Isolation and functional characterization of CsLsi1, a silicon transporter gene in Cucumis sativus

Abstract

Cucumber (Cucumis sativus) is a widely grown cucurbitaceous vegetable that exhibits a relatively high capacity for silicon (Si) accumulation, but the molecular mechanism for silicon uptake remains to be clarified. Here we isolated and characterized CsLsi1, a gene encoding a silicon transporter in cucumber (cv. Mch-4). CsLsi1 shares 55.70 and 90.63% homology with the Lsi1s of a monocot and dicot, rice (Oryza sativa) and pumpkin (Cucurbita moschata), respectively. CsLsi1 was predominantly expressed in the roots, and application of exogenous silicon suppressed its expression. Transient expression in cucumber protoplasts showed that CsLsi1 was localized in the plasma membrane. Heterologous expression in Xenopus laevis oocytes showed that CsLsi1 evidenced influx transport activity for silicon but not urea or glycerol. Expression of cucumber CsLsi1-mGFP under its own promoter showed that CsLsi1 was localized at the distal side of the endodermis and the cortical cells in the root tips as well as in the root hairs near the root tips. Heterologous expression of CsLsi1 in a rice mutant defective in silicon uptake and the over-expression of this gene in cucumber further confirmed the role of CsLsi1 in silicon uptake. Our results suggest that CsLsi1 is a silicon influx transporter in cucumber. The cellular localization of CsLsi1 in cucumber roots is different from that in other plants, implying the possible effect of transporter localization on silicon uptake capability.

Tomato roots have a functional silicon influx transporter but not a functional silicon efflux transporter

“…Uptake of Si in the roots is mediated by influx transporter Lsi1 and efflux transporter Lsi2. Because the first Si transporter Lsi1 was identified in rice (Ma et al, ), its homologs have been cloned in barley (Chiba et al, ), maize (Mitani, Yamaji, & Ma, ), pumpkin (Mitani et al, ), horsetail (Grégoire et al, ), wheat (Montpetit et al, ), soybean (Deshmukh et al, ), and cucumber (Sun et al, ). Lsi1 is responsible for transport of Si from soil solution to the root cell, the first step of Si uptake.…”

“…Our functional analysis of SlLsi1 revealed distinct expression pattern and localization of Lsi1 among different plant species. For example, SlLsi1 was expressed in both root tips and basal root regions (Figure ), but CsLsi1 in cucumber was mostly expressed in the root tips and Lsi1 in rice and barley was mainly localized in the basal region of the roots (Chiba et al, ; Sun et al, ; Yamaji & Ma, ). The cellular localization of Lsi1 also differs among different plant species.…”

“…In barley, Lsi1 is localized at epidermal and cortical cells in seminal roots and hypodermal cells in lateral roots (Chiba et al, ), whereas ZmLsi1 in maize is found at epidermal and hypodermal cells in seminal and crown roots and at cortical cells in lateral roots (Mitani, Yamaji, & Ma, ). CsLsi1 in cucumber was localized at the distal side of the endodermis and at the cortical cells (Sun et al, ). Furthermore, unlike Lsi1 in rice, barley, and maize (Ma et al, ; Chiba et al, ; Mitani, Yamaji, & Ma, ), SlLsi1 did not show polar localization (Figure ).…”

“…A cooperative transport mediated by Lsi1 and Lsi2 is required for high Si uptake in rice (Ma & Yamaji, ). Homologous genes of Lsi1 have then been identified in other plant species including barley (Chiba, Mitani, Yamaji, & Ma, ), maize (Mitani, Yamaji, & Ma, ), pumpkin (Mitani, Yamaji, Ago, Iwasaki, & Ma, ), horsetail (Grégoire et al, ), wheat (Montpetit et al, ), soybean (Deshmukh et al, ), and cucumber (Sun et al, ). Homologs of Lsi2 have also been identified in barley (Mitani, Chiba, Yamaji, & Ma, ), maize (Mitani, Chiba, et al, ), pumpkin (Mitani‐Ueno, Yamaji, & Ma, ), and cucumber (Sun et al, ).…”

Silicon-mediated Improvement in Plant Salinity Tolerance: The Role of Aquaporins

“…Although, in general, Lsi1 and Lsi2 are localized in the plasma membrane, the distribution differs among species; in fact, in rice they are found in the exodermis and endodermis in the mature regions of the main and lateral roots (Ma et al, 2006), while in other monocotyledons such as maize and barley they are localized in epidermal, hypodermal, and cortical cells (Chiba et al, 2009; Mitani et al, 2009). In dicots, the pumpkin CmLsi1 is found in all root cells (Mitani et al, 2011) and the cucumber, CsLsi1 has been recently localized in endodermal and cortical cells in root tips and in root hairs (Sun et al, 2017). Therefore, the localization of Si transporters in the roots could be an important factor that determines how Si influences water uptake and, therefore, the sensitivity of the plant to salinity.…”