SMALL AUXIN UP RNA Genes Research Articles

Characterization of a Small Auxin-Up RNA (SAUR)-Like Gene Involved in Arabidopsis thaliana Development

The root of Arabidopsis thaliana is used as a model system to unravel the molecular nature of cell elongation and its arrest. From a micro-array performed on roots that were treated with aminocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene, a Small auxin-up RNA (SAUR)-like gene was found to be up regulated. As it appeared as the 76th gene in the family, it was named SAUR76. Root and leaf growth of overexpression lines ectopically expressing SAUR76 indicated the possible involvement of the gene in the division process. Using promoter::GUS and GFP lines strong expression was seen in endodermal and pericycle cells at the end of the elongation zone and during several stages of lateral root primordia development. ACC and IAA/NAA were able to induce a strong up regulation of the gene and changed the expression towards cortical and even epidermal cells at the beginning of the elongation zone. Confirmation of this up regulation of expression was delivered using qPCR, which also indicated that the expression quickly returned to normal levels when the inducing IAA-stimulus was removed, a behaviour also seen in other SAUR genes. Furthermore, confocal analysis of protein-GFP fusions localized the protein in the nucleus, cytoplasm and plasma membrane. SAUR76 expression was quantified in several mutants in ethylene and auxin-related pathways, which led to the conclusion that the expression of SAUR76 is mainly regulated by the increase in auxin that results from the addition of ACC, rather than by ACC itself.

옥신 반응 SMALL AUXIN UP RNA 유전자의 최근 연구 동향 및 작물 개량을 위한 적용

Auxin is a key plant hormone which regulates overall plant growth development. A number of researches to investigate auxin signaling identified three major classes of early auxin response genes: AUX/IAA, GH3 and SMALL AUXIN UP RNA (SAUR). Among these genes, in planta functions of SAUR gene family are largely ambiguous, while both AUX/IAA and GH3 genes are analyzed to mediate negative feedback on auxin response. SAUR genes encode small plant-specific proteins. SAUR gene products are highly unstable and transiently expressed in the tissue- and developmental-specific manners in response to auxin and various environmental stimuli. In the decades, molecular and genetic approaches to elucidate in planta functions of SAURs have been hampered by several factors such as the unstable molecular features and functional redundancy among them. However, a series of recent studies focusing on several subgroups of SAUR gene family made significant progress in our understanding of its biochemical and physiological functions. These works suggest that many SAUR proteins mainly regulate auxin-related cell expansion and auxin transport. In this review, the recent progress in SAUR research and prospects for crop improvement through its genetic manipulation are discussed.

Tissue-Specific Expression of SMALL AUXIN UP RNA41 Differentially Regulates Cell Expansion and Root Meristem Patterning in Arabidopsis

Among the three primary auxin-induced gene families, Auxin/Indole-3-Acetic Acid (Aux/IAA), Gretchen Hagen3 (GH3) and SMALL AUXIN UP RNA (SAUR), the function of SAUR genes remains unclear. Arabidopsis SAUR genes have been phylogenetically classified into three clades. Recent work has suggested that SAUR19 (clade II) and SAUR63 (clade I) promote cell expansion through the modulation of auxin transport. Herein, we present our work on SAUR41, a clade III SAUR gene with a distinctive expression pattern in root meristems. SAUR41 was normally expressed in the quiescent center and cortex/endodermis initials; upon auxin stimulation, the expression was provoked in the endodermal layer. During lateral root development, SAUR41 was expressed in prospective stem cell niches of lateral root primordia and in expanding endodermal cells surrounding the primordia. SAUR41-EGFP (enhanced green fluorescent protein) fusion proteins localized to the cytoplasm. Overexpression of SAUR41 from the Cauliflower mosaic virus 35S promoter led to pleiotropic auxin-related phenotypes, including long hypocotyls, increased vegetative biomass and lateral root development, expanded petals and twisted inflorescence stems. Ectopic SAUR41 proteins were able to promote auxin transport in hypocotyls. Tissue-specific expression of SAUR41 from the PIN1, WOX5, PLT2 and ACR4 promoters induced the formation of new auxin accumulation/signaling peaks above the quiescent centers, whereas tissue-specific expression of SAUR41 from the PIN2 and PLT2 promoters enhanced root gravitropic growth. Cells in the root stem cell niches of these transgenic seedlings were differentially enlarged. The distinctive expression pattern of the SAUR41 gene and the explicit function of SAUR41 proteins implied that further investigations on the loss-of-function phenotypes of this gene in root development and environmental responses are of great interest.

