<p></p><p></p><p>Abiotic stresses, such as cold, are serious agricultural
problems resulting in substantial crop and revenue losses. Soybean (<i>Glycine max</i>) is an important worldwide
crop for food, feed, fuel, and other products. Soybean has long been considered
to be cold-intolerant and incapable of cold acclimation. In contrast to these
reports, this study demonstrates that cold acclimation improved freezing
tolerance in the domestic soybean cultivar ‘Williams 82’ with 50% enhancement
of freezing tolerance after 5.2 +\- 0.6 days of cold exposure. Decreases in
light dependent photosynthetic function and efficiency accompanied cold
treatment. These decreases were due to an increase in photon dissipation likely
driven by a decrease in plastoquinone (PQ) pool size limiting electron flow
from photosystem II (PSII) to photosystem I (PSI). Cold-induced damage to
operational photosynthesis began at 25 minutes of cold exposure and maximal
photosynthesis was disrupted after 6 to 7 hours of cold exposure. Cold exposure
caused severe photodamage leading to the loss of PSII reaction centers and
photosynthetic efficiency.</p>
<p>Comparisons of eight cultivars of <i>G. max</i> demonstrated a weak correlation between cold acclimation and
northern cultivars versus southern cultivars. In the non-domesticated soybean
species <i>Glycine soja</i>, the germination
rate after cold imbibition was positively correlated with seedling cold
acclimation potential. However, the overall cold acclimation potential in <i>G. soja</i> was equal to that of domestic
soybean <i>G. max</i> reducing the
enthusiasm for the “wild” soybean as an additional source of genetic diversity
for cold tolerance. </p>
<p> </p>
<p>Despite being relatively cold intolerant, the soybean
genome possesses homologs of the major cold responsive CBF/DREB1 transcription
factors. These genes are cold-induced in soybean in a similar pattern to that
of the cold tolerant model plant species Arabidopsis thaliana. In Arabidopsis,
EIN3, a major component of the ethylene signaling pathway, is a negative
transcriptional regulator of CBF/DREB1. In contrast to <i>AtEIN3</i> transcript levels which do not change during cold treatment
in Arabidopsis, we observed a cold-dependent 3.6 fold increase in <i>GmEIN3 </i>transcript levels in soybean. We
hypothesized that this increase could prevent effective CBF/DREB1 cold
regulation in soybean. Analysis of our newly developed cold responsive reporter
(<i>AtRD29Aprom::GFP/GUS</i>) soybean
transgenic lines demonstrated that inhibition of the ethylene pathway via
foliar sprays (AVG, 1-MCP, and silver nitrate) resulted in significant cold-induced
GUS activity. Transcripts of <i>GmEIN3A;1</i>
increased in response to ethylene pathway stimulation (ACC and ethephon) and
decreased in response to ethylene pathway inhibition in the cold. Additionally,
in the cold, inhibition of the ethylene pathway resulted in a significant
increase in transcripts of <i>GmDREB1A;1</i>
and <i>GmDREB1A;2</i> and stimulation of the
ethylene pathway led to a decrease in <i>GmDREB1A;1</i>
and <i>GmDREB1B;1</i> transcripts. To assess
the physiological effects of these transcriptional changes; electrolyte
leakage, lipid oxidation, free proline content, and photosynthesis were
examined. Improvement in electrolyte leakage, a measure of freezing tolerance,
was seen only under silver nitrate treatment. Only 1-MCP treatment resulted in
significantly decreased lipid oxidation. Transcripts for CBF/DREB1 downstream
targets (containing the consensus CRT/DRE motifs) significantly decreased in
plants treated with ethylene pathway stimulators in the cold; however, ethylene
pathway inhibition generally produced no increase over basal cold levels. </p>
<p> </p>
<p>To identify if GmEIN3A;1 was capable of binding to <i>GmDREB1</i> promoters, the negative
regulator GmEIN3A;1 and the positive regulator GmICE1A were cloned and expressed
in Escherichia coli (E. coli). Preliminary binding results indicated that
GmEIN3A;1 can bind to a double stranded section of the GmDREB1A;1 promoter
containing putative EIN3 and ICE1 binding sites. GmICE1A is capable of binding
to the same section of the <i>GmDREB1A;1</i>
promoter, though only when single stranded. Additional experiments will be
required to demonstrate that GmEIN3A;1 and GmICE1A are capable of binding to
the <i>GmDREB1A;1</i> promoter and this work
provides the tools to answer these questions. </p>
<p> </p>
<p>Overall, this work provides evidence that the ethylene
pathway transcriptionally inhibits the CBF/DREB1 pathway in soybean through the
action of GmEIN3A;1. Yet when <i>GmCBF/DREB1</i>
transcripts are upregulated by ethylene pathway inhibition, no consistent
change in downstream targets was observed. These data indicate that the
limitation in cold tolerance in soybean is due to a yet unidentified target
downstream of CBF/DREB1 transcription.</p><p></p><p></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8029055 |
| Date | 10 June 2019 |
| Creators | Jennifer Dawn Robison (6623789) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY-NC-SA 4.0 |
| Relation | https://figshare.com/articles/Molecular_and_Physiological_Response_of_Soybean_Glycine_max_to_Cold_and_the_Stress_Hormone_Ethylene/8029055 |
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