Identifying genes based on
patterns of gene expression in specific organs or at specific
stages of development is a useful approach to improving our
understanding of complex biological processes. Scientists Vivian
Irish at Yale University in
Connecticut, Rob Martienssen at
Cold Spring Harbor Laboratory in New York, and their
colleagues used a strategy known as "gene trapping" to identify
numerous genes involved in the regulation of flower development
in the model plant Arabidopsis thaliana. The research is
reported in a paper by Nakayama et al. in the September issue of
The Plant Cell. The gene
trap technique involves genetic transformation of Arabidopsis
plants with a reporter gene whose activity is visualized in a
simple assay, leading to the rapid identification of genes that
show specific patterns of expression. In this case, the
researchers isolated 80 different gene trap Arabidopsis
lines identifying genes that show distinct patterns of
expression in flower petals and/or stamens (the pollen-bearing
organs). The research is one of the first large-scale gene trap
studies in the area of flower development, and provides
extensive information on many genes likely to have critical
roles in this essential stage of plant reproduction.
Genes provide the blueprints for proteins that carry out the
functions of living cells. In any particular organ or tissue at
any particular stage of development, gene activity may be "on"
(expressing the messenger RNA transcripts that lead to
production of the corresponding protein) or "off" (no
expression). Examining gene expression patterns therefore
provides information on gene function. Gene trapping is an
alternative to methods such as DNA microarray analysis for the
detection of differentially expressed genes, and has the
advantage of identifying subtle differences in expression
patterns within target organs. For example, genes expressed only
in stamen tissue during the early stages of pollen development
are likely to have an important function in controlling pollen
formation.
The gene trap technique used by Drs. Irish and
Martienssen involved genetic transformation of Arabidopsis
plants with the reporter gene β-glucuronidase (GUS) lacking an
external promoter sequence to drive gene expression. Each
transformation event leads to insertion of the GUS gene at a
random site within the plant genome. All endogenous genes
contain promoter sequences that determine where and when they
will be expressed in an organism. The reporter GUS gene, lacking
its own promoter, will only be expressed and produce the GUS
protein if it happens to be inserted into the plant genome in
the immediate vicinity of an endogenous gene promoter. GUS
activity is assayed in transformed plants by treating harvested
seedlings with a stain that turns blue in the presence of GUS.
Successful "gene trapped" plants will show the characteristic
blue stain in specific patterns in the organs or tissues of
interest. The endogenous gene corresponding to the trapped
promoter can be fished out of the genome and sequenced based on
its proximity to the inserted reporter gene. Further experiments
can then be conducted, for example, to examine the expression of
the native gene in wild type plants and to investigate gene
function by creating mutant plants that either lack expression
of or overproduce the native protein.
As noted by Dr. Martienssen "gene traps are
powerful tools to examine both gene expression and gene function
in animal and plant systems. Large scale studies like this are
going to provide valuable information concerning regulatory
networks and target genes". Dr. Irish added "using the gene
trapping strategy, we have identified a host of new genes
involved in floral development, as well as illuminating some of
the processes involved in establishing different tissues and
organs. This general approach is very effective in providing
novel insights into development that are not easily gleaned
using other available techniques."
Many of the trapped genes were sequenced and
identified, giving clues about how they might function in petal
and stamen development. Floral organ development depends on
appropriate specification and differentiation of the unique
organ identities (e.g. petals, stamens, ovules). An interesting
aspect of this research is the finding that the expression of
many trapped genes is restricted to particular subdomains of the
proximodistal axis of petals and stamens, implying that
intensive regulation of patterning along this axis is critical
for floral organ development.
This research is an excellent example of how
modern molecular biology techniques help to increase our
understanding of complex biological processes.
The Plant
Cell is published by the
American Society of Plant Biologists.
