Plant gravitropism, the directional growth of a plant or plant parts in response to the gravity, is controlled by growth-regulating substances and light. Auxin is the primary plant hormone responsible for stimulating plant cellular growth and plays a central role in regulating gravitropism. When a stem is placed in a horizontal position, auxin is transported to the lower side of the stem stimulating growth and pushing the stem upright. While stem gravitropism is caused by auxin accumulated on the lower side, the gaseous hormone ethylene also increases after horizontal placement and plays a modulating role in regulating the process. Our current research projects use the genetic model plant, mouse-ear cress (Arabidopsis thaliana), to characterize the gravitropic responses and to analyze expression of specific forms of the ethylene biosynthetic enzyme, 1-aminocyclopropane-1-carboxylic acid synthase (ACS).
One recent research project focuses on whether ethylene biosynthetic genes are expressed in larger amounts on the lower compared to the upper side of horizontally-placed Arabidopsis stems. Our results show that gravitropic curvature is increased in seedling stems of mutants that also exhibit increased ethylene production, and that ACS is highly expressed on the lower side of the stem indicating a stimulatory role for ethylene in upward curvature in Arabidopsis stems.
Light also modulates gravitropic curvature, an effect attributed to phytochrome (a plant photoreceptor) regulation, possibly through its interaction with ethylene biosynthesis and response. Our research on dark-grown pea stems demonstrated that ethylene production is inhibited by a short-term red-light pulse, but increased after horizontal placement. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of the amount of mRNA produced by ACS did not support strong transcriptional regulation of these genes by red light.
Harrison, M.A. (2012) Cross-talk between phytohormone signaling pathways under optimal and stressful conditions. Chapter 2 In: Phytohormones and Abiotic Stress Tolerance in Plants, Professor Dr. Nafees A. Khan (ed.), Springer-Verlag GmbH Berlin Heidelberg
Gilkerson*, J.G., J.A. Kelley**, and M.A. Harrison. 2009. Evaluation of ethylene production in tobacco and Arabidopsis induced by particle bombardment. Bio-Rad Bulletin 5847 Hogan**, J., E. Murray, and M.A. Harrison. 2006. Evaluation of ethylene as an indicator of plant stress in hydroponic systems. Scientia Horticulturae 110: 311-318
Harrison, M.A. 2006. Role of ethylene in the regulation of stem gravitropic curvature. In: Ethylene Action in Plants, Professor Dr. Nafees A. Khan (ed.), Springer-Verlag GmbH Berlin Heidelberg, pp 135-149
Harrison, M.A. 2006. Plant hormones and signal transduction. In: Plant Cell Biology, W.V. Dashek and M.H. Harrison (eds), Science Publishers, Enfield, NH, pp 451-487
Harrison, M.A. 2005. Role of ethylene in the regulation of stem gravitropic curvature In: Ethylene Action in Plants, Professor Dr. Nafees A. Khan (ed.), Springer-Verlag GmbH Berlin Heidelberg (invited article, in press).
Harrison, M.A. 2005. Plant Cell Signaling in Plant Cell Biology, W.V. Dashek (ed), Science Publishers, Enfield, NH (in press).
Jenski, L.J., N.D. Lees, and M.A. Harrison. 2005. Getting Attention for New Initiatives without Making It “Just More Work”! The Department Chair 16: 15-17.
Steed C.L., L.K. Taylor, and M.A. Harrison. 2004. Red-light regulation of ethylene biosynthesis and gravitropism in etiolated pea stems. Plant Growth Regulation 43: 117-125.
Bassett, C.L., Nickerson, M.L., Farrell, R.E. and Harrison, M. 2004. Multiple transcripts of a leucine-rich repeat receptor kinase from morning glory originate from different TATA boxes in a tissue-specific Manner. Molecular & General Genetics 271: 752-760.