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INTRODUCTION

Peppers are commercially grown as a spice and vegetable crop. Hot pepper is a Solanaceous crop, originated in Central and South America, and is introduced to India over 500 years ago.Among the domesticated species, Capsicum annuum L is one of the most extensively cultivated pepper species in India. In India, 75% of chilli production is from the southern states viz., Andhra Pradesh, Telangana, Karnataka, Tamil Nadu and Maharashtra. Concerted efforts in thecrop improvement program in pepper resulted in release of many improved varieties and F₁ hybrids for commercial cultivation. Utilization of male sterile systems in F₁ hybrid seed production of peppers is exceptionally economical.

Male sterility in crops is due to a failure to produce functional pollen or anthers (Grelonet al., 1994, Pruitt and Hanson, 1991; Budar and Pelletier, 1994). CMS/ CGMS is exploited for the development of F₁ hybrids in many crops around the world (Hanson, 1991; Hanson and Bentolia, 2004; Miller and Bruns, 2016). Generally,CMS resulted due to the rearrangements in the mitochondrial genome sequences,which in turn results in the arrangement ofnew open reading frames (ORF) which alter the expression of normal genes of the mitochondrial ATP synthesis complex (Pruitt and Hanson,1991; Budar and Pelletier, 1994). The rearrangements within the sub unit genes of ATP synthesis, such as atp 4, 6, 8 and 9 (Pruitt and Hanson, 1991; Hanson and Bentolia, 2004; Schanable and Wise, 1998Pruitt and Hanson, 1989) are responsible for the CMS in crops and other gene rearrangements observed in pepper lines will be contributed by coxII and nad9.

Hot pepper genotype PI164835, a collection from India was the first CMS line reported (Peterson, 1958), and is being used in production of F. hybrid seeds all over the world (Reddy et al., 2002). In this CMS line, a new ORF viz., orf456 was found as flanking region of the coxII gene at the 3’ end. The atp6-2 gene is believed to be regulated through restoration-of-fertility (Rf) loci at the transcriptional level and the orf456is regulated at post transcriptional or translational level (Kim et al., 2006; Kim et al., 2007), are responsiblef or CMS trait. In the present study, the four stable CGMS lines developed and being use dinpepper improvement program at ICAR-IIHR, Bangalore are validated with the two male sterile cytoplasm (S-cytoplasm) trait linked markers, atp6-2 and orf456 and one restoration-of-fertility(Rf) loci linked to CRFmarker.

MATERIALS AND METHOD

Plant material

An aggregate of nine male sterile and their comparing nine maintainer lines were utilized in the current study are referenced in the Table 2.

Phenotypic evaluation of male sterility

The phenotypic evaluation of male sterility and fertility in lines were carried out by the visual observation at the flowering stage.The male sterile plants showed no pollen grains with shriveled anther lobes, whereas the male fertile plants have bulged anther lobes with abundant pollen grains (Plate 1).

Pollen morphologyandsize

The freshly unopened flower samples of male sterile and male fertile plants were gathered from the field in the early dawn,and put away in impenetrable zip lock polythene covers over the ice package to keep up the freshness. The pollen grains were collected from the dehisced anther lobes independently from individual flowers, frozen on the liquid nitrogen and stored them at -1950 for further studies. For morphological examinations, the individual pollen grains were directly dusted on to the slides and length and breadth of the individual grains were measured using scanning electronmicroscope (TM3000, Hitachi, Japan). The reproductive parts of both male sterile and male fertile flowers and the cross section of the anther lobes were additionally seen under the scanning electronmicroscope (TM3000, Hitachi, Japan),in order to study the morphological difference between the male sterile and male fertile flowers.The stereo microscopy images of the dehisced flowers were additionally examined .ZEISS Stereo zoom microscope Stemi 508 doc, Germany).

DNA extraction:

The total genomic DNA was isolated from the leaves of one month old seedling using 4% CTAB plant extraction protocol (Doyle and Doyle, 1990). The genomic DNA samples were qualitatively checked in 0.8% agarose gel and quantitatively by using UV- spectrophotometer. The concentrated DNA was diluted to 20ng/µL according to the spectrophotometer reading and thus diluted DNA is used as the template in PCR for genotyping with specific molecular markers.

