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Structure and expression analysis of early auxin-responsive Aux/IAA gene family in rice (Oryza sativa).

Auxin exerts pleiotropic effects on plant growth and development by regulating the expression of early auxin-responsive genes of auxin/indoleacetic acid (Aux/IAA), small auxin-up RNA, and GH3 classes. These genes have been studied extensively in dicots like soybean and Arabidopsis. We had earlier characterized a cDNA of the first monocot member of Aux/IAA family from rice. The achievement of the large scale rice genome sequencing combined with the availability of full-length cDNA sequences from Knowledge-based Oryza Molecular Biological Encyclopedia provided us the opportunity to draw up the first comprehensive list of Aux/IAA genes in a monocot. By screening the available databases, we have identified 31 Aux/IAA genes having high sequence identity within the conserved domains I, II, III, and IV. The genomic organization as well as chromosomal location of all the Oryza sativa indoleacetic acid (OsIAA) genes is reported. The rice Aux/IAA proteins can be classified in two groups (A and B) on the basis of their phylogenetic relationship with Arabidopsis Aux/IAA proteins. An evolutionary pattern of the rice Aux/IAA genes has been discussed by analyzing their structure (exon/intron organization) and duplications. Interestingly, the duplication of rice Aux/IAA genes was found to be associated with chromosomal block duplication events in rice. The in-silico analysis has been complemented with real-time polymerase chain reaction analysis to quantify transcript levels of all Aux/IAA family members. OsIAA genes showed differential and overlapping organ-specific expression patterns in light- and dark-grown seedlings/plants. Although auxin enhanced the transcript abundance of most of the OsIAA genes, the effect was more pronounced on OsIAA9, 14, 19, 20, 24, and 31. These results provide a foundation for future studies on elucidating the precise role of rice Aux/IAA genes in early steps of auxin signal transduction.

Amino Acid Sequence↗

Acceleration of Aux/IAA proteolysis is specific for auxin and independent of AXR1.

Aux/IAA proteins are short-lived transcriptional regulators involved in auxin signaling. Using Aux/IAA luciferase (LUC) fusion proteins expressed in Arabidopsis thaliana, we previously showed that rapid degradation of these proteins requires conserved Aux/IAA domain II and that exogenous auxin accelerates their degradation. To further examine auxin-mediated increases in proteolysis, the degradation of two other LUC fusion proteins, a non-cleavable ubiquitin LUC fusion (UB1-72::LUC) and SAUR15::LUC was determined in vivo in seedlings. Their half-lives were 20 +/- 4 and 104 +/- 10 min, respectively. SAUR15::LUC half-life was not affected by pre-incubation with 2,4-D. Auxin did not have an equivalent effect on UB(1-72)::LUC steady-state levels as compared to PsIAA6:LUC. LUC fused to an Aux/IAA domain II degraded more rapidly following auxin application, demonstrating that this region is sufficient for auxin-mediated acceleration of proteolysis. Hormonal cross-talk at the level of Aux/IAA proteolysis was examined. 1-aminocyclopropane-1-carboxylic acid (ACC), benzyladenine (BA), abscisic acid (ABA), and brassinolide (BL) did not affect the degradation rate of IAA1::LUC, and gibberellic acid (GA3) and salicylic acid (SA) did not specifically affect the steady-state levels of Aux/IAA::LUC proteins. An Aux/IAA::LUC transgene was crossed into the auxin resistant-1 (axr1-12) background. In axr1-12, the half-life of PsIAA6(1-73)::LUC increased 4.5-fold, but proteolysis still accelerated in response to exogenous auxin. These data suggest that auxin is the only phytohormone that accelerates Aux/IAA proteolysis, and that this acceleration is specific for Aux/IAA proteins. In addition, AXR1 plays an important role in rapid basal proteolysis of Aux/IAA proteins, but is not required for auxin-mediated acceleration of their degradation.

Amino Acids, Cyclic↗

Mutation in domain II of IAA1 confers diverse auxin-related phenotypes and represses auxin-activated expression of Aux/IAA genes in steroid regulator-inducible system.

