Microsatellites, or simple sequence repeats (SSRs), contain repetitive DNA sequence where tandem repeats of one to six base pairs are present number of times. Chloroplast genome sequences have been  shown to possess extensive variations in the length, number and distribution of SSRs. However, a comparative analysis of chloroplast microsatellites is not available. Considering their potential importance in generating genomic diversity, we have systematically analysed the abundance and distribution of simple and compound microsatellites in 164 sequenced chloroplast genomes from wide range of plants. The key findings of these studies are (1) a large number of mononucleotide repeats as compared to SSR(2-6)(di-, tri-, tetra-, penta-, hexanucleotide repeats) are present in all chloroplast genomes investigated, (2) lower plants such as algae show wide variation in relative abundance, density and distribution of microsatellite repeats as compared to flowering plants, (3) longer SSRs are excluded from coding regions of most chloroplast genomes, (4) GC content has a weak influence on number, relative abundance and relative density of mononucleotide as well as SSR(2-6). However, GC content strongly showed negative correlation with relative density (R (2) = 0.5, P < 0.05) and relative abundance (R (2) = 0.6, P < 0.05) of cSSRs. In summary, our comparative studies of chloroplast genomes illustrate the variable distribution of microsatellites and revealed that chloroplast genome of smaller plants possesses relatively more genomic diversity compared to higher plants.

Capsicum annuum (Chilli) is a perennial herbaceous plant that is cultivated as an annual crop throughout the world, including India. Chilli leaf curl disease (ChiLCD) is a major biotic constraint, causing major losses in chilli production. During 2014, leaf samples of chilli plants displaying leaf curl disease were collected from the Ahmedabad district of Gujarat, India. These samples were used to isolate, clone and sequence viral genomic DNA and an associated betasatellite DNA molecule. Sequence analysis showed 90.4 % nucleotide sequence identity to the previously reported chilli leaf curl virus-[India:Guntur:2009] (ChiLCV-[IN:Gun:09]. As per ICTV nomenclature rules, ChiLCV-Ahm represents a new species of begomovirus, and we therefore propose the name chilli leaf curl Ahmedabad virus-[India:Ahmedabad:2014] (ChiLCAV-[IN:Ahm:14]). The associated betasatellite DNA showed a maximum of 93.5 % nucleotide sequence identity to a previously reported tomato leaf curl Bangladesh betasatellite and may be named tomato leaf curl Bangladesh betasatellite-[India:Ahmedabad:Chilli:2014].

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In December 2012, tomato leaf curl disease (ToLCD) (2) was observed in tomato-growing areas of Gandhinagar District of Gujarat, a state in northwestern India. Incidence of ToLCD was estimated to be between 40 and 70% depending on the cultivars used. Infected plants exhibited symptoms consisting of leaf rolling, leaf curling, and yellowing typical of begomoviruses. Total DNA was isolated from a single affected tomato plant (2). Begomovirus infection in this sample was established by amplification of the expected-size 550-bp DNA fragment from this extract by PCR with degenerate DNA-A primers (3). Rolling circle amplification (RCA) using ϕ29 DNA polymerase was carried out on the total DNA, followed by digestion with Bam HI. An amplicon of ~2.8 kb was gel-eluted and cloned into Bam HI linearized pBluescript II KS(+). Restriction enzyme digestion of plasmid DNA from the resulting clones indicated the presence of one type of molecule. Using PCR and universal betasatellite primers, the expected 1.3-kb fragment was amplified from the DNA extract (1). An amplicon of ~1.3 kb was gel-eluted and cloned into pTZ57RT vector. Sequence analysis revealed that DNA-A (GenBank Accession No. KC952005) is composed of 2,753 nt and showed the highest identity (87.8%) with Tomato leaf curl Kerala virus[India:Kerala:2008] (GenBank Accession No. EU910141). An analysis for recombination showed this begomovirus DNA likely to have originated by recombination between Tomato leaf curl Kerala virus and Tomato leaf curl Karnataka virus. The satellite DNA-β (GenBank Accession No. KC952006) is composed of 1,365 nt and showed the highest identity (75.6%) with Tomato leaf curl betasatellite[India:Ludhiana:2004] (ToLCB-[IN:Lud:04]) (GenBank Accession No. AY765255). On the basis of DNA-A sequence analysis, the ICTV species demarcation criteria of 89% DNA-A sequence identity, and genome organization, the present isolate was considered as a new begomovirus species and named Tomato leaf curl Gandhinagar virus (ToLCGNV). The betasatellite shares less than 78% identity with (ToLCB-[IN:Lud:04]), it is considered a new species of betasatellite and the name, Tomato leaf curl Gandhinagar betasatellite (ToLCGNB) is proposed. Multimeric clones of the begomovirus and betasatellite DNAs were generated in a binary vector and these plasmids transformed into Agrobacterium tumefaciens. Nicotiana benthamiana and tomato plants agroinoculated with the cloned begomovirus DNA developed leaf curl symptoms, whereas plants co-agroinoculated with the cloned begomovirus and betasatellites developed more severe symptoms, including leaf rolling, leaf curling, and yellowing. The symptoms induced by the begomovirus and betasatellite DNAs were indistinguishable from those observed in the field. Thus, ToLCGNV is a new monopartite begomovirus which, in association with a new species of betasatellite, causes ToLCD in Gandhinagar, India. The presence of ToLCGNV needs to be considered, along with the already reported begomoviruses infecting tomatoes in this state, e.g., Tomato leaf curl Gujarat virus (2), in studies aimed to developing tomato cultivars with stable resistance to these tomato-infecting begomoviruses in India. References: (1) R. W. Briddon et al. Mol. Biotechnol. 20:315, 2002. (2) C. Reddy et al. Arch Virol. 150:845, 2005. (3) S. D. Wyatt and J. K. Brown. Phytopathology 86:1288, 1996.