Glyphosate Herbicides – How Do They Kill Plants?
Jim Rahe
Professor Emeritus, Simon Fraser University
Burnaby B.C. Canada
rahe@sfu.ca
Introduction. - Commercial herbicides with glyphosate as the active ingredient
(Roundup, et al.) were introduced in the 1970’s and soon became widely used for
agricultural, forestry and industrial applications. Their use expanded markedly in the
mid-1990’s with the advent of genetically engineered glyphosate tolerant (Roundup
Ready) crop cultivars.
In field use, glyphosate is absorbed by foliage and unprotected shoot tissues. It is nonselective and relatively few plant species are naturally tolerant. Glyphosate is rapidly and
strongly adsorbed to soil colloids and glyphosate herbicides have negligible residual
activity in most soils. It is water-soluble and of low mammalian toxicity.
With this combination of characteristics, glyphosate herbicides are used in diverse ways.
As directed sprays, glyphosate herbicides are widely used in orchard, berry, and nursery
crops, in ornamental landscapes and for vegetation management on rights of way and in
diverse industrial settings. They are used as broadcast sprays in agriculture for pre-plant
cleanup, vegetation management in fallow rotations, pre-harvest desiccation of cereals,
and for general weed control in Roundup Ready cultivars. In forestry, glyphosate
herbicides are used as both directed and broadcast sprays for conifer release.
The biochemical mode of action of glyphosate was documented in the published
literature soon after introduction of glyphosate herbicides. Glyphosate inhibits the
activity of enolpyruvyl-shikimate-3-phospahte synthase (EPSPS), an enzyme of the
shikimic acid pathway. The shikimic acid pathway occurs in plants and microorganisms.
Through this mode of action, glyphosate interferes with the synthesis of aromatic amino
acids phenylalanine and tryptophan end products of the shikimic acid pathway.
Although growth ceases within hours following application of glyphosate herbicides,
death of treated plants occurs only after 1-3 weeks when glyphosate herbicides are
applied during periods of active growth, and considerably longer after application when
applied at other times. Although glyphosate herbicides are applied to shoot portions of
plants, death of treated plants appears to require translocation of glyphosate to, and
effects that occur in roots. Indeed, glyphosate herbicides are among the few that are
effective on rhizomatous species such as Agropyron repens. Perennial plants that receive
sublethal doses of glyphosate herbicides often show suppressed shoot growth and foliar
symptoms that appear in the year following the year of exposure to glyphosate, and
symptoms may persist for two or more years. Many of these effects of glyphosate
suggest that the cause of death in plants treated with glyphosate herbicides may involve
more than direct metabolic consequences of the effect of glyphosate on synthesis of
aromatic amino acids.
Soil Fungi and Herbicidal Activity of Glyphosate. – I began research in the early
1980s that was based on the hypothesis that root pathogens naturally present in soil might
contribute to the herbicidal efficacy of glyphosate herbicides.
Initial experiments showed that seedlings of several plant species growing in either
pasteurized soil or an artificial growth medium (vermiculite) were considerably less
sensitive to glyphosate than were similar seedlings growing in untreated field soils. In
contrast, bean seedlings were equally sensitive to either paraquat or 2,4-D herbicides
when growing in heat treated and untreated soils. Roots of seedlings growing in
untreated field soil were colonized by Pythium and Fusarium spp. within 2-3 days
following foliar application of glyphosate. Amendment of heat treated soils with
Pythium or Fusarium restored full sensitivity of seedlings growing in amended soils to
glyphosate, as did addition of Pythium or Fusarium via seed treatment, root dips or
wound inoculation. When sensitivity of seedlings growing in pasteurized soil or
vermiculite to glyphosate was restored by addition of Pythium, foliar treatment with the
fungicide metalaxyl (Ridomil) protected seedlings from the herbicidal effect of
glyphosate. Metalaxyl is a systemic fungicide that is translocated to roots and has
selective activity against Oomycete fungi such as Pythium (Johal and Rahe, 1984).
