**3. Phosphorus in plants**

In terms of its importance in most organisms, P has been considered an essential, irreplaceable element in all living cells. P stands second to N in terms of its essentiality as a macronutrient in plants/crops. P has been widely considered as a key regulatory element for plant growth and metabolism. Apart from providing an anchor for the plant in the soil, roots take up water and nutrients from the soil solution. In fact, the interception of nutrients available in the soil solution occurs in the rhizosphere. Eventually, the movement of nutrients toward the root involves mass flow and diffusion, where the former process contributes only 2–3% of the total amount of P (transported), usually required by many crops to produce acceptable yields. Interestingly, Pi, the only form of P that can be assimilated by plant, exhibits uneven distribution, relative immobile (as its diffusion coefficient is very low: 10–12 to 10–15 m2 s−1), and high fixation. Hence, the concentration of Pi is very low (<2.0 μM) in soil solution, and even in fertile soil (10 μM). In plants, P represents 0.1–0.5% of the dry weight. Therefore, Pi-availability has been a worldwide constraint for crop growth and productivity [3].

#### *Introductory Chapter: Phosphorus in Soils and Plants DOI: http://dx.doi.org/10.5772/intechopen.113397*

The knowledge of the plant Pi-acquisition and distribution mechanism may help in enhancing the plant/crop P-use efficiency. The list of the major strategies adopted by most plants/crops under P-deficiency includes: (i) increase in root surface area by formation of finer roots, aerenchyma, and root hairs, and eventual improved soil exploration; (ii) secretion of organic acids to release Pi through complexation reaction of organic acids with Al3+, Fe3+, and Ca2+; (iii) the complexation reaction of organic acids with phosphatases to release Pi from the organic sources and thus to enhance the availability of Pi in soils; (iv) arbuscular mycorrhizal fungi (AMF) colonization; and (v) employing Pi transporters to facilitate Pi-uptake [3, 9, 10]. Arbuscular mycorrhizal fungi (AMF; phylum Glomeromycota) contribute to plant P-nutrition. Notably, AMF-colonization process is modulated by Pi-status and PHT1 transporter. Interestingly, this AMF-colonization is usually not well developed in soils with adequate, plant-available P. Both terrestrial plant species (90%) and in particular vascular plants, including main crops (>80%), exhibit AMF-colonization. Molecular genetic studies have unveiled various Pi-transporters, which facilitate the plant uptake and translocation of available Pi. Five phylogenetically distinct classes of families of plant Pi-transporters are known: PHT1, PHT2, PHT3, PHT4, and PHT5. PHT1s are, in general, plasma membrane-located and mainly function in Pi acquisition from soil. PHT2, PHT3, PHT4, and PHT5 family members are known to contribute in Pi distribution within the plant such as translocation against chloroplasts, mitochondria, Golgi, and vacuole. PHT1 transporters, the best studied plant Pi transporters, belong to multigene family and are involved in transport of Pi from apoplast/soil solution to the root cell through the plasma membrane [11]. Under P-deficiency/starvation, P-starvation response (PSR) pathway is activated, which involves PHOSPHATE TRANSPORTER TRAFFIC FACILITATOR1 (PHF1) and PHT1, and several transcription factors including PHOSPHATE STARVATION RESPONSE 1 (PHR1) and PHR1-LIKE 1 (PHL1), WRKY74, WRKY75, MYB2, MYB4, and ARF16 have been associated with the PSR pathway and implicated in P uptake [2, 12, 13]. Organic esters (such as phosphomonoesters and diesters) are the organic forms of P in plants. Phytase-mediated release of phosphates from phytates is possible during seed germination. Organic forms of P (namely, phosphomonoesters and diesters) are broken in plants by enzymes, namely, phosphatases and diesterases, in order to make P available to plants [4]. Plant responses to the low P (or P-starvation) also involve several hormones and signaling molecules (including cytokinins, CKs; abscisic acid, ABA; gibberellin, GA; and strigolactones, SLs) [14].

Interestingly, essential biomolecules such as DNA, RNA, ATP, NADPH, and membrane phospholipids involve P. In fact, nucleic acids, phosphorylated proteins, various phosphorylated metabolites, and phospholipids present the major pool of organic-P. Hence, plant tissues exhibit relatively high (about 5–20 mM) phosphate concentration. Phosphatases, nucleases, and phosphoesterases have been reported to contribute in the release of Pi from the organic forms during senescence in plants. Additionally, photosynthesis, respiration, and activation of proteins *via* phosphorylation, the life-sustaining processes in plants, strictly involve P. P is irreplaceable in many P-containing biomolecules. ATP and NADPH are the major high energetic molecules and are prerequisites to ensure photosynthesis functioning during the Calvin cycle [15]. Literature is full on the role of P in plant cell metabolism. The regulatory mechanisms during the developmental processes as well as response to stress conditions in plants involve phosphorylation of proteins. Moreover, intermediates in Calvin–Benson cycle are phosphorylated with Pi. Pi is released in the cytosol and is cycled back to plastids. The availability of Pi regulates photosynthesis, which is inhibited with limited Pi-supply. A plethora of literature supports the significant roles of P in imparting plant/crop tolerance to various abiotic stresses including drought [16–18], heavy metals [19–22], soil salinity [23–26], temperature [27–30], and water logging [31, 32].