Most of Aux/IAA genes are rapidly induced by auxin. The Aux/IAA proteins are short-lived nuclear proteins sharing the four conserved domains. Domain II is critical for rapid degradation of Aux/IAA proteins. Among these gene family members, IAA1 is one of the earliest auxin-inducible genes. We used a steroid hormone-inducible system to reveal putative roles and downstream signaling of IAA1 in auxin response. Arabidopsis transgenic plants were generated expressing fusion protein of IAA1 (IAA1-GR) or IAA1 with a mutation in domain II (iaa1-GR) and the glucocorticoid hormone-binding domain (GR). IAA1-GR transgenic plants did not exhibit any discernable phenotypic differences by DEX treatment that allows nuclear translocation of the fusion protein. In contrast, diverse auxin-related physiological processes including gravitropism and phototropism were impaired by DEX treatment in roots, hypocotyls, stems, and leaves in iaa1-GR transgenic plants. Auxin induction of seven Aux/IAA mRNAs including IAA1 itself was repressed by DEX treatment, suggesting that IAA1 functions in the nucleus by mediating auxin response and might act as a negative feedback regulator for the expression of Aux/IAA genes including IAA1 itself. Auxin induction of Aux/IAA genes in the presence of cycloheximide can be repressed by DEX treatment, showing that the repression of transcription of the Aux/IAAs by the iaa1 mutant protein is primary. Wild-type IAA1-GR could not suppress auxin induction of IAA1 and IAA2. These results indicate that inhibition of auxin-activated transcription of Aux/IAA genes by the iaa1 mutant protein might be responsible for alteration of various auxin responses.

Arabidopsis↗

Auxin modulates the degradation rate of Aux/IAA proteins.

Aux/IAA gene family members were first identified by their rapid transcriptional increase in response to auxin. Auxin/indole-3-acetic acid protein (Aux/IAA) luciferase (LUC) fusions expressed in Arabidopsis under control of a non-auxin-responsive promoter were used to monitor the effect of auxin on protein abundance independent of transcriptional regulation by auxin. After 2 hr in the presence of 1 microM exogenous dichlorophenoxyacetic acid (2,4D), a synthetic auxin, the levels of pea IAA6 (PSIAA6) and Arabidopsis IAA1 LUC activity were 35% and 67%, respectively, of mock-treated genetically identical seedlings, whereas the activity of LUC alone from equivalently treated seedlings remained unaltered. The steady-state level of an Aux/IAA fusion protein lacking domain II, one of the conserved domains found in all Aux/IAA proteins, was not reduced in the presence of auxin. Higher levels of exogenous auxin were required to affect the steady-state level of the PSIAA6LUC fusion with a point mutation in domain II. A 13-aa consensus sequence from domain II fused to LUC created an auxin-responsive fusion protein. The change in steady-state levels in response to auxin is extremely rapid, with a decrease in LUC activity detectable by 2 min after auxin application. Direct half-life measurements show that the decrease caused by exogenous auxin is due to the decrease in fusion protein half-life. These results suggest that auxin rapidly modulates the degradation rate of Aux/IAA proteins, with higher levels of auxin increasing the proteolytic rate of Aux/IAA family members.

Arabidopsis↗

Contrasting modes of diversification in the Aux/IAA and ARF gene families.

The complete genomic sequence for Arabidopsis provides the opportunity to combine phylogenetic and genomic approaches to study the evolution of gene families in plants. The Aux/IAA and ARF gene families, consisting of 29 and 23 loci in Arabidopsis, respectively, encode proteins that interact to mediate auxin responses and regulate various aspects of plant morphological development. We developed scenarios for the genomic proliferation of the Aux/IAA and ARF families by combining phylogenetic analysis with information on the relationship between each locus and the previously identified duplicated genomic segments in Arabidopsis. This analysis shows that both gene families date back at least to the origin of land plants and that the major Aux/IAA and ARF lineages originated before the monocot-eudicot divergence. We found that the extant Aux/IAA loci arose primarily through segmental duplication events, in sharp contrast to the ARF family and to the general pattern of gene family proliferation in Arabidopsis. Possible explanations for the unusual mode of Aux/IAA duplication include evolutionary constraints imposed by complex interactions among proteins and pathways, or the presence of long-distance cis-regulatory sequences. The antiquity of the two gene families and the unusual mode of Aux/IAA diversification have a number of potential implications for understanding both the functional and evolutionary roles of these genes.

