stuff from Fox et al.
et al. show that several synthetic compounds that bind with
the estrogen receptorincluding DDT, bisphenol A and methyl
parathion interfere with the ability of nitrogen-fixing bacteria
to form a symbiotic relationship with their leguminaceous hosts
(plants like beans, peas and alfalfa). This symbiosis is the basis
for a key ecological process, nitrogen fixation, which is essential
for life on earth.
Rhizobial bacteria form symbiotic relationships with legumes, living
in nodules within the plant's roots and converting nitrogen from
one chemical form to another. The conversion, called nitrogen fixation,
is the principal natural process by which nitrogen is made available
for use by living organisms. Life doesn't work without it.
plant-bacteria symbiosis is initiated when the bacterium detects
a chemical signal exuding naturally from the roots of the legume.
The signals belong to a class of compounds, phytoestrogens, which
are so described because of their coincidental ability to interact
with vertebrate estrogen receptors.
detect the signal, the bacteria employs receptors analogous to hormone
receptors. The phytoestrogen binds to the bacterial receptor and
the resulting complex then activates a gene in the bacterium. Activated,
the gene initiates an exchange of chemicals between plant and bacteria
that stimulates and maintains the nodules in which the bacteria
did they do? Fox et al. reasoned that if phytoestrogens
were able to interact with the estrogen receptor, then synthetic
compounds that interact with the estrogen receptor might be capable
of binding with the bacterium's phytoestrogen receptor and reducing
et al. worked with alfalfa and its symbiotic bacterium Sinorhizobium
meliloti. The plant exudes a phytoestrogen, the flavenoid, luteolin,
which activates the Nod gene in the bacterium.
created an in vitro testing system in which they could measure
Nod gene induction with luteolin alone and then when a series
of endocrine disrupting compounds (EDCs) were added to the experiment.
induction by luteolin at 1µ Molar concentration was set as
the standard for the experiment, or 100% induction. Adding EDCs
separately in different concentrations then allowed Fox et al. to
determine the potency of EDCs in suppressing Nod induction.
performed a second set of experiments with alfalfa roots to determine
whether the EDC impact on Nod induction would occur in whole
organisms. To do this, they inoculated the roots with a bacterium
that turns blue upon exposure to one of the biochemical products
of Nod gene activation.
did they find? Contaminants with estrogenic activity decreased
gene expression by up to 90%. In addition to DDT and bisphenol A,
methyl parathion, pentachlorophenol and two plant flavonoids (chrysin
and genistein ) also interfered with phytoestrogen signaling.
in Nod induction caused by adding different EDCs to
an in vitro assay with 1µM luteolin.
the experiment, the standard for Nod induction (100%)
is set by the impact of 1µM luteolin alone. As the concentration
of contaminants is increased (from left to right in the graph),
Nod induction is reduced.
Fox et al. 2001.
photographs of alfalfa roots show the impact of several EDCs
on Nod gene expression. In the control, the root tip is exposed
only to the phytoestrogen luteolin at 1 µM concentration.
It is blue over much of its length, reflecting the presence
or one of the biochemical products of gene activation.
the lower three photographs, three different compounds are
added to the experiment, each at 50 µM. Each decreases
the amount of blue, indicating that gene expression has been
Fox et al. 2001.
does this mean?
are two important lessons from this study.
first is that it demonstrates conclusively that the symbiosis
between legumes and rhizobial bacteria is vulnerable to signal disruption
by synthetic contaminants. How extensively this is occuring in the
real world becomes an important question, as this symbiosis is crucial
to one of the main biogeochemical cycles that makes life on earth
possible, the nitrogen cycle. Several of the compounds Fox et
al. tested are widespread contaminants in soils. Other contaminants,
not tested, share chemical characteristics with those shown in these
experiments to have effects, and these others are both likely to
interfere with the same process and widely distributed in soils.
Thus it is likely that nitrogen fixation has been affected.
perhaps, the global nitrogen cycle is already being affected
by human activities through large-scale anthropogenic production
of nitrogen, especially for fertilizers and as pollutant byproducts
of burning fossil fuels. Anthropogenic nitrogen fixation now
surpasses all natural processes combined and thus there
is much more fixed nitrogen circulating in the global nitrogen
cycle. This excess has had profound effects on many ecosystems,
causing eutrophication in lakes and estuaries and altering
may turn out that this excess nitrogen pollution has masked
the types of impacts in natural ecosystems discovered by Fox
et al. Moreover, it is unlikely that the anthropogenic fixed
nitrogen is simply substituting for whatever losses are being
caused by disruption of the symbiosis, because nitrogen fixed
by human action is distributed in space and time differently
than the nitrogen fixed by natural processes.
second important lesson from this work is that it reinforces the
need to consider endocrine disruption as just one type of chemical
impact within a broader framework of signal, or message, disruption.
Many of life's crucial processes are controlled by chemical signals.
Some of these, hormones, mediate events within and among cells,
for example, the activation of specific genes. Fox et al.
demonstrate that signal disruption can also take place in chemical
message systems controlling relationships between organisms, in
this case the two participants in a symbiotic relationship: legumes
and rhizobial bacteria.
of the major trends of endocrine
disruption research has been to gradually broaden the range
of systems thought to be vulnerable. This field started with a tight
focus on contaminants that interfere with estrogen signaling. It
broadened to other steroid hormones like testosterone and progesterone,
and to non-steroidal hormones like thyroid. Fox et al.'s
results take it one major step farther: communication among organisms.
results should encourage researchers to begin to look at other
chemically-mediated symbioses for signs of chemical disruption.
Foremost among these, in my opinion, should be two:
bleaching threatens coral reefs world-wide. It involves the expulsion
of symbiotic algae (zooxanthellae) from their coral hosts. While
the bleaching has been widely attributed to increases in sea-water
temperature associated with global warming, the changes in temperature
that have been experienced are small. An important hypothesis
to consider is that the signals mediating this symbiosis have
been chemically disrupted, either by chemicals alone or by an
interaction between chemicals and rising temperatures. Such interactions
would not be unprecedented: for example, temperature and contamination
can interact to alter the sex
ratio of turtles.
have noticed widespread
forest decline involving trees in Europe and North America.
These declines are at least in part associated with changes in
the abundance of mycorrhizal fungi, which exist symbiotically
with tree roots and are essential for nutrient absorbtion by tree
roots. Investigation into possible disruption
of the signals that mediate these symbioses might prove very useful
to understand the declines.
findings from Fox et al.
the October 2001 ehormone meeting at Tulane University in New Orleans,
Jennifer Fox and colleagues presented new data from their studies
of the impact of EDCs on symbiosis. In this new set of experiments,
they exposed growing plants to EDCs and examined the numbers of
nodules formed per plant and the mass of the plants. EDCs suppressed
nodule number and plant biomass, as predicted by the study reported
in Nature (above). Thus the impact of EDCs on Nod
gene activation is likely to have real world effects.