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  • From: Hilborn.E@epamail.epa.gov
  • To: North American Fruit Explorers <nafex@lists.ibiblio.org>
  • Subject: [NAFEX] Pesticides Disrupt Nitrogen Fixation
  • Date: Thu, 3 Jan 2008 16:20:43 -0500

Interesting news from the journal Environmental Health Perspectives

http://www.ehponline.org/docs/2007/115-12/forum.html#pest







Pesticide
s Disrupt
Nitrogen
Fixation
Talk
about a
vicious
cycle.
Some
organochl
orine
pesticide
s
suppress
nitrogen-
fixing
bacteria
from
replenish
ing
natural
nitrogen
fertilize
r in
soil,
resulting
in lower
crop
yields,
stunted
growth,
and an
ever-grea
ter need
for
additives
to boost
productio
n. This
previousl
y
unrecogni
zed
effect
stems
from
pesticide
s
interferi
ng with
flavonoid
signaling
from
leguminou
s plants
such as
alfalfa,
peas, and
soybeans
to soil
bacteria
that fix
nitrogen.
"People
assume
that
endocrine
disruptio
n by
pesticide
s occurs
only in
humans
and
animals
with
estrogen
receptors
, but we
find
there are
nontradit
ional
targets
affected
by
pesticide
s," says
Jennifer
Fox, a
postdocto
ral
researche
r at the
Center
for
Ecology
and
Evolution
ary
Biology
at the
Universit
y of
Oregon.




(Embedded image moved to file: pic10069.jpg)Rhizobium infects
alfalfa roots

image: Dan Gage/courtesy of Journal of Bacteriology

Plants
produce
chemicals
, many
that are
structura
lly
similar
to
phytoestr
ogens,
that
attract
Rhizobium
soil
bacteria
to their
root
systems
to form
nodules
for
nitrogen
fixation.
Inside
the
nodules,
the
bacteria
convert
atmospher
ic
nitrogen
into the
natural
fertilize
r
ammonia.
These
symbiotic
soil
bacteria
respond
to
plant-pro
duced
flavonoid
molecules
through
nodulatio
n D
(NodD)
receptors
, which
act
similarly
to
estrogen
receptors
in
vertebrat
es. In
the May
2004
issue of
EHP, Fox
and
colleague
s
reported
a study
using
Sinorhizo
bium
meliloti,
a soil
bacterium
that
fixes
nitrogen
in
symbiosis
with
alfalfa.
The
researche
rs found
that the
organochl
orine
pesticide
s
pentachlo
rophenol
(PCP) and
methyl
parathion
at levels
found in
farm
soils
inhibited
NodD
signaling
by 90%
and that
DDT cut
signaling
by 45%.
In recent
greenhous
e
experimen
ts, Fox
and
colleague
s at
Tulane
Universit
y treated
alfalfa
seeds
inoculate
d with S.
meliloti
with PCP,
methyl
parathion
, and
DDT,
which are
found in
soil
after
farmers
spray
crops,
especiall
y in
countries
where
these
pesticide
s have
not been
banned.
Six weeks
after
treatment
,
PCP-treat
ed
alfalfa
plants
produced
no
nodules,
and plant
yields
fell to
17%
compared
with
control
plants
not
treated
with
pesticide
s. Methyl
parathion
and DDT
reduced
nodule
numbers
and plant
yields by
about
half. The
researche
rs
estimate
that the
amount of
inhibitio
n
measured
in this
experimen
t could
translate
in
real-worl
d
condition
s to a
one-third
loss of
plant
yield
each
growing
season.
The
results,
published
12 June
2007 in
the
Proceedin
gs of the
National
Academy
of
Sciences,
indicate
that
pesticide
residues
in soil
could not
only
reduce
harvest
yields,
but also
increase
the need
for
synthetic
fertilize
rs,
thereby
raising
costs for
farmers
and
contribut
ing to
environme
ntal
pollution
. The
observati
ons also
may
explain a
trend in
the past
40 years
toward
stagnant
crop
yields
despite
record
high use
of
pesticide
s and
synthetic
fertilize
rs
worldwide
.
In
unpublish
ed
experimen
ts,
co-author
John
McLachlan
,
director
of the
Tulane
Center
for
Bioenviro
nmental
Research,
showed
that the
same
three
pesticide
s disrupt
NodD
signaling
in
Rhizobium
sp.
strain
NGR234, a
bacterium
that
fixes
nitrogen
in
symbiosis
with more
than 100
leguminou
s plants
growing
in
tropical
and
subtropic
al soils.
"Many of
the
species
that
NGR234
nodulates
are trees
and
bushes,
such as
teak and
rosewood,
that
promote
vital
soil
improveme
nt within
nutrient-
poor
tropical
soils,"
he says.
McLachlan
says
farmers
in poorer
countries
especiall
y cannot
afford to
lose
natural
fertilize
r because
synthethi
c
fertilize
rs cost
so much.
The
connectio
n between
pesticide
s and
nitrogen
fixation
shows
that
"pristine
and
natural
interacti
ons
between
bacteria
and
plants
are being
jeopardiz
ed by
what we
put into
the
soil,"
says Ann
Hirsch, a
plant
molecular
biologist
at the
Universit
y of
Californi
a, Los
Angeles.
Similarly
, the
effects
of
pesticide
s on
human
disease
continue
to be
documente
d, with a
report in
the
October
2007
issue of
EHP
linking
childhood
exposure
to DDT
with a
fivefold
higher
risk of
breast
cancer in
women.
"Now we
are
affecting
our
agricultu
ral
assets,"
Hirsch
says,
"somethin
g people
take for
granted."
Fox plans
to test
the
effects
of
pesticide
s on
alfalfa
and
soybeans
under
real-life
field
condition
s. "We
want to
screen
pesticide
s to see
which
ones
cause
minimal
damage to
certain
types of
crops and
maximize
the
amount of
natural
fertilize
r made,"
she says.
Carol
Potera



Attachment: pic10069.jpg
Description: JPEG image




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