Os11g0523800

The rice Os11g0523800 was reported as OsARF1 in 2002 and 2008 by researchers from Japan and Egypt respectively.

Annotated Information

Figure 1. Mutant VS. WT(from reference) .

Figure 2. Transcript abundance of OsARF1 in wild type rice from .

Figure 3. Auxin concentration dependence of OsARF1 expression..

Function

OsARF1 is the first full-length member of auxin response factor (ARF) gene family to be cloned from monocot plant. It was shown to be essential for developmental growth stages of the life cycle of rice. The knock down of OsARF1 turned off some key switches for transformation from vegetative stage to the reproductive stage. As an early response gene in auxin signaling, OsARF1 could be associated with embryogenesis and may play a key role in fertility and regulate quite a few downstream genes associated with growth and reproduction. Further elucidation of the downstream targets of Os-ARF1 will help to understand the regulation of plant growth by auxin-mediated gene expression.

GO assignment(s): GO:0003677, GO:0005634, GO:0006445, GO:0009725, GO:0045449, GO:0046983

Wild Type VS. Mutant

An antisense OsARF1 recombinant plasmid was transformed into rice embryogenic calli through Agrobacterium-mediated transformation by Attia et al and they found that the growth of transgenic rice was inhibited.

• The AS-OsARF1 transformed plants showed significantly lower growth and vigor that included smaller leaves and shorter heights, compared to nontransformed plants. Interestingly, the length of the tillering node didn’t change significantly in transgenic plants (Figure 1). Also, the leaves curled across the width.
  
  
• Eight of 11 transgenic plants failed to head and the remaining three were sterile, although they were able to develop to the heading stage 13–15 days later than the non-transformed plants. Hence, no F2 progeny was obtained.

Expression

• RT-PCR Analyses by Attia et al. showed that OsARF1 was transcript accumulated in callus and young panicle at much higher amount than in leaf and root(Figure 2). This indicates that OsARF1 may be associated with embryogenesis, because of the higher transcript abundance in young panicles and calli, which both are embryonic tissues.(1.embryogenic callus; 2. differentiating callus; 3. young panicles; 4. leaves;5. roots. Lower lanes: Actin mRNAas a control.)
  
  
• The study by Frank Waller et al. showed that OsARF1 mRNA levels are rapidly induced by auxin. They extracted total RNA from coleoptile segments which had been incubated in different auxin concentrations after depletion of internal auxin. Then they measured the abundance of OsARF1 mRNA in northern blot experiments and found that the highest level of OsARF1 transcript was detected after incubation with either 1 or 3 μM auxin, whereas incubation in 100 μM auxin produced low OsARF1 transcript levels. OsARF1 mRNA steady-state levels therefore follow an optimum curve with a maximum of between 1 and 3 μM auxin.
  
  
• Microarray analysis of the transgenic plants by Attia et al. showed that 435 genes were differently expressed, 255 of them were down regulated and 180 were up regulated. The annotated genes were located in 9 sub-categories of molecular functions in gene ontology. Most of the differently expressed genes were categorized among those encoding proteins with catalytic activity.

Localization

The study by Frank Waller et al. showed that OsARF1 was localized to the nucleus in vivo.

Figure 4. Analysis of the nuclear localization signal (NLS) in OsARFs.

A monopartite NLS in the DBD is responsible for nuclear localization of OsARFs.In OsARFs, the first NLS with a bipartite NLS structure is predicted to be in the middle of DBD, and the second, which resembles the monopartite NLS of simian virus 40, is predicted to be at the end of the DBD(DNA binding domain). The fluorescence of the 35S: OsARF19–sGFP fusion protein was observed only in the nucleus of onion epidermal cells, while the control 35S: sGFP fluorescence was observed throughout the entire cell (Fig. 2A, B, positive and negative control). Proteins fused to a bipartite NLS containing element II were detected throughout transformed cells, not only in the nucleus (Fig. 2A, aII–dII). Elements I, III, V, or VI fused to OsARF–sGFP caused expression throughout the entire cell, similar to 35S:sGFP (Fig. 2A, aI–dI and aIII–dIII; 2B, aV–dV and aVI–dVI). The fluorescence of sGFP fused to a monopartite NLS containing element IV accumulated exclusively in the nucleus (Fig. 2B, aIV–dIV). These data indicate that a monopartite NLS in the DBD had the capacity to direct nuclear localization of OsARF.

