- Research article
- Open Access
AtMND1 is required for homologous pairing during meiosis in Arabidopsis
© Panoli et al; licensee BioMed Central Ltd. 2006
Received: 22 May 2006
Accepted: 27 July 2006
Published: 27 July 2006
Pairing of homologous chromosomes at meiosis is an important requirement for recombination and balanced chromosome segregation among the products of meiotic division. Recombination is initiated by double strand breaks (DSBs) made by Spo11 followed by interaction of DSB sites with a homologous chromosome. This interaction requires the strand exchange proteins Rad51 and Dmc1 that bind to single stranded regions created by resection of ends at the site of DSBs and promote interactions with uncut DNA on the homologous partner. Recombination is also considered to be dependent on factors that stabilize interactions between homologous chromosomes. In budding yeast Hop2 and Mnd1 act as a complex to promote homologous pairing and recombination in conjunction with Rad51 and Dmc1.
We have analyzed the function of the Arabidopsis orthologue of the budding yeast MND1 gene (AtMND1). Loss of AtMND1 did not affect normal vegetative development but caused fragmentation and missegregation of chromosomes in male and female meiosis, formation of inviable gametes, and sterility. Analysis of the Atmnd1 Atspo11-1 double mutant indicated that chromosome fragmentation in Atmnd1 was suppressed by loss of Atspo11-1. Fluorescence in situ hybridization (FISH) analysis showed that homologous pairing failed to occur and homologues remained apart throughout meiosis. AtMND1 showed strong expression in meiocytes as revealed by RNA in situs.
We conclude that AtMND1 is required for homologous pairing and is likely to play a role in the repair of DNA double strand breaks during meiosis in Arabidopsis, thus showing conservation of function with that of MND1 during meiosis in yeast.
The formation of at least one crossover between pairs of homologous chromosomes is necessary for their correct segregation at meiosis I. The stages of interactions between homologous chromosomes that lead to crossover formation have been broadly grouped as: an initial localization of homologous chromosomes within the same region, mediated by interstitial interactions; close pairing and strand exchange at the DNA level as a part of recombination; and synapsis between homologous chromosomes together with completion of recombination .
Recombination at the DNA level in yeast and in other organisms is initiated by double strand breaks (DSBs) made by Spo11 [2, 3]. Interaction between DSBs and a homologous intact chromosome can lead to crossover and noncrossover recombination products which are formed by two different pathways . Processing of DSBs by 5' end resection yields 3' single-stranded ends that asymmetrically invade a homologous chromosome and lead to the formation of a double-Holliday junction intermediate which has been proposed to account for the majority of crossovers [5, 6]. Interaction between homologous chromosomes at the sites of DSBs is promoted by the action of the RecA-like strand exchange proteins Rad51 and Dmc1 [7, 8]. Several lines of evidence suggest that Rad51 and Dmc1 have different but overlapping functions [9, 10] and interact with distinct sets of proteins in promoting recombination [11–13]. Rad51 acts in mitosis and in meiosis  whereas Dmc1 is meiosis specific .
MND 1 was identified in Saccharomyces cerevisiae using three different screens based on genetic and functional genomic approaches that were directed at identifying genes that played a role in meiotic recombination and/or chromosome segregation [16–18]. The mnd1 mutant shows defects in nuclear division, meiotic recombination, and repair of DSBs. Mnd1 has been shown to act as a complex with Hop2 [18, 19] and the Mnd1/Hop2 complex localizes to chromosomes independently of Rad51 and Dmc1 [18, 20]. Genetic studies have provided evidence that Hop2 and Mnd1 act in the same pathway as Dmc1 and Rad51 [17–21]. Biochemical studies using yeast, human, and mouse orthologues have provided evidence that Mnd1/Hop2 stimulates the strand exchange activity of Dmc1 and that of Rad51 [19, 22, 23]. The interaction of Mnd1 with Hop2 has been shown to promote the interaction of Hop2 with Dmc1 and stimulate the strand exchange activity of Dmc1 . Additional roles for Mnd1/Hop2 that have been proposed are in promoting interhomologue associations at DSBs through interaction with the axial elements or other proteins perhaps by relieving structural constraints [18, 20, 25] and in the designation of DSBs for noncrossover recombination .
