Cosuppression of the chloroplast localized molecular chaperone HSP90.5 impairs plant development and chloroplast biogenesis in Arabidopsis
© Oh et al.; licensee BioMed Central Ltd. 2014
Received: 14 January 2014
Accepted: 11 September 2014
Published: 13 September 2014
HSP90.5 is a chloroplast localized HSP90 family molecular chaperone in Arabidopsis, and it has been implicated in plant abiotic stress resistance, photomorphogenesis and nuclear-encoded protein import into the chloroplast. However, how these processes are controlled by HSP90 is not well understood. To understand the role of HSP90.5 in chloroplast function and biogenesis, in this study, we generated transgenic Arabidopsis plants that overexpress a C-terminally FLAG-tagged HSP90.5. By characterizing three HSP90.5 cosuppression lines, we demonstrated the essential role of HSP90.5 in plant growth and chloroplast biogenesis.
Immunoblotting and quantitative PCR analyses revealed three independent HSP90.5 cosuppressing transgenic lines. All three cosuppression lines displayed a certain degree of variegated phenotype in photosynthetic tissues, and the cosuppression did not affect the expression of cytosolic HSP90 isoforms. HSP90.5 cosuppression was shown to be developmentally regulated and occurred mostly at late developmental stage in adult leaves and inflorescence tissues. HSP90.5 cosuppression also caused significantly reduced rosette leaf growth, transient starch storage, but did not affect rosette leaf initiation or inflorescence production, although the fertility was reduced. Isolation of chloroplasts and size exclusion chromatography analysis indicated that the FLAG at the HSP90.5 C-terminus does not affect its proper chloroplast localization and dimerization. Finally, transmission electron microscopy indicated that chloroplast development in HSP90.5 cosuppression leaves was significantly impaired and the integrity of chloroplast is highly correlated to the expression level of HSP90.5.
We thoroughly characterized three HSP90.5 cosuppression lines, and demonstrated that properly controlled expression of HSP90.5 is required for plant growth and development in many tissues, and especially essential for chloroplast thylakoid formation. Since the homozygote of HSP90.5 knockout mutant is embryonically lethal, this study provides transgenic lines that mimic the conditional knockout line or siRNA line of the essential HSP90.5 gene in Arabidopsis.
In higher plants, non-photosynthetic proplastids divide and differentiate into specialized plastids in response to developmental and environmental cues[1–3]. Chloroplasts, the photosynthetic plastids, exist in all green tissues and are derived either directly from proplastids in the meristems or from dark-grown intermediates known as etioplasts. In photosynthetic plant cells, chloroplasts also undergo division via a mechanism similar to that used by cyanobacteria, from which the chloroplast originated by symbiosis[4–6]. Chloroplasts facilitate the absorption of solar energy and fix inorganic carbon into organic molecules, thus providing food for all animals.
Different from cyanobacteria genomes which generally encode several thousand proteins, the chloroplast genome contains only 60–200 genes. Chloroplasts in green plant, however, contain several thousand proteins and as many as 95% of them are encoded by nuclear genes and imported from the cytoplasm where they are synthesized. Protein import into the chloroplast is mediated by the Toc/Tic complexes residing in the outer and inner chloroplast membranes[9, 10]. Since proteins are mainly in non-native states during import, refolding is required after translocation. To ensure the proper protein folding and turnover of these proteins as well as newly synthesized ones from its own de novo protein synthesis machinery, a comprehensive protein quality control system exists in the chloroplast. This complex system includes chaperone families of HSP40, HSP60, HSP70, HSP90 and HSP100, and proteases ClpP, FtsH, DegP, and SPP[11, 12]. Malfunction of protein quality control components have been shown to impair chloroplast function and plant development[13–15].
