With increasing demand of agricultural production and as the peak in global production will occur in the next decades, phosphorus (P) is receiving more attention as a nonrenewable resource . Maintaining a proper P-supplying level at the root zone can maximize the efficiency of plant roots to mobilize and acquire P from the rhizosphere by an integration of root morphological and physiological adaptive strategies. Furthermore, P uptake and utilization by plants plays a vital role in the determination of final crop yield. A holistic understanding of P dynamics from soil to plant is necessary for optimizing P management and improving P-use efficiency, aiming at reducing consumption of chemical P fertilizer, maximizing exploitation of the biological potential of root/rhizosphere processes for efficient mobilization, and acquisition of soil P by plants as well as recycling P from manure and waste.

Responses of root architecture development to low phosphorus availability: a review ( Annals of Botany 112: 391–408, 2013 doi:10.1093/aob/mcs285, available online at www.aob.oxfordjournals.org )

Background: Phosphorus (P) is an essential element for plant growth and development but it is often a limiting nutrient in soils. Hence, P acquisition from soil by plant roots is a subject of considerable interest in agriculture, ecology and plant root biology. Root architecture, with its shape and structured development, can be considered as an evolutionary response to scarcity of resources.

† Scope :This review discusses the significance of root architecture development in response to low P availability and its beneficial effects on alleviation of P stress. It also focuses on recent progress in unravelling cellular, physiological and molecular mechanisms in root developmental adaptation to P starvation. The progress in a more detailed understanding of these mechanisms might be used for developing strategies that build upon the observed explorative behaviour of plant roots.

†Conclusions The role of root architecture in alleviation of P stress is well documented. However, this paper describes how plants adjust their root architecture to low-P conditions through inhibition of primary root growth, promotion of lateral root growth, enhancement of root hair development and cluster root formation, which all promote P acquisition by plants. The mechanisms for activating alterations in root architecture in response to P deprivation depend on changes in the localized P concentration, and transport of or sensitivity to growth regulators such as sugars, auxins, ethylene, cytokinins, nitric oxide (NO), reactive oxygen species (ROS) and abscisic acid (ABA). In the process, many genes are activated, which in turn trigger changes in molecular, physiological and cellular processes. As a result, root architecture is modified, allowing plants to adapt effectively to the low-P environment. This review provides a framework for understanding how P deficiency alters root architecture, with a focus on integrated physiological and molecular signalling. PDF enclosed

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Plant roots absorb P from soil solution as H2PO4 ions. In strongly acidic soils H2PO4 dominates, while in alkaline soils, P is largely present as HPO42- form. The mechanisms of P uptake by plants are through root interception, mass flow and diffusion. Root surfaces can intercept P during the displacement process (Barber, 1984). The second mechanism is the mass flow of water and dissolved P to the root surface, which is driven by transpiration. The quantity of nutrient supplied to plants by mass flow are based on nutrient concentration in the soil solution and the amount of water transpired. The contribution of diffusion relating to the supply of nutrients to the root surface. Diffusion is the main mechanism for the movement of P under P deficient soil to the root surface. The driving force of diffusion is a concentration gradient.In soil grown plants a concentration gradient between the adjacent soil and the root surface is formed when the uptake rate of ions exceeds the supply of mass flow. Depletion profiles develop with time and their shape depends mainly on the balance between uptake by roots, replenishment from soil, and mobility of ions by diffusion.

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Thanks doctor Anoop Kumar Srivastava, but which is the key mechanism among root architecture, root exudates and microorganisms, in the phosphorus availability for plants?

What are the mechanisms that govern the phosphorus availability for plant in deficient phosphorus soils?

Many of the global soils used for agriculture are not optimized for phosphorus availability. Firstly, Phosphorus is only available at a small window of soil pH and majority of production soils are not necessarily in the window of 6.0 to 7.3 and many producers are not adjusting their soil pH or even measuring it consistently. Besides pH I believe that the main mechanisms that can play a significant role in phosphorus availability and increase phosphorus uptake by plants in soil is the presence and activity of arbuscular mycorrhizae. Compared to naked non colonized plant root the mycorrhizal extension effectively increases the effective root system surface interaction area by hundreds to thousand times. The problem with so many studies is that the root system is studied without focusing that for 80% of all crop plants will acquire mycorrhizal associations. Thirdly, in tropical soils particularly most of the available phosphorus is in organic factions and in both most tropical and temperate soils the soils are not optimized in soil organic matter content. In addition, many of our modern agricultural practices including pesticide use, monoculture, row cropping, tillage, method of fertilization and lack of organic amendments can have cumulative negative effects on mycorrhizal association. When crop plants optimized in their early mycorrhizal association the optimizing of plant performance can result at less than 25% of the amount needed in nonmycorrhizal conditions. Mycorrhizal inoculation is still a practice not generally employed. Considering concerns of P deficiency and its cost and issues mycorrhizae deserve intensive research in their applied applications in commercial agriculture environments. Finally, the use of Silicates are being increasingly recognized as stimulating phosphorus metabolism and to aid in acid soil infertility issues including toxicities and deficiencies of other nutrients.

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Dear Touhami , there will always be much higher phosphorous -uptake-efficiency when root architecture is better equipped to generate /synthesize exudates that trigger microbial proliferation. so its the cascading effect of having better root architecture , considering the phosphorous as most immobile nutrient in soil. Any of these alone in field , cannot be held responsible for phosphorous uptake...

Good question to be follow.

Mechanism governing availability of P in soil

(1) Phosphate solubilizing and mineralizing microorganisms present in rhizosphere improve the availability of inorganic and organic phosphate by production of organic acid and phosphatase/phytase enzyme respectively

(2) Phytase enzyme hydrolyses phytate P and phosphatase hydrolyzes organic P (Po) to plant available orthophosphate compounds and thus increases the availability of soil P through the mineralization of Po

(3) Secondary organic metabolites such as siderophore, enzymes, phenol, amino acids, sugar, organic acid anion reduce metals with variable oxidation state (bound to phosphate) to lower oxidation state resulting in more soluble P.

(4) Microbial assimilation of ammonium ion results in excretion of proton that decreases soil pH and act as solvent for P solubilization and improving plant available P.

(5) Carbondioxide released from microbial respiration dissolves in soil water to make carbonic acid that solubilizes P through reduced rhizosphere pH

(6) Microbial biomass P makes a major component of immobilized P that is potentially available for plant nutrition.

(7) Production of acid phosphatase by plant roots mineralize organically bound P

(8) Symbiotic association of plant roots with mycorrhiza. The mycelium network is able to mineralize P through the production of enzymes phosphatase and phytase.

(9) Root architecture (Cluster root formation, enhanced root hair, length of lateral roots)contribute towards enhanced P uptake by plant

High organic matter content of soil improves the availability of organic P

Soil pH

Soil moisture

Root exudates

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