The mechanisms of Si-mediated alleviation of P deficiency in wheat grown in acid soils polluted by mine tailings

Abstract

Pollution from industrial activities is increasingly creating marginal conditions for crop production worldwide. Nutrient deficiency, and in particular phosphorus (P) deficiency is an often overlooked factor which can be a severe constraint for plant growth in soils affected my mining activates (Nikolic et al., 2011). The root exudation of carboxylates (mainly citrate and malate) has been considered as the major root response to mobilize sparingly soluble P in the rhizosphere. Therefore the enhanced activity of phosphoenolpyruvate carboxylase (PEPC) appears to be a key P-independent metabolic bypass reaction of malate/citrate biosynthesis in P deficient plants. Although the beneficial role of silicon (Si) on plant growth has been reported on several plant species grown under P deficient conditions the underlying mechanism is still unknown. The pot experiments were conducted with winter wheat, including Si fertilizers in addition to the conventional reclamation amendments (e.g. cow manure, NPK and lime) in the acid soil polluted by sulphidic mine tailings (collected form the Timok floodplain). The main focus of this study was on the dynamics of P pools in wheat rhizosphere, and on the molecular mechanism of root response to P availability in the rhizosphere (expression of TaPT1 and TaPT2, encoding Pi transporters, TaPEPC encoding PEPC, and MATE-family genes encoding Al-activated citrate efflux transporter). All the amendments induced a significant change in the rhizosphere P fractions (readily available, Al- and Fe-bound P). For instance, Si supply has caused significant increase in readily available P, and reduction in Al-P and Fe-P. This can be attributed to synergetic effect of pH increase and reduction of P sorption by Al- and Fe-oxides. The leaf P concentration in wheat plants treated with Si significantly increased and was in the range of leaf P concentration in P-fertilized plants. While the expressions of the root TaPT2 and TaPEPC were down-regulated by P availability in the rhizosphere and plant P status, TaPT1 and TaMATE showed different pattern with markedly enhanced expression at Si treatment irrespectively of the P supply. In conclusion, Si nutrition effectively alleviates P deficiency in wheat by 1) increased P availability in rhizosphere, most probably due to MATE-mediated citrate exudation, and 2) enhanced P acquisition as a consequence of Si-promoted expressions of PT1 transporter in root plasma membrane.