Silicon accumulated in the shoots results in down-regulation of phosphorus transporter gene expression and decrease of phosphorus uptake in rice

Silicon (Si) as a beneficial element can improve nutrient imbalance, but the molecular mechanism for this effect is poorly understood. The objective of this study is to examine the mechanism underlying Si-induced decrease of phosphorus (P) uptake in rice (Oryza sativa) at adequate/high P supply. A rice mutant (lsi1) defective in Si uptake and its wild type (cv. Oochikara) were used. The P uptake was compared in the presence and absence of Si and the expression of Pi transporter genes was quantified. Si addition in the nutrient solution significantly decreased shoot P concentration and uptake in the WT, but not in lsi1 mutant at two P levels, adequate (90 mu M) and high (210 mu M). Neither the root-to-shoot translocation of P nor the P distribution in different organs was altered by Si in both WT and lsi1. Heterogeneous expression of Lsi1 in Xenopus oocyte did not show transport activity for Pi. The expression of Pi transporter genes (OsPT1, 2 and 8) in the roots was hardly affected by Si in both WT and lsi1, but that of OsPT6 was down-regulated by Si in the WT roots, but not in the lsi1 roots. Furthermore, a split root experiment showed that Si accumulated in the shoots suppressed the expression of OsPT6. In rice grown in paddy field, lsi1 showed higher P concentration in the straw, husk and brown rice than the WT. Si decreased P uptake through down-regulating the expression of P transporter gene, OsPT6 in rice and Si accumulated in the shoot is required for this down-regulation.