Plant Physiology Preview
Published on February 17, 2006; 10.1104/pp.105.075671
Received December 15, 2005
Returned for revision January 14, 2006
Accepted February 5, 2006
Ethylene Modulates Flavonoid Accumulation and Gravitropic Responses in Roots of Arabidopsis thaliana
Charles S. Buer , Poornima Sukumar , and Gloria K. Muday *
Department of Biology, Wake Forest University, Winston-Salem, NC 27109
* Corresponding author; email: muday{at}wfu.edu.
Plant organs change their growth direction in response to reorientation relative to the gravity vector. We explored the role of ethylene in Arabidopsis thaliana root gravitropism. Treatment of wild-type Columbia seedlings with the ethylene precursor, 1-aminocyclopropane carboxylic acid (ACC), reduced root elongation and gravitropic curvature. The ethylene-insensitive mutants, ein2-5 and etr1-3, had wild-type root gravity responses, but lacked the growth and gravity inhibition by ACC found in wild type. We examined the effect of ACC on tt4(2YY6) seedlings, which have a null mutation in the gene encoding chalcone synthase (CHS), the first enzyme in flavonoid synthesis. The tt4(2YY6) mutant makes no flavonoids, has elevated IAA transport, and exhibits a delayed gravity response (Brown et al., 2001; Buer and Muday, 2004). Roots of tt4(2YY6), the backcrossed line, tt4-2, and two other tt4 alleles had wild-type sensitivity to growth inhibition by ACC, while the root gravitropic curvature of these tt4 alleles were much less inhibited by ACC than wild-type roots, suggesting that ACC may reduce gravitropic curvature by altering flavonoid synthesis. ACC treatment induced flavonoid accumulation in root tips, as judged by a dye that becomes fluorescent upon binding flavonoids, in wild type but not ein2-5 and etr1-3. ACC also prevented a transient peak in flavonoid synthesis in response to gravity. Together, these experiments suggest that elevated ethylene levels negatively regulate root gravitropism, using EIN2- and ETR1-dependent pathways, and that ACC inhibition of gravity response occurs through altering flavonoid synthesis.
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