HSP90 is a molecular chaperone that has been implicated in playing roles at the late stage of de novo protein folding. The HSP90 orthologues have been identified within all known plant chloroplasts and prokaryotic photosynthetic bacteria, and they belong to the HSP90C subfamily[17, 18]. However, since a previous analysis of Arabidopsis genome identified seven HSP90 isoforms, and the chloroplast HSP90 was termed as HSP90.5, we hereafter specifically refer to Arabidopsis chloroplast HSP90 as HSP90.5. HSP90 is composed of three highly conserved domains, an N-terminal ATP-binding domain, a middle domain which has been implicated in binding client proteins, and a C-terminal dimerization domain which helps HSP90 form a homodimer (see reviews by[20–22]). All known eukaryotic cytosolic HSP90s contain a MEEVD motif at the C-terminus that is essential for the binding of proteins with tetratricopeptide repeat (TPR) domains[23, 24], such as cochaperones HOP/Sti1, Cpn6, and Cpn7 which modulate cytosolic HSP90 function[25, 26]. However, this MEEVD pentapeptide motif is lacking in prokaryotic HSP90 isoforms, chloroplast, mitochondrion, and ER-localized HSP90 orthologues. Therefore, the cochaperones that modulate cytosolic HSP90 activity are likely not all conserved in endosymbiont originated organelles.
While it is absolutely required for eukaryotic cells and is generally encoded by a multiple-gene family in higher organisms[19, 27], HSP90 is not essential for prokaryotic cells. HtpG, the prokaryotic HSP90 family member, is absent in many bacteria and Archaea. In E. coli, HtpG is dispensable and knockout of HtpG only causes slight temperature sensitivity. It is generally believed that the role of HSP90 in prokaryotic cells could be complemented by other chaperones such as the HSP60 family chaperonins.
Nevertheless, HSP90 orthologue in chloroplast is indispensable for eukaryotic photosynthetically active cells. In C. reinhardii, a unicellular eukaryote, chloroplast localized HSP90C was identified to form a four-protein ‘foldosome’ with HSP70B, CDJ1 and CGE1. HSP90C is strongly induced by heat shock, implying a role in stress management. It was proposed that CDJ1 may bring substrate proteins to the HSP90C-HSP70B complex, and the functional cycle of HSP90C, if there is one, likely mimics that of cytosolic HSP90, which requires cytosolic HSP70, HOP, p23/Sba1, and other cochaperones. A chlorate-resistant mutant, cr88, in Arabidopsis has been shown to result from a point mutation (G646R) in the chloroplast localized HSP90.5. cr88 mutant plants have impaired gene expression for photosynthesis associated genes such as nitrate reductase 2 (NR2) and chlorophyll a/b binding protein (CAB). The mutant plants also display long hypocotyls in red light, suggesting its role in the regulation of photomorphogenesis. HSP90.5 gene has been shown to be constitutively expressed in young plants while its mRNA level is almost undetectable in mature plants. In a recent study by using isolated chloroplasts, HSP90.5 was reported to interact with Tic110, a component of Tic, and cpHSP70, suggesting a role of HSP90.5 in facilitating the import of nuclear encoded proteins into chloroplast and likely in the formation of a foldosome in high plant chloroplast.
In an effort to understand the role of HSP90 in plant abiotic stress resistance, we previously generated transgenic Arabidopsis plants that overexpress wild type HSP90.5 and showed that overexpression of HSP90.5 reduces plant tolerance to salt and drought stresses. HSP90.5 overexpressing plants also have impaired resistance to oxidative stress. To further understand the function of HSP90.5 in chloroplast biogenesis and function, in this study, we generated new transgenic Arabidopsis lines that express a C-terminal FLAG-tagged HSP90.5, in which the expression of endogenous and transgenic genes could be differentially monitored. In addition to HSP90.5 overexpression lines, three independent transgenic lines that have their endogenous and transgenic HSP90.5 expressions cosuppressed at certain development stages were identified. It should be noted that during the revision of this manuscript, Li and colleagues published a work showing that HSP90.5 was also able to be cosuppressed by transforming an untagged HSP90.5 into Arabidopsis, and that HSP90.5 may be involved in the disassembly of VIPP1. Since analysis of HSP90.5 knockout in Arabidopsis suggested that it is an essential gene and knockout of the gene is embryonically lethal, the cosuppression lines obtained in this study mimics a partial loss of function mutant. Our analysis of the cosuppression lines indicated that properly controlled expression of HSP90.5 is important for plant growth and chloroplast biogenesis.