Arabidopsis↗

AUX/IAA proteins are active repressors, and their stability and activity are modulated by auxin.

Aux/IAA genes are early auxin response genes that encode short-lived nuclear proteins with four conserved domains, referred to as I, II, III, and IV. Arabidopsis Aux/IAA proteins repressed transcription on auxin-responsive reporter genes in protoplast transfection assays. Mutations in domain II resulted in increased repression, whereas mutations in domains I and III partially relieved repression. Aux/IAA proteins fused to a heterologous DNA binding domain were targeted to promoters of constitutively expressed reporter genes and actively repressed transcription in an auxin-responsive and dose-dependent manner. In comparison with an unfused luciferase protein, luciferase fused to Aux/IAA proteins displayed less luciferase activity, which further decreased in the presence of auxin in transfected protoplasts. Domain II mutations increased and domain I mutations decreased luciferase activity with the fusion proteins. These results suggested that Aux/IAA proteins function as active repressors by dimerizing with auxin response factors bound to auxin response elements and that early auxin response genes are regulated by auxin-modulated stabilities of Aux/IAA proteins.

Arabidopsis↗

Roles and activities of Aux/IAA proteins in Arabidopsis.

Auxin induces various distinct developmental responses, partly by regulating gene expression. The Aux/IAA genes are a large gene family, many of which are induced by auxin. Work on Arabidopsis Aux/IAA genes has begun to reveal that they can regulate development and auxin-induced gene expression. Furthermore, auxin responses require Aux/IAA protein turnover. Finally, recent evidence suggests that Aux/IAA proteins can mediate light responses. Work in the near future should test whether Aux/IAA proteins are antennae that connect auxin and light signals to endogenous developmental responses.

Arabidopsis↗

Auxin regulates SCF(TIR1)-dependent degradation of AUX/IAA proteins.

The plant hormone auxin is central in many aspects of plant development. Previous studies have implicated the ubiquitin-ligase SCF(TIR1) and the AUX/IAA proteins in auxin response. Dominant mutations in several AUX/IAA genes confer pleiotropic auxin-related phenotypes, whereas recessive mutations affecting the function of SCF(TIR1) decrease auxin response. Here we show that SCF(TIR1) is required for AUX/IAA degradation. We demonstrate that SCF(TIR1) interacts with AXR2/IAA7 and AXR3/IAA17, and that domain II of these proteins is necessary and sufficient for this interaction. Further, auxin stimulates binding of SCF(TIR1) to the AUX/IAA proteins, and their degradation. Because domain II is conserved in nearly all AUX/IAA proteins in Arabidopsis, we propose that auxin promotes the degradation of this large family of transcriptional regulators, leading to diverse downstream effects.

Amino Acid Motifs↗

Degradation of Aux/IAA proteins is essential for normal auxin signalling.

The growth substance auxin mediates many cellular processes, including division, elongation and differentiation. PSIAA6 is a member of the Aux/IAA family of short-lived putative transcriptional regulators that share four conserved domains and whose mRNAs are rapidly induced in the presence of auxin. Here PSIAA6 was shown to serve as a dominant transferable degradation signal when present as a translational fusion with firefly luciferase (LUC), with an in vivo half-life of 13.5 min in transgenic Arabidopsis seedlings. In a transient assay system in tobacco protoplasts using steady-state differences as an indirect measure of protein half-life, LUC fusions with full-length PSIAA6 and IAA1, an Aux/IAA protein from Arabidopsis, resulted in protein accumulations that were 3.5 and 1. 0%, respectively, of that with LUC alone. An N-terminal region spanning conserved domain II of PSIAA6 containing amino acids 18-73 was shown to contain the necessary cis-acting element to confer low protein accumulation onto LUC, while a fusion protein with PSIAA6 amino acids 71-179 had only a slight effect. Single amino acid substitutions of PSIAA6 in conserved domain II, equivalent to those found in two alleles of axr3, a gene that encodes Aux/IAA protein IAA17, resulted in a greater than 50-fold increase in protein accumulation. Thus, the same mutations resulting in an altered auxin response phenotype increase Aux/IAA protein accumulation, providing a direct link between these two processes. In support of this model, transgenic plants engineered to over-express IAA17 have an axr3-like phenotype. Together, these data suggest that rapid degradation of Aux/IAA proteins is necessary for a normal auxin response.