PCR conditions and validation of molecular markers:

The polymerase chain reaction master mixture contained 2µL of 10X buffer, 2µL 25 mM MgCl., 2.5µL 1mM dNTP, (3b Blackbio, Spain ) 1.5µL of 10µM of forward and reverse primer, 0.5µL 1U Taq DNAPolymerase (3b Blackbio, Spain) and 2µL of 20ng template DNA. The PCR conditions for the validation of the three SCAR markers were carried out as mentioned here. Initial denaturation at 950 for 5 minutes accompanied with 30 repeated cycles of denaturation at 940 for 60 seconds, annealing as given in the Table 1 for 60 seconds, extension at 720 for 60 seconds and final extension at 720 for 5 minutes. The reactions were carried out in the thermocycler (Eppendorf, Germany). PCR amplified fragments were separated on 1.5% agarose gel/1X TBE (w/ vol), stained with ethidium bromide dye and documented under neath the ultra violet light (UVIPro Platinum, Cambridge, U.K). The experimentswere repeated for three consecutive times with eachmarker for confirmation of results.

Cloning and sequencing:

The PCR amplified fragments of atp6-2 gene in male sterile lines were separated on 1% agarose stained with EtBr gel, excised and purified the fragments using Nucleospin® Gel and PCR Clean-Up Kit (Macherey-Nagel, Germany). Five µL of the eluted product was ligated into pTZ57RT cloning vector system. The pTZ57RT vector containing the ligated DNA was successfully transformed into DH5α strain of E.coli. Transformed colonies were spread on Luria Bertani agar/Ampicillin/X-gal/IPTG plates and were identified through blue white screening after incubation at 370 overnight. Recombinant colonies were confirmed using colony PCR, further plasmid was isolated using alkaline lysis method. The isolated plasmids were confirmed for the presence of insert (atp6-2 gene) by digestion with the restriction enzyme, EcoRI and the restriction digested products were separated on 1% agarose/ EtBr gel to differentiate two distinct bands of vector and the 850bp insert respectively. Before sequencing, PCR product clean-up was performed using Nucleospin® Gel and PCR Clean-Up Kit (Macherey- Nagel, Germany). The sequencing was carried out in ABI-3710 Prison automated DNA analyzer (Europhins, India).

RESULTS AND DISCUSSION

We used two male sterile cytoplasm (S-cytoplasm) trait linked markers, atp6-2875 and orf456(Ji et al.,2013and Kim et al., 2005, 2007) and one restoration-of-fertility (Rf) loci linked marker CRF (Gulyas et al.,2006) to validate nine male sterile and their corresponding nine maintainer lines. CMS linked SCAR markerorf456 amplified in allthe male sterile genotypes (S-cytoplasm), at an expected base pairs of 456 as shown in the Fig. 2 and this 456bp amplicon size was absent in all corresponding maintainer lines (.-cytoplasm) (Fig.2c, Table 2).Instead of amplifying at expected amplicon size of 875bp, atp6-2 marker amplified at 850 bp in all the nine male sterile genotypes (S-cytoplasm) (Fig.2b, Table 2).In order to confirm the 25bp difference in the amplicon size, further cloning and sequencing was undertaken. Five clones each of the male sterile lines were selected, plasmid isolated, purified and further sequenced (ABI- 3710 Prisom automated DNA analyzer). Sequence obtained from ABI-3710 Prisom automated DNA analyzer was analysed from NCBI site (www.ncbi.nlm.nih.gov) and checked for nucleotide sequence identity of the observed sequences and found that there is almost 99% identity for Capsicum annuum atp6-2 subunit.The presence of the expected amplicon pattern in all nine male sterile genotypes (S-cytoplasm) proved that the mitochondrial gene associated atp6-2 subunit is responsible for the transcription of the orf456 novel gene which indeed responsible for the cause of CMS in the cultivar varieties of hot peppers. Meanwhile, the nine corresponding male fertile/ maintainer lines (S-cytoplasm) failed to amplify at the expected amplicon size. The CMS lines which are phenotypically male sterile are genotypically carrying a sterile cytoplasm, . with rfrf loci and all the maintainer or fertile B lines are genotypically carrying a normal cytoplasm, N with rfrf loci.The one CRF-SCAR marker specific to restoration-of-fertility (Rf) locus as expected failed to amplify the 550bp fragment in any of the nine cytoplasmic male sterile (A) lines or cytoplasmic male fertile/maintainer (B) lines (Fig 2,Table 2).The complete absence of the CRF-SCAR marker in all genotypes used for the current study proves that these samples didn’t carry a restoration-of-fertility, Rf loci, indicating that the cytoplasm looks genotypically normal, N or sterile, S. Even though there are markers for identification of restoration-of-fertility (Rf) in hot pepper (Kumar et al., 2009, Kim et al., 2005, Zhang et al., 2000),CRF-SCAR marker (Gulyas et al.,2006) is the most commonly and widely used molecular marker for the detection of presence or the absence of restoration-of-fertility in CMS lines of hot pepper.

