Chemistry Reference
In-Depth Information
crystalline silica is not observed in the many silicified organisms examined to date and
must be prevented by the organism as a crystalline material is inherently more difficult to
mold than an amorphous covalent polymer.
The completion of genome mapping for silicon accumulating plant species including
rice (Goff et al. 2002; Yu et al. 2002) will make it possible to identify which genes are
silicon dependent and perhaps also to identify any genes involved with the mineralization
process. It will also be possible to look for synergy or not with other cellular processes
including lignin synthesis.
Unanswered questions, a selection
In diatoms, the silica deposition vesicle is the location where silica is produced and
molded to generate species-specific structures. It provides a specialized microenvironment
that requires transport of silicic acid (most likely) and all required organic moieties across
the silicalemma in order to generate the silicified structures that are then molded by the
“expandable reaction vessel” in conjunction with other cytoskeletal structures such as
microtubules and microfilaments. Energy is required for the process and mitochondrial
centers are located close to points of silicification. The composition of the membrane is not
known, nor is the mechanism by which it is able to expand and maintain its integrity during
frustule formation. When the daughter cells separate with their new valve components
(N.B. the system knows to make the correct “half” of the frustule!) it is thought that the
silicalemma becomes all or part of an organic casing surrounding the silicified components
of the mature wall (Pickett-Heaps et al. 1990). Other components that may be present in the
outer coating include sulphated polysaccharides and protein or proteoglycans (McConville
et al. 1999) and mucilage (Hoagland et al. 1993). A range of techniques including electron
microscopy, atomic force microscopy and biochemical separation procedures have been
used to try to elucidate the structure and chemical composition of the silicalemma but with
little success and much work is required in this area. The availability of genomic sequences
for diatoms will be invaluable in this study as specific genes can be “switched off” and the
consequences of this investigated so that structural information specific to the silicalemma,
its composition, its three dimensional structure and function can be elucidated. N.B. the
first steps in gene manipulation have been achieved (Zaslavskaia et al. 2001) but much
remains to be done. Gene mutation techniques and techniques that enable the “addition” of
specific genes will both be needed in order to unequivocally establish the functions
encoded by specific genes.
Similarly in sponges, the availability of genomic sequence data will enable the
identification of genes specifically involved in all stages of the biosilicification process and
by mutation and removal/insertion of specific genes and a measure of the organisms
response to external stimuli (e.g., silicon in some form). Eventually these experiments
should be possible for plants although it is likely that more immediate success in the field
will be achieved by the route, extract protein, find function with model
in vitro
studies,
clone gene etc.
Although there is now more information on the likely chemical environment in
which silica is formed in nature, the relationship between biochemical structure and their
precise function
in vivo
are unknown. We still do not know if the identified molecules are
the only ones involved in the formation of nanoscale silica structures. We also do not
know how such silica particles might be manipulated to produce structures on higher
length scales. Probes to investigate reactions
in situ
are required (for example see
Shimizu et al. 2001) as are approaches that will look at the whole cell system and find the
relationships between chemistry at the molecular level and cytoskeletal behavior.
Patterning of silica is still an area that is little understood.
