45 | P a g e
ENHANCING PHOSPHORUS USE EFFICIENCY THROUGH BREEDING
Sheelamary S* and Lakshmi K
Division of Crop Improvement,
ICAR-Sugarcane Bredding Institute, Coimbatore-641007
*Corresponding author email: [email protected]
ABSTRACT
Phosphorus (P) is a nutrient that restricts crop productivity and is expected to become increasingly common in the future. To boost phosphorus use efficiency, phosphate absorption efficiency and productivity per unit P taken up can be improved. Plant P levels could be reduced significantly by reducing ribosomal RNA and replacing phospholipids with sulfolipids and galactolipids. P-use efficiency research that considers physiological, metabolic, molecular biology, genetic, and phylogenetic elements is critically needed to advance our knowledge of this complicated feature.
INTRODUCTION
To avoid nutrient depletion and soil deterioration, it is an absolute requirement for sustainable food production that nutrients withdrawn with crop harvest be supplied. Nitrogen (N), phosphorus (P), and potassium (K) are the most abundant nutrients required by plants, and a deficit in any of these minerals significantly affects crop yield. The global need for N and P fertiliser is continually increasing, owing to the necessity to enhance food production to keep up with the growing population. P fertiliser is made from phosphate rock, and some people are concerned that this natural resource will be depleted soon. Phosphate rock reserves are expected to last another 300–400 years, and investments are being made to find new supplies and develop alternative technologies for separating P from marine sediments or human and animal waste.
Crops take up approximately 15–30% of the fertiliser P in the year of application; increasing P acquisition can result in significant efficiency advantages. This component of P efficiency has gotten much attention recently, and it was recently examined, revealing intriguing opportunities for better crop characteristics and agronomic metrics. On the other hand, physiological PUE has received far less attention, and comparative investigations of PUE have frequently been hampered by confounding effects of variation in P-acquisition efficiency. P inputs must balance p exports, and agricultural systems with high yields per unit P taken up, or PUE, will be the most sustainable and productive.
PROBLEMS IN IMPROVING PHOSPHOROUS USE EFFICIENCY
46 | P a g e The phosphate cycle in soils is a complicated process governed by various physical, chemical, and biological factors. For optimal crop growth, it is difficult to determine the quantity of P fertilizer to be applied. It is believed that no more than 8% of phosphorus applied to soil in fertilizers is retrieved in crops, depending on soil type, physical condition, and chemical status. The rest is either adsorbed to soil particles and organic matter, taken up by soil bacteria, or lost to surface waters. The P is accumulated due to the over applying of P by farmers to meet the yield. This lead to leaching and eutrophication and ultimately inefficient use of P by the plants. So, we have to breed the varieties for Phosphorous use efficiency.
FIG 1. PHOSPHOROUS CYCLE IN ENVIRONMENT
IMPORTANT PLANT TRAITS RELEVANT FOR PUE BREEDING
Phosphorus is found in various places in plants, including energy carriers, nucleic acids, and signaling pathway proteins. Plants have a tough time absorbing phosphate from the soil because it is linked to calcium in alkaline soils and aluminum and/or iron in acidic soils, organic material from manure or crop debris found in the soil-bound to phytate compounds. Crops have to mobilize the fertilizer from soil and utilize it for yield. PUE is based on a diverse range of plant characteristics. Breaking down these complex traits into component qualities that can be tested cheaply for a specific crop species and showing consistent PUE contributions is critical for improved breeding success.
Furthermore, these features should have as simple an inheritance as feasible to efficiently introduce PUE into elite plant material. Optimizing P scavenging would be an appealing breeding goal,
47 | P a g e especially for soils with high bound P. Internal P consumption efficiency would be a useful breeding aim in the long run, despite the fact that it is likely to be more difficult. Root features related to P uptake are architectural traits that are visibly important to external P efficiency. There are likely to be qualities present that are relevant to internal P efficiency among these. Therefore, increasing the internal P use efficiency could be a valuable breeding goal (Rose and Wissuwa, 2012).
AGRONOMIC AND PHYSIOLOGICAL TRAITS
Root characteristics are important for effective P uptake. P deficiency causes a larger root-to-shoot ratio as well as alterations in root architecture. Root extension plays a vital role in scavenging P from the soil, as the rhizosphere is quickly depleted of P, and P replenishment through diffusion and mobilization does not always keep up with absorption. Higher PUE from roots has been linked to a number of architectural alterations. A greater number of lateral roots increases the possibility of scavenging for P.
Root hair growth modification can be accomplished at a minimal carbon cost. P uptake capacity is increased as densities and lengths are increased. Enhanced axial root lengths were also reported in maize and common bean, but without increased lateral branching, which was interpreted as exploratory behavior for soil patches enriched in P, where lateral root production would eventually become functional.
Increased root density in the upper parts of the soil favor P uptake efficiency. Not all crops, it appears, have the ability to boost root hair development in order to increase P absorption capability. The changes in the density and growth angle of lateral roots helps to increases the scavenging capacity of the plants in the upper layer of the soil (Ao et al., 2010). However, the preference for root surface investments in the upper soil layer may compromise water usage efficiency, as water is normally more available in deeper layers under water-stressed situations. The PUE can be improved by improving the root architectural traits such as lateral branching and root hair density.
