**3.6 Phosphorus mobilization and transporters**

Plants can mobilize and transport P within their tissues to improve their ability to acquire P from the soil. P can be mobilized within plant tissues by phosphatase enzymes, which break down organic P-compounds into inorganic forms that are more readily available for plant uptake. Plants also have specific transporters that can uptake inorganic P from the soil and transport it into their tissues [68].

Membrane proteins are called P-transporters to facilitate P transport across plants' plasma membranes and other intracellular membranes [69]. Several P-transporters exist in plants including the PHT1, PHT2, PHT3, PHO1 transporters [70]. The PHT1 transporter family is the most extensively studied and is involved in inorganic phosphate uptake from the soil [71]. This transporter family comprises 9 to 13 members in different plant species and is expressed in the root epidermis and cortex, where they play a crucial role in the uptake of inorganic phosphate from the soil [71]. They have been shown to have a high affinity for inorganic phosphate and can transport it against a concentration gradient [72]. PHT2 transporters are expressed in the plasma membrane of root hairs and are involved in phosphate uptake and translocation [54].

PHT3 transporters are localized in the chloroplast and are involved in phosphate transport from the cytoplasm to the chloroplast, which is required for photosynthesis [73]. PHO1 transporters translocate inorganic phosphate from the root to the shoot in plants [74, 75]. They are localized in the plasma membrane of the root endodermis and are responsible for loading inorganic phosphate into the xylem for transport to the shoot [74]. PHO1 transporters have also been shown to play a role in the secretion of phosphate-containing compounds into the rhizosphere, which can increase the availability of P in the soil [69]. The regulation of P transporters is critical for

*Regulation of Plant-Microbe Interactions in the Rhizosphere for Plant Growth and Metabolism… DOI: http://dx.doi.org/10.5772/intechopen.112572*


#### **Table 2.**

*Major plant transport proteins involved in phosphorus uptake.*

maintaining P-homeostasis in plants [2]. Several factors can influence the expression and activity of P-transporters, including P availability, plant age, and environmental stresses [76]. Under low P-conditions, plants can upregulate the expression of PHT1 transporters, leading to an increase in phosphate uptake [76]. Similarly, under drought stress, the expression of PHT1 transporters can be downregulated, leading to a decrease in phosphate uptake [76]. **Table 2** lists the major transport proteins and highlights their important functions.

#### **3.7 Organic phosphorus mineralization**

According to Sharpley [90], organic P-compounds comprise a significant proportion of total P in many soils. However, these organic P-compounds are often not readily available to plants. Microorganisms play a crucial role in the mineralization of organic P, which is the process by which organic P-compounds are converted into inorganic P-forms. Several factors influence the rate of organic P-mineralization, including soil pH, moisture content, temperature, and the availability of nutrients such as nitrogen and P. For example, studies have shown that organic P-mineralization rates increase with increasing soil pH [91, 92]. Plants can also play a role in organic P-mineralization by releasing organic acids or other compounds that can stimulate microbial activity and increase the availability of inorganic P. For instance, in one experiment the release of root exudates by maize plants increased the mineralization of organic P in the soil [93].

Overall, organic P-mineralization is a crucial process in P-cycling in soils and is essential to plant nutrition. Understanding the factors influencing this process can help optimize fertilizer management strategies and improve crop productivity.
