7 Uptake of Silicon in Different Plant Species Jian Feng Ma Abstract Silicon (Si) is the second most abundant element in soil, and has a wide array of functions in the growth and development of plants. Silicon is able to alleviate var- ious stresses including diseases, pests, lodging, drought, and nutrient imbalance. Although all plants contain some Si in their tissues, the concentrations of Si in the shoots differ greatly with plant species, and this difference is attributed to the capacity of the roots to take up Si. At least two steps are involved in Si uptake, including radial transport from external solution to the root cells, and subsequent release from the root cells to the xylem. Currently, the latter process seems more important for high Si accumulation. The first gene encoding Si transporter has recently been identified in rice, a typical Si-accumulating plant. The transporter encoded by this gene shows a high specificity for Si, and is localized at the distal side of both exodermis and endodermis. The future cloning of more genes will help in our understanding of the molecular mechanisms of Si uptake in different plant species. Key words: plant species, radial transport, silicon, transporter, xylem loading. 7.1 Silicon in Plants Silicon (Si) is the second most abundant element in the Earth’s crust. In fact, sil- icon dioxide comprises about 60% of the Earth’s crust mass, and more than 50% of the soil mass. The Si concentration in soil solution is normally between 3.5 and 40 mg L 1 , in the form of silicic acid [1]. Although all plants grown in soil will contain some Si in their tissues, the Si concentration in the shoots varies con- siderably among plant species, ranging from 0.1% to 10% Si of the tissue dry weight [2]. Takahashi and coworkers have conducted an extensive survey on Si concentrations in almost 500 plant species from Bryophyta to Angiospermae, 113 Handbook of Biomineralization. Edited by E. Ba ¨uerlein Copyright 8 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31804-9
Transcript
7
Uptake of Silicon in Different Plant Species
Jian Feng Ma
Abstract
Silicon (Si) is the second most abundant element in soil, and has a wide array of
functions in the growth and development of plants. Silicon is able to alleviate var-
ious stresses including diseases, pests, lodging, drought, and nutrient imbalance.
Although all plants contain some Si in their tissues, the concentrations of Si in
the shoots differ greatly with plant species, and this difference is attributed to the
capacity of the roots to take up Si. At least two steps are involved in Si uptake,
including radial transport from external solution to the root cells, and subsequent
release from the root cells to the xylem. Currently, the latter process seems more
important for high Si accumulation. The first gene encoding Si transporter has
recently been identified in rice, a typical Si-accumulating plant. The transporter
encoded by this gene shows a high specificity for Si, and is localized at the distal
side of both exodermis and endodermis. The future cloning of more genes will
help in our understanding of the molecular mechanisms of Si uptake in different
Silicon (Si) is the second most abundant element in the Earth’s crust. In fact, sil-
icon dioxide comprises about 60% of the Earth’s crust mass, and more than 50%
of the soil mass. The Si concentration in soil solution is normally between 3.5
and 40 mg L�1, in the form of silicic acid [1]. Although all plants grown in soil
will contain some Si in their tissues, the Si concentration in the shoots varies con-
siderably among plant species, ranging from 0.1% to 10% Si of the tissue dry
weight [2]. Takahashi and coworkers have conducted an extensive survey on Si
concentrations in almost 500 plant species from Bryophyta to Angiospermae,
113
Handbook of Biomineralization. Edited by E. BauerleinCopyright 8 2007 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31804-9
grown under similar soil conditions (for a summary, see Ref. [2]). The results
showed that there is a characteristic distribution of Si accumulation in the plant
kingdom (Fig. 7.1). For example, plants belonging to the Bryophyta, Lycopsida,
and Equisetopsida of Pteridophyta show high Si accumulation, whereas those be-
longing to the Filicopsida in Pteridophyta, Gymnospermae, and Angiospermae
show low Si accumulation. In higher plants, only plants in the Gramineae and
Cyperacea show high Si accumulation. Recently, it was reported that plants in
the Balsaminaceae family also accumulate Si [3]. Plants in the Cucurbitales, Urti-
cales, and Commelinaceae show intermediate Si accumulation, whereas most
other plants species show low Si accumulation.
