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 "PSB 1 ", moderate "PSB 2 and PSB 4 " and high "PSB 3 and PSB 5 " P-solubilizing capacity "PSC") were used to investigate aboveand 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 PSB 5 which increased root biomass and PSB 3 that had greater effect on root diameter in 7-, 15-and 42day 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 PSB 4 and PSB 2. Shoot biomass significantly increased with both PSB 2 and PSB 5. Root and shoot physiology significantly improved with PSB 1 (lowest PSC) and PSB 4 (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 (PSB 2 , PSB 4) 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.

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Aims 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. Methods and Results 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). Conclusions 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.

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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.