Arabidopsis↗

Yokonolide B, a novel inhibitor of auxin action, blocks degradation of AUX/IAA factors.

Yokonolide B (YkB; also known as A82548A), a spiroketal-macrolide, was isolated from Streptomyces diastatochromogenes B59 in a screen for inhibitors of beta-glucoronidase expression under the control of an auxin-responsive promoter in Arabidopsis. YkB inhibits the expression of auxin-inducible genes as shown using native and synthetic auxin promoters as well as using expression profiling of 8300 Arabidopsis gene probes but does not affect expression of an abscisic acid- and a gibberellin A3-inducible gene. The mechanism of action of YkB is to block AUX/IAA protein degradation; however, YkB is not a general proteasome inhibitor. YkB blocks auxin-dependent cell division and auxin-regulated epinastic growth mediated by auxin-binding protein 1. Gain of function mutants such as shy2-2, slr1, and axr2-1 encoding AUX/IAA transcriptional repressors and loss of function mutants encoding components of the ubiquitin-proteolytic pathway such as axr1-3 and tir1-1, which display increased AUX/IAAs protein stability, are less sensitive to YkB, although axr1 and tir1 mutants were sensitive to MG132, a general proteasome inhibitor, consistent with a site of action downstream of AXR1 and TIR. YkB-treated seedlings displayed similar phenotypes as dominant AUX/IAA mutants. Taken together, these results indicate that YkB acts to block AUX/IAA protein degradation upstream of AXR and TIR, links a shared element upstream of AUX/IAA protein stability to auxin-induced cell division/elongation and to auxin-binding protein 1, and provides a new tool to dissect auxin signal transduction.

Anti-Bacterial Agents↗

An early auxin-responsive Aux/IAA gene from wheat (Triticum aestivum) is induced by epibrassinolide and differentially regulated by light and calcium.

The plant hormone auxin plays a central role in regulating many aspects of plant growth and development. This largely occurs as a consequence of changes in gene expression. The Aux/IAA genes are best characterized among the early auxin-responsive genes, which encode short-lived transcriptional repressors. In most plants examined, including Arabidopsis, soybean, and rice, the Aux/IAA genes constitute a large gene family. By screening the available databases, at least 15 expressed sequence tags (ESTs) have been identified from wheat (Triticum aestivum), which exhibit high sequence identity with Aux/IAA homologues in other species. One of these Aux/IAA genes, TaIAA1, harbouring all the four conserved domains characteristic of the Aux/IAA proteins, has been characterized in detail. The expression of TaIAA1 is light-sensitive, tissue-specific, and is induced within 15-30 min of exogenous auxin application. Also, the TaIAA1 transcript levels increase in the presence of a divalent cation, Ca(2+), and this effect is reversed by the calcium-chelating agent, EGTA. The TaIAA1 gene qualifies as the primary response gene because an increase in its transcript levels by auxin is unaffected by cycloheximide. In addition to auxin, the TaIAA1 gene is also induced by brassinosteroid, providing evidence that interplay between hormones is crucial for the regulation of plant growth and development.

Blotting, Southern↗

Aux/IAA proteins are phosphorylated by phytochrome in vitro.