Within a family, the degree of Si accumulation differs among subfamilies.
For example, the degree of Si accumulation in Gramineae follows the order
of Bambusoideae > Pooideae > Panicoideae > Eragrostoideae [2]. Furthermore,
there also are genotypical variations in Si concentration; for example, a variation
from 1.53% to 2.71% Si was reported in the hull of barley cultivars [4].
Fig. 7.1 Distribution of Si-accumulating plants in plant kingdom. (Adapted from Ref. [2].)
114 7 Uptake of Silicon in Different Plant Species
7.2
Beneficial Effects of Silicon on Plant Growth
Numerous beneficial effects of silicon on plant growth and development have
been observed in both Si-accumulating and non-Si-accumulating plants, includ-
ing barley, cucumber, rice, and strawberry. The effects are especially characterized
by the alleviation of multiple stresses, both biotic and abiotic in nature [5].
7.2.1
Disease Control
Silicon is effective in controlling (biotic) diseases caused by both fungi and bacte-
ria in different plant species. For example, Si increases the resistance of rice to
leaf and neck blast, sheath blight, brown spot, leaf scald, and stem rot [6]. Silicon
also decreases the incidence of powdery mildew in barley, cucumber, strawberry,
and wheat, ring spot in sugarcane; rust in cowpea; leaf spot in Bermuda grass
and gray leaf spot in St. Augustine grass and perennial ryegrass [7]. Silicon also
enhances the resistance of plants to insect pest such as stem borer and planthop-
per [8].
7.2.2
Alleviation of Stress
Silicon also alleviates many abiotic stresses, including chemical and physical
stress [5]. Typically, Si alleviates water stress by reducing transpiration – by up to
30% in the case of rice, which has a thin cuticle [2]. Under water-stressed condi-
tions (low humidity), the effect of Si on rice growth was more pronounced than
on rice that was cultivated under non-stressed conditions (high humidity) [2].
When rice leaves were exposed to a solution containing polyethylene
glycol (PEG), electrolyte leakage (which is an indicator of membrane lesion)
from the leaf tissues was decreased as the level of Si in the leaves increased [9].
The level of polysaccharides in the cell wall was higher in leaves containing Si
than in those lacking Si. These results suggest that, in rice leaves, the Si is in-
volved in cell water relationships such as mechanical properties and water perme-
ability.
Silicon application in rice is also effective in alleviating the damage caused by
climatic stress such as typhoons, low temperature and insufficient sunshine dur-
ing the summer season [2]. A typhoon attack usually causes lodging and sterility
in rice, with a resultant considerable reduction in rice yield. A high deposition of
Si in rice enhances the resistance to lodging.
The beneficial effects of Si under phosphorus (P)-deficiency stress have been
observed in many plants, including barley and rice. In an experiment using a nu-
trient solution, Si supply resulted in a larger increase in the dry weight of a rice
shoot at a low P level (14 mM P) than at a medium level (210 mM) [10]. The larger
beneficial effect of Si on plant growth under P-deficiency stress may be attributed
7.2 Beneficial Effects of Silicon on Plant Growth 115
to the enhanced availability of internal P through reductions in excess Fe and Mn
uptake. Silicon can also alleviate the damage caused by P excess by decreasing the
excessive uptake of P, which results in a decreased internal inorganic P concen-
tration. An alleviative function of Si on Mn toxicity has been observed in hydro-
ponically cultured barley, bean, pumpkin, and rice [2].
The beneficial effect of Si under salt stress has been observed in barley, rice,
and wheat [2]. In rice, both shoot and root growth were inhibited by 60% in the
presence of 100 mM NaCl for three weeks, but Si addition significantly alleviated
salt-induced injury [11]. The Na concentration in the shoot was reduced by 50%
by Si addition. Alleviative effects of Si on Al toxicity have been observed in barley,
maize, rice, sorghum, soybean, and teosinte [2].
7.2.3
Plant Growth
In addition to Si alleviating a variety of stresses, it also improves light interception
by keeping leaves erect, thereby stimulating canopy photosynthesis in rice [2].