Evolution

Auxin is one of the two most important plant hormones, and regulates various growth and developmental processes by controlling the expression of auxin-response genes (Ulmasov et al., 1995). Auxin responsiveness is conferred to several genes by conserved promoter elements, termed ‘auxin-responsive elements'(AuxRE) AuxRE promoter elements are bound by a new class of plant-specific transcription factors, named auxin response factors (ARFs) (Ulmasov et al., 1997a). Because of the extremely low expression of genes encoding ARFs, none were isolated until 1997. The first auxin response factor (ARF1) was isolated from Arabidopsis (Ulmasov, et al., 1997a). It was found that all members of the ARF gene family have an amino-terminal DNA-binding domain and most contain a C-terminal region with two conserved domains, which are involved in homo-and hetero-dimerization (Ulmasov et al., 1999). In Arabidopsis, it has been reported that the ARF proteins are encoded by a gene family with 23 members and some of them have been shown to repress or to activate expression of reporter genes with an AuxRE promoter element (Ulmasov et al., 1999a; Remington et al., 2004; Okushima et al., 2005). Recently, the first full-length ARF gene of a monocot plant was cloned from rice (Frank et al., 2002). Several rice ARF family transcriptional regulators were also identified homologous to Arabidopsis ARF1 (Waller et al., 2002). From genome sequences many ARF genes in Arabidopsis and in rice have been isolated (Akila et al., 2004; Wang et al., 2007).

Knowledge Extension

Figure 5. The modular structure of the OsARF family.(from reference) .

• Auxin signaling plays a vital role in plant growth and development processes like, in apical dominance, tropic responses, lateral root formation, vascular differentiation, embryo patterning and shoot elongation. The auxin response factor (ARF) and auxin/indole acetic acid (Aux/IAA) protein families are required for transcriptional regulation of auxin response genes, and they are very important in auxin signalling and plant development.
  
  
• An ARF protein contains a DNA-binding domain (DBD) in the N-terminal region, a middle region that functions as an activation domain (AD) or repression domain (RD), and a carboxyl-terminal dimerization domain (CTD) that are similar to those found in the C terminus of Aux/IAAs, which is a protein-protein interaction domain that mediates the homo- and heterodimerization of ARFs and also the hetero-dimerization of ARF and Aux/IAA proteins (Figure 5).
  
  
• The ARF proteins are encoded by a large gene family, with 23, 25 and 31 members in Arabidopsis, Rice and Maize respectively. Genetic divergence between Arabidopsis and rice ARF gene family investigated by genome-wide analysis revealed that most of the rice OsARFs and maize ZmARFs are related to Arabidopsis ARFs and fall into sister pairs as in Arabidopsis.

Labs working on this gene

• Institut für Biologie II, Albert-Ludwigs-Universität, Schänzlestrasse 1, 79104 Freiburg
• Hitachi Advanced Research Laboratory, Hatoyama, Saitama 350-0395
• Present address: Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan
• Rice Biotechnology Lab., Rice Research & Training Center, Sakha, Kafr EL-Sheikh, 33717, Egypt
• Institute of Genetic Engineering, School of Life Science, Fudan University, Shanghai, 200433, China
• Plant Biotechnology and Genomics Core-Facility, Department of Plant, Soil, and Agricultural Systems,Southern Illinois University, Carbondale, IL 62901–4415, USA
• Department of Genetics, Faculty of Agriculture, Alexandria University, Alexandria, Egypt
• Faculty of Agriculture Research Park (FARP) and Biochemistry Department, Faculty of Agriculture, Cairo University,12613 Giza, Egypt