SAUR36, a SMALL AUXIN UP RNA Gene, Is Involved in the Promotion of Leaf Senescence in Arabidopsis

Small Auxin Up RNA genes (SAURs) are early auxin-responsive genes, but whether any of them are involved in leaf senescence is not known. Auxin, on the other hand, has been shown to have a role in leaf senescence. Some of the external application experiments indicated that auxin can inhibit leaf senescence, whereas other experiments indicated that auxin can promote leaf senescence. Here, we report the identification and characterization of an Arabidopsis (Arabidopsis thaliana) leaf senescence-associated gene named SAG201, which is highly up-regulated during leaf senescence and can be induced by 1-naphthaleneacetic acid, a synthetic auxin. It encodes a functionally uncharacterized SAUR that has been annotated as SAUR36. Leaf senescence in transfer DNA insertion saur36 knockout lines was delayed as revealed by analyses of chlorophyll content, F(v)/F(m) ratio (a parameter for photosystem II activity), ion leakage, and the expression of leaf senescence marker genes. In contrast, transgenic Arabidopsis plants overexpressing SAUR36 (without its 3' untranslated region [UTR]) displayed an early leaf senescence phenotype. However, plants overexpressing SAUR36 with its 3' UTR were normal and did not exhibit the early-senescence phenotype. These data suggest that SAUR36 is a positive regulator of leaf senescence and may mediate auxin-induced leaf senescence and that the 3' UTR containing a highly conserved downstream destabilizes the SAUR36 transcripts in young leaves.

The SAUR19 subfamily of SMALL AUXIN UP RNA genes promote cell expansion

The plant hormone auxin controls numerous aspects of plant growth and development by regulating the expression of hundreds of genes. SMALL AUXIN UP RNA (SAUR) genes comprise the largest family of auxin-responsive genes, but their function is unknown. Although prior studies have correlated the expression of some SAUR genes with auxin-mediated cell expansion, genetic evidence implicating SAURs in cell expansion has not been reported. The Arabidopsis SAUR19, SAUR20, SAUR21, SAUR22, SAUR23, and SAUR24 (SAUR19-24) genes encode a subgroup of closely related SAUR proteins. We demonstrate that these SAUR proteins are highly unstable in Arabidopsis. However, the addition of an N-terminal GFP or epitope tag dramatically increases the stability of SAUR proteins. Expression of these stabilized SAUR fusion proteins in Arabidopsis confers numerous auxin-related phenotypes indicative of increased and/or unregulated cell expansion, including increased hypocotyl and leaf size, defective apical hook maintenance, and altered tropic responses. Furthermore, seedlings expressing an artificial microRNA targeting multiple members of the SAUR19-24 subfamily exhibit short hypocotyls and reduced leaf size. Together, these findings demonstrate that SAUR19-24 function as positive effectors of cell expansion. This regulation may be achieved through the modulation of auxin transport, as SAUR gain-of-function and loss-of-function seedlings exhibit increased and reduced basipetal indole-3-acetic acid transport, respectively. Consistent with this possibility, SAUR19-24 proteins predominantly localize to the plasma membrane.

