The TIR1 protein of Arabidopsis functions in auxin response and is related to human SKP2 and yeast Grr1p

Abstract

Genetic analysis in Arabidopsis has led to the identification of several genes that are required for auxin response. One of these genes, AXR1, encodes a protein related to yeast Aos1p, a protein that functions to activate the ubiquitin-related protein Smt3p. Here we report the identification of a new gene called TRANSPORT INHIBITOR RESPONSE 1 (TIR1). The tir1 mutants are deficient in a variety of auxin-regulated growth processes including hypocotyl elongation and lateral root formation. These results indicate that TIR1 is also required for normal response to auxin. Further, mutations in TIR1 display a synergistic interaction with mutations in AXR1, suggesting that the two genes function in overlapping pathways. The TIR1 protein contains a series of leucine-rich repeats and a recently identified motif called an F box. Sequence comparisons indicate that TIR1 is related to the yeast protein Grr1p and the human protein SKP2. Because Grr1p and other F-box proteins have been implicated in ubiquitin-mediated processes, we speculate that auxin response depends on the modification of a key regulatory protein(s) by ubiquitin or a ubiquitin-related protein.[Key Words: F-box protein; Arabidopsis; auxin response; TIR1; human SKP2; yeast Grr1p] Received September 10, 1997; revised version accepted November 14, 1997.Physiological studies have implicated the plant hormone indole-3-acetic acid (IAA or auxin) in the regulation of diverse developmental processes including stem elongation, apical dominance, photo-and gravitropism, and lateral root initiation (Klee and Estelle 1991). At the cellular level, auxin acts to regulate these processes through changes in cell division and cell expansion (Evans 1984). Although the mechanism or mechanisms of auxin action are largely unknown, the existing evidence suggests that the hormone acts both at the plasma membrane and within the cell (Vesper and Kuss 1990; Venis and Napier 1995). Auxin treatment results in rapid hyperpolarization of the plasma membrane and induction of specific gene expression (Barbier-Brygoo 1995;Abel and Theologis 1996).In Arabidopsis, genetic studies have resulted in the identification of a number of genes that are required for normal auxin response (Hobbie and Estelle 1994, Leyser 1997). For three of the genes, AXR1, AXR4, and AUX1, recessive mutations result in reduced auxin response as well as an array of auxin-related growth defects (Hobbie and Estelle 1994). By genetic criteria, AXR1 and AUX1 appear to act in distinct pathways. AUX1 is a membrane protein with similarity to amino acid permeases from plants and fungi (Bennett et al. 1996). Because IAA is an indolic compound structurally related to tryptophan, AUX1 may function in cellular auxin uptake and not response per se. In contrast, the axr1 and axr4 mutations display a synergistic interaction, suggesting that the wild-type genes function in the same or overlapping pathways (Hobbie and Estelle 1995). Genetic experiments indicate that another gene, called SAR1, can also be placed in this group. The sar1 muta...

Ubiquitination-mediated protein degradation and modification: an emerging theme in plant-microbe interactions

“…Mutation of RCE1 (RUB-conjugating enzyme1) resulted in defects in the expression of defense-related genes' basic chitinase and PDF1.2, inferring that the SCF E3 complex it modified is required for plant defense [87]. RCE1 was shown in Arabidopsis to be part of a stable SCF TIR ubiquitin E3 ligase complex [88], which is a critical component in plant auxin signaling [89][90][91]. It will be intriguing to see if RCE1 is also associated with SCF COI1 complex, which has been shown be involved in plant defense signaling (see above).…”

COS1: An Arabidopsis coronatine insensitive1 Suppressor Essential for Regulation of Jasmonate-Mediated Plant Defense and Senescence

“…We then attempted to investigate the role for the cos1 mutation in transport inhibitor response1 (tir1), an auxin-insensitive mutant, which defines the F-box protein TIR1 that is most closely related to COI1 (Ruegger et al, 1998;Xie et al, 1998). The tir1-1 cos1 coi1-2 mutant was identified through sequence verification of the tir1-1 allele among the cos1 coi1-2 homozygous plants from F2 progeny of a cross between tir1-1 and cos1 coi1-2.…”

“…The tir1-1 mutant, which harbors a point mutation from guanine 440 to adenine (g440a) leading to an amino acid replacement from G147 to D (G147D) (Ruegger et al, 1998), was crossed to the cos1 coi1-2 mutant plant. The cos1 homozygous plants were identified from the F2 progeny based on cos1-conferred lesion-like yellow spots, and all of the cos1 homozygous plants were also homozygous for the coi1-2 allele because cos1 has close genetic linkage with coi1-2.…”

Fine tuning of auxin signaling by miRNAs

“…A genetic screen for auxin-resistant mutant identified TIR1 (transport inhibitor response 1) as a receptor for auxin. TIR1 encodes an F-box protein (Ruegger et al, 1998) and is a component of SCF-type ubiquitin-ligases, which are responsible for targeting specific proteins for degradation after tagging them with multiple ubiquitin residues (Vierstra, 2003). In fact the name SCF itself is derived from three foremost identified members: Skp1, Cullin, and F-box protein.…”

“…A number of genes in auxin signaling cascade are confirmed as targets of miRNAs. miR393 targets four closely related F-box genes, including the auxin receptor TIR1 (Ruegger et al 1998;Gray et al 2001). miR393 also targets At3g23690, a basic helix-loop helix transcription factor that is homologous to GBOF-1 from tulip, which Genbank annotates as an auxininducible gene.…”