Silicon also promotes cell elongation but not cell division, probably due to Si-
enhanced extensibility of the cell wall in rice [12]. Recently, Si was found to
increase the extensibility of the cell wall in the growing zone, and to reduce cell-
wall extensibility in the basal zone of isolated stellar tissues covered by endoder-
mal inner tangential walls in the roots of sorghum; this implied a role for Si in
enhancing root elongation and in protecting the stele as a mechanical barrier by
hardening the cell wall of the stele and endodermal tissues [13].
Most of the beneficial effects of Si are expressed through its deposition in the
cell walls of the epidermal surfaces of leaves, stems, and hulls. Silicon is depos-
ited in cell wall material as a polymer of hydrated, amorphous silica, forming
silica-cuticle double layers and silica-cellulose double layers [2]. Silicon is also de-
posited in the bulliform cells, dumbbell cells, and in the long and short cells
on the surface of leaves and hulls. Therefore, the beneficial effects of Si are quan-
titatively related to the amount of Si accumulated in the shoots. Silicon is also
thought to induce an active defense system in the suppression of plant disease;
an example of this is the enhancement by Si of phytolexin production in cucum-
ber and rice [14].
7.3
Uptake Systems of Si in Different Plant Species
The differences in Si accumulation described above have been attributed to the
ability of the roots to take up Si [15]. Silicon is taken up in the form of an un-
charged molecule, silicic acid [16]. Three different modes of Si uptake have been
proposed for plants with different degrees of Si accumulation, namely active, pas-
sive, and rejective uptake [2]. Plants which employ an active mode take up Si
faster than water uptake, and this results in a depletion of Si in the uptake solu-
116 7 Uptake of Silicon in Different Plant Species
tion. Plants which employ a passive mode take up Si at a rate that is similar to
water uptake, and thus no significant changes in Si concentration are observed
in the uptake solution. In contrast, plants which employ a rejective mode of up-
take tend to exclude Si, and this is demonstrated by an increasing concentration
of Si in the uptake solution. These phenomena suggest that different mecha-
nisms are involved in the Si uptake by different plant species.
The uptake of Si involves at least two processes: (i) the radial transport of Si
from the external solution to cortical cells; and (ii) the release of Si from cortical
cells into the xylem (xylem loading). These processes have been examined in rice,
cucumber, and tomato, which represent high, medium, and low accumulations,
respectively [17]. The results of a time-course experiment showed that whilst the
Si concentration in the root-cell symplast increased with time in all species, the Si
concentration in the symplast was much higher in rice, followed by cucumber
and tomato; this occurred despite the Si concentration in the symplast in all spe-
cies being higher than in the external solution. A kinetic study showed that Si
concentrations in the root-cell symplast increased with increasing external Si con-
centration, but was saturated at a higher concentration, although the Si level in
the root-cell symplast differed widely between the three species (Fig. 7.2). Based
on these curves, the Km values were estimated as 0.16, 0.15, and 0.16 mM for
rice, cucumber, and tomato, respectively. In the same study, the values of Vmax
were 34.5, 26.9 and 13.3 ng Si root�1 8 h�1, respectively, for rice, cucumber, and
tomato. These results suggested that the radial transport of Si from the external
solution to the cortical cells in the three plant species is mediated by a transporter
which shows a similar affinity to silicic acid. However, the differences in Vmax
suggested that the density of the Si transporter on the root cell membranes dif-
fers among plant species, following the order of rice > cucumber > tomato.
A higher Si concentration in the root-cell symplast than in the external solution
suggests that silicic acid is transported against a concentration gradient from the
external solution to the cortical cells, and that this process would be energy-
dependent. This is supported by the finding that presence of a metabolic inhibitor
(2,4-dinitrophenol; 2,4-DNP) or exposure to low temperature resulted in a de-
creased radial transport of Si [17]. Furthermore, the Si concentration in the root-
cell symplast was reduced to a level similar to that in the apoplast and the exter-
nal solution in the presence of 2,4-DNP and under low temperature. These
results further suggest that the radial transport of Si involves two components:
first, a transporter-mediated component (as described above); and second, passive
transport by diffusion.
