Chemistry Reference
In-Depth Information
serves to strengthen the plant and to provide rigidity. The silica that is deposited in
the phytoliths effectively acts as a skeletal system for the plant.
Not only does the silicon taken up by plants impart structural reinforcement for
the plant, research has also demonstrated that silicon aids the plant in resisting biotic
(e.g., insects, herbivores, bacteria, fungi) and abiotic (e.g., wind, cold, heat, salinity,
droughts) stressors.[
21
] Related to this phenomenon is the roles that silicon plays
in carbohydrate synthesis (Chap. 2) and phenolic synthesis, especially in rice [
22
].
Since plants lack the capacity for locomotion that allows animals to evade preda-
tors they have evolved a strategy of physical defense with which silicon plays an
integral role. Physical features such as thorns, spines, and rough surface cell lay-
ers, all of which have been shown to be rich in silica, are all examples of strategies
that plants have evolved to protect themselves against insect pests and herbivores
[
21
,
23
]. These physical characteristics afford plants physical barriers against the
ingress of insects and pathogens while thorns, spines, and “chewy” exteriors may
convince herbivores to seek easier meals elsewhere. Research has also demonstrat-
ed that the ingestions of silicon-rich plants by insects may limit the ability of the
insects to ingest sufficient quantities of other nutrients and/or water [
23
,
24
].
Silicon is also believed to be responsible for the production of secondary me-
tabolites in plants as a response to biotic stressors [
12
,
21
,
25
]. These secondary
metabolites are believed to help the plant defend itself against predators but also
microorganisms. Studies have shown that some plants do contain genes whose ex-
pression is regulated by silicon [
26
]. Plants (
Arabidopsis
) infected with powdery
mildew demonstrated a clear difference in the expression of certain genes and the
expression of these genes was regulated by silicon. In control plants there was little
difference in gene expression [
27
].
1.2.2
Marine Organisms
Perhaps the most widely studied groups of organisms that utilize silicon are found
fresh and salt water systems. It has been estimated that the Earth's oceans con-
tain 10
17
moles of dissolved silicic acid (Si(OH)
4
) at an average concentration of
approximately 70 μM [
28
]. Given these relatively high values of Si(OH)
4
in the
world's bodies of water it stands to reason that some aspect of Nature would evolve
a use for such an abundant compound.
1.2.2.1
Diatoms
Diatoms are single-celled photosynthetic eukaryotes and constitute the largest class
of protists (> 10,000 species) living in the planet's bodies of water [
29
-
31
]. A great
deal of scientific effort has been committed to exploring how these organisms take in
silicic acid from their environment and use it to construct ornate, species-specific sil-
icon-based skeletal systems (Fig.
1.1
) [
32
-
35
]. Diatoms utilize polycationic peptides