Auxin-responsive SAUR39 gene modulates auxin level in rice

The signaling molecule auxin plays a central role in several aspects of plant growth and developmental processes. Underlying optimization of these processes are complex mechanisms orchestrating the expression of genes involved in controlling auxin level, movement and signalling. The small auxin-up RNA (SAUR) family comprises a large set of genes whose expressions are early auxin-responsive. However, the function of these genes is largely unknown. Loss-of-function mutants in a number of Arabidopsis SAUR genes did not show any marked phenotypic differences to wild-type plants likely due to compensatory functions of conserved members of the SAUR gene family. We have recently shown that a rice SAUR39 gene negatively regulates auxin synthesis and transport in rice. Here we propose a model that constitutive induction of SAUR39 gene expression reduces growth and seed yield in rice plants due to the presence of a lower auxin level, reduced polar auxin transport, less chlorophyll and increased sugar and anthocyanin contents. In wild-type plants, the SAUR39 gene is expressed at a low level and is transiently induced by changes in external auxin or other environmental stimuli, but within hours of this change its expression is reduced to the low constitutive level. This homeostatic mechanism is essential for optimal plant growth and seed yield and its disruption due to the constitutive overexpression of SAUR39 leads to a set of negative pleiotropic phenotypes.

SAUR39, a small auxin-up RNA gene, acts as a negative regulator of auxin synthesis and transport in rice.

The phytohormone auxin plays a critical role for plant growth by regulating the expression of a set of genes. One large auxin-responsive gene family of this type is the small auxin-up RNA (SAUR) genes, although their function is largely unknown. The expression of the rice (Oryza sativa) SAUR39 gene showed rapid induction by transient change in different environmental factors, including auxin, nitrogen, salinity, cytokinin, and anoxia. Transgenic rice plants overexpressing the SAUR39 gene resulted in lower shoot and root growth, altered shoot morphology, smaller vascular tissue, and lower yield compared with wild-type plants. The SAUR39 gene was expressed at higher levels in older leaves, unlike auxin biosynthesis, which occurs largely in the meristematic region. The transgenic plants had a lower auxin level and a reduced polar auxin transport as well as the down-regulation of some putative auxin biosynthesis and transporter genes. Biochemical analysis also revealed that transgenic plants had lower chlorophyll content, higher levels of anthocyanin, abscisic acid, sugar, and starch, and faster leaf senescence compared with wild-type plants at the vegetative stage. Most of these phenomena have been shown to be negatively correlated with auxin level and transport. Transcript profiling revealed that metabolic perturbations in overexpresser plants were largely due to transcriptional changes of genes involved in photosynthesis, senescence, chlorophyll production, anthocyanin accumulation, sugar synthesis, and transport. The lower growth and yield of overexpresser plants was largely recovered by exogenous auxin application. Taken together, the results suggest that SAUR39 acts as a negative regulator for auxin synthesis and transport.

Functional characterization of a small auxin-up RNA gene in apical hook development in Arabidopsis

Auxin functions, at least in part, by regulating a set of early auxin response genes: Aux/ IAAs, GH3s and small auxin-up RNAs ( SAURs). The SAUR genes encode short transcripts that accumulate rapidly after auxin treatment. There are 72 SAUR genes in Arabidopsis. However, none of these genes have been functionally characterized, although some members have been implicated in calcium/calmodulin-mediated auxin responses and etiolated seedling growth. Here, we investigated the function of a SAUR gene, abolished apical hook maintenance 1 ( AAM1), through molecular genetic and transgenic studies. Transgenic Arabidopsis seedlings overexpressing AAM1 exhibited short, hookless hypocotyls with partially open cotyledons in darkness. The hookless phenotype was completely rescued by exogenous auxin. In contrast, hypocotyls of a T-DNA insertion mutant aam1 were slightly longer than wild-type hypocotyls and exhibited normal hook curvature. Notably, AAM1 is predominantly expressed on the inner side of the apical hook, similar to the DR5::GUS reporter. However, on illumination, the AAM1 transcripts were evenly distributed in the apical region, demonstrating that the apical hook development is intimately related to the localized expression of AAM1.

