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BioMed Research International
Volume 2014 (2014), Article ID 465739, 13 pages
http://dx.doi.org/10.1155/2014/465739
Research Article

Basal Transcription Factor 3 Plays an Important Role in Seed Germination and Seedling Growth of Rice

1Institute of Crop Science, College of Agriculture and Biotechnology, Zhejiang University, Yu-Hang-Tang Road 388, Hangzhou 310058, China
2Department of Biotechnology and Genetic Engineering, Kohat University of Science and Technology, Kohat 26000, Pakistan

Received 30 January 2014; Revised 14 April 2014; Accepted 28 April 2014; Published 29 May 2014

Academic Editor: Henry Wong

Copyright © 2014 Wenyi Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

BTF3 has been recognized to be involved in plant growth and development. But its function remains mostly unknown during seed germination and seedling stage. Here, we have analyzed OsBTF3-related sequences in Oryza sativa L. subspecies, japonica, which resembles with the conserved domain of a nascent polypeptide associated complex (NAC) with different homologs of OsBTF3 and human BTF3. Inhibition of Osj10gBTF3 has led to considerable morphological changes during seed germination and seedling growth. Germination percentage was not influenced by the application of GA3, ABA, and NaCl but all concentrations caused wild-type (WT) seeds to germinate more rapidly than the RNAi (Osj10gBTF3Ri) transgenic lines. Seedling inhibition was more severe in the Osj10gBTF3Ri seedlings compared with their WT especially when treated with 100 or 200 M GA3; 50% reduction in shoots was observed in Osj10gBTF3Ri seedlings. The expression of Osj3g1BTF3, Osj3g2BTF3 and Osj10gBTF3 was primarily constitutive and generally modulated by NaCl, ABA, and GA3 stresses in both Osj10gBTF3Ri lines and WT at the early seedling stage, suggesting that Osj3g1BTF3 and Osj10gBTF3 are much similar but different from Osj3g2BTF3 in biological function. These results show that OsBTF3 plays an important role in seed germination and seedling growth gives a new perception demonstrating that more multifaceted regulatory functions are linked with BTF3 in plants.

1. Introduction

Different transcription factors (TFs) play an important role in multiple physiological mechanisms such as cell cycle progression, metabolism, growth, development, and reproduction [1, 2]. Among these TFs, the basal transcription factors 3 (BTF3) is one of the extremely important transcription factors due to their role in various biotic and abiotic stress processes [36] and different physiological and developmental mechanisms such as ionic homeostasis [7], photosynthetic rate [8], and pollen development [6] in plants.

BTF3 was basically found in HeLa cell that is essential for RNA polymerase II dependent transcription [9]. The BTF3 codes two isoforms, BTF3a and BTF3b, in the human, as an outcome of alternative splicing. BTF3a encodes for a protein with complete characteristics of BTF3 and stimulates transcription, while BTF3b encodes a shortened form of BTF3a, as it is missing the first 44 amino-terminal extension. Afterward, BTF3b was documented as a component of the nascent polypeptide associated complex (NAC). It consists of two subunits, that is, αNAC and βNAC [10]. BTF3 is a β-subunit of the NAC that has been participating in regulating protein localization during translation [11].

BTF3 in plant lacks the N-terminals which exist in the human BTF3a form. But, the N-terminals extension of the plant proteins interacts with that of the transcriptionally dormant BTF3b isoform [12]. In Arabidopsis, AtBTF3 coincides with the translation initiation factor (iso) 4E (eIFiso4E) and formed the translation initiation complex eIF(iso)4F [12]. NbBTF3 silenced plants show an abnormally developed phenotype in Nicotiana benthamiana. Similarly, in TaBTF3-silenced wheat transgenic plants, Ma et al. [13] demonstrated that the transcripts of the mitochondrial and chloroplast encoded genes were noticeably decreased. In Capsicum annuum, BTF3 gene was observed to play a role in hypersensitive response (HR) cell death and might function as TFs in the nucleus via transcriptional regulation of HR-related gene expression [14].

