Putative Silicon Transporters and Effect of Temperature Stresses and Silicon Supplementation on Their Expressions and Tissue Silicon Content in Poinsettia

Silicon (Si) is a beneficial element for plants. To understand Si uptake and accumulation in poinsettia, the Si transporters and their expression patterns were investigated. Nodulin 26-like intrinsic membrane proteins (NIPs) act as transporters of water and small solutes, including silicic acid. In this study, one NIP member, designated EpLsi1, was identified. Additionally, a protein from the citrate transporter family, designated EpLsi2, was identified. Sequence analyses indicated that EpLsi1 belonged to the NIP-I subgroup, which has a low Si uptake capacity. Consistently, the measured tissue Si content in the poinsettia was less than 1.73 ± 0.17 mg·g⁻¹ dry weight, which was very low when compared to that in high Si accumulators. The expressions of <i>EpLsi1</i> and <i>EpLsi2</i> in poinsettia cuttings treated with 0 mg·L⁻¹ Si decreased under temperature stresses. A short-term Si supplementation decreased the expressions of both <i>EpLsi1</i> and <i>EpLsi2</i> in the roots and leaves, while a long-term Si supplementation increased the expression of <i>EpLsi1</i> in the leaves, bracts, and cyathia, and increased the expression of <i>EpLsi2</i> in the roots and leaves. Tissue Si content increased in the roots of cuttings treated with 75 mg·L⁻¹ Si at both 4 and 40 °C, indicating that the transport activities of the EpLsi1 were enhanced under temperature stresses. A long-term Si supplementation increased the tissue Si content in the roots of poinsettia treated with 75 mg·L⁻¹ Si. Overall, poinsettia was a low Si accumulator, the expressions of Si transporters were down-regulated, and the tissue Si content increased with temperature stresses and Si supplementation. These results may help the breeding and commercial production of poinsettia.