<p>Tall waterhemp management
in agronomic crops continues to be an increasing problem due to widespread
resistance to herbicides, including protoporphyrinogen oxidase (PPO)-inhibitors.
With limited effective postemergence herbicides, especially in soybeans,
research to further understand the selection of PPO-resistant (PPO-R) tall
waterhemp and identification of new herbicide resistance mechanisms is crucial
for improving weed management decisions in order to slow selection for
herbicide resistance and prolong the effectiveness of PPO-inhibiting
herbicides.</p>
<p> Previous research has shown that soil-applied
applications of PPO-inhibiting herbicides can increase the frequency of the PPO
resistance trait (∆G210)
in surviving tall waterhemp plants, even when applied in combination at the
same ratio with the very long chain fatty acid inhibitor (VLCFA), <i>s-</i>metolachlor. Field experiments were
conducted to determine if selection for tall waterhemp resistant individuals to
PPO-inhibitors could be reduced when the soil residual activity of <i>s</i>-metolachlor persisted longer than the
PPO-inhibitor herbicide. The frequency of ∆G210 in surviving individual plants increased as the
fomesafen rate increased, but was independent of the rate of <i>s</i>-metolachlor. Additionally,
heterozygosity of ∆G210
in surviving individuals did not change with any rate or combination of
fomesafen and <i>s</i>-metolachlor. However,
saflufenacil, standard PPO-inhibitor with relatively short soil residual
activity, applied alone increased the number of homozygous PPO-R tall waterhemp
by 15% compared to the high rate of <i>s</i>-metolachlor
and the combination of saflufenacil and <i>s</i>-metolachlor.
Furthermore, this research demonstrated that end of season control of tall
waterhemp plays a more vital role in delaying a large-scale shift towards
herbicide resistance through reduced seed production. This can be achieved
through the combination of multiple effective herbicide sites of action,
including soil residual PPO-inhibitors. Tall waterhemp control and density were
greatest with the high rates of fomesafen plus <i>s</i>-metolachlor, which resulted in the lowest number of PPO-R tall
waterhemp that survived herbicide treatment at the end of season.</p>
<p> Prior to the research conducted in this thesis, the only
known resistance mechanism to PPO-inhibiting herbicides in tall waterhemp has
been the ∆G210
target site mutation. A previously developed TaqMan assay used to determine the
presence or absence of the ∆G210
mutation has allowed accurate, high throughput screening of this mutation.
However, suspected PPO-R tall waterhemp do not always receive positive
confirmation indicating the presence of an alternative resistance mechanism.
Identification of additional resistance mechanisms can provide valuable insight
in regards to resistance to PPO-inhibiting herbicides as well as cross
resistance to other herbicide modes of action, which can lead to improved tall
waterhemp management decisions. Of 148 tall waterhemp populations collected
across the Midwestern U.S., 84% of the populations sampled contained at least
one PPO-R biotype with the ∆G210
mutation, although several individual plants across the Midwest U.S. exhibited
phenotypic resistance to fomesafen that could not be explained by ∆G210. The percentage of
PPO-R tall waterhemp without ∆G210
was 19, 5, 2, 1, and 2% for Iowa, Illinois, Indiana, Minnesota, and Missouri,
respectively. Following the initial greenhouse screening, subsequent tall
waterhemp populations were selected that exhibited low-, mid-, and high-level
resistance to fomesafen that resulted in resistance ratios from 0.6 to 17X in
response to fomesafen. This research documents the variability in fomesafen
response to multiple tall waterhemp populations in addition to revealing the
presence of additional resistance mechanism(s), other than the previously known
∆G210 mutation that
has been the benchmark for resistance to PPO-inhibiting herbicides in tall
waterhemp.</p>
<p> Lastly, greenhouse and lab experiments were conducted to
investigate the role of antioxidant enzymes with PPO-R tall waterhemp via ∆G210. The objectives of
this research were to determine if the variability in resistance ratios for
PPO-R tall waterhemp documented in greenhouse and field scenarios could be due
to an enhanced antioxidant enzyme pathway. Basal levels of antioxidant enzymes
in PPO-S populations were not different from PPO-R populations when pooled
together by respective phenotype. However, enzyme activity of tall waterhemp
populations varied at the individual level, but independent of the ∆G210 mutation. This
indicates that an inherent enhanced antioxidant enzyme pathway does not cause
the variability in fomesafen response in tall waterhemp. With the exception of glutathione
reductase, antioxidant enzyme activity following fomesafen application was
generally the same for PPO-R and PPO-S populations by increasing, decreasing,
or remaining unchanged. Glutathione reductase activity in PPO-S populations
decreased compared to PPO-R populations from 9 to 36 HAT. By 36 HAT, all antioxidant
enzyme activity for PPO-S populations was lower compared to PPO-R populations
most likely a consequence of more lipid peroxidation. This research shows that
antioxidant enzyme activity correlated with fomesafen application and documents
the variability observed within tall waterhemp populations with and without the
∆G210 mutation. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/14573514 |
| Date | 03 August 2021 |
| Creators | Brent Coy Mansfield (10782717) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Characterization_of_protoporphyrinogen_oxidase_PPO_herbicide_resistance_in_tall_waterhemp_Amaranthus_tuberculatus_/14573514 |
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