Browsing by Author "Monroe-Augustus, Melanie"
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Item Genetic approaches to elucidating the mechanisms of indole-3-acetic acid and indole-3-butyric acid function in Arabidopsis thaliana(2004) Monroe-Augustus, Melanie; Bartel, BonnieAuxin is an important plant hormone that plays significant roles in plant growth and development, influencing apical dominance, hypocotyl elongation, lateral root initiation, gravitropism, and phototropism. There are two forms of endogenous auxin, indole-3-butyric acid (IBA) and the more abundant indole-3-acetic acid (IAA). Auxin signal transduction pathways remain to be fully elucidated, though numerous auxin-response mutants have been identified. IBR5 is a gene potentially involved in auxin signal transduction. The ibr5 mutant has pleiotropic defects including decreased sensitivity to the inhibitory effects of auxin, synthetic auxins, auxin transport inhibitors, and the phytohormone abscisic acid. Like certain other auxin-response mutants, ibr5 has a long root and short hypocotyl when grown in the light. Additionally, ibr5 displays aberrant vascular patterning, increased leaf serration, and reduced accumulation of an auxin-inducible reporter. Positional information was used to determine that the gene defective in ibr5 encodes an apparent dual-specificity phosphatase. Using immunoblot and promoter-reporter gene analyses, we found that IBR5 is expressed throughout the plant. The identification of IBR5 relatives in other flowering plants suggests that IBR5 function is conserved throughout angiosperms. The results suggest that IBR5 is a phosphatase modulating phytohormone signal transduction and support a link between auxin and abscisic acid signaling pathways. Less is known concerning IBA function, though this auxin is widely used for rooting in commercial and agricultural settings. IBA functions primarily via its conversion to IAA by a process similar to fatty acid beta-oxidation in the peroxisomes. Additionally, IBA may act via its own signaling pathway, separate from IAA. Screens for mutants resistant to the inhibitory effects of IBA that remain sensitive to IAA have revealed 32 mutants to date. These IBA-response mutants have been placed into five distinct classes based on phenotypic analyses of root and hypocotyl elongation following growth on various hormones and unsupplemented medium. Here I characterize eight IBA-response mutants and use positional information to localize the genes defective in these mutants. Analyzing genes involved in IAA and IBA responses will provide a better understanding of the function of auxin in Arabidopsis thaliana and may eventually allow manipulation of the pathways involved to enhance agricultural production.Item Matrix proteins are inefficiently imported into Arabidopsis peroxisomes lacking the receptor-docking peroxin PEX14(Springer, 2011) Monroe-Augustus, Melanie; Ramón, Naxhiely Martínez; Ratzel, Sarah E.; Lingard, Matthew J.; Christensen, Sarah E.; Murali, Chaya; Bartel, BonnieMutations in peroxisome biogenesis proteins (peroxins) can lead to developmental deficiencies in various eukaryotes. PEX14 and PEX13 are peroxins involved in docking cargo-receptor complexes at the peroxisomal membrane, thus aiding in the transport of the cargo into the peroxisomal matrix. Genetic screens have revealed numerous Arabidopsis thaliana peroxins acting in peroxisomal matrix protein import; the viable alleles isolated through these screens are generally partial loss-of-function alleles, whereas null mutations that disrupt delivery of matrix proteins to peroxisomes can confer embryonic lethality. In this study, we used forward and reverse genetics in Arabidopsis to isolate four pex14 alleles. We found that all four alleles conferred reduced PEX14 mRNA levels and displayed physiological and molecular defects suggesting reduced but not abolished peroxisomal matrix protein import. The least severe pex14 allele, pex14-3, accumulated low levels of a C-terminally truncated PEX14 product that retained partial function. Surprisingly, even the severe pex14-2 allele, which lacked detectable PEX14 mRNA and PEX14 protein, was viable, fertile, and displayed residual peroxisome matrix protein import. As pex14 plants matured, import improved. Together, our data indicate that PEX14 facilitates, but is not essential for peroxisomal matrix protein import in plants.Item The IBR5 phosphatase promotes Arabidopsis auxin responses through a novel mechanism distinct from TIR1-mediated repressor degradation(BioMed Central, 2008) Strader, Lucia C.; Monroe-Augustus, Melanie; Bartel, BonnieBackground: In Arabidopsis, INDOLE-3-BUTYRIC ACID RESPONSE5 (IBR5), a putative dual-specificity protein phosphatase, is a positive regulator of auxin response. Mutations in IBR5 result in decreased plant height, defective vascular development, increased leaf serration, fewer lateral roots, and resistance to the phytohormones auxin and abscisic acid. However, the pathways through which IBR5 influences auxin responses are not fully understood. Results: We analyzed double mutants of ibr5 with other mutants that dampen auxin responses and found that combining ibr5 with an auxin receptor mutant, tir1, enhanced auxin resistance relative to either parent. Like other auxin-response mutants, auxin-responsive reporter accumulation was reduced in ibr5. Unlike other auxin-resistant mutants, the Aux/IAA repressor reporter protein AXR3NT-GUS was not stabilized in ibr5. Similarly, the Aux/IAA repressor IAA28 was less abundant in ibr5 than in wild type. ibr5 defects were not fully rescued by overexpression of a mutant form of IBR5 lacking the catalytic cysteine residue. Conclusion: Our genetic and molecular evidence suggests that IBR5 is a phosphatase that promotes auxin responses, including auxin-inducible transcription, differently than the TIR1 auxin receptor and without destabilizing Aux/IAA repressor proteins. Our data are consistent with the possibility that auxin-responsive transcription can be modulated downstream of TIR1-mediated repressor degradation.