A Larger Root System of Foxtail Millet (Setaria italica L.) under Phosphate Limitation is Coupled with Contrasting Expression Patterns of Phosphate and Nitrate Transporters

Foxtail millet (Setaria italic L.), a widely cultivated food and fodder crop, develops a smaller root system while enlarges the root diameter facilitating nutrient transport under nitrogen limitation. How foxtail millet responds to phosphate limitation (LP) remains unaddressed. LP seedlings of the standard variety Yugu1 significantly reduced P concentrations in both shoots and roots and displayed higher levels of anthocyanin accumulation in leaves, indicating that the seedlings suffered from P limitation under hydroponic culture. One obvious and adaptive phenotype of LP plants was the larger root system mostly as the result of stimulation of lateral root proliferation in terms of the number, density, and length. Preferential biomass accumulation in the root under LP ensured carbon provision for root expansion and resulted in significant increases in the total and specific root length, which substantially extended the absorptive surface of P in the growth medium. Elevation of auxin and GA3 concentrations might serve as an internal boost underpinning root architectural repatterning under LP. Not just morphological adaptation, up-regulation of expression of SiPHT1;1 and SiPHT1;4 in roots and that of SiPHT1;2 in roots and shoots favored enhancement of P uptake and translocation under LP. On the basis of a larger root system of LP plants, internal nitrogen surpluses occurred as indicated by higher concentrations of nitrogen and dramatic increases in free amino acids in both shoots and roots. Such nitrogen surplus ‘signals’ tended to switch down expression of nitrate transporters SiNRT2.1 and SiNAR2.1 in the root and that of SiNRT1.11 and SiNRT1.12 in the shoot to reduce nitrate mobilization towards and within the shoot. Together, our work provided new insights into adaption of a critical cereal crop to LP and its innate connection with nitrogen nutrition.