Some transgenic plants expressing FLAG-tagged HSP90.5 display variegated phenotype in photosynthetic tissues
Segregation of the three HSP90.5 cosuppression lines in T2 and T3 generations
Variegated plant (%)b
Variegated phenotype is associated with HSP90.5 cosuppression
For some juvenile leaves developing albino tissues, the leaf apex remained green (Figure 2, A and B). To examine whether HSP90.5 is differentially suppressed within a leaf, we cut a typical variegated leaf into two regions, the proximal albino/yellow region and the distal green region. HSP90.5 was expressed well in both regions of wild type leaves, however, at a much higher level in the distal region than in the proximal region where albino tissues appeared in cosuppression lines (Figure 5C). This result supports the hypothesis that variegated phenotype is induced by HSP90.5 cosuppression. Since albino tissues also appeared in other organs such as inflorescence stems and sepals (Figure 3), we examined the expression of HSP90.5 in these tissues. As shown for no. 57 variegated plants, albino flower buds and inflorescence stems had reduced HSP90.5 expression compared to wild type (Figure 5D). Interestingly, HSP90.5 was well expressed in roots of wild type plants, though there is no photosynthesis activity in these tissues, and the expression of HSP90.5 in the roots of variegated plants was noted to be significantly reduced (Figure 5D), suggesting that HSP90.5 cosuppression likely also occurred in root tissues.
FLAG-tag at HSP90.5 C-terminus does not affect its chloroplast localization and dimerization
To test whether the C-terminal FLAG-tag affects the dimerization of HSP90.5, we expressed and purified both FLAG-tagged and wild type HSP90.5 from E. coli. The purified proteins were subjected to size exclusion chromatography. FLAG-tagged HSP90.5 was eluted with a peak at the molecular mass of about 160kDa, very similar to the elution peak of wild type HSP90.5 (Figure 7B). This indicated that FLAG-tag at the C-terminus did not affect the HSP90.5 dimerization. Additionally, since the C-termini of cytosolic HSP90 in eukaryotes contain extremely conserved MEEVD motifs that are required for the tetratricopeptide repeat domain binding[23, 24], we examined chloroplast HSP90 C-termini that are derived from different species. Alignment of chloroplast HSP90 homologues from green algae, moss, Selaginella, Arabidopsis, rice, maize, soybean, and Populus indicated that the C-terminal amino acids of chloroplast HSP90 are not absolutely conserved (Additional file1: Figure S4). Combined with its proper chloroplast localization (Figure 7A), and the fact that high level expression of FLAG-tagged HSP90.5 in the heterozygotes of lines no. 3 and no. 57 (Figures 4,5A) did not affect the plant growth or show variegated phenotype, it is unlikely that the FLAG-tag at C-terminus significantly affects HSP90.5 function in vivo. The variegated phenotype in lines no. 3, 8 and 57 is likely only due to reduced protein expression level.
Cosuppression of HSP90.5 impairs plant growth and chloroplast development
Mis-regulation of molecular chaperone HSP90 expression in Arabidopsis leads to pleitropic effects
In this study, we identified and analyzed three HSP90.5 cosuppression lines and showed that reduced expression of HSP90.5 impairs chloroplast development (Figure 9) and plant growth (Figure 8B), and induces an abnormal variegated phenotype in some rosette leaves and inflorescence tissues where they are supposed to be green and photosynthetically active (Figures 2A and3). Because of abnormal chloroplast development and function, the transient starch contents in HSP90.5 cosuppression leaves are also significantly reduced (Figure 8, E and F). This study therefore establishes the essential role of the HSP90 family protein in chloroplast biogenesis, and development. It should be noted that the variegated phenotype of HSP90.5 cosuppression lines is different from the delayed greening phenotype observed for the cr88 mutant. The typical cr88 mutant phenotype is the slower greening process than wild type throughout the life span. For HSP90.5 cosuppression plants, the albino tissues mainly appear in the late developmental stage and never develop in the cotyledons, while cr88 seedlings have yellowish cotyledons. In agreement with this phenotype, we did not observe any significant repression of HSP90.5 expression in cotyledons of the variegated lines by immunoblotting (data not shown). More importantly, whenever the variegated phenotype appears in leaves, stems or sepals, it remains and does not become green at the later time.