Genome-wide analysis, evolutionary expansion, and expression of early auxin-responsive SAUR gene family in rice ( Oryza sativa)

Small auxin-up RNAs ( SAURs) are the early auxin-responsive genes represented by a large multigene family in plants. Here, we report the identification of 58 OsSAUR gene family members from rice ( Oryza sativa japonica cv Nipponbare), the model monocot plant, by a reiterative database search and manual reannotation; 2 of these are pseudogenes. The coding sequences of OsSAURs do not possess any intron. Most of the predicted OsSAUR protein sequences harbor a putative nuclear localization signal at their N-terminus. Localized gene duplications appear to be the primary genetic event responsible for SAUR gene family expansion in rice. Interestingly, the duplication of OsSAURs was found to be associated with the chromosomal block duplication as well. The phylogenetic analysis revealed that the SAUR gene family expanded in rice and Arabidopsis due to species-specific expansion of the family in monocots and dicots. The auxin-responsive elements and downstream element are conserved in the upstream and downstream sequences, respectively, of OsSAURs. In addition to the 21 OsSAURs with full-length cDNA sequences and 20 with expressed sequence tags, gene expression analyses of at least 7 OsSAURs by RT-qPCR indicated that the majority of identified OsSAURs most likely are expressed in rice. The transcript abundance of the OsSAURs examined increased within a few minutes of exogenous auxin application with varying kinetics. The present study provides basic genomic information for the rice SAUR gene family and will pave the way for deciphering the precise role of SAURs in plant growth and development.

Isolation and Characterization of an Auxin-InducibleSAURGene from Radish Seedlings

A gene homologue to the auxin-inducible SAUR (Small Auxin Up RNAs) of soybean was isolated from a cDNA and a genomic library of radish. The cDNA clone was about 470 bp in length and contained a DST (Down STream) element that was involved in mRNA instability in the 3'-untranslated region. The genomic clone contained two short sequences that related to auxin-responsible core sequences (AGTCTC and TATCCCAC) in its promoter region. The radish SAUR transcript was accumulated by auxin and cycloheximide treatments in the same way as that in other plant species. However, the NDE and DUE elements that were reported in all of the other SAUR gene promoters were not conserved in this radish SAUR gene. The results suggest that the promoter activity of the SAUR gene will be regulated by only two short sequences.

Multiple regions of the Arabidopsis SAUR-AC1 gene control transcript abundance: the 3′ untranslated region functions as an mRNA instability determinant.

The small-auxin-up-RNA (SAUR) transcripts are rapidly induced by auxin and are among the most short-lived mRNAs in higher plants. In this study, we investigate the regulation of SAUR-AC1, a well characterized SAUR gene of Arabidopsis. Be examining the expression of chimeric genes in transgenic tobacco, we demonstrate that the promoter region of SAUR-AC1 mediates auxin induction. Sequences downstream of the promoter region were found to limit mRNA accumulation in a manner that was independent of auxin treatment. Both the coding region and the 3' untranslated region (UTR) of SAUR-AC1 independently contribute to this limitation. Effects on mRNA stability were assayed using chimeric genes under the control of the tetracycline-repressible Top10 promoter. mRNA half-life analysis following tetracycline treatment showed that the SAUR-AC1 coding region does not contain elements that decrease mRNA stability. In contrast, the 3' UTR was found to act as a potent mRNA instability determinant. This finding and the general utility of the Top10 system should provide the means to elucidate mRNA decay pathways that are potentially novel and specific for certain unstable transcripts.

8 Isolation and characterization of radish SAUR (Small Auxin Up RNA) gene

8 Isolation and characterization of radish SAUR (Small Auxin Up RNA) gene

An auxin-inducible element in soybean SAUR promoters.