Orthologues of MND1 have been identified in protists, fungi, plants, and animals and some of these have been characterized and shown to have closely related functions . In yeast an mnd1 disruption has been reported to cause defects only in meiosis and does not result in sensitivity to radiation induced DNA damage . However, an Arabidopsis mutant, Atmnd1-Δ1 has been recently shown to be sensitive to gamma radiation indicative of a role in mitotic repair, and also to undergo chromosome fragmentation during meiosis . Here we have used the same mutant allele to analyze the role of the AtMND1 gene in meiosis. We show that AtMND1 is required for homologous pairing, an early step in the recombination process and that chromosome fragmentation in the Atmnd1 mutant is likely to be due to defective repair of meiotic DSBs. We also show that consistent with its role in meiosis, AtMND1 is strongly expressed in meiocytes.
AtMND1 shows increased expression in reproductive tissues
Mutation of AtMND1 causes male and female sterility due to production of defective gametes
Reciprocal crosses between wild type and Atmnd1 mutant.
No. of seeds per silique
1 +/- 1
58 +/- 5
Female gametophytic defects in the Atmnd1 mutant.
AtMND1 is strongly expressed at meiosis
AtMND1 is required for homologous pairing
Chromosome fragmentation in Atmnd1 is suppressed by a mutation in AtSPO11-1
Pairing and recombination between homologous chromosomes at meiosis relies on search for homology using resected ends that are created at the sites of DSBs. This search is mediated by the action of RecA-like strand exchange proteins Rad51 and Dmc1 which bind to single stranded DNA and promote the formation of joint molecules [7, 8]. The strand exchange activity of Dmc1 and Rad51 is stimulated by Hop2 and Mnd1 which cooperate together as a complex [18, 19]. Both Hop2 and Mnd1 are required in yeast for homologous pairing and meiotic DSB repair [16–18, 20]. We have shown in this study that the Arabidopsis orthologue of MND1, AtMND1 is required for homologous pairing during meiosis in Arabidopsis where it is likely to play a role in the repair of meiotic DSBs. AtMND1 also shows strong expression in meiocytes.
The early defects in the Atmnd1 mutant with respect to overall appearance of chromosomes during meiosis were a lack of thickening during zygotene and absence of subsequent synapsis at pachytene. Fragmentation of chromosomes became apparent at diplotene and isolated univalents and fragments were first visible at diakinesis. FISH analysis using a centromere 1 specific probe indicated that homologous pairing did not take place in the mutant during zygotene and homologous chromosomes remained apart throughout meiotic prophase and meiosis I.
The meiotic phenotype of Atmnd1 is similar to that caused by a mutation in AHP2 which encodes the Arabidopsis orthologue of HOP2 . In both cases there is chromosome fragmentation and a defect in homologous pairing. The failure to synapse and the appearance of fragmented chromosomes late in meiotic prophase I is a feature of several Arabidopsis mutants that are implicated in processing and repair of DSBs [39, 40]. The observation that Atspo11-1 suppressed the chromosome fragmentation phenotype of Atmnd1 supports the interpretation that Atmnd1 is also defective in meiotic DSB repair. A major difference between the meiotic phenotype of yeast mnd1 and that for Atmnd1 in Arabidopsis is the absence of meiotic arrest in Arabidopsis whereas mnd1 shows prophase arrest which is alleviated by a mutation in MEC1, a major regulator of DNA damage induced checkpoints . A lack of arrest is also seen in the case of ahp2 whereas hop2 shows prophase arrest . The failure to arrest in the case of Arabidopsis is likely to be due to the absence or leakiness of meiotic DNA damage checkpoints and has also been observed for other Arabidopsis meiotic mutants for which the yeast counterparts show prophase arrest .
The yeast MND1 gene is expressed and functions only in meiosis and is not considered to play a role in mitotic DNA repair . The AtMND1 gene is dispensable for somatic development, however the Atmnd1 mutant is defective in mitotic DNA repair and AtMND1 is induced in response to gamma irradiation  pointing to an evolutionary difference between yeast and plants with respect to the role of MND1. In plants the major pathway for repair of DSBs in somatic cells is non-homologous end joining (NHEJ) whereas in yeast the homologous recombination pathway predominates, which may explain the requirement for Hop2/Mnd1 in promoting efficient repair of DSBs and the maintenance of genome integrity in somatic cells . Orthologues of MND1 and HOP2 are not present in C. elegans and Drosophila melanogaster both of which do not require DSBs for homologous synapsis at meiosis .
