Nodulin 26-like intrinsic protein CsNIP2;2 is a silicon influx transporter in Cucumis sativus L.

Nodulin 26-like intrinsic proteins (NIPs) are a family of channel-forming transmembrane proteins that function in the transport of water and other small molecules. Some NIPs can mediate silicon transport across plasma membranes and lead to silicon accumulation in plants, which is beneficial for the growth and development of plants. Cucumber is one of the most widely consumed vegetables; however, the functions of NIPs in this crop are still largely unknown. Here, we report the functional characteristics of CsNIP2;2. It was found that CsNIP2;2 is a tandem repeat of CsNIP2;1, which had been demonstrated to be a silicon influx transporter gene. CsNIP2;2 has a selectivity filter composed of cysteine, serine, glycine and arginine (CSGR), which is different from all previously characterized silicon influx transporters in higher plants at the second helix position. Xenopus laevis oocytes injected with CsNIP2;2 cRNA demonstrated a higher uptake of silicon than the control, and the uptake remained unchanged under low temperature. CsNIP2;2 was found to be expressed in the root, stem, lamina and petiole, and exogenous silicon treatment decreased its expression in the stem but not in other tissues. Transient expression of CsNIP2;2-eGFP fusion sequence in onion epidermal cells showed that CsNIP2;2 was localized to the cell nucleus, plasma membrane and an unknown structure inside the cell. The results suggest that CsNIP2;2 is a silicon influx transporter in cucumber, and its subcellular localization and the selectivity filter are different from those of the previously characterized silicon influx transporters in other plants. These findings may be helpful for understanding the functions of NIPs in cucumber plants.