Auxin/indole-3-acetic acid (Aux/IAA) genes encode short-lived transcription factors that are induced as a primary response to the plant growth hormone IAA or auxin. Gain-of-function mutations in Arabidopsis genes, SHY2/IAA3, AXR3/IAA17, and AXR2/IAA7 cause pleiotropic phenotypes consistent with enhanced auxin responses, possibly by increasing Aux/IAA protein stability. Semidominant mutations shy2-1D, shy2-2, axr3-1, and axr2-1 induce ectopic light responses in dark-grown seedlings. Because genetic studies suggest that the shy2-1D and shy2-2 mutations bypass phytochrome requirement for certain aspects of photomorphogenesis, we tested whether SHY2/IAA3 and related Aux/IAA proteins interact directly with phytochrome and whether they are substrates for its protein kinase activity. Here we show that recombinant Aux/IAA proteins from Arabidopsis and pea (Pisum sativum) interact in vitro with recombinant phytochrome A from oat (Avena sativa). We further show that recombinant SHY2/IAA3, AXR3/IAA17, IAA1, IAA9, and Ps-IAA4 are phosphorylated by recombinant oat phytochrome A in vitro. Deletion analysis of Ps-IAA4 indicates that phytochrome A phosphorylation occurs on the N-terminal half of the protein. Metabolic labeling and immunoprecipitation studies with affinity-purified antibodies to IAA3 demonstrate increased in vivo steady-state levels of mutant IAA3 in shy2-2 plants and phosphorylation of the SHY2-2 protein in vivo. Phytochrome-dependent phosphorylation of Aux/IAA proteins is proposed to provide one molecular mechanism for integrating auxin and light signaling in plant development.

Arabidopsis↗

Aux/IAA proteins contain a potent transcriptional repression domain.

Aux/IAA proteins are short-lived nuclear proteins that repress expression of primary/early auxin response genes in protoplast transfection assays. Repression is thought to result from Aux/IAA proteins dimerizing with auxin response factor (ARF) transcriptional activators that reside on auxin-responsive promoter elements, referred to as AuxREs. Most Aux/IAA proteins contain four conserved domains, designated domains I, II, III, and IV. Domain II and domains III and IV play roles in protein stability and dimerization, respectively. A clear function for domain I had not been established. Results reported here indicate that domain I in Aux/IAA proteins is an active repression domain that is transferable and dominant over activation domains. An LxLxL motif within domain I is important for conferring repression. The dominance of Aux/IAA repression domains over activation domains in ARF transcriptional activators provides a plausible explanation for the repression of auxin response genes via ARF-Aux/IAA dimerization on auxin-responsive promoters.

Amino Acid Sequence↗

Changes in auxin response from mutations in an AUX/IAA gene.

Transcription of the AUX/IAA family of genes is rapidly induced by the plant hormone auxin, but evidence that AUX/IAA genes mediate further responses to auxin has been elusive. Changes in diverse auxin responses result from mutations in the Arabidopsis AXR3 gene. AXR3 was shown to be a member of the AUX/IAA family, providing direct evidence that AUX/IAA genes are central in auxin signaling. Molecular characterization of axr3 gain-of-function and loss-of-function mutations established the functional importance of domains conserved among AUX/IAA proteins.

Amino Acid Sequence↗

Biochemical characterization of recombinant polypeptides corresponding to the predicted betaalphaalpha fold in Aux/IAA proteins.

The plant hormone indoleacetic acid (IAA or auxin) transcriptionally activates a select set of early genes. The Aux/IAA class of early auxin-responsive genes encodes a large family of short-lived, nuclear proteins. Aux/IAA polypeptides homo- and heterodimerize, and interact with auxin-response transcription factors (ARFs) via C-terminal regions conserved in both protein families. This shared region contains a predicted betaalphaalpha motif similar to the prokaryotic beta-ribbon DNA binding domain, which mediates both protein dimerization and DNA recognition. Here, we show by circular dichroism spectroscopy and by chemical cross-linking experiments that recombinant peptides corresponding to the predicted betaalphaalpha region of three Aux/IAA proteins from Arabidopsis thaliana contain substantial alpha-helical secondary structure and undergo homo- and heterotypic interactions in vitro. Our results indicate a similar biochemical function of the plant betaalphaalpha domain and suggest that the betaalphaalpha fold plays an important role in mediating combinatorial interactions of Aux/IAA and ARF proteins to specifically regulate secondary gene expression in response to auxin.

Amino Acid Sequence↗

OsIAA1, an Aux/IAA cDNA from rice, and changes in its expression as influenced by auxin and light.

