**3. Phosphorus availability in soil and mechanisms underlying its acquisition in plants**

#### **3.1 Production of phosphatase enzymes**

Plants have developed a sophisticated network of mechanisms to cope with various environmental stresses, such as nutrient deficiency, drought, and high salinity. One crucial strategy plants employ is through the production of phosphatase enzymes, which play a crucial role in mobilizing P from various organic and inorganic sources in the soil [4]. The secretion of phosphatase enzymes by plant roots into the rhizosphere enables the breakdown of a wide range of organic P-based compounds, making inorganic P readily available for plant uptake [48, 49]. Under low P-conditions, plants induce the expression of genes encoding phosphatases, increasing enzyme activity in various tissues [50]. This upregulation of phosphatase activity helps plants to efficiently scavenge phosphate from the soil and maintain their growth and development. Moreover, mycorrhizal fungi can stimulate phosphatase activity, enhancing P uptake and translocation in plants [4].

The major phosphatases identified in plants include acid phosphatases, alkaline phosphatases, and purple acid phosphatases [4]. These enzymes are involved in the hydrolysis of different types of P-compounds, such as nucleotides, phospholipids, and phytate. Phosphatase enzymes have been shown to have multiple functions in plants, such as regulating iron homeostasis and improving the availability of minerals like iron, zinc, and calcium for plant uptake [4, 14].

#### **3.2 Secretion of organic acids**

Plants also secrete organic acids, such as citrate and malate, into the rhizosphere to solubilize inorganic P-compounds such as iron and aluminum phosphates [51]. These acids chelate metal ions bound to P, releasing P into the soil solution and making it available for plant uptake [52]. Through this mechanism plants cope with P deficiency and low availability in the soil [51]. The secretion of organic acids by plant roots facilitate the solubilization and uptake of essential nutrients, including P, iron, and aluminum [53]. Organic acids are low-molecular-weight compounds that chelate cations and dissolve insoluble mineral compounds, making them more available for plant uptake [51].

Plant roots, including citrate, malate, and oxalate, secrete several organic acids [15]. Citrate is the most commonly secreted organic acid, and is involved in the solubilization and uptake of various nutrients, such as iron, aluminum, and P [54]. On the other hand, malate is involved in the uptake of aluminum, while oxalate helps in the uptake of calcium and aluminum [15]. These organic acids are *Regulation of Plant-Microbe Interactions in the Rhizosphere for Plant Growth and Metabolism… DOI: http://dx.doi.org/10.5772/intechopen.112572*

secreted into the rhizosphere by specialized cells in the root, known as root border cells, and root hairs [53].

Various factors, including nutrient deficiency, pH, and root exudates, regulate the secretion of organic acids in plants [55]. Under nutrient-deficient conditions, plants induce the expression of genes encoding enzymes involved in organic acid synthesis and transport, increasing their secretion [51]. Additionally, the pH of the rhizosphere can also influence organic acid secretion, with lower pH values generally leading to increased secretion [55].

Releasing organic acids by plant roots may also substantially affect the soil microbial ecology. Organic acids can act as a carbon source for soil microorganisms, promoting their growth and activity [56]. Additionally, the solubilization of nutrients by organic acids can also increase microbial activity and diversity in the rhizosphere. Several studies have reported on the secretion of organic acids by different plant species. For instance, a study by Palomo, Claassen [57] showed that maize plants secrete high citrate levels under P-deficient conditions, enhancing their P uptake. In another similar study soybean plants were reported to secrete malate and oxalate under aluminum stress conditions, which was argued to increase aluminum tolerance [58].

#### **3.3 Mycorrhizal symbiosis**

P, an essential nutrient for plant growth and development. However, it is often limited in soil due to low availability and solubility. The mycorrhizal symbiosis enables plants to overcome P-limitation by accessing P from a larger soil volume and increasing P-uptake efficiency by forming highly branched arbuscules. Mycorrhizal symbiosis is a mutually beneficial relationship between plants and fungi, where the plant provides the fungus with carbohydrates in exchange for nutrients, including P. The arbuscules provide a large surface area for P uptake and translocation from the soil to the plant.

Mycorrhizal fungi can also release enzymes and organic acids that help to solubilize P-compounds in soil, making them available for plant uptake. The release of organic acids by mycorrhizal fungi, such as citrate, malate, and oxalate, can increase the solubility of insoluble P compounds, such as calcium phosphates, by forming complexes with metal ions that bind to P, thus freeing it for plant uptake [59]. Additionally, mycorrhizal fungi can release enzymes such as acid phosphatase, which can hydrolyze organic P-compounds, such as phytate, and release inorganic P for plant take up [4].

#### **3.4 Change in root morphology**

One common strategy plants employ for increasing P-uptake is the production of longer and more branched roots, which can increase the surface area of the root system and improve the plant's ability to explore the soil for nutrients. For example, several studies have shown that plants grown under low P-conditions produce longer and more branched roots than those grown under high P-conditions [60, 61].

In addition to changes in root length and branching, plants can also alter the distribution of root hairs, which are small projections from the root surface that increase the surface area of the root system. Under low P-conditions, some plant species produce more root hairs per unit length of root than under high P conditions [62]. Plants can also alter the morphology of their root tips to improve P-uptake. For example, some plants produce cluster roots, which are highly branched structures that form at

the tips of roots and increase the surface area of the root system. These structures are prevalent in plants that grow in soils with low P-availability, such as Proteaceae and Casuarinaceae species [63]. Another strategy that plants use to increase P-uptake is the production of exudates, which are organic compounds released by plant roots that can increase P-availability in soil. For example, some plants release organic acids that can solubilize mineral-bound P and make it available for uptake [4]. In addition to these morphological changes, some plant species have also evolved symbiotic relationships with mycorrhizal fungi, which can improve P-uptake by extending the root system and increasing the surface area of the root system [5].
