Phosphate Uptake Process

Phosphate use ProcessIntroductionWhilst reproach moisture and nitrogen (N) are study limitations to agricultural production systems in the SAT, phosphorus (P) deficiency alike limits rationalize growth on many lands. The cost and availability of phosphatic fertilizers to the majority of farmers in the region restrict their use. Attention has, therefore, turned to making more in force(p) use of the soil ortho inorganic inorganic inorganic inorganic phosphate reserves by seeking rate genotypes and management systems that result in more effective use of goods and services and physical exercise of soil-P. A bit of promising strategies are being explored, many of which are presented in this Workshop. To be effectively developed, all of them require an under- standing of the mechanisms of phosphate uptake and utilization by crop plants. Use of molecular tools by nutritional physiologists in recent old age has consider- ably enhanced the brain of these mechanisms and provid ed new op mannerunities for manipulating nutrient uptake and utilization. Key genes mingled in the functioning mformer(a) been identified and information on their role and regulation is accumulating. This motif provides a summary of the phosphate uptake process and highlights some of the authoritative molecular mechanisms mingled.The external phosphate concentrationPlant reports recrudesce their phosphate from the external soil theme where it is in equilibrium with phosphate sorbed onto soil minerals and colloids. These sorption reactions maintain embarrassed concentrations of phosphate in soil declaration whilst buffering the amount of phosphate in solution. The movement of phosphate ions to the sites through and through which it is taken up into root cells occurs by diffusion. This is a relatively slowly process and, in P-deficient soils, results in the concentration of phosphate in solution being depleted around plant roots. Thus, many of the strategies for improving phosphate uptake are aimed at reducing this depletion zone and increasing the solution phosphate concentration immediately adjacent to the sites of phosphate uptake in the roots.Extension of roots into undepleted regions of soil provides the root eyeshade with external P concentrations quasi(prenominal) to those in the bulk soil solution. Further cover song on the root axis extension of root hairs from epidermal cells in many plant species considerably increases the volume of soil explored for phosphate. Still further back, the soil volume explored by some species growing in low phosphate soils may be enhanced by the presence of hyphae of mycorrhizal fungi which smoke extend several centimeters from the root stand up. A cone of soil in which the concentration of phosphate in solution is depleted thus develops back from the root achievement. Within this zone the equilibrium of the phosphate sorption will require shifted towards release of sorbed phosphate ions into solution. Distance to the uptake sites within the root and any barriers to phosphate diffusion determine whether the plant can glide path these ions.The root apoplasmThe walls of root epidermal and cortical cells and the associated intercellular spaces make up the apoplasm. In young roots, these walls are composed of inter- laced fibres that form an render latticework (Peterson and Cholewa, 1998). Soil solution can therefore, move radially towards the primordial stellar region of the root through the pores in this latticework and the intercellular spaces. The suberised Casparian band around the tangential walls of endodermal cells prevents radial movement into the central stela of nutrients in the soil solution. The band also restricts nutrients within the stele from leaking surface into the apoplasm. Older areas of some roots have another layer of suberised cells in the outer layers of cortical cells that form the exodermis. This layer further restricts apoplastic movement of external so il solution in these regions of the root. In slower growing roots, much(prenominal) as those on plants subjected to stress, the exodermis may be formed closer to the tip than in rapidly growing roots (Perumalla and Peterson, 1986). Movement of solutes through the apoplasm also appears to be certified near the meristematic region close to the root tip where the smallfibrils of the cell walls appear densely packed (Peterson and Cholewa, 1998).The interlacing fibres of cell walls in the apoplasm serve to filter soil solution. They also increase the path space over which phosphate ions must diffuse to the underlying uptake sites on the plasmalemma. The presence of carboxyl groups associated with the pectic polysaccharides of the cell wall fibres results in an boilers suit negative charge. Anions such as phos- phate are repelled by this charge and restricted to the larger pores