Identification of Two Auxin Biosynthesis Mutants Which Result in Root-specific Ethylene Insensitivity

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Title: Identification of Two Auxin Biosynthesis Mutants Which Result in Root-specific Ethylene Insensitivity
Author: Robertson-Hoyt, Joyce Marie
Advisors: Steven Spiker, Committee Member
Steve Clouse, Committee Member
Ralph Dewey, Committee Member
Jose Alonso, Committee Chair
Abstract: The objective of this research was to identify genes involved in ethylene-auxin crosstalk that are required to maintain wild-type ethylene signaling and response in the roots but not hypocotyls of Arabidopsis seedlings. Using this root-specific phenotype, two genes related to Indole-3-Acetic Acid (IAA) biosynthesis were identified that are required for proper ethylene response. WEAK ETHYLENE INSENSITIVE7 (WEI7) codes for the beta subunit of anthranilate synthase, which catalyzes the first committed step in the tryptophan biosynthesis pathway, the conversion of chorismate to anthranilate, a tryptophan, therefore, IAA precursor. WEAK ETHYLENE INSENSITIVE8 (WEI8) codes for an aminotransferase, which is believed to convert tryptophan to indole-3-pyruvic acid (IPA) in the IAA biosynthesis pathway. WEI7 works in conjunction with WEI2 (the anthranilate synthase α-subunit protein) and both are up-regulated by ethylene and responsible, at least in part, for the ethylene-mediated stimulation of auxin biosynthesis. This increase in IAA then is required for a normal response to ethylene in the root. Since tryptophan treatments do not induce auxin phenotypes of elongated hypocotyls and epinastic cotyledons in Arabidopsis, this regulatory scenario is likely to exist for other genes of the tryptophan and IAA biosynthesis pathway, such as WEI8. wei8, a loss of function mutant of an auxin biosynthetic gene, results in root-specific ethylene insensitivity. WEI8 has aminotransferase activity in vitro, can use tryptophan as a substrate, and therefore, it could be responsible for the in vivo conversion of tryptophan into IPA. Double mutant studies support this model. When wel2, a mutant of the WEI8 homolog, WEL2, is used to generate wei8 wel2 double mutants, auxin defects such as agravitropism and loss of apical dominance were seen. Double mutant seedling roots were agravitropic, and adult plants had severe loss of apical dominance as well as reduction in fertility due to defects in flower anatomy. Additionally, analysis of the auxin reporter DR5-GUS expression suggests that WEI8 is required for maintaining normal levels of auxin in the roots of Arabidopsis. WEI7 and WEI8 regulate IAA production and are unexpected interaction points between ethylene response and IAA biosynthesis. Identification of these genes will enable further research on the role of specific pathways of IAA biosynthesis in different tissues and during specific developmental programs. Moreover, it will contribute to a better understanding of hormonal crosstalk in plants.
Date: 2008-02-14
Degree: PhD
Discipline: Genetics

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