These and subsequent experiments conclusively demonstrated that soil fungi such as
Pythium and Fusarium that colonize plant roots can contribute to the herbicidal efficacy
of glyphosate (glyphosate synergistic interaction, GSI) (Rahe et al. 1990, Levesque and
Rahe 1992, Levesque et al. 1992, 1993a, 1993b). In contrast, fungi such as Rhizopus and
Penicillium that do not colonize plant roots do not contribute to efficacy of glyphosate.
Fungi that contribute to the herbicidal efficacy of glyphosate occur in diverse soil types.
The magnitude of difference in glyphosate sensitivity for seedlings growing in untreated
and heat treated soils ranges from 2- to 3-fold to 30- to 50-fold in different plant species.
Most of the experiments described above were done by Guri Johal and Andre Levesque,
as graduate students in my laboratory who did their PhD research on the interactions
between soil fungi and the roots of glyphosate treated plants.
Ecology of glyphosate synergistic Pythium spp. (GSP). – Rolando Descalzo, a third
student in my laboratory to do his PhD research on glyphosate – plant root – soil fungus
interactions, studied aspects of the ecology of the phenomenon. He observed reduced
sensitivity of seedlings growing in heat treated soil without exception for each of several
soils that he tested (Descalzo, et al. 1996). These soils differed widely with regard to
type, climate of the geographic region from which they were obtained, and agricultural
practices (humid coastal, interior semi dessert; irrigated, non-irrigated; cultivated, virgin).
He then made isolations of Pythium spp. from roots of glyphosate treated bean seedlings
growing in these soils. Recombinant tools for differentiation and identification of fungal
species were not yet available in the early 1990’s when Rolando did this work. Using
conventional morphological analysis and restriction fragment length polymorphism
(RFLP) analyses, he was able to assign 65 isolates to six species and a total of either 14
or 15 RFLP groupings depending on the restriction enzyme used. Rolando then evaluated
isolates representative of each of the 15 RFLP groups with regard to their efficacy as
glyphosate synergists on beans, sunflower and pepper. An isolate representing one of the
three RFLP groups of P. ultimum was able to enhance glyphosate efficacy on bean by
nearly 300-fold when it was added to heat treated soil in which the seedlings were grown.
Eleven of the Pythium isolates enhanced efficacy of glyphosate by 20- to 30-fold, and the
remaining three isolates had relatively little effect of efficacy of glyphosate.
When these same isolates were tested using sunflower, each of the 15 isolates enhanced
efficacy of glyphosate on sunflower seedlings growing in heat treated soil by 15- to 20
fold. When tested on pepper seedlings, enhancement of glyphosate efficacy ranged from
2-fold to 60-fold, with most of the isolates enhancing efficacy by approximately 20 fold.
These results showed that while there were some differences in efficacy of different
isolates as glyphosate synergists, these differences were specific to the plant species
being used for testing (Descalzo et al, 1997).
Overall, the results supported the conclusion that diverse Pythium species and genotypes
can act as glyphosate synergists on several different plant species, and that glyphosate
synergistic Pythium species occur in diverse soil types. This conclusion was further
supported by an experiment in which Rolando tested the efficacy of isolates of 10
different Pythium species obtained from type cultures as glyphosate synergists on bean
seedlings. Although the efficacy of these different species as glyphosate synergists on
bean seedlings varied considerably (7-fold to 80-fold), all had synergistic activity.
Physiology of predisposition by glyphosate of plant roots to colonization by root rot
fungi. - Lixing Liu, a fourth student whose PhD research in my lab dealt with aspects of
the interaction of soil fungi with roots of glyphosate treated plants, studied mechanisms
by which glyphosate predisposes roots to colonization by Pythium spp. She tested four
hypotheses: (1) that glyphosate directly stimulates growth of Pythium, (2) that glyphosate
enhances root exudation, (3) that glyphosate blocks phytoalexin production by bean
roots, and (4) that glyphosate interferes with induced lignification in bean roots.
