Identification and phylogenetic analysis of ABI3 family genes in rice
In the present investigation, using “Abscisic acid insensitive 3”, “ABI3”, and Pfam ID “PF02362” as keywords, we identified 2, 6 and 83 ABI3 genes in rice from Ensemble Plant database. Domain analysis was carried out for these 91 ABI3 family genes and the ones not containing complete B3 domain were excluded. Eight genes were eliminated and remaining 83 containing B3 domains were used for further analysis (Supplementary Table 1). As the characteristic domain present in ABI3 family genes is primarily B3 and hence its presence is necessary for its expression in plants. The eliminated genes contained other functional domains but B3 domain was absent, therefore they were excluded for further analysis and only genes showing presence of complete B3 domain were selected for conducting expression studies in rice. Phylogenetic tree analysis revealed that OsABI3 proteins can be divided into eight major groups (Group I to VIII). Group I formed the largest clade with 21 members followed by group III which has 15 members (Fig. 2).
Chromosome mapping of ABI3 family genes
Chromosomal distribution of ABI3 genes was carried out and the results showed wide scattering of the genes throughout the genome of rice plants. The genes are distributed on the 12 chromosomes with maximum probable density on chromosome 1, 3, 4 and 12 and are not randomly equally distributed on all chromosomes (Fig. 3). Genes present on chromosme 1, 3, 4 and 12 are 21, 18, 13 and 11 respectively and other chromosomes showed lesser numbers of genes. Very high probable gene density is indicative of regions conserved for the gene presence in the rice genome during the course of evolution.
Identification of potential miRNAs and their target ABI3 genes
For identification of potential rice miRNA, a total of 10898 plant miRNAs were retrieved from PMRD (Plant micro RNA database)., CD-HIT-v4.5.4 With identity value of 90, was used to remove the redundancy in miRNA sequences. After removing redundant sequences, a set of 5025 miRNA sequences (reference set of miRNA sequences) were analysed for sequence similarity (Local BLAST by using Blast2GO-v5.2) with the OsABI3 genes assembly (Fig. 1). BLASTx analysis (E-value ≤ 1e-10) showed that out of 6 miRNA identified, sequences of only 5 miRNA i.e., ath-miR5021, csi-miR3948, osa-miRf11773-akr, osa-miRf12029-akr, and ptc-miRf10053-akr were found to be non-coding, while sequences of 1 miRNA i.e., ath-miRf10989-akr was coding for protein, which was not used for further analysis. Further, in order to identify miRNA-targeted OsABI3 genes, local BLAST was performed using Blast2GO. The potential ABI3 specific targets of ath-miR5021 was BGIOSGA028020, csi-miR3948 was BGIOSGA027821, osa-miRf11773-akr was BGIOSGA037707, osa-miRf12029-akr was BGIOSGA011738 and ptc-miRf10053-akr was BGIOSGA014119, respectively (Table 1).
Table 1
Identified potential miRNAs and their target ABI3 specific genes in Indica rice.
S. No. | miRNAs | ABI3 specific target genes |
1 | ath-miR5021 | BGIOSGA028020 |
2 | csi-miR3948 | BGIOSGA027821 |
3 | osa-miRf11773-akr | BGIOSGA037707 |
4 | osa-miRf12029-akr | BGIOSGA011738 |
5 | ptc-miRf10053-akr | BGIOSGA014119 |
Prediction of the secondary structure of potential miRNAs
The five non-coding miRNA sequences i.e., ath-miR5021, csi-miR3948, osa-miRf11773-akr, osa-miRf12029-akr, and ptc-miRf10053-akr, were further used for secondary structure analysis including hairpin stem-loop structure using MFOLD version 3.1 (Fig. 4A-E). The putative miRNAs obtained varied in their lengths ranging from 20 to 24 nucleotides. The negative MFE (-ΔG) of the miRNA precursors were also calculated to study the stability of the hairpin stem-loop structure (Table 2). In comparison to the length of miRNAs, the length of putative precursor miRNAs of rice also varied, ranging from 80 to 171 nucleotides. The stability of the secondary hairpin structure of pre-miRNA was determined by MFE (-ΔG). The distribution of G, C, A, and U nucleotides in the pre-miRNA were found to be different, where it ranged from 5-32.02% for A, 18.6-41.25% for U, 22.22–41.86% for G and 7.19–27.9% for C, respectively (Table 3).
Table 2
Determination of minimal free folding energy (MFE) of the identified potential miRNA from Indica rice.
miRNAs | Mature miRNA sequence | ST | Loc | LP | LM | NM | (G + C) % | MFE (∆G) | AMFE | MFEI |
ath-miR5021 | UGAGAAGAAGAAGAAGAAAA | + | 5’ | 171 | 20 | 1 | 36.257 | 42.7 | 24.971 | 0.689 |
csi-miR3948 | UGGAGUGGGAGUGGGAGUAGGGUG | + | 5’ | 153 | 24 | 1 | 29.412 | 27 | 17.647 | 0.600 |
osa-miRf11773-akr | GCCAUUGCCAUGGCCAUGGCCUCG | + | 3’ | 83 | 24 | 1 | 60.241 | 36.9 | 44.458 | 0.738 |
osