**4. Breeding or crop modification strategies**

#### **4.1 Selection of improved genotypes**

Selection and breeding nutrient-efficient species or genotypes within a species are justified in terms of reduction in fertilizer input cost of crop production and also

reduced risk of contamination of soil and water. Many plants have evolved morphological, physiological, biochemical, and molecular adaptive systems to cope with Pdeficiency stress, such as altered root architecture to explore more soil volume and increased carboxylate exudation containing phosphatases, nucleases, and various organic acids [29]. These mechanisms and strategies are necessary to liberate or solubilize Pi from organic and other insoluble pools [30], enhance Pi uptake capacity [31], recycle internal Pi remobilize/retranslocate P from mature to young developing organs [32, 33], and reprioritize metabolic P utilization [34]. Under the current situation, farmers need P-efficient genotypes that perform better than other genotypes with equivalent P inputs. Therefore, selection/identification of cultivars that can absorb and use P efficiently is a promising strategy to cope with environments deficient in bio-available P. Due to the diverse functional and structural roles of P in plants, P-use efficiency (PUE) is a complex trait to dissect [24].

#### **4.2 Modification of root morphology and physiology**

The root morphological factors such as length, thickness, surface area, and volume have profound effects on the plant's ability to acquire and absorb nutrients in soil [35]. These parameters are influencing the ability of the roots to penetrate high density soil layers, to extremes tolerate temperature, moisture, toxicities, and deficiencies of elements. Additionally, they have the ability to modify the rhizosphere pH and the nutrient uptake kinetics. Efficient acquisition will depend first on root architecture in terms of transporters and exudates and often the presence of symbiotic associations such as mycorrhiza. Hence, improving early root establishment, high-affinity transporter systems, association of microorganisms (mycorrizha), proliferation of roots, and enhanced mechanisms for increasing bio-availability of nutrients and then enhancing NUE [5]. Improvement of transporters plays essential roles particularly in conjunction with effective root proliferation in contributing to nutrient use efficiency. The other important attribute for uptake efficiency is having adequate sinks to store acquired nutrients, which will prevent negative feedback regulation on the initial acquisition/assimilatory processes and should provide important remobilizable storage [5]. The second component of uptake efficiency is root physiological activity such as differing uptake kinetics, i.e., maximum net influx (Imax), affinity of the transporter **(**Km) and the roots depletion ability (Cmin), which result in different nutrient uptake rates per unit root and time due to their effect on P diffusion [36]. Lower Km values (higher affinity) and higher Imax values indicate a higher uptake rate of plants for a determined nutrient at low concentration [11].

A recent study further showed that root tips also play an important role and, despite their small size, accounted for approximately 20% of the total seedling Pi uptake [37], mainly increasing organic acid exudation strategies [38]. Plants increase total soil exploration by increasing root length, increasing root branching, increasing specific root length (i.e., roots with smaller diameter), and modifying branching angle [39–41]. The findings of Bates and Lynch [39] suggested that increased root growth is associated with improved plant performance under low P by exploring a larger volume of soil. Consequently, root: shoot ratio increases significantly in low-P environments and is an excellent index for partitioning photosynthesized carbon between above- and below-ground plant parts. Root density and root: shoot ratio generally increased under P deficiency, thus favoring P acquisition by plants [29].

Genetic variation for root hair traits, particularly root hair length, can be exploited in breeding for improved P uptake efficiency and P fertilizer use efficiency in crops.

*Toward the Recent Advances in Nutrient Use Efficiency (NUE): Strategies to Improve… DOI: http://dx.doi.org/10.5772/intechopen.102595*

Moreover, a deeper root with more aerenchyma tissues in the cortex of the roots can also be an important trait that contributes to efficient N uptake with lower carbon input in root growth [42]. This root architecture may also be efficient in the uptake of deep water and therefore help to increase drought resistance [43]. However, Miguel et al. [44] showed in field trials that shallow and hairy root traits are synergistic in their effects on Pi uptake by bean. However, modifying root growth in response to nutrient deficiency, it is a challenge and complex to identify key regulators that are sufficiently upstream and robust to be suitable for developing plants with optimized root systems for nutrient uptake [8].

#### **4.3 Improving translocation (partitioning/remobilization)**

Levels of fertilizer applications influence the total dry matter accumulation, thereby affecting the nutrient demand (uptake/utilization) [9]. Improved nutrient utilization efficiency from agrochemicals through PGPR and (or) AMF can contribute to the protection of water resources against agro-pollution and reduce the growing cost of fertilizers [10]. After inorganic phosphate (Pi) acquisition from rhizosphere, Pi should be efficiently transported to shoot for the requirement of plant growth by phosphate transporters (*Pht1, Pht2, Pht3,* and *Pht4*), which are located on the plasma membrane, plastidial membrane, mitochondrial membrane, and Golgi compartment, respectively [45]. In crops, a large fraction of the Pi present in vegetative organs is remobilized to the grain during the reproductive growth, and soil Pi availability at this stage has a relatively small effect on grain yield. Enhanced expression of high-affinity, plasma-membrane-bound Pi transporters in roots and a concomitantly increased Pup- take capacity were reported as a typical P-starvation response [46]. Moreover, enhanced metabolic activities of young tissues make them stronger sinks for the already absorbed P. Remobilization of stored P in the stem and older leaves to metabolically active sites may supplement the restricted P supply under P deficiency [29].

Another promising area for improvement of crop NUE is to enhance the efficiency of nutrient remobilization from senescing organs to young, developing organs, particularly immature leaves, and developing seeds [47]. The senescence process, that is, the dying-off of vegetative plant parts during seed maturation, is at the core of the nutrient use efficiency issue, as the nutrients need to be remobilized from these parts and translocated into the developing seed [48]. Maximizing the effectiveness of Premobilization from senescing organs could make an important contribution to the development of crops that can tolerate Pi deficiency, because senescing organs of most "modern" crop varieties exhibit low P-remobilization efficiencies of <50% [30]. An integral understanding of P remobilization would facilitate development of effective biotechnological strategies to improve crop PUE, thereby reducing the rate of depletion of nonrenewable rock P reserves [30, 47]. Therefore, mobilization and redistribution of P from the old tissues to the young tissues will also contribute to high P use efficiency. Better distribution of nutrients in parts of plant (root, shoot, and grain) reflects their use efficiency [11].

#### **4.4 Improving internal utilization**

In the plant, uptake and utilization efficiency of nutrients are governed by different physiological mechanisms and their response to deficiency, tolerance, and toxicity of element(s) and climatic variables [49]. Efficient internal utilization of nutrient is generally attributed because of high photosynthetic activity per unit of nutrient (P)

and more efficient P remobilization from older to young leaves [47]. Acid phosphatase contributes to the increased P utilization efficiency in bean through P remobilization from old leaves [50]. Therefore, improving higher total chlorophyll concentration [51], enhancing phosphorylase stimulation [52], and improving partitioning of carbon between glycolytic and pentose phosphate pathways [53] also provide an effective approach to improve phosphorus use efficiency and crop productivity simultaneously.

P-utilization efficient cultivars produce high yield per unit of absorbed P under P deficient conditions, since they have low internal P demand for normal metabolic activities and growth. Hence, they have low requirement for mineral P fertilizer inputs to produce reasonably high yield. Moreover, they remove less P from soil during growth and therefore the quantity of P removed along with the harvestable parts of the crop would obviously be less, consequently reducing the quantity of mineral P fertilizer inputs required for maintenance fertilization [54].