Further, using the scanning electron microscope (SEM) the morphological variation in pollen grain size among the nine male sterile and their corresponding maintainer lines (Fig.1& Plate 2) was studied measuring the length and breadth of pollen grains (Plate 2). Maximum variation in pollen grain length was observed among A lines compared to B lines, and it ranged from 12.2 to 46.2µM and 36.4 to 45.7µM, respectively. Similarly, maximum variation in pollen grain breadth was observed among A lines and ranged from 5.24 to 21.9 µM, whereas it ranged from 19.4 to 27. 8μM a mong B lines (Fig. 1 & Plate 2),respectively.

The SEM images of the reproductive parts of the male sterile flowers morphologically found to be very shorter in size compared to the male fertile plants. The anther lobes of the male sterile flowers appeared to be shrivelled with less or shrunken pollen grains, whereas the male fertile plants have bulged anther lobes with abundant pollen grains . Plate 3b.. So as to see the distribution of the pollen grains inside the anther lobe, the cross section of the anther lobe was studied. The SEM images clearly distinguished the male sterile plants had no visible pollens inside the tetrad pollen chambers, rather the male fertile plants produced numerous functional pollens . Plate 3c. attached to the tetrad anther chambers. The stereo microscopy images of the dehisced flowers clearlyshowed the a bsence of pollens a t differ entmagnification in male sterile plants where as presence of abundant pollen grains were visible in and out ofthe anther lobes of male fertile plants as shown on Plate 3a.

CONCLUSION

CMS in crops is caused due to a failure to produce functional pollen or anthers (Gómez 1999, Pruitt and Hanson, 1991). Previously, the two male sterile cytoplasm (S-cytoplasm) trait linked molecular markers viz., atp6-2 and orf456 (Ji et al.,2013 and Kim et al. ,2005, 2007)were identified and characterised in CMS lines of hot pepper, were further used for the hybrid seed production in a commercial scale. The CMS pepper lines, were validated with the existing SCAR markers linked to the male sterility in pepper.The eight hot pepper lines namely IIHR 3285, IIHR 3226 , IIHR 3287, and IIHR 3228 (four CMS and 4 maintainer lines) developed at ICAR- IIHR, Bangalore and the other ten hot pepper lines (5 CMS and 5 maintainer lines) received from AVRDC, Taiwan, are having common sterile cytoplasm and restoration-of-fertility genes as were successfully validated using the three already known SCAR markers i.e., two male sterile cytoplasm (S-cytoplasm) trait linked to atp6-2 and orf456 (Ji et al.,2013 and Kim et al., 2005, 2007) and one restoration-of-fertility (Rf) loci linked marker CRF (Gulyaset al.,2006) and these molecular markers are highly reproducible at the genotypic level. Thus, these molecular markerscan be effectively used to recognize CMS from maintainer lines and fertility restorer lines and helps to fasten the breeding work to incorporate the CGMS system with varied fruit types and to incorporate disease resistant genes into A, B and R lines.

Acknowledgments

The authors thank Indian Councilfor Agricultural Research (ICAR), New Delhi, India for providing funds under the Flagship programme on Studies on male sterility system to increase the efficiency of F1 hybrids in horticultural crops. The authors also express a word of thanks to the Director, ICAR-IIHR for constant encouragement and support.

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