Subcellular localization of silicon and germanium in grass root and leaf tissues by SIMS: evidence for differential and active transport

Abstract

Silicon transport and incorporation into plant tissue is important to both plant physiological function and to the influence plants have on ecosystem silica cycling. However, the mechanisms controlling this transport have only begun to be explored. In this study, we used secondary ion mass spectrometry (SIMS) to image concentrations of Si in root and shoot tissues of annual blue grass (Poa annua L.) and orchard grass (Dactylis glomerata L.) with the goal of identifying control points in the plant silica uptake pathway. In addition, we used SIMS to describe the distributions of germanium (Ge); the element used to trace Si in biogeochemical studies. Within root tissue, Si and Ge were localized in the suberized thickwalled region of endodermal cells, i.e. the proximal side of endodermal cells which is in close association to the casparian strip. In leaves, Si was present in the cell walls, but Ge was barely detectable. The selective localization of Si and Ge in the proximal side of endodermal cell walls of roots suggests transport control is exerted upon Si and Ge by the plant. The absence of Si in most root cell walls and its presence in the cell walls of leaves (in areas outside of the transpiration terminus) suggests modifications in the chemical form of Si to a form that favors Si complexation in the cell walls of leaf tissue. The low abundance of Ge in leaf tissue is consistent with previous studies that suggest preferential transport of Si relative to Ge.

High-Resolution Secondary Ion Mass Spectrometry Reveals the Contrasting Subcellular Distribution of Arsenic and Silicon in Rice Roots

“…For the wild-type samples, Si can be transported across the endodermis by Lsi1 and Lsi2, and if the Si concentration is very high, strong proximal accumulation of Si in the endodermal cell walls could be caused if Si efflux from the endodermis is greater than Si loading into the xylem, as observed in the mature root zone (Fig. 5) and the previous studies of Parry and Soni (1972), Lux et al (1999), Gong et al (2006), and Sparks et al (2010). For the lsi2 mutant roots in this study, an accumulation on the distal side of the endodermis may not be observed due to the low concentration and short duration of Si supplied.…”

“…However, in these studies, with the exception of Parry and Soni (1972), Si appears to localize only to the proximal side of the endodermis rather than completely surrounding the cell, as observed in our studies. Similarly, Sparks et al (2010) mapped the Si distribution in the roots of two grass species (P. annua and D. glomerata) and found the same distribution, with the Si localized only to the proximal side of the endodermis. Gong et al (2006) stated that in their study, the Si was found to be a component of the root cell walls, and Parry and Soni (1972) concluded that more Si was localized on the inner tangential walls, which is consistent with our observations.…”

“…Recent examples of biological studies by nano-SIMS include high-resolution imaging of the nickel distribution in a nickel hyperaccumulator plant (Smart et al, 2010), localization of As and Se in cereal grains (Moore et al, 2010), lipid distributions in phaseseparated membranes (Kraft et al, 2006), and in situ mapping of nitrogen uptake in the rhizosphere (Clode et al, 2009). The SIMS technique has also recently been used to map, at subcellular resolution, Si and germanium in root and leaf tissues of annual blue grass (Poa annua) and orchard grass (Dactylis glomerata; Sparks et al, 2010).…”

Biogenic and pedogenic controls on Si distributions and cycling in grasslands of the Santa Cruz soil chronosequence, California

“…While dominated by wellformed phytoliths ( Fig. 3a and b), biogenic Si also occurs as coating along cell walls and in other fine-scale structures (Sparks et al, 2011) that may be selectively reacted during short-term experiments. Sesquioxides and organics may also become sorbed and precipitated on soil phytoliths, increasing their chemical resistance and slowing their weathering rates (Piperno, 2006).…”

“…The mechanism and reaction rates associated with weathering, a focus of intensive research over the last several decades (see reviews in Brantley et al, 2007), have direct applications to buffering acid precipitation, long-term atmospheric CO 2 drawdown and climate change (Walker et al, 1981;White and Blum, 1995;Berner and Berner, 1997;Driscoll et al, 2005). Terrestrial biogenic silica is produced when soluble Si is extracted from soils by biopumping of plants and transformed into opaline silica principally as phytoliths (Epstein, 1999;Sparks et al, 2011). Mechanisms by which grasses accumulate Si may include passive uptake, enzyme polymerization and species-specific efflux transporters (Sangster and Hodson, 1986;Ma et al, 2007).…”