In addition to being strongly induced in meiocytes at the time of meiosis we found that AtMND1 is also expressed in the tapetum at the same time. We have earlier noticed this to also be the case for the DUET gene which has a male meiosis specific phenotype . It is possible that the tapetal expression at the same time as in meiocytes may reflect an overlap in the expression profile between tapetal cells and microspore mother cells which form adjacent layers and are both descended from the archesporial cell. Indeed the secondary parietal cells that are the precursors of the tapetum appear to retain the developmental potential to form meiocytes as revealed by mutations in EXS/EMS1 [45, 46] and TPD1  where microsporocytes are formed in place of tapetal cells. We also note that the onset of increased expression of AtMND1 appears to be at anther stage 4 in a region occupied by sporogenous cells that are the precursors of male meiocytes. This stage is prior to the formation of meiocytes and initiation of recombination. These observations would suggest that the regulatory mechanisms responsible for increased expression of AtMND1 in reproductive tissues may be distinct from those for DNA damage inducible expression .
In summary we have shown that AtMND1 is required for homologous pairing and repair of DSBs during meiosis in Arabidopsis. Loss of AtMND1 does not affect normal vegetative development but causes male and female sterility due to fragmentation and defective segregation of chromosomes in meiosis.
Plant material and growth conditions
All the plants described in this study were Arabidopsis ecotype Col-0. The T-DNA insertion lines SALK_110052 and SALK_146172 used in this study were obtained from the Arabidopsis Biological Resources Centre, Ohio State University. Plants were grown as described previously .
Characterization of the T-DNA Lines
Genomic DNA was extracted from the SALK T-DNA insertion lines using the method of Dellaporta et al.,1983 . Presence of the T-DNA insert in Atmnd1 was confirmed by PCR using a left border outwardly directed primer (LB1) in combination with a gene-specific primer (AtMND1F1) flanking the site of insertion. Based on our analysis, there are at least two tandemly placed T-DNA inserts placed next to each other such that the orientation in the genome is LB-RB-LB. Junction fragments on either ends of T-DNA were amplified using primer combinations AtMND1F1 and LB1 and AtMND1R1 and LB2. Sequencing of the AtMND1R1-LB2 product revealed the presence of a 86 bp deletion within intron 7. Primers AtMND1F1 and AtMND1R1 were utilized to amplify the wild type allele. For the Atspo11-1 line the presence of the T-DNA insert was confirmed using the primer TSPO11R in combination with LB1 and wild type copy using TSPO11F in combination with TSPO11R. Homozygous insertion lines showed a phenotype that was the same as that of Atspo11-1-1 .
A full-length genomic clone spanning 3891 bp (AGI coordinates 14382003–14385894) was amplified using primers MndF1 and MndR1 that incorporates restriction sites, Bam H1 at 5' and Eco R1 at 3' end respectively. The amplification was done using TripleMaster PCR system (Eppendorf) as per the manufacturer's instructions. The resulting 3.89 kb fragment was cloned into the pGEM-T vector (Promega) followed by sequencing of the fragment. Restriction digestion with enzymes Bam H1 and Eco R1 was performed to release the fragment, which was sub-cloned into the binary vector pCAMBIA1300. The fragment was mobilized into Agrobacterium strain AGL1 by tri-parental mating. The in planta transformation was carried out on heterozygous Atmnd1 plants by vacuum infiltration as reported earlier . The transformants were selected on medium comprising of 1% bacto agar, 1% sucrose, 1 mM KNO3 and 50 μg/ml of hygromycin B and genotyped by PCR using Atmnd_out_FG_Rev1 and Atmnd_out_FG_For1 primers.
Expression analysis by Real Time RT-PCR
Total RNA was isolated using Trizol (Sigma) as per the manufacturers protocol. cDNA was synthesized from 5–7 μg of RNA using the Superscript first strand synthesis system (Invitrogen) with Oligo dT primers. Real Time PCR reactions were done in a 10 μl volume comprising of primer, cDNA template and 1× SYBER Green PCR master mix (Applied Biosystems). GAPC was used as the internal normalization control. PCR was performed on the ABI Prism 7900 HT Sequence Detection System (Applied Biosystems) in a 384 well reaction plate according to the manufacturer's recommendations. Primers were MNDRTF1 and MNDRTR1 for AtMND1 and GAPRTF1 and GAPRTR2 for GAPC. Cycling parameters consisted of 2 minutes incubation at 50°C, 10 minutes at 95°C and 40 cycles of 95°C for 15 seconds, 57°C for 30 seconds and 67°C for 30 seconds. Each PCR reaction was performed in triplicate and the experiment were repeated twice. Specificity of the amplifications was verified at the end of each PCR run using ABI prism dissociation curve analysis software. Results from the ABI Prism 7900 HT Sequence Detection System were analyzed further using Microsoft Excel. Quantification of mRNA was calculated from threshold points (Ct values) located in the log-linear range of real time PCR amplification plots.
