ASSIMILATION OF PHOSPHORUS AND ITS PHYSIOLOGICAL FUNCTION

ASSIMILATION OF PHOSPHORUS

NUTRIENT AND ITS PHYSIOLOGICAL

ROLE

Dr. Alka Narula

Ruchi Rani19M.Sc Biotechnology,IInd Semester

PHOSPHORUSPhosphorus is an important plant macronutrient, making up about 0.2% of a plant’s dry weight.

It is a component of key molecules such as nucleic acids, phospholipids, and ATP, and, consequently, plants cannot grow without a reliable supply of this nutrient.

Pi is also involved in controlling key enzyme reactions and in the regulation of metabolic pathways .

After N, P is the second most frequently limiting macronutrient for plant growth.

ASSIMILATION OF PHOSPHORUS

PHOSPHORUS IN SOILAlthough the total amount of P in the soilmay be high, it is often present inunavailable forms or in forms that are onlyavailable outside of the rhizosphere. Fewunfertilized soils release P fast enough tosupport the high growth rates of crop plantspecies. In many agricultural systems inwhich the application of P to the soil isnecessary to ensure plant productivity, therecovery of applied P by crop plants in agrowing season is very low, because in thesoil more than 80% of the P becomesimmobile and unavailable for plant uptakebecause of adsorption, precipitation, orconversion to the organic form (Holford,1997).

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Soil P is found in different pools, such asorganic and mineral P .

It is important to emphasize that 20 to 80% of Pin soils is found in the organic form, of whichphytic acid (inositol hexaphosphate) is usuallya major component .

The remainder is found in the inorganicfraction containing 170 mineral forms of P.

Soil microbes release immobile forms of P tothe soil solution and are also responsible for theimmobilization of P.

The low availability of P in the bulk soil

limits plant uptake. More soluble minerals suchas K move through the soil via bulk flow anddiffusion, but P is moved mainly by diffusion.

Since the rate of diffusion of P is slow, highplant uptake rates create a zone around the rootthat is depleted of P.

Plant root geometry andmorphology are importantfor maximizing P uptake,because root systems thathave higher ratios ofsurface area to volume willmore effectively explore alarger volume of soil(Lynch, 1995).In certain plant species,root clusters (proteoidroots) are formed inresponse to P limitations.These specialized rootsexude high amounts oforganic acids (up to 23% ofnet photosynthesis), whichacidify the soil and chelatemetal ions around theroots, resulting in themobilization of P and somemicronutrients(Marschner, 1995).

Proteoid roots produced by white lupin with (A) andwithout (B) phosphate. Plants were grown in a solution culture for 3 weeks. Plasma membrane was isolated from different types of roots: proteoid roots of P-sufficient plants, marked as proteoid (P); lateral roots of P-sufficient plants, marked as lateral (P); active proteoid roots (the youngest, fully developed proteoid root) of P-deficient plants, marked as proteoid (P); lateral roots of P-deficient plants, marked as lateral (P).

P UPTAKE ACROSS THE PLASMA MEMBRANE The uptake of P poses a problem for plants,

since the concentration of this mineral in thesoil solution is low but plant requirements arehigh.

The form of P most readily accessed by plantsis Pi, the concentration of which rarely exceeds10 mm in soil solutions.

Therefore, plants must have specializedtransporters at the root/soil interface forextraction of Pi from solutions of micromolarconcentrations, as well as other mechanismsfor transporting Pi across membranes betweenintracellular compartments, where theconcentrations of Pi may be 1000-fold higherthan in the external solution.

There must also be efflux systems that play arole in the redistribution of this preciousresource when soil P is no longer available oradequate.

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The form in which Pi exists in

solution changes according to pH. The

pKs for the dissociation of H3PO4-

into H2PO4- and then into HPO4 2- are

2.1 and 7.2, respectively. Therefore,

below pH 6.0, most Pi will be present

as the monovalent H2PO4- species,

whereas H3PO4- and HPO4 2- will be

present only in minor proportions.

Most studies on the pH dependence of

Pi uptake in higher plants have found

that uptake rates are highest between

pH 5.0 and 6.0, where H2PO4-

dominates, which suggests that Pi is

taken up as the monovalent form.

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Pi does not enter simply as H2PO4 2- or HPO4 2-, both ofwhich would lead to membrane hyperpolarization.

From these results it is likely that Pi is cotransported withpositively charged ions. Cotransport of Pi with a cationinvolving a stoichiometry of more than 1 C+/H2PO4 2- ormore than 2 C+/HPO4 2- would result in a net influx ofpositive charge and hence lead to the observed membranedepolarization.

The cytoplasmic acidification associated with Pi transportwould suggest that the cation is H1, but acidification wouldoccur regardless of the nature of the cation if thetransported species were H2PO4 2-, since it would undergoa pH-dependent dissociation in the cytoplasm to HPO4 2-and H+.