By immunoblotting analysis, we showed that the appearance of albino tissues is highly correlated to reduced HSP90.5 protein levels (Figure 5). However, this does not indicate that HSP90.5 is not required for non-photosynthetic tissues. Since HSP90.5 is well expressed in root and flower tissues (Figure 5D) of wild type plants, this suggests that molecular chaperone HSP90 may also play a role in the function of other types of plastids such as amyloplasts and chromoplasts. Our results agree with the microarray analysis of HSP90.5 transcript and imply that plastid localized molecular chaperone HSP90 may play a global role for plastid division, differentiation and physiological function, as has been observed for the global roles of cytosolic HSP90 in fungi and mammalian cells[47, 48]. Based on a recent study on HSP90.5 in protein import into chloroplast, and our observation that HSP90.5 is also cosuppressed in the root tissues of line no. 57A plants (Figure 5D), it would be interesting to examine whether and how the development of the other type plastids such as amyloplast in HSP90.5 cosuppressing plants is affected.
Role of HSP90.5 in thylakoid formation and retrograde signaling pathway
Thylakoid membrane formation within the plastid is a hallmark of chloroplast differentiation. The biogenesis of the thylakoid network is a complex process that is poorly understood. While the insertion of photosynthetic proteins into existing thylakoids has been noted, little is known about the formation of the thylakoid membrane itself. Nonetheless, we now know that thylakoid biogenesis requires coordination of protein synthesis, translocation of lipid and pigment synthesis[50, 51]. In Arabidopsis, VIPP1, a vesicle-inducing plastid protein, has been shown to play important roles in thylakoid formation. A vipp1 mutation in Arabidopsis also displays variegated phenotypes with very few internal membranes or only distorted/degraded thylakoids. VIPP1 does not display any features of an intrinsic membrane protein, but is associated with chloroplast membranes, suggesting its role in transport of lipids from the inner membrane to the thylakoids. Studies in C. reinhardti have shown that chloroplast HSP90 forms a complex with HSP70 and VIPP1, implicating HSP90.5 in thylakoid membrane formation. Electron microscopy analysis in our study showed that HSP90.5 cosuppression albino regions in rosette leaves lack thylakoid membranes, and that only partially developed thylakoid membranes exist in the yellowish-green transition region (Figure 9), thus in agreement with previous studies on roles of HSP90.5 and VIPP1 in thylakoid membrane formation[35, 52].
The cr88 mutant developed longer hypocotyls in red light, but not in far-red or blue light, suggesting that HSP90.5 plays a role in photomorphogenesis, likely by affecting phytochrome PhyB signaling but not PhyA signaling. By analyzing cr88cop1 double mutant, it was demonstrated that HSP90.5 likely functions downstream of COP1, a master regulator in photomorphogenesis. Impaired expression of photosynthesis-associated genes in the cr88 mutant supports the role of HSP90.5 in photomorphogenesis. However, it is intriguing that HSP90.5 is localized in plastids while all studied photomorphogenesis-related proteins such as COP1, DET1 and photoreceptors[56, 57] are localized in the cytoplasm and nucleus. Whether the observed effect of cr88 on photomorphogeneis results from an indirect consequence of impaired chloroplast development, or whether cr88 plants directly exerts a retrograde signal to regulate photomorphogenesis-related gene expression is unclear. The role of HtpG, a molecular chaperone HSP90 orthologue, in regulating the activity of uroporphyrinogen decarboxylase and thus regulating tetrapyrrole biosynthesis has been noted in cyanobacteria. A potential retrograde signal, the chlorophyll biosynthesis intermediate Mg-protoporphyrin IX has been shown to bind cytosolic HSP90, chloroplast HSP90.5, and even HSP70 in Arabidopsis. Therefore, it is very likely that reduction of HSP90.5 expression in cosuppression lines directly impairs the retrograde signaling pathway mediated by Mg-protoporphyrin IX. The three HSP90.5 cosuppression lines with distinct variegated phenotype would allow deciphering the role of HSP90 in chlorophyll biosynthesis and in the chloroplast retrograde signaling pathway in the future, in addition to its known role in nuclear encoded protein import into the chloroplast.
