Is the Ethylene Precursor 1-aminocyclopropane-1-carboxylic Acid Involved in Maize Primary Root Growth Regulation under Water Stress?

Undergraduate #154
Discipline: Biological Sciences
Subcategory: Plant Research

Deshawn Toliver - Fort Valley State University
Co-Author(s): Dante Smith and Robert E. Sharp, Ph.D , Division of Plant Sciences and Interdisciplinary Plant Group, University of Missouri, Columbia, Missouri

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Drought is one of the main factors limiting crop growth and yield globally. It is known that under water deficit stress, the primary root of maize (Zea mays L.) accumulates the hormone abscisic acid (ABA), which helps to maintain root elongation (Saab et al. 1990). It was shown that root growth was inhibited in ABA-deficient mutants under water stress and was restored when ABA was added back to wild-type levels (Sharp et al. 1994). Subsequent studies showed that ABA-deficient mutants under water stress exhibited increased production of the gaseous hormone ethylene. To test the hypothesis that the increase in ethylene was a cause of the inhibition of root elongation, the effects of two different ethylene synthesis inhibitors, aminooxyacetic acid (AOA) and aminoethoxyvinylglycine (AVG), and an inhibitor of ethylene binding, 1-methylcyclopropene (1-MCP) were examined (Spollen et al. 2000; Smith and Sharp, unpublished). Interestingly, root growth of the ABA-deficient mutants were restored by treatment with AOA and AVG, but not with 1-MCP. AOA and AVG block ethylene synthesis by inhibiting the activity of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, a key enzyme of ethylene synthesis. Therefore, these results lead to the hypothesis that accumulation of the ethylene precursor ACC, rather than ethylene itself, may be the cause of growth inhibition in ABA-deficient roots under water stress. Consistent with this hypothesis, recent studies have reported that ACC can act as a regulatory signal independently of ethylene (Van de Poel and Van Der Straeten 2014). In the present study, this theory was tested by examining the effects of treatment with α-aminoisobutyric acid (AIBA), a structural analog and therefore competitive inhibitor of ACC, on the growth of ABA-deficient mutant roots under water stress conditions. Our results showed that treatment with AIBA had no effect on the growth of VP-14 mutant roots; furthermore it caused growth inhabitation when applied in combination with the ethylene synthesis inhibitor AOA. Therefore, the results do not support our hypothesis. In fact, the results suggest that ACC may be necessary for root growth maintenance. The explanation for the independent action of AOA in the restoring growth requires further investigation.

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Funder Acknowledgement(s): This work was supported by NSF HRD (#1238789) HBCU-UP Targeted Infusion grant to Prof. Seema Dhir at Fort Valley State University, Fort Valley, GA.

Faculty Advisor: Prof. Seema Dhir, dhirs@fvsu.edu

Role: I germinated maize seedlings and tested the effect of two inhibitors of ethylene biosynthesis to determine the cause of inhibition of root growth under stress.