Physiological and molecular insights involved in silicon uptake and transport in ryegrass

Abstract

The silicon (Si) uptake system of two ryegrass (Lolium perenne L.) cultivars was characterised by assessing the concentration- and time-dependent kinetics. Additionally, a Si transporter gene was isolated from ryegrass and their expression pattern was analysed. The concentration-dependent kinetics was examined in Jumbo and Nui cultivars supplied with 0, 0.5, 1.0, 2.0, and 4.0 mM Si and harvested at 24 h and 21 d. The time-dependent kinetics was evaluated at 0, 0.5, or 2 mM Si doses after 0, 3, 6, 9, 12, and 24 h. RACE-PCR was performed to isolate a full-length sequence codifying for a Si transporter, and semi-quantitative and quantitative RT-PCR was used to analyse its expression pattern. Differential Si uptake between ryegrass cultivars was found. Moreover, Lineweaver-Burk linearization showed similar V_max values between cultivars; however, different K_m suggested that Jumbo and Nui may have different affinities for silicic acid. The dissimilarities in K_m between cultivars might involve either the differential contribution of known proteins responsible for Si uptake and transport or the involvement of undiscovered Si transporters. We identified a putative Si transporter from ryegrass Nui (LpLsi1), which was only expressed in roots and down-regulated by Si supply. The predicted amino acid sequence of LpLsi1 did not only show a high similarity and close phylogenetic relationship with monocot Si influx transporters but also indicated that it is a membrane protein possessing a high conservation of domains essential for silicic acid selectivity. Our findings provide evidence of LpLsi1 in ryegrass, which supports its high Si accumulation capacity.

Keywords: Gene expression; Kinetics; Lolium perenne L; Lsi1; Silicon uptake.