The soybean SAUR (Small Auxin-Up RNA) genes are transcriptionally induced by exogenous auxins within a few minutes after hormone application. This response is specifically induced by auxins primarily in epidermal and cortical cells within elongation zones of hypocotyls and epicotyls. We have previously shown that an 832-bp soybean SAUR promoter/beta-glucuronidase (GUS) reporter gene fusion is responsive to auxin in transgenic tobacco plants (Y. Li, G. Hagen, T.J. Guilfoyle [1991] Plant Cell 3: 1167-1175). Similar results were obtained with an 868-bp SAUR 15A promoter-GUS reporter gene in transgenic tobacco (Y. Li, unpublished results). We have now analyzed a soybean SAUR 15A promoter in transgenic tobacco plants using 5' unidirectional deletions, internal deletions and mutations, and gain-of-function assays with a minimal cauliflower mosaic virus 35S promoter. Our results indicate that the distal upstream element/NdeI restriction endonuclease site element (NDE) (B.A. McClure, G. Hagen, C.S. Brown, M.A. Gee, T.J. Guilfoyle [1989] Plant Cell 1: 229-239) in the SAUR 15A promoter is necessary and sufficient for auxin induction. Our results also show that the 30-bp NDE portion of this element is responsible for most, if not all, of the auxin inducibility of the SAUR 15A promoter. The NDE contains two adjacent sequences, TGTCTC and GGTCCCAT, which have been previously identified as putative auxin-responsive elements. We propose that these elements might function independently or together, possibly with an additional element(s), to confer auxin inducibility to the SAUR promoters.

Characterization of the auxin-inducible SAUR-AC1 gene for use as a molecular genetic tool in Arabidopsis.

The small auxin up RNA (SAUR) genes were originally characterized in soybean, where they encode a set of unstable transcripts that are rapidly induced by auxin. In this report, the isolation of a SAUR gene, designated SAUR-AC1, from Arabidopsis thaliana (L.) Heynh. ecotype Columbia is described. The promoter of the SAUR-AC1 gene contains putative regulatory motifs conserved among soybean SAUR promoters, as well as sequences implicated in the regulation of other genes in response to auxin. The transcribed region is approximately 500 bp in length and contains no introns. Highly conserved sequences located within the SAUR-AC1 transcript include the central portion of the coding region and a putative mRNA instability sequence (DST) located in the 3' untranslated region. Accumulation of SAUR-AC1 mRNA is readily induced by natural and synthetic auxins and by the translational inhibitor cycloheximide. Moreover, several auxin- and gravity-response mutants of Arabidopsis exhibit decreased accumulation of the SAUR-AC1 mRNA in elongating etiolated seedlings. In particular, in the axr2-1 mutant the SAUR-AC1 transcript accumulates to less than 5% of wild-type levels. These studies indicate that SAUR-AC1 will be a useful probe of auxin-induced gene expression in Arabidopsis and will facilitate the functional analysis of both transcriptional and posttranscriptional regulatory elements.

Transgenic tobacco plants that overproduce cytokinins show increased tolerance to exogenous auxin and auxin transport inhibitors

Transgenic tobacco plants expressing the Agrobacterium tumefaciens cytokinin biosynthetic ipt gene under the control of an auxin-inducible SAUR (Small Auxin-Up RNA) gene promoter were used to study interactions between exogenously applied auxins or auxin transport inhibitors and endogenously produced cytokinins. The transgenic plants used in this study had cytokinin levels about 10-fold higher than non-transformed tobacco plants. In aseptic culture, the transgenic tobacco plants exhibited increased tolerance to the toxic effects of high concentrations of exogenously applied auxins. This tolerance is exemplified by increased plant height and fresh weight in transgenic plants treated with auxin compared to similarly treated non-transformed plants. In addition to increased tolerance to exogenous auxins, the transgenic plants showed increased tolerance to applied auxin transport inhibitors.