to high concentrations of toxic substances like heavy metals and organic pollutants. This jeopardizes soil ...fertility and ecological balance. In this study, we adopted a novel approach based on the use of low-cost amendments that can be used in small to tailing sites for better plant growth and reducing health risks. On a 37-ha-old abandoned pyrrhotite mine site (Kettara) which was operated for 34 years near Marrakesh a high amount of tailing material low in organic C, high in salt contents, and contaminated by various heavy metals still exists. This material is low in fertility and is only patchily discovered by vegetation which should hinder the material of aeolian erosion. To increase the fertility of the material and improve vegetation growth, the soil material has to be improved by different organo-mineral amendments and their combinations as Marble waste (Mw), Clay (Cy), and Compost (Cp). To determine the impact of these amendments on plant growth the fast-growing Lupinus angustifolius L. a legume plant found close to the mining site, identified with the Regional Herbarium 'MARK' code: MARK-14861. This plant serves as a pivotal component in the comprehensive approach aimed at rehabilitating the soils surrounding the mining site. To identify the most effective organo-mineral amendment, we examined 65 combinations by mixing mine tailing soils with varying proportions (2.5%, 5%, 7.5%, or 10%) of the three distinct amendments and its combinations (Com), to identify the optimal rate for effective seeds germination and plant growth. Preliminary results on the germination index of L. angustifolius seeds revealed that four combinations (Com1: Cy2.5-Cp2.5-Mw10; Com2: Cy2.5-Cp5-Mw5; Com3: Cy7.5-Cp2.5-Mw7.5; and Com4: Cy10-Cp10-Mw10) maintained a high index due to improved soil properties. These combinations were used as the soil substrate material for a greenhouse experiment where plant growth, heavy metals accumulation, soil characteristics, and available metal content were determined. Our findings demonstrated that incorporating these four organo-mineral soil amendments into Kettara mine tailings led to a significant enhancement in plant growth. Notably, L. angustifolius plants exhibited a preferential accumulation of heavy metals in the root’s biomass (Cu: 565.60, Zn: 433.52, and Pb: 301.44 mg kg−1) with limited translocation to shoot parts (Cu: 37.44, Zn: 28.40, and Pb: 19.36 mg kg−1), particularly following the application of Com4. Furthermore, the properties of the mine soil were improved, including both neutralization of acidic soil pH (3 to ∼ 7) and reduction of metal elements bioavailability. Specifically, Com1, Com3, and Com4 effectively decreased the mobile fraction of metals in the tested mine tailings to exceptionally low levels achieving a reduction of 99% for copper, lead, and zinc, and 95% for arsenic. Our findings from this study indicated that our amendment holds promising potential for enhancing both plant growth and soil fertility, while simultaneously mitigating the mobility of heavy metals in heavy metal-contaminated acidic mine soils.
•The synergistic use of marble waste with compost's buffering properties and the binding capabilities of organo-mineral materials (compost and clay), led to a significant reduction in the mobile fraction of heavy metals for Cu, Pb and Zn by 99% and 95% for arsenic declining health risk of former mining soils.•The germination index of L. angustifolius seeds revealed that four combinations (Com1: Cy2.5-Cp2.5-Mw10; Com2: Cy2.5-Cp5-Mw5; Com3: Cy7.5-Cp2.5-Mw7.5; and Com4: Cy10-Cp10-Mw10) maintained a high index of germination due to improved soil properties.•L. angustifolius plants exhibited a preferential accumulation of heavy metals in the root’s biomass (Cu: 565.60 mg kg−1, Zn: 433.52 mg kg−1, and Pb: 301.44 mg kg−1) with limited translocation to shoot parts.
This study aims to investigate the effect of isolated drought-tolerant rhizobacteria, spanning various groups, such as nitrogen-fixing bacteria (NFB), phosphate solubilizing bacteria (PSB), and other ...plant growth promoting rhizobacteria (PGPR), on the growth of wheat (Triticum durum) plants, focusing on various morphological and physiological responses under moderate drought and low-P availability. Among 343 rhizobacterial morphotypes, 16 exhibited tolerance to NaCl and PEG-6000. These included 8 PSB, 4 NFB, and 4 osmotolerant-PGPR groups, distributed across 14 different genera. Biochemical characterization showcased diverse PGP capabilities, particularly in P solubilization. The dynamic responses of drought-tolerant PSB to salt and PEG-6000-induced drought stress involved variations in organic acid (OA) secretion, with specific acids, including palmitic, lactic, and stearic, playing crucial roles in enhancing available P fractions. Inoculation with rhizobacteria significantly increased both shoot (SDW) and root (RDW) dry weights of wheat plants, as well as rhizosphere available P. PSB11 (Arthrobacter oryzae) emerged as the most effective strain, plausibly due to its positive impact on root morphological traits (length, surface, and volume). Other isolates, PSB10 (Priestia flexa), PSB13 (Bacillus haynesii), and particularly PGPR2 (Arthrobacter pascens) significantly increased shoot P content (up to 68.91 %), with a 2-fold increase in chlorophyll content. The correlation analysis highlighted positive associations between SDW, shoot P content, chlorophyll content index (CCI), and leaf area. Additionally, a negative correlation emerged between microbial biomass P and root morphophysiological parameters. This pattern could be explained by reduced competition between plants and rhizobacteria for accessible P, as indicated by low microbial biomass P and strong plant growth. Our investigation reveals the potential of drought-tolerant rhizobacteria in enhancing wheat resilience to moderate drought and low-P conditions. This is demonstrated through exceptional performance in influencing root architecture, P utilization efficiency, and overall plant physiological parameters. Beyond these outcomes, the innovative isolation procedure employed, targeting rhizobacteria from diverse groups, opens new avenues for targeted isolation techniques. This unique approach contributes to the novelty of our study, offering promising prospects for targeted bioinoculants in mitigating the challenges of drought and P deficiency in wheat cultivation.
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