Recently, in rice, Wang et al. [6] have demonstrated that BTF3 plays important role in plant growth and development. However, the function of different variants of OsBTF3 in seed germination and seedling growth is not yet documented. In the present work, a cDNA sequence of a BTF3-like gene (Osj10gBTF3) was extracted, and its function examined via transgenic strategies to recognize the role it played in seed germination and seedling growth in rice. We also attempt to investigate that how the OsBTF3s were transcriptionally regulated by salt, GA3, and ABA stresses during seed germination and seedling stage in Osj10gBTF3Ri lines and wild type.

2. Materials and Methods

2.1. Isolation of Osj10gBTF3 and Generation RNAi Repression Lines

RNAs extractions, the full length of cDNA synthesis, were carried out using the BTF3 gene-specific primers via procedures as described by Wang et al. [6]. The amplified products of cDNA were cloned into a pMD18-T vector (TaKaRa). RNAi repression vector was constructed as suggested by Wang et al. [6]. The resultant vector was incorporated into the EHA105 (A. tumefaciens strain), which was used to infect rice embryogenic calli raised from mature seeds of Nipponbare to produce transformed calli. Transformants were screened using primers specific for the hygromycin B phosphor transferase gene (Hpt) by PCR amplification as demonstrated by Wang et al. [6].

2.2. Seed Germination and Seedling Growth

Seeds of T1 Osj10gBTF3Ri and wild-type (WT) of Oryza sativa L. subspecies japonica line Nipponbare were germinated on double filter paper in Petri plates treated with 0 (distilled water), 50 and 100 mM NaCl, 100 and 200 μM GA3, and 5 and 10 μM ABA. The seeds were incubated at °C in dark condition in growth chamber. Seed germination was counted at an interval of 12 hours up to 7 days, up to emergence of ~2 mm radical and germination percentage was calculated when there was 50% of seed germination. Seedlings after 15 days were taken and separated into shoots and roots for measuring root and shoot lengths.

2.3. RNA Isolation from Different Tissues

RNA was extracted from the fresh roots and shoots of Osj10gBTF3Ri lines and WT under normal and stress conditions with Trizol reagent (Invitrogen) by following the manufacturer’s instructions. Full length of cDNA synthesis of first-strand was done with the M-MLV first-strand synthesis system.

2.4. Expression Analysis Using Real-Time PCR

To check the expression of Osj3g1BTF3, Osj3g2BTF3, and Osj10gBTF3, real-time quantitative PCR (q-PCR) was used. The target gene names and their primers for the q-PCR are listed in Supplementary Table 2 (see Supplementary Material available online at http://dx.doi.org/10.1155/2014/465739). Q-PCR was done using the method of Wang et al. [6]. The relative expression levels of Osj3g1BTF3, Osj3g2BTF3, and Osj10gBTF3 in root and shoot were calculated using the method [15]. Here, . The (cycle threshold) value was the average value of the three different independent samples for both target and reference genes.

2.5. Statistical Analysis

The data were analyzed by one-way analysis of variance (ANOVA), and mean values were separated by least significant difference (LSD) at the 5% and 1% probability level using Statistical software (Sigmaplot 10.0.). Sequences of different variants of OsBTF3 were aligned by using ClustalX, while MEGA 5.0 was used for construction of phylogenetic tree.

3. Results

3.1. OsBTF3 Sequence and Phylogenetic Tree

The protein sequences of different variants of OsBTF3 were aligned with the related sequences from Arabidopsis (AtBTF3, AEE29647), wheat (TaBTF3, AFV31408), sorghum (SbBTF3, EER93008), maize (ZmBTF3, ACG28870), Ricinus communis (RcBTF3, EEF34688), Capsicumvannum (CaBTF3, ABM55742), Solanum lycopersicum (SlBTF3, NP_001234229), and Nicotiana benthamiana (NbBTF3, ABE01085) (Supplementary Table 1). Protein sequence analysis showed that different variants of OsBTF3 such as Osj10gBTF3 and Osj3g1BTF3 contained a putative mitochondrial matrix targeting sequence (MMTS) (residues 1–15, MNVDKLKKMAGAVRT), a putative nuclear localization signal (NLS) (residues 22–25, RRKK), a conserved NAC domain (residues 33–97), and a putative ER-retention/retrieval signal (ERRS) (residues 161–164, AEEK), while Osj3g2BTF3 lack MMTS or ERRS (Figure 1(a)). It means that these two genes having different genomic loci are either cytosolic or a nucleus proteins. These results show that Osj10gBTF3 and Osj3g1BTF3 are probably differing from Osj3g2BTF3 in subcellular localization and other biological function.