The subsequent process – that is, the release of Si from the cortical cells to the
xylem (xylem loading) – was also compared among rice, cucumber, and tomato
[17]. When the roots were exposed to a nutrient solution containing 0.5 mM Si,
the concentration of Si in the xylem sap of rice reached 6.0 mM Si in 30 min, and
18 mM in 8.5 h. This suggested that Si release to the xylem is a very rapid pro-
cess, and that in rice, Si is loaded against a concentration gradient. In contrast, Si
concentrations in the xylem sap of cucumber and tomato were much lower: in
cucumber, the concentration was only 0.6 mM after 30 min, and remained stable
7.3 Uptake Systems of Si in Different Plant Species 117
Fig. 7.2 Kinetic study of radial transport of Si in rice (A), cucumber (B), and tomato (C).
118 7 Uptake of Silicon in Different Plant Species
during the experiment period, whilst in tomato it was lower than that of the exter-
nal solution.
A kinetics study of xylem loading of Si showed that, in rice, the process is medi-
ated by a transporter (Fig. 7.3) [17], whereas in cucumber and tomato it is medi-
ated by passive diffusion. Si concentrations in the xylem sap increased gradually
Fig. 7.3 Kinetic study of xylem loading of Si in rice (A), cucumber (B), and tomato (C).
7.3 Uptake Systems of Si in Different Plant Species 119
with increasing Si concentration in the external solution in both cucumber and
tomato (Fig. 7.3), though the concentration was lower than that in the external
solution. These results indicated that xylem loading is a very important determi-
nant of high-level Si accumulation in the shoots of rice. In contrast, the much
lower accumulation of Si in cucumber and tomato might be due to a lower den-
sity of the transporter from the external solution to the cortical cells, and a defec-
tive (or even absent) transporter from the cortical cells to the xylem.
Recently, a study using different plant species confirmed the coexistence of both
passive and active Si-uptake components, and that their relative contributions
were dependent on the plant species and external Si concentration [18]. Whilst
the active component serves as the major mechanism in rice and maize (see
above), passive uptake prevails in sunflower and wax gourd at high Si concentra-
tions, though active uptake becomes important at low concentrations.
7.4
Genes Involved in Si Uptake
Although a range of physiological studies have shown that Si transport is medi-
ated by transporters in Si-accumulating plants (see above), the role of the genes
encoding these transporters is not well understood. Recently, a gene responsible
for Si uptake was identified in rice [19], and cloned by utilizing a rice mutant
which is defective in Si uptake [20]. The mutant was isolated from sodium
azide-treated M2 seeds of rice using Ge tolerance as a selection parameter [20].
The mutant had a plant type similar to wild-type, except that the leaf blade of
lsi1 remained droopy when Si was supplied in solution culture. The Si concentra-
tion of the shoot tissue was found to be much lower in the mutant than in the
wild-type, but concentrations in the roots of each plant were similar. A relatively
long-term uptake experiment showed Si uptake by the mutant to be significantly
lower than that of the wild-type, whereas there was no difference in the uptake of
other nutrients such as P and K. The Si concentration in xylem sap of the wild-
type rice was also much higher than that of lsi1 [21].
The gene was cloned by a map-based cloning technique using a mapping pop-
ulation derived from a cross between the mutant and an Indica cultivar, Kasalath.
The gene Lsi1 is localized on chromosome 2, and consists of five exons and four
introns [19]. The cDNA of this gene is 1409 base pairs in length, and the deduced
protein consisted of 298 amino acids. The gene is predicted to encode a mem-
brane protein similar to water channel proteins, aquaporins. The predicted amino
acid sequence has six transmembrane domains and two Asn-Pro-Ala (NPA) mo-
tifs, which are well conserved in typical aquaporins. Blast search and ClustalW
analyses revealed that Lsi1 belongs to a Nod26-like major intrinsic protein (NIP)
subfamily. A single nucleotide substitution (G ! A) occurred in the lsi1 mutant,
and this resulted in an amino acid change from alanine in the wild-type to threo-
nine in the mutant, at a position of 132 aa. The substitution of Thr132 for Ala132
provoked severe steric interactions with Val55 and Val59 in helix 1 (H1), facilitat-
ing a movement of H1. This unfavorable interaction would affect the conforma-
120 7 Uptake of Silicon in Different Plant Species
tion of Asn108, the pore-forming residue in the P-loop according to computer
modeling, and thus change the conformation of the transporter.
