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The CO2-induced Up- and Down-regulation of Cucumber Metabolites to Alleviate Drought Stress

Paper Reviewed
Li, M., Li, Y., Zhang, W., Li, S., Gao, Y., Ai, X., Zhang, D., Liu, B. and Li, Q. 2018. Metabolomics analysis reveals that elevated atmospheric CO2 alleviates drought stress in cucumber seedling leaves. Analytical Biochemistry 559: 71-85.

Writing as background for their study, Li et al. (2018) state that "although previous investigators have considered the combined effects of elevated [CO2] on various physiological responses to drought stress, much less is known about the combined effects of elevated [CO2] and drought stress on plant metabolism." And so it was that this team of nine Chinese researchers set out to investigate the metabolic mechanisms involved in the CO2-induced alleviation of drought stress under elevated levels of this important atmospheric trace gas.

In accomplishing this objective, Li et al. grew cucumber (Cucumis sativus, cv Jinyou 35) seedlings in open-top greenhouses in China under two CO2 concentrations and three water treatments. The CO2 concentrations included ambient (400 ppm) and elevated (800 ppm) while the three water treatments consisted of control (no water stress), moderate drought stress and severe drought stress. As best as can be determined from their paper, the short-term experiment lasted only 7 days. At the end of this period, several plant growth-related parameters were measured by the authors, who also utilized a metabolomics approach based on "ultra-high performance liquid chromatography quadrupole-time of flight mass spectrometry (UHPLC-Q-TOF MS) to explore the combined effects of elevated [CO2] and drought stress on the leaves of [the] cucumber seedlings."

And what did the scientists learn?

Not surprisingly, the stress of drought (whether moderate or severe) significantly reduced plant photosynthesis, plant height, stem diameter, leaf area, relative water content, stomatal conductance and transpiration rate under both ambient and elevated CO2 conditions. However, elevated CO2 ameliorated the severity of these reductions and the metabolomics approach provided some insight as to how it did so.

Results indicated that multiple metabolites were differentially expressed (either up-regulated or down-regulated) in plants subjected to the two drought treatments under elevated CO2. Regarding these metabolites, Li et al. report the following:
1. Several carbohydrate- and alcohol-related metabolites were up-regulated, many of which are known to help protect cell membranes or to regulate osmotic pressure to alleviate drought stress.
2. Multiple organic acids and amino acids that help improve stress tolerance were down-regulated under elevated CO2, indicating that cucumber leaves "no longer required high levels of [these] metabolites that are components of the metabolic pathways involved in resistance to drought stress."
3. Elevated CO2 improved the antioxidant activity of cucumber leaves, helping to facilitate drought resistance by increasing the levels of phenolic compounds.
4. Elevated CO2 enhanced the "photosynthetic and osmotic adjustment capacity by accumulating glutamate, which can then be used to synthesize more photosynthetic pigments and to increase proline synthesis; thereby, enhancing cucumber adaptions to severe drought stress."

The results of this analysis demonstrate that the benefits of atmospheric CO2 enrichment typically reported in most studies (e.g., leaf, stem and/or root biomass enhancements) begin at a metabolic level, where multiple metabolites are differentially expressed to help improve growth under both stressful and non-stressful growing conditions.

Posted 29 April 2019