Lixing found that glyphosate at concentrations within the range that could occur as a
result of application of glyphosate herbicides to field soils did not affect mycelial growth
of Pythium ultimum or P. sylvaticum. Germination and growth of germ tubes was
significantly greater in root exudates from bean seedlings whose primary leaves had been
treated with glyphosate than in root exudates from control seedlings, although the effect
was not large. Comparison of phytoalexins in roots of bean seedlings grown in different
media supported the conclusion that presence of phytoalexins was probably induced by
soil microorganisms. Glyphosate did not significantly affect the accumulation of
phytoalexins induced by inoculation with Pythium spp., but did interfere with
lignification induced by inoculation with either P. ultimum or P. sylvaticum. Overall,
Lixing’s research supported the hypothesis that enhanced colonization by Pythium in
roots of bean seedlings whose foliage has been treated with glyphosate occurs as a result
of interference with lignin-based defense mechanisms. Foliar application of glyphosate
may also alter root exudation in a manner that enhances colonization of roots by Pythium
(Liu et al. 1995, 1997). Glyphosate was also shown by Johal to interfere with defense
mechanisms in shoot tissues of bean seedlings (Johal and Rahe 1988, 1990).
Conclusions. - The results of the research that I have just described provide definitive
evidence that Pythium, Fusarium and perhaps other root pathogenic fungi resident in
diverse soils have the potential to contribute to the herbicidal effect of glyphosate. The
rates of application of glyphosate that we used in these experiments were well below
those that would occur with recommended rates of application of glyphosate herbicides in
the field. At recommended rates of application, it is likely that glyphosate would kill
most plants of most species directly, irrespective of colonization of roots by root
pathogenic fungi. The kinds of effects that we have documented are likely to occur in
non-target plants that receive inadvertent exposures when glyphosate herbicides are
being applied as directed sprays, and in plants growing in soils where adsorption of
glyphosate is relatively low. If populations of root pathogenic fungi in soil are enhanced
over time by use of glyphosate herbicides, it is possible that subsequent crops might be at
increased risk of damping off and root rot (Descalzo et al., 1998).
What is the evidence that such effects occur under field conditions? Levesque examined
fungi in roots of six weed species following application of Roundup in the field (loamy
sand soil type) and found that Fusarium spp, were the most prevalent colonizers, and that
soil populations of Fusarium spp. were enhanced following use of Roundup (Levesque
and Rahe 1987). Smiley et al. (1992) reported enhanced levels of Rhizoctonia root rot in
barley where glyphosate herbicides had been used prior to planting to control volunteer
barley and other vegetation just prior to planting. More recently, Fernandez et al. (2005,
2006a, 2006b) reported that use of glyphosate herbicides was associated with increased
Fusarium head blight in wheat and barley, as well as root rot in both crops.
Several other published reports also describe or suggest enhanced root disease associated
with use of glyphosate herbicides, but the number of such reports is small in relation to
the magnitude of usage of glyphosate herbicides. Hopefully, the incidence of enhanced
root disease associated with use of glyphosate herbicides is proportional to the small
number of published reports of such associations. Glyphosate is the world’s most widely
used herbicide for many good reasons. It is the herbicide of choice in a wide range of
applications, is effective, has low environmental impact and has very low mammalian
toxicity. It may be a long time if ever before another herbicide with as many positive and
as few negative attributes as glyphosate comes along. It is in the interest of agriculture
and society in general to be alert to possible disease interactions associated with use of
glyphosate herbicides and to develop an understanding of these situations so that their
impact can be minimized.
Literature cited
Descalzo, R.C., Punja, Z.K., Levesque, C.A. and Rahe, J.E. 1996. Identification and role
of Pythium species as glyphosate synergists on bean (Phaseolus vulgaris) grown in
different soils. Mycological Research 100: 971-978.
Descalzo, R.C., Punja, Z.K., Levesque, C.A. and Rahe, J.E. 1997. Assessment of host
specificity among different species of glyphosate synergistic Pythium. Mycological
Research 100: 1445-1453.
Descalzo, R.C., Punja, Z.K., Levesque, C.A., and Rahe, J.E. 1998. Glyphosate treatment
of bean seedlings causes short term increases in Pythium populations and damping off
potential in soils. Applied Soil Ecology 8: 25-33.
Fernandez, M.R., Selles, F., Gehl, D., DePauw, R.M., and Zentner, R.P. 2005. Crop
production factors associated with fusarium head blight in spring wheat in eastern
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eastern Saskatchewan, Canada. Crop Science (accepted for publication).
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