COMPARTMENTATION OF P Maintenance of stable cytoplasmic Pi concentrations is essential

for many enzyme reactions.This homeostasis is achieved by a combination of membrane

transport and exchange between various intracellular pools of P.These pools can be classified in a number of different ways.

First, according to their location in physical compartments suchas the cytoplasm, vacuole, apoplast, and nucleus. The pH ofthese compartments will determine the form of Pi.

The second pKa for H3PO4- is 7.2, so Pi in the cytoplasm will beapproximately equally partitioned between the ionic formsH2PO4 -and HPO4 2-, whereas in the more acidic vacuole andapoplast, H2PO4- will be the dominant species.

Third , by the chemical form of P, such as Pi, P-esters, P-lipids,and nucleic acids. The proportion of the total P in each chemicalform (except P in DNA) changes with tissue type and age and inresponse to P nutrition. Third, according to physiologicalfunction, as metabolic, stored, and cycling forms.

REGULATION OF Pi UPTAKE

When the supply of Pi is limited, plants grow more roots, increase the rate of uptake by roots from the soil, retranslocate Pi from older leaves, and deplete the vacuolar stores of Pi.

Conversely, when plants have an adequate supply of Pi and are absorbing it at rates that exceed demand, a number of processes act to prevent the accumulation of toxic Pi concentrations.

These processes include the conversion of Pi into organic storage compounds (e.g. phytic acid), a reduction in the Pi uptake rate from the outside solution , and Pi loss by efflux, which can be between 8 and 70% of the influx .Any or all of these processes may be strategies for the maintenance of intracellular Pi homeostasis.

P TRANSLOCATION IN WHOLE PLANT In P-sufficient plants most of the Pi absorbed by the roots is

transported in the xylem to the younger leaves. Concentrations of Pi in the xylem range from 1 mM in Pi-starved

plants to 7 mM in plants grown in solutions containing 125 mMPi .

There is also significant retranslocation of Pi in the phloem fromolder leaves to the growing shoots and from the shoots to theroots.

In Pi-deficient plants the restricted supply of Pi to the shootsfrom the roots via the xylem is supplemented by increasedmobilization of stored P in the older leaves and retranslocationto both the younger leaves and growing roots.

This process involves both the depletion of Pi stores and thebreakdown of organic P in the older leaves.

In the xylem P is transported almost solely as Pi, whereassignificant amounts of organic P are found in the phloem.

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PART OF ALL LIVING CELLS

All living cells require a continual supply of energy for all the processes which keep the organism alive i.e.; ATP ( ADENOSINE TRIPHOSPHATE)

ESSENTIAL PART OF PHOTOSYNTHESIS

AND RESPIRATION

Metabolic process involved a series of chemical interaction that give NTPs ,NADH and FADH2 as an energy source.

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PHOTOSYNTHESIS RESPIRATION

COMPONENT OF CELL MEMBRANESThe cell membrane is a biologicalmembrane that separate the interior of allcells from outside environment . It consistsof PHOSPHOLIPID BILAYER withembedded proteins.

Basic component of phospholipids are -

PHOSPHATE ,ALCOHOL, FATTY ACIDS,GLYCEROL/ SPHINGOSINE.

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Cell membranes are involved in a variety of cellular processes such ascell adhesion , ion conductivity and cell signaling and also serves as theattachment surface for several extracellular structures.

CELL ADHESION

CELL SIGNALING

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CELL ATTACHMENT TO EXTRACELLULAR MATRIX ION CONDUCTIVITY

Genetic Transfer Phosphorus is a vital

component of the substancesthat are building blocks ofgenes and chromosomes.

So, it is an essential part of theprocess of carrying the geneticcode from one generation tothe next, providing the“blueprint” for all aspects ofplant growth and development.

An adequate supply of P isessential to the development ofnew cells and to the transfer ofthe genetic code from one cellto another as new cells areformed

Nutrient TransportPlant cells can accumulatenutrients at much higherconcentrations than arepresent in the soil solutionthat surrounds them.

This allows roots to extractnutrients from the soilsolution where they arepresent in very lowconcentrations.

Movement of nutrientswithin the plant dependslargely upon transportthrough cell membranes,which requires energy tooppose the forces of osmosis.Here again, ATP and otherhigh energy P compoundsprovide the needed energy.

ROLE IN SIGNAL TRANSDUCTION

Phosphorus has an important role insignal transduction.

Inositol trisphosphate together withDAG, is a secondary messengermolecule used in signal transductionand lipid signalling in biologicalmolecules.

While DAG stays inside themembrane, IP3 is soluble and diffusesthrough the cell.

It is made by hydrolysis of PIP2, aphospholipid that is located in theplasma membranes, by PLC.

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Adaptation of H-Pumping and Plasma Membrane H+ ATPase Activity in Proteoid Roots of

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