RNA In situ Hybridizations
In situ hybridizations were carried out as described earlier . Anti-sense RNA specific to the AtMND1 gene was used as probe along with sense control. We used the full-length AtMND1 cDNA amplified from the cDNA clone using Nco 1F and Eco R1R primers and subcloned into pGEM-T vector for strand specific probe synthesis. Floral stages are according to .
Developmental analysis of whole mount anthers and ovules was done after fixing and clearing the inflorescence in methyl benzoate as described previously . The slides were observed on a Zeiss Axioplan 2 Imaging microscope under DIC optics using a 40× oil immersion objective. Pollen viability was examined using the method of Alexander staining . Meiotic chromosome spreads were prepared, analyzed, and staged based on chromosomal morphology and with respect to the stage of the surrounding tapetal cells, according to Ross et al., 1996  with minor modifications as described in . Chromosomes were stained with DAPI (1 μg/ml) and observed on a Zeiss Axioplan 2 Imaging microscope using a 365 nm excitation and 420 nm long pass emission filter and a 100× oil objective. Images were captured on an Axioplan CCD camera using Axiovision (version 3.2) and processed using Adobe Photoshop 6.0.
Fluorescence In situ Hybridization (FISH)
Meiotic spreads were carried out as described above and FISH analysis was done according to  with incorporation of minor modifications. The hybridization mix was prepared with 5' FITC labeled probe FITC-(CCCTAAA)6  at a concentration of 5 μg/ml in 50% deionised formamide, 2× SSC and 10% dextran sulphate. The hybridization mix was denatured on a hot block for 3 minutes at 100°C and immediately cooled on ice. The slides were denatured separately with 100 μl denaturation mix comprising of 70% deionised formamide, 2× SSC and 50 mM sodium phosphate buffer pH 7.0 mounted under a 24 × 50 mm2 cover slip and incubated at 80°C for 5 minutes. After the incubation, the slides were washed in ice-cold 70% ethanol for two minutes followed by dehydration in 70%, 90% and 100% ethanol respectively (2 minutes each) and air dried. Denatured probe (100 μl) was then applied to the slides and covered with a 24 × 50 mm2 cover slip. Hybridization was carried out in a moist chamber for 18 hours at 37°C. Post-hybridization washes were performed in 2× SSC, pH 7.0 (two washes each for 5 minutes at room temperature) followed by 2× SSC for 5 minutes at 42°C. Chromosomes were counter stained with DAPI (1 μg/ml) in Vectashield (Vector Laboratories). Fluorescence detection was done on a Zeiss Axioplan 2 Imaging microscope equipped with epifluorescence illumination and distinct filters for DAPI and FITC using a 100× oil immersion objective. The images were captured with a Axioplan CCD camera using Axiovision software (Zeiss, version 3.2) and processed using Adobe Photoshop 6.0.
Primers used in this study
1. AtMND1F1 ACCGAAGAAGGGTGTAATTAGTCAGTC
2. AtMND1R1 ATTGTCGCAGTGTGAAGATGTTATCTG
3. MndF1 CAGGAGAATTCAAACCGAGAACATGAAACAGATCC
4. MndR1 GACGAGGATCCAATCATAGAAACAGACTTGGACC
5. Atmnd_out_FG_Rev1 CCTGGACCAGAAGAAGGTAAGGGTTTTG
6. Atmnd_out_FG_For1 GAGCTATTCACATGCTTAACAAGTTGCTAACAG
7. Nco1 F GCTCGCCCATGGCTATGTCTAAGAAACGGGGAC
8. EcoR1 R GCGGAGAATTCCTAAGCTTCATCTTGTACTAGCT
9. LB1 AACCAGCGTGGACCGCTTGCTGCAACTC
10. LB2 CAGGGCCAGGCGGTGAAGG
11. MNDRTF1 TCGATGATGATCTTGTTGCGAA
12. MNDRTR1 TCACACTGATCAACAAGTTCTGCt
13. GAPRTR2 CAGTCTTCTGAGTAGCAGTGATTGA
14. GAPRTF1 AGCACGAATACAAGTCCGACCT
15. TSPO11R ACTGTGATAACAATGCAGCGGTTCG
16. TSPO11F CAGCACAATCCATTGTGGACCGTGC
Note added in proof
While this manuscript was under review, a paper was published by Kerzendorfer et al., also describing work on the role of AtMND1 in homologous pairing and recombination .
This work was supported by the Council for Scientific and Industrial Research (CSIR), Govt. of India and by a grant from the Department of Biotechnology (DBT) to IS. AP, JS, MM, TR were supported by fellowships from CSIR, MR by a fellowship from the University Grants Commission, and BN was supported by a fellowship from DBT. We acknowledge ABRC, Ohio for supply of seed material and DNA clones.
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