Abstract
Phosphorus (P) is an essential macronutrient for optimum productivity of several crops including soybean, but it is largely limited in most arable lands. The persistent depletion of phosphate rocks is a major constraint for plant growth and development. Improved P efficiency (PE, uptake and use efficiency) drives an enhanced biological nitrogen fixation in soybean, which could consequently lead to an increased grain and biomass production. Thus, to mitigate P limitation in soybean production, a wide range of genetic approaches have been deployed. These approaches have unravelled the morphological, genetic, phytohormonal and metabolic mechanisms involved in soybean adaptations to P-deficient conditions. Most of the approaches are targeted at root modifications due to the importance of roots in the mediation of early responses to P-deficient conditions. PE among soybean genotypes in P-deficient conditions is mostly dependent on the root plasticity, development of shallow root architecture, symbiosis with arbuscular mycorrhizal fungi and exudation of phosphatases and organic acid anions. Genetic manipulation of soybean has revealed a number of important genes (GmPAPs, GmPTs, GmPLDZ2, GmIPS1 and GmExPB2), transcription factors (C2H2 zinc finger protein, WRKY and MYB) and quantitative trait loci (q14-2 and q19-2) modulating P mobilisation, uptake and utilization under P limiting conditions. Through the modification of root traits, the hormonal (biosynthesis, transport and secretion of ethylene, auxin and jasmonic acid) and metabolic (phenylpropanoid biosynthesis and phenylalanine metabolisms) pathways also modulate PE in soybean. Recent advances in genetic manipulations of root traits offer promising ways for enhancing soybean production, particularly in a P limiting environment.