fig1
Figure 1: Amino acid sequence comparison (a) and phylogenetic tree (b) of different variants of OsBTF3. A deduced amino acid sequence of different variants of OsBTF3 and alignment with related sequences of different plants. The residues conserved among the compared sequences are in red colour. The overline (---) indicates a putative nuclear localization signal sequence, while the double dashed line (==) indicates the NAC domain. Gene Bank accession numbers for the proteins in the alignment are as follows: Arabidopsis thaliana BTF3 (AEE29647), Nicotiana benthamiana BTF3 (ABE01085), Oryza sativa 10gBTF3 (NP_001064883), Oryza sativa 3g1BTF3 (NP_001048709), Oryza sativa 3g2BTF3 (NP_001051911), Capsicum annuum BTF3 (ABM55742), Ricinus communis BTF3 (EEF34688), Solanum lycopersicum BTF3 (NP_001234229), Triticum aestivum BTF3 (AFV31408), Zea mays BTF3 (ACG28870), and Sorghum bicolor BTF3 (EER93008). The tree was constructed using MEGA5 software.

Phylogenetic analysis (Figure 1(b)) was performed between different variants of OsBTF3 and BTF3-like members from other plant species. The analysis showed that Osj10gBTF3 is more strongly associated to Osj3g1BTF3, followed by other BTF3 homologs of the plant species. The BTF3 homologs of monocot and dicot species form different clades. Additionally, Osj3g2BTF3 has a closer association to the homologs of dicot species AtBTF3 and RcBTF3 than Osj3g1BTF3 and Osj10gBTF3. These results indicate that there may be functional variation between Osj3g1BTF3, Osj10gBTF3, and Osj3g2BTF3 of BTF3 proteins.

3.2. Inhibition of Osj10gBTF3 Causes Phenotypic Changes

The functional analysis of BTF3 was investigated using repression and overexpression strategies. Different independent Osj10gBTF3 overexpressed lines were produced before the application of the RNAi construct for functional analysis of Osj10gBTF3 using the overexpression constructs. But most of these lines did not show any significant change in phenotype; therefore, we mainly focused on RNAi inhibition analysis. Osj10gBTF3Ri transgenic seeds were investigated for their capability to germinate, mainly in the presence of different concentrations of GA3, ABA, and NaCl. Though the final germination percentage was not affected by the application of GA3, ABA, and NaCl but all the levels of GA3, ABA, and NaCl caused WT seeds to germinate more rapidly than the Osj10gBTF3Ri transgenic lines and their mean germination rate reduced with increased GA3, ABA, and NaCl concentrations (Figure 2). It is worth mentioning here that the decrease in germination rate was more severe in the GA3 treated seeds as compared to ABA and NaCl. These results demonstrate that Osj10gBTF3Ri transgenic lines are more sensitive to GA3 than ABA and NaCl.

fig2
Figure 2: Germination of Osj10gBTF3Ri lines and wild-type (WT) rice seeds treated with different concentrations of GA3, ABA, and NaCl. Germination readings were taken after 4th, 5th, 6th, 7th, and 8th days. Error bars indicate the S.D. (). , (Student’s -test).

The seedling growth of WT and Osj10gBTF3Ri lines showed a different trend of variations in response to GA3, ABA, and salt treatments (Figure 3). The root and shoot lengths decreased in response to increasing concentrations of ABA and NaCl (Figure 4). However, NaCl had a more negative effect on root and shoot than ABA. On the other hand, GA3 did not induce significant changes in root with increasing concentration of GA3; however, a considerable increase in the shoot was observed with the increasing level of GA3. It was worth observing that the repression was more serious in the Osj10gBTF3Ri seedlings compared with their WT. When treated with 100 or 200 μM GA3, 50% reduction in shoots was observed in Osj10gBTF3Ri seedlings compared to their WT (Figure 4). However, shoot length was increased in Osj10gBTF3Ri seedlings when treated with 50 mM NaCl. These results demonstrate that inhibition of BTF3 caused significant phenotypic changes during seed germination and seedling growth; however, these changes were more serious when treated with GA3 as compared to ABA and NaCl.