In this study, we showed that the plastid-localized HSP90.5 is widely expressed in Arabidopsis plants, and transgene-induced silencing of HSP90.5 significantly impairs plant growth and chloroplast biogenesis in the leaves. By analyzing the differential silencing levels of HSP90.5, we demonstrated that properly controlled expression of HSP90.5 is required for plant growth and development. This study provided further evidence showing the essential role of HSP90.5, and generated transgenic lines that mimic the conditional knockout line of the essential HSP90.5 gene in Arabidopsis, thus allowing us to further investigate the function of HSP90.5 in the future.
FLAG-tagged HSP90.5 gene construction and plant materials
C-terminally FLAG-tagged HSP90.5 coding sequence was amplified by PCR from an HSP90.5 cDNA clone with primers HSP90.5GWFullF and HSP90.5GWFullR (Additional file1: Table S1). The FLAG-tag (DYKDDDDK) coding sequence was in primer HSP90.5GWFullR and the amplified fragment was cloned using gateway cloning strategy and finally cloned into destination vector pGWB402Ω, generating pGWB402Ω-HSP90.5FLAG binary vector, with HSP90.5 controlled by 2xCaMV 35S promoter and NPTII gene for screening of transgenic plants. Arabidopsis thaliana ecotype Columbia-0 (Col-0) was transformed with A. tumefaciens GV3101 carrying pGWB402Ω-HSP90.5FLAG using the floral dip method. All plants were grown within Conviron ATC60 growth chamber under 16/8 hr photoperiod at 110 μmol.m2.sec-1 and 22°C unless specified. To select transgenic plants or test kanamycin resistance, seeds were sterilized, sown onto ½ Murashige and Skoog salt (MS salt, Sigma) agar medium with 1% sucrose and 25 μg/ml kanamycin and grown under 120 μmol.m2.sec-1 at 16/8 hr photoperiod and 22°C in Adaptis 1000 growth incubator.
Protein expression and purification from E. coli and antibodies
The coding region of predicted mature HSP90.5 (D62-D780) was amplified from the cDNA construct and cloned into p11 vector and that of FLAG-tagged mature form was amplified from pGWB402Ω-HSP90.5FLAG and cloned into pProEXHTb, generating expression plasmids p11-HSP90.5 and pProEXHTb-HSP90.562-780FLAG, respectively. Primers used for cloning and the relative positions of primers are shown in Additional file1: Table S1 and Additional file1: Figure S1. N-terminally His6-tagged mature forms of HSP90.5 and HSP90.5FLAG were expressed in and purified from E. coli using Ni-NTA (QIAGEN) according to the manufacturer’s manual. The purified proteins were used either for size exclusion chromatography or for polyclonal antibody production by Signalway AntiBody (College Park, USA). Other primary antibodies used in this study include anti-FLAG monoclonal antibody (Sigma), and anti-HSP90.2 antibody, which was originally raised using purified S. cerevisiae HSP82, but also specifically recognized Arabidopsis cytosolic HSP90.1 and HSP90.2, while not HSP90.5.
Chloroplast isolation and quantification of chlorophyll
The protocol for chloroplast isolation from Arabidopsis rosette leaves was adapted from Kley et al.. Briefly, approximately 10 g of 2-week-old Arabidopsis seedlings were collected and crushed in 1X Clp buffer (330 mM sorbitol, 50 mM HEPES/KOH, pH 7.5, 5 mM EDTA, 5 mM EGTA, 1 mM MgCl2, 10 mM NaHCO3, and 0.5 mM DTT). The homogenate was filtered through two layers of Miracloth (Calbiochem) before it was loaded on the top of 40% Percoll™ layer (made with 1X Clp buffer) and centrifuged at 2,000 × g for 10 minutes. The chloroplasts in the pellets were washed with 10 mL 1X Clp buffer and centrifuged at 1,000 × g for 5 min. The pellet was frozen in liquid nitrogen and stored at -80°C until further use.
