Molecular and Physiological Responses of Soybean (Glycine max) to Cold and the Stress Hormone Ethylene

Jennifer Dawn Robison, Purdue University

Abstract

Abiotic stresses, such as cold, are serious agricultural problems resulting in substantial crop and revenue losses. Soybean (Glycine max ) 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). Coldinduced 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. Comparisons of eight cultivars of G. max demonstrated a weak correlation between cold acclimation and northern cultivars versus southern cultivars. In the nondomesticated soybean species Glycine soja, the germination rate after cold imbibition was positively correlated with seedling cold acclimation potential. However, the overall cold acclimation potential in G. soja was equal to that of domestic soybean G. max reducing the enthusiasm for the “wild” soybean as an additional source of genetic diversity for cold tolerance. Despite being relatively cold intolerant, the soybean genome possesses homologs of the major cold responsive CBF/DREB1 transcription factors. These genes are coldinduced 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 AtEIN3 transcript levels which do not change during cold treatment in Arabidopsis, we observed a cold-dependent 3.6 fold increase in GmEIN3 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 (AtRD29Aprom::GFP/GUS) 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 GmEIN3A;1 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 GmDREB1A;1 and GmDREB1A;2 and stimulation of the ethylene pathway led to a decrease in GmDREB1A;1 and GmDREB1B;1 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.

Degree

Ph.D.

Subject Area

Plant sciences|Agronomy

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