The Aux/IAA class of genes are rapidly induced by exogenous auxins and have been characterized extensively from many dicot species like Arabidopsis, Glycine max and Pisum sativum. We report here the isolation and characterization of rice (Oryza sativa L. subsp. Indica) OsIAA1 cDNA as a monocot member of the Aux/IAA gene family. The predicted amino acid sequence of OsIAA1 corresponds to a protein of ca. 26 kDa, which harbors all four characteristic domains known to be conserved in Aux/IAA proteins. The conservation of these Aux/IAA genes indicates that auxins have essentially a similar mode of action in monocots and dicots. Northern blot analysis revealed that the OsIAA1 transcript levels decrease in the excised coleoptile segments on auxin starvation, and the level is restored when auxin is supplemented; the increase in OsIAA1 transcript level was apparent within 15 to 30 min of auxin application. Auxin-induced OsIAA1 expression appears to be correlated with the elongation of excised coleoptile segments. In light-grown rice seedlings, OsIAA1 is preferentially expressed in roots and basal segment of the seedling, whereas in the etiolated rice seedlings, the OsIAA1 transcripts are most abundant in the coleoptile. A comparative analysis in light- and dark-grown seedling tissues indicates that the OsIAA1 transcript levels decrease on illumination.

Amino Acid Sequence↗

Aux/IAA gene family is conserved in the gymnosperm, loblolly pine (Pinus taeda).

We isolated five members of the Aux/IAA gene family in loblolly pine (Pinus taeda L.). Degenerate primers complementary to conserved regions of angiosperm Aux/IAA genes were used to amplify fragments that were, in turn, used as probes to screen a cDNA library constructed from auxin-treated hypocotyls. The five unique clones, named PTIAA1-5, contain the four highly conserved domains that are characteristic of the Aux/IAA proteins. All clones contain the bipartite nuclear localization signal (NLS) between Domains I and II that is predicted in most angiosperm Aux/IAA genes, but only one, PTIAA2, contains the conserved NLS in Domain IV. The five invariant residues in Domain II that have been found to constitute part of a protein destabilization element in Arabidopsis thaliana (L.) Heynh. are conserved in all the PTIAAs. A postulated phosphorylation site located between Domains I and II and proximal to the conserved bipartite NLS was conserved in 20 out of 36 genes in this analysis, including the pine genes. Transcripts of all five PTIAAs accumulated specifically in the hypocotyls in response to exogenous auxin treatment and were induced by all auxins tested. Transcript abundance above basal levels in response to 1-naphthaleneacetic acid treatment was first detected after 10 min (PTIAA3) to 3 h (PTIAA2) in the different genes and remained above basal levels throughout 7 days. Induction of PTIAA2 was inhibited by the protein synthesis inhibitor cycloheximide, indicating that PTIAA2 is a secondary response gene. Phylogenetic analysis showed that all five pine genes clustered within a single class (Class I) of the dendrogram. Clone PTIAA2 has a sequence that is relatively distinct from the other four and is the most closely related to the angiosperm genes of Class I. Class I contains both primary and secondary auxin response genes, suggesting that it is the original lineage and that other gene classes have evolved subsequent to the angiosperm/gymnosperm divergence.

Amino Acid Sequence↗

Differential accumulation of Aux/IAA mRNA during seedling development and gravity response in cucumber (Cucumis sativus L.).

The plant hormone auxin transcriptionally activates Aux/IAA genes. We have isolated three Aux/IAA cDNA from cucumber, two cDNAs (CS-IAA1 and CS-IAA2) containing the complete open reading frame (ORF), and one partial cDNA (CS-IAA3). Northern blotting analysis showed that Aux/IAA mRNAs were induced during the emergence of radicles from seed coats. After radicle emergence, their mRNAs accumulated in the basal part of the hypocotyl much more than in the apical part, and later in elongating region of hypocotyls. CS-IAAI and CS-IAA3 mRNA significantly accumulated in response to auxin, although the increment of the former mRNA accumulation by auxin application was much greater than that of the latter. CS-IAA2 did not show an apparent change by auxin treatment in our experiment. In horizontally germinating seedlings, the transition zone between hypocotyl and root curves was due to downward gravitropic growth. On the other hand, vertically germinating seedlings of cucumber do not curve in the early stage of seedling development. The CS-IAA1 mRNA accumulation in horizontally germinating seedlings was more than that in vertically germinating ones during radicle emergence. Furthermore, asymmetric distribution of CS-IAA1 mRNA was detected in the transition zone in in situ hybridization analysis. These results suggest that the CS-IAA1 gene product may be involved in the gravity response during early development of seedlings.

Amino Acid Sequence↗