fig3
Figure 3: Comparison of Osj10gBTF3Ri lines and wild-type (WT) rice seeds grown under various concentrations of GA3, ABA, and NaCl.
fig4
Figure 4: Response of Osj10gBTF3Ri lines and wild-type (WT) seedlings to different concentrations of GA3, ABA, and NaCl after 15 d of germination. Error bars indicate the S.D. (). , (Student’s -test).

To assess whether these phenotypic changes in Osj10gBTF3Ri seedling were caused by RNAi inhibition, real-time quantitative PCR analyses were carried out using the OsBTF3-specific primers (Supplementary Table 2) to check the regulation of different variants of OsBTF3 in root and shoot at seedling stage under ABA (5 and 10 μM), GA3 (100 and 200 μM), and (50 and 100 mM) salt stresses. The ubiquitine mRNA level was used as a positive control.

The results showed that the expression of Osj3g1BTF3 was induced by low and high concentrations of GA3 in root and shoot but was repressed by high concentrations of GA3 in shoot (Figure 5). In contrast, the number of transcripts increased with GA3 in Osj10gBTF3Ri seedling at 100 μM but decreased at higher concentrations. In Osj3g2BTF3, samples treated with 100 μM GA3 had less transcripts compared with controls in root. However, the transcript level significantly increased in root and shoot at 100 and 200 μM GA3 (Figure 5). For Osj10gBTF3Ri lines, the transcript level increased with GA3 concentration both in root and in shoot but the transcript increased by about 50% in shoot and 100% in root treated with 100 μM GA3, respectively, as compared to controls (Figure 5) and the expression of transcript increased with the increasing concentration of GA3 in both root and shoot (Figure 5). In contrast, the transcript level decreased in both root and shoot with GA3 in Osj10gBTF3Ri seedling particularly in root at 200 μM (Figure 5).

fig5
Figure 5: The expression analyses of Osj10gBTF3 (LOC_Os10g34180) and its two homologous genes Osj3g1BTF3 (LOC_Os03g01910) and Osj3g2BTF3 (LOC_Os03g63400) in root and shoot of Osj10gBTF3Ri lines and wild-type (WT) under different concentrations of GA3. The detection was done based on three independent samples and ubiquitine was used as internal control.

For the ABA treatment, the number of Osj3g1BTF3 transcripts increased with increasing level from 5 to 10 μM in the root. However, the level appeared to be significantly decreased in shoot at 5 μM, after which the expression level increased at 10 μM (Figure 6). However, the transcript level decreased in root and shoot of Osj10gBTF3Ri lines after treatment with 5 μM ABA, but the expression level significantly increased in root and shoot at 10 μM ABA (Figure 6). In Osj3g2BTF3, the expression decreased with increasing level of ABA in the root and the transcript level increased significantly in shoot at 10 μM ABA. In Osj10gBTF3Ri seedling, the expression level significantly decreased in root and shoot with increasing ABA concentration (Figure 6). Similarly, in Osj10gBTF3, the expression of transcript increased with increasing concentration of ABA in the root. However, the transcript level did not change significantly in shoot after treatment with ABA. In contrast, the number of transcripts decreased about 50% with ABA in Osj10gBTF3Ri seedling as compared to WT. These results show simply that OsBTF3 expression is mostly constitutive but slightly modulated by ABA in shoot at higher concentrations.

fig6
Figure 6: The expression analyses of Osj10gBTF3 (LOC_Os10g34180) and its two homologous genes Osj3g1BTF3 (LOC_Os03g01910) and Osj3g2BTF3 (LOC_Os03g63400) in root and shoot of Osj10gBTF3Ri lines and wild-type (WT) under different concentrations of ABA. The detection was done based on three independent samples and ubiquitine was used as internal control.