When the expression of Lsi1 was suppressed by RNA interference (RNAi), Si
uptake was significantly reduced [19]. Furthermore, when the cRNA encoding
Lsi1 was injected into Xenopus laevis oocytes, an increased transport activity for
silicic acid was observed [19], which supported the idea that Lsi1 is a major trans-
porter for Si into rice roots. The transporter also shows high specificity for Si,
whereas a non-charged molecule, glycerol, is not transported by Lsi1. A kinetic
study also showed that Si transport activity increased with increasing Si concen-
trations in the external solution.
Lsi1 is mainly expressed in the roots, and the expression is constitutive. How-
ever, the expression is regulated by Si level, and will decrease by 25% with a con-
tinuous supply of Si [19]. The results of in-situ hybridization showed that the
mRNA of Lsi1 was localized at the exodermis and endodermis. Interestingly, Lsi1is expressed in the main roots and lateral roots, but not in root hairs (Fig. 7.4).
Fig. 7.4 Localization of the Si transporter in rice. (a,b) Fluorescence of
the Lsi1-GFP fusion protein in transgenic plants. (a) Distribution of Lsi1
in the intact roots; (b) localization of Lsi1 in the longitudinal section;
(c) cellular localization of Lsi1, detected by immunolocalization staining
with antibody of Lsi1; (d) computer modeling of Lsi1; the molecule at
the center shows silicic acid.
7.4 Genes Involved in Si Uptake 121
This is consistent with the results of a previous physiological study which showed
that root hairs do not play any demonstrable part in Si uptake, but that lateral
roots contribute significantly to Si uptake [22]. Further investigations showed
that the transport protein is localized on the plasma membrane of the distal side
of both the exodermis and endodermis cells (Fig. 7.4), where the Casparian strips
exist [19]. As solutes are unable to pass through the Casparian strips freely, trans-
porters are needed to reach the stele for translocation from the roots to the shoot.
There are three close Lsi1 homologues in maize, namely ZmNIP 2-1, 2-2, and
2-3 m, with identity of 77 to 83%, and one homologue in rice (Os06g12310,
named Lsi6) with identity of 77% (Fig. 7.5). Maize also can accumulate Si, which
suggests that ZmNIP 2-1 to 2-3 might also be involved in Si uptake. It is interest-
ing to notice that genes close to Lsi1 are also found in zucchini and chickpea
Fig. 7.5 The phylogenetic relationship of Lsi1, a rice Si transporter, in
rice, maize, Arabidopsis, and other plant species. Os, rice genes; At,
Arabidopsis gene; Zm, maize genes. The scale bar indicates the genetic
distance.
122 7 Uptake of Silicon in Different Plant Species
(both of which are dicotyledonous), and the function of these genes remains to be
examined in relation to Si accumulation in these plants.
In the marine diatom which requires Si as an essential element, a gene family
encoding Si transporters has been identified from Cylindrotheca fusiformis [23].
However, these genes do not have any similarity to the genes identified from
rice, indicating that the Si uptake system is different between diatoms and higher
plants.
A model of Si uptake in rice roots is illustrated in Figure 7.6. Many genes are
thought to be involved in Si uptake in rice, and Lsi1 appears to be responsible for
the influx of Si from the external solution to the root cells. Genes for efflux and
distribution of Si in rice remain to be identified, and homologues of these genes
must also be examined in different plant species. Furthermore, factors regulating
Si uptake need also to be identified in order to provide a better understand of the
molecular mechanisms of Si uptake in different plant species.
Acknowledgments
The author thanks Drs. Lawrence Datnoff and Naoki Yamaji for their critical read-
ing of this manuscript.
References
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