To measure chlorophyll contents from whole leaf lysate, the leaves were homogenized in liquid N2 and then suspended in 80% acetone. The solution was centrifuged for 5 minutes at 15,000 g at 4°C. The supernatant was collected and absorptions at 663 nm and 646 nm were measured using Nanodrop 2000c (Thermo Scientific). The total chlorophyll (a + b) level, the chlorophyll a and chlorophyll b contents were estimated according to previously described.
Total soluble protein extraction from fresh leaves and immunoblotting analysis
Leaf or inflorescence tissues were ground in an eppendorf tube using a micropestle with 5× fresh weight of buffer containing 100 mM Tris–HCl, pH7.5, 20 mM EDTA, 100 mM NaCl, 0.1% β-mecaptoethanol, 1× protease inhibitor cocktail (Bioshop). The mixtures were centrifuged for 10 min at 14,000 rpm and the supernatants were collected. Protein concentrations were determined by Bradford reagents (Bio-Rad) and samples were subjected to SDS-PAGE, transferred to nitrocellulose membrane and immunoblotted with specific antibodies.
Size exclusion chromatography
The protein samples were run on a size exclusion column, Superdex™ 200 10/30 (GE Healthcare), with the assistance of ÄKTApurifier 10 (GE Healthcare). The column was pre-equilibrated with running buffer (25 mM Tris–HCl, pH 8.0, 100 mM NaCl, 10% glycerol) and 600 μg of HSP90.562–780 or 200 μg of C-terminally FLAG-tagged HSP90.562–780 was run. Fractions in 0.5 ml were collected and analyzed by SDS-PAGE. Protein standards used are as previously described.
Complementary DNA synthesis and Real-Time PCR analysis
Total RNA was extracted with RNeasy® Mini Kit (QIAGEN) combined with QIAshredder (QIAGEN) from approximately 100 mg (fresh weight) of leaf samples that were frozen in liquid N2 and ground to a fine powder using a micropestle. Superscript first-strand synthesis system (Invitrogen) was used for the synthesis of cDNA from 250 ng of total RNA. cDNA (6.3 ng) was added to iQ SYBRE Green qPCR supermix (Bio-Rad). Forward primer HSP90.5-qF was used in combination with reverse primer HSP90.5-qEndo-R to detect endogenous mRNA, or reverse primer HSP90.5-qTotal-R to detect the total mRNA. The primer sequences and their relative positions in HSP90.5 gene are indicated in Additional file1: Table S1 and Additional file1: Figure S1. The mean Ct (cycle threshold) value of three replicates was normalized against the Ct for ACTIN7, an internal control (Additional file1: Table S1). Data analysis was performed with Bio-Rad Gene Expression Analysis for iCycler iQ® Real-Time PCR Detection System (version 1.10).
Hexose, sucrose and starch analyses
To measure the soluble sugar and transient starch contents in Arabidopsis, fresh rosette leaves were collected at the end of the light cycle and snap frozen in liquid nitrogen followed by lyophilisation for 5 hr. The soluble hexose, sucrose and starch in the rosette leaves were measured following the procedure described by Wang et al..
Transmission electron microscopy (TEM)
Leaves from 23-day-old plants were dissected and prepared for transmission electron microscopy analysis at the Centre for Neurobiology and Stress (CNS), University of Toronto Scarborough (UTSC). TEM sample preparation was performed according to Hyman & Jarvis. Hitachi H7500 Transmission Electron Microscope was used with Olympus SIS Megaview II digital camera. The samples were visualized on iTEM software.
We thank Bhavank Shah for his help in the construction of binary vector and Yao Wang for her help in generation of transgenic plants. This work was supported by an NSERC Discovery Grant 371789–2009 (to RZ).
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