For salt treatment, the expression level of Osj3g1BTF3 increased with increasing level of salt in the root and shoot particularly at 50 mM. In contrast, the transcript level decreased significantly in Osj10gBTF3Ri seedling at 100 mM NaCl (Figure 7), but the level of transcript was dramatically decreased about 80% in root at 50 and 100 mM as compared to control (Figure 7). In Osj3g2BTF3, the transcript level increased in both root and shoot with salt particularly at 50 mM; however, in Osj10gBTF3Ri lines, the level of expression gradually decreased in both root and shoot with increasing concentration as compared with WT (Figure 7). The samples treated with salt significantly upregulated the transcript level of Osj10gBTF3 at 50 mM both in root and in shoot, after which the expression level decreased at 100 mM. In Osj10gBTF3Ri seedling, the number of transcripts decreased mainly in roots with salt as compared to control (Figure 7). These results show that the downregulation of the BTF3 gene in Osj10gBTF3Ri seedling may reflect the compromised protein translation and importing activity under salt stress. For hormone treatment, the induction effect showed by ABA was much more effective than the repression effect caused by GA3. There was a difference between the two hormones, indicating that BTF3 is more sensitive to GA3 than ABA.

fig7
Figure 7: The expression analyses of Osj10gBTF3 (LOC_Os10g34180) and its two homologous genes Osj3g1BTF3 (LOC_Os03g01910) and Osj3g2BTF3 (LOC_Os03g63400) in root and shoot of Osj10gBTF3Ri lines and wild-type (WT) under different concentrations of salt. The detection was done based on three independent samples and ubiquitine was used as internal control.

4. Discussion

Recently, it has been documented that BTF3 plays a vital role in plant growth and development [6]. However, there are no reports available about the role of OsBTF3 and how they are regulated by GA3, ABA, and NaCl during seed germination and seedling growth. In the present study, BTF3-like gene Osj10gBTF3 was cloned and functionally analyzed and the results may help to understand OsBTF3 protein function more comprehensively.

There are numerous BTF3 genes in rice genomes and in other plant species that are highly similar to each other (Figure 1(a)). The amino acid sequences of different variants of OsBTF3 are similar to other plant species excluding variations in the lengths of the exon and intron. We have observed three signal systems which include important MMTS, NLS, and ERRS in Osj3g1BTF3 and Osj10gBTF3, used to target the correct destination of new synthesized proteins [6]. It has been reported that these specific addressing systems such as MMTS [16], NLS [17], and ERRS [18] have been used by the new synthesized proteins to specifically target the correct destination [19]. A NLS “RRKKK” is putatively positioned between amino acid residues 22–26 in the N-terminal (Figure 5(a)), demonstrating that the OsBTF3 protein might be located in both cytoplasmic membrane system and nucleus [6]. It is well established that BTF3 is not only a cytoplasmic membrane protein that functions in the targeting and translocation of nascent polypeptides [20, 21] but may also play an important role in the nucleus, affecting the transcription levels of some genes in rice [2224].

It was observed that Osj10gBTF3 and Osj3g1BTF3 are probably similar but different from Osj3g2BTF3 in biological function. To verify this, BTF3-like gene Osj10gBTF3 was cloned and functionally analyzed during seed germination and seedling growth. In the present study, it was observed that the Osj10gBTF3Ri lines were sensitive to GA3, ABA, and NaCl treatment, particularly to GA3 since their applications did not affect final germination percentage but caused WT seeds to germinate more rapidly than the Osj10gBTF3Ri transgenic lines, and their mean germination rate decreased with increased concentrations of GA3, ABA, and NaCl. Similarly, seedling growth was inhibited compared with that of WT control, particularly when treated with 100 or 200 uM GA3; 50% reduction in shoots were observed in Osj10gBTF3Ri seedlings (Figure 5). RNAi inhibition of Osj10gBTF3 reduced the expression of Osj10gBTF3 transcripts, as a result reducing the manufacturing of the essential amount of NAC complex for normal plant growth. Hence, once the transcript level of Osj10gBTF3 is decreased, it disturbed the function of this monomeric subunit plus the complete NAC, resulting in severe vegetative defects in Osj10gBTF3Ri lines [6]. It has been reported that the repression of Osj10gBTF3 caused serious plant growth and development defects [6]. These results are similar to some other studies demonstrating that BTF3 take part in cell cycle regulation via interaction with cyclins [25]. Mutants lacking NAC also showed visible growth and morphological problem in yeast and tobacco [20, 26]. Furthermore, the inhibition of BTF3 participated in the repression of transcription and protein synthesis in apoptotic K562 cells [27], while change in BTF3 expression is linked with apoptosis in BL60 Burkitt lymphoma cells [28]. On the basis of these results, we can conclude that BTF3 plays an important role in seed germination and seedling growth.