Phosphorus is involved in so many facets of plant metabolism, PUE is expected to affect a wide range of physiological features. Root exudates are hypothesized to aid in mobilizing P from fixed sources in the soil to improve P absorption. Although exudates can reach levels in the rhizosphere that potentially release P from soil particles, their efficiency is unknown. In reaction to low P, hydrolytic enzymes such as acid phosphatases and ribonucleases are activated, and upon exudation, they can release P fixed in organic forms in the soil, such as phytate.
Internal P usage efficiency is hypothesized to be influenced by a variety of metabolic changes.
The effective mobilization of P within the plant, for example, recycling P from mature/senescing plant sections to actively growing tissue and re-using phosphate from vacuoles that have a buffering function in holding P when metabolic needs in the cytoplasm are in excess, is an important element. Phosphatases are engaged in P mobilisation in the soil and in plant internal re-allocation, which requires certain types of phosphatases. Specific inwardly localized phosphatases are poorly understood. Similarly, high-affinity transporters appear to play a role in internal P mobilization by upregulating particular transporter genes in ageing tissues.
Low P induces quick increases in potentially damaging reactive oxygen species (ROS) and higher ROS scavenging enzyme activities similarly to other abiotic and biotic stresses in plants. Auxin and
48 | P a g e ethylene signaling is associated with lateral root initiation, which is a pivotal response to low P. Auxin's effects on lateral root growth are counteracted by gibberellin. Low P inhibits cytokinin production, which is known to suppress P starvation-induced (PSI) genes. Even when the vacuolar P pools are not depleted, low cytokinin stimulates root growth, and a decrease in root export inhibits shoot growth. Sugar signaling also interacts with P responsiveness. Decreased photosynthesis in response to low P results into increased starch formation in shoots. In addition, sugar loading into the phloem is increased and thus allocated to roots, in turn increasing the root/shoot ratio and leading to activation of P-responsive genes. For breeding, many of these physiological reactions are complex and difficult to quantify. Some are simple, such as the examination of exudates in response to low P. Genomic approaches such as transcriptomics and metabolomics have been used to target the more complicated features.
BREEDING FOR PUE
Breeding for PUE is a complicated process due to the complexity of this trait and the influence of the environment. Under stressful conditions, narrow-sense heritability was poor. Furthermore, because of the typically high regional variation of P availability in the soil, testing for P usage efficiency in the field might be difficult. The phosphorus use efficiency was heritable under glasshouse conditions, but this turned out not to be the case under field conditions in some crops. Successful selection for increased PUE was more in case by focussing on root architecture. Recent advances in molecular approaches, like as marker-assisted selection (MAS) and genomics, have tremendously aided traditional breeding studies.
QTLs for PUE could be used through introgression into elite varieties, although this has hardly been done in breeding programs. PUE-related genes include transcription factors, genes involved in signal transduction, proteins directly involved in P scavenging (e.g., acid phosphatases and high-affinity P transporters), hormone-responsive factors, and metabolic factors (e.g., phosphatases) (Zhang et al., 2010).
Alternative approaches, like mutagenesis and gene expression microarrays, can track out genes linked to PUE. When transgenic plants function well under agronomic settings with low P availability and, preferably, without P stress, they can be used immediately for cultivar development. These criteria may not always be met since PUE, like other abiotic stress-related features, is complicated, and single gene effects may come with trade-offs, especially in environments when P is abundant. Plant fungal combinations should exhibit considerable yield benefits relative to the non-mycorrhizal condition for the breeders' elite plant genotypes in actual field situations when breeding crops for PUE. The mutant clearly showed high responsiveness to mycorrhiza, but as such, it would not be a reasonable basis for cultivar development (Sawers et al., 2008). Plant responsiveness to mycorrhiza has been studied in a variety of methods. Furthermore, mycorrhizal performance in the field may be much more difficult to quantify and forecast from controlled experimental work than basic plant PUE attributes.
CONCLUSION
Separating PUE into qualities accessible to genetic study and manipulation appears to make it possible to breed for it. For this, an optimized phenotyping method for measuring root architectural features, such as P scavenging via root exudations and mycorrhiza, is required. Based on the developmental and physiological aspects of phosphorus use efficiency, underpinned by molecular biology
49 | P a g e and genetics that suggest significant improvement in internal PUE is possible. The magnitude of PUE gains obtained through different mechanisms, and their variation associated with genetic and environmental factors, should be quantified through targeted research. More efficient use of P within the plant adds to the gains that can be made by improving P-acquisition efficiency and reduces P fluxes on cropland and in the environment.
REFERENCES
Ao, J., Fu, J., Tian, J., Yan, X., and Liao, H. 2010. Genetic variability for root morph-architecture traits and root growth dynamics as related to phosphorus efficiency in soybean. Functional Plant Biology 37:304–312
Rose, T.J., and Wissuwa, M. 2012. Rethinking internal phosphorus utilization efficiency: a new approach is needed to improve PUE in grain crops. Advances Agronomy 116:185–217
Sawers, R. J. H., Gutjahr, C., Paszkowski, U. 2008. Cereal mycorrhiza: an ancient symbiosis in modern agriculture. Trends Plant Science 13:93–97
Zhang, D., Liu, C., Cheng, H., Kan, G., Cui, S., Meng, Q., Gai, J., Yu, D. 2010. Quantitative trait loci associated with soybean tolerance to low phosphorus stress based on flower and pod abscission.
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