within the apoplasm. Mucilages, ex- creted into cell walls and surrounding many roots, carry negatively ch arged hydroxyl groups which can further modify the flow of anions. These, and other root excretions, provide substrates for rhizosphere micro-organisms that can influence nutrient concentrations close to the uptake sites. The can effect is that movement of phosphate may be prevent within the apoplast, further modifying the concentration of phosphate at the outer surface of the plasmalemma, particularly in cells in the inner cortex. Even in soils comfortably supplied with phosphate this concentration is likely to be less than 2 micro molar. In the P-deficient soils of the SAT, the concentration will be much lower than this. white plague of phosphate into the symplasmThe plasmalemma of root epidermal and cortical cells provides the boundary between the apoplasm and the symplasm. at one time inside the symplasm, nutrient ions in the cytoplasm can move radially through to the stele via plasmodesmata connections without encountering further membrane barriers (Clarkson, 1993). Trans- port of ions crosswise the permeable plasmalemma is, therefore, a critical step that mediates and regulates the uptake of nutrients into the plant. The physiology and kinetics of post of nutrients across the plas- malemma has been known for a long time. Epstein and colleagues (Epstein and Hagen, 1952 Epstein, 1953) conducted classical experiments over 40 years ago that showed that ion uptake by plant roots could be draw by first order kinetics in a similar manner to many enzyme reactions. They also showed that, for the major nutrients studied, the process could be describe by two phases a high-affinity system operating at low external nutrient concentrations and a low-affinity system operating at higher(prenominal) external concentrations. An implication arising from these experiments was that uptake through the plasmalemma was mediated by proteins plant in this membrane. However, closing off and identification of the specific proteins involved proved to be very difficult until nutritional physiologists began to apply molecular techniques to the study of the mechanisms of ion transport in plants. With the aid of this new technology over the past 8 years, many of the specific proteins involved in transport of a number of nutrient ions in plants have been characterized, the genes convert these proteins identified, and the complex regulatory systems involved have begun to be untangled. Genes encoding the phosphate transporter proteins responsible for inflow of phosphate into the cells of roots and some other tissues have been detached, and the roles of some of these have been defined.Uptake of phosphate into the root symplasm involves transport from concentrations less than 2 micro molar in the surrounding apoplasm across the membrane to the cytoplasm where phosphate concentrations are maintained in the mill molar range. This, together with the net negative charge on the inside of the plasmalemma, necessitates that strong electro- chemical gradients cho ose to be overcome for successful transfer of phosphate anions into root cells. Trans- port of phosphate across the plasmalemma, therefore, requires a high-affinity, energy driven transport mechanism. The genes encoding such transporters have been isolated from a number of plant species during the past 4 years and the sequence and topology of the encoded transporter proteins inferred from the DNA sequences. acknowledgment of plant phosphate transportersAn Expressed Sequence Tag from an Arabidopsis clon containing similarities to the sequences of genes encoding phosphate transporters that had been isolated from yeast and fungi led to the isolation of the first reported genes encoding plant phosphate transporters (Muchhal et al., 1996 Smith et al., 1997a). These genes were isolated from Arabidopsis. They now form part of the rapidly growing Pht1 family of plant phosphate transporters which includes members isolated from tomato (Daram et al., 1998 Liu et al., 1998a), potato (Leggewie et al., 1997), Catharanthus (Kai et al., 1997), Medicago (Liu et al., 1998b), barley (Smith et al., 1999) and redundant genes from Arabidopsis (Mitsukawa et al., 1997a). Eight different members of this family of phosphate transporters have been isolated from the barley genome to date (Smith et al., 1999). A member of a second family of phosphate transporters, Pht2, that has similarities to the instead different family of phosphate transporters represented by some mammalian Na+/phosphate cotransporters has recently been isolated from Arabidopsis (Daram et al., 1999). This transporter, which functions as an H+/H2PO4 cotransporter in plants, is primarily expressed in Arabidopsis shoot tissues. It appears to be involved in the internal cycling of phosphorus within the plant.