To further verify these results, we observed expression of different variants of OsBTF3 in root and shoot in Osj10gBTF3Ri and WT seedlings. We observed that the expression of different variants of OsBTF3 was primarily constitutive, being generally modulated by different GA3, ABA, and salt stress treatments in Osj10gBTF3Ri and WT seedlings (Figures 5, 6, and  7). The expression of Osj3g1BTF3 and Osj10gBTF3 was induced by low and high concentrations of GA3 and ABA in root and shoot (Figures 5 and 6), but the transcript level decreased in shoot at high concentrations (Figures 5 and 6). In Osj10gBTF3Ri seedling, the transcript level increased both in root and in shoot with increasing GA3 and ABA concentration, whereas the transcript level of Osj10gBTF3 decreased with increasing GA3 concentration (Figures 5 and 6). In Osj3g2BTF3, seedling treated with 100 μM GA3 had less transcripts, but the level significantly increases in root at 100 μM GA3 (Figure 5), whereas, in Osj10gBTF3Ri lines, the transcript level increased both in root and shoot with GA3 concentration (Figure 5). In Osj3g2BTF3 the transcript levels decreased with increasing level of ABA in the root but increased in shoot at 10 μM ABA. In Osj10gBTF3Ri seedling, the expression level significantly decreased in root and shoot with increasing ABA concentration (Figure 6). Wang et al. [6] observed a 62% increase in Osj10gBTF3 expression level with ABA, compared to a 22% reduction with GA3 after treatment with the same concentration for 12 h. In previous reports, it has been observed that the transcript level of NbBTF3 remained unaffected when exposed to 100 μM of ABA and GA3 [26]. Similarly, He et al. [29] did not find any significant change in SmBTF3, when treated with ABA for 24 hrs. By using 100 μM of ABA to treat maize (Zea mays L.) plants, Zhang et al. [30] found that ZmBTF3 (Zea mays BTF3) expressions were significantly decreased. The salt concentration upregulated Osj3g1BTF3, Osj3g2BTF3, and Osj10gBTF3 expression at 50 mM in both root and shoot, but, in Osj10gBTF3Ri seedling, the number of transcripts decreased mainly in roots with salt as compared to control (Figure 7). Wang et al. [6] have observed that the expression of Osj10gBTF3 was downregulated under high salt concentration, while low salt concentrations upregulated Osj10gBTF3 expression in rice. Similarly, Li et al. [4] demonstrated that transcript level of OsBTF3 was significantly reduced in rice plants under salt stress and the salt resistance was improved in the seedlings of OsBTF3 transgenic lines. In maize, Zhang et al. [30] observed that expression of BTF3 was downregulated at 250 mM NaCl. In contrast, BTF3 transcript level was significantly increased by salt stress in Suaeda asparagoides [31], while expression level of SabBTF3 was differentially regulated by various abiotic stresses, such as drought, salinity, ABA, and temperature in roots [7]. These results also suggested that Osj3g1BTF3 and Osj10gBTF3 are much similar in their response to GA3, ABA, and NaCl during seedling stage.

This is a first report demonstrating the involvement of different variants of BTF3 in seed germination and seedling stage. It was observed that Osj3g1BTF3 and Osj10gBTF3 are a similar type of genes which plays an important role in plant growth, especially in seed germination and seedling growth. Functional analysis revealed that both of these genes are much similar in biological functions. Further investigation is required to provide the insight into the functions of BTF3 and the connected mechanisms responsible for improving abiotic stress tolerance in plants.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Authors’ Contribution

Wenyi Wang and Muhammad Jamil contributed equally to this work.

Acknowledgment

This work was financially supported by the China Postdoctoral Science Foundation (Grant no. 2013M541788).

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