Methylation analysis of selected promoter regions of OsGLP genes under control condition
To analyze the cytosine methylation status in the promoter regions of OsGLP genes in control leaves and roots samples of KS282 and Super Basmati, bisulfite sequencing was performed in selected promoter regions of 3 OsGLP genes (OsGLP4-1, OsGLP8-10 and OsGLP8-12). Promoter region was selected within 1kb upstream region of transcription start site for OsGLP4-1 (from − 153 to -455 bp), OsGLP8-10 (from − 436 to -794 bp) and OsGLP8-12 (from − 528 to -787 bp). As a result of methylation analysis in control samples using CyMATE software, among three OsGLP genes, highest number of methylated cytosine residues was identified in OsGLP8-12 whereas no methylated cytosine residues were identified in promoter regions of OsGLP4-1 and OsGLP8-10 in both rice varieties (Fig. 1a). However, the level of methylation was found to be different depending upon CGN, CHG and CHH sequence context as well as rice varieties. Overall, the level of methylation was found to be higher in CGN context than CHG and CHH contexts, In this aspect, in both leaves and roots, promoter region of OsGLP8-12 exhibited 90.90% methylated CGN positions and 9.09% CHG positions in KS282 whereas 78.6% methylated CGN positions and 21.4% CHG positions were observed in Super Basmati (Fig. 1b). Interestingly, no methylated residues were observed at CHH position in any analyzed promoter of the OsGLP genes from control sample of both varieties.
Drought induced differential methylation patterns in promoter regions of OsGLP in leaves and roots of rice varieties
To investigate the location, class and methylation status of cytosine residue in the selected promoter region of OsGLP genes in response to drought stress, comparative methylation analysis of bisulfite sequences was performed in promoter regions of 3 OsGLP genes (OsGLP4-1, OsGLP8-10 and OsGLP8-12) in leaves and roots of KS282 and Super Basmati under control and drought conditions by utilizing cytosine methylation map generated by CyMATE and multiple sequence alignment by CLUSTALX/BioEdit. For promoter region of OsGLP4-1, overall analysis indicated no differentially methylated sites in any of the bisulfite converted sequences from control and drought treated leaves and roots of KS282 and Super Basmati (Fig. 2, Online Resource 2). However, the analyzed promoter segment of OsGLP 8–10 was observed with differentially methylated sites in roots of KS282 and Super Basmati under drought stress with KS282 exhibiting higher methylation rate (22.22%) at CGN positions as compared to Super Basmati (11.11%) (Fig. 3, Online Resource 3). Among 3 OsGLP genes, bisulfite sequence of OsGLP8-12 promoter that was observed with most methylated sites in both KS282 and Super Basmati exhibited differentially methylated sites with respect to a particular variety, tissue and stress type. For example, in control samples, 23.40% methylated cytosine residues in KS282 and 29.17% in Super Basmati were observed with differential methylation status at CHG position whereas single CGN site was found with differential methylation status (de-methylation) in roots of Super Basmati under drought stress (Fig. 4, Online Resource 4).
Salt induced differential methylation patterns in promoter regions of OsGLP in leaves and roots of rice varieties
To understand the impact of salt stress on methylation pattern in promoter regions of OsGLP genes, bisulfite sequences of promoter regions of 3 OsGLP genes (OsGLP4-1, OsGLP8-10 and OsGLP8-12) were analyzed in control and salt treated leaves and roots of KS282 and Super Basmati by utilizing cytosine methylation map generated by CyMATE and multiple sequence alignment by CLUSTALX/BioEdit. Similar to drought stress, the comparative methylation map and multiple sequence alignment of control and salt treated samples from both rice varieties showed no differentially methylated positions in the bisulfite converted selected promoter region of OsGLP4-1 (Fig. 5, Online Resource 5). However, in contrast to drought stress, salt stress induced no change in methylation status of cytosine residues in the selected promoter segment of OsGLP8-10 and all of the possible methylation sites were found to be un-methylated in leaves and roots samples of both rice varieties under control and salt stress conditions (Fig. 6, Online Resource 6). The comparative analysis of bisulfite converted sequences of OsGLP8-12 promoter from control and salt treated samples revealed the methylation patterns similar to drought treated samples with single but same salt stress induced de-methylated CGN site in roots of Super Basmati. Interestingly, in addition to differentially methylated sites, OsGLP8-12 promoter sequence from KS282 and Super Basmati showed difference in their nucleotide sequence at two different sites where cytosine was replaced with adenine at one mutated site and guanine was replaced with adenine at the other site in KS282 while master sequence and sequences from Super Basmati shared same sequence (Fig. 7, Online Resource 7).
Stress induced cytosine methylation changes around CREs in promoter regions of OsGLP genes
To better understand the importance of modulated methylation patterns at specific cytosine residues on the functionality of selected genes with respect to a particular stress treatment, the location and methylation status, analysis of CREs was performed in the promoter regions of OsGLP genes. Selected promoter sequences for OsGLP4-1, OsGLP8-10 and OsGLP8-12 were analyzed in detail by PLACE and PlantCare, and significant CREs for abiotic stress and transcription regulation were identified. For instance, the analyzed sequence of OsGLP4-1 consisted of various CREs mainly including MYB, MYC, W-box, CTCC-motif, TATA-Box and CAAT-box. However, all the possible sites of methylation were found to be un-methylated within or around these CREs (Fig. 8). Furthermore, several important CREs (MYB, W-box, GT1-motif, ABRE/ACGT, CTCC-motif, TATA-Box and CAAT-box) were also determined in promoter region of OsGLP8-10 and one of the two differentially methylated sites (identified in bisulfite sequence analysis) was found in close proximity with W-box CTCC, MYB and GT1 motif while the other site was observed near GT1 motif and CAAT-box (Fig. 9). Characterization of CREs in the promoter region of OsGLP8-12 also indicated the presence of MYB, W-box, LTR, CTCC-motif, CAAT-box, TATA-box and ABRE/ACGT-motif and several methylated and differentially methylated sites were observed near or within these CREs. Investigation of differentially methylated sites with respect to CREs showed that one of the differentially methylated sites at CHG position was present nearly after CTCC-motif, MYB and W-box motifs indicating that these motifs were followed by un-methylated cytosine in KS282 while methylated cytosine in Super Basmati in all samples. In addition to this, single site that was differentially methylated in response to salt and drought stress (de-methylated in roots of Super Basmati under salt and drought stress) was identified within ABRE/ACGT-motif. Interestingly, one of the two mutated sites observed in OsGLP8-12 promoter sequence in KS282 was determined near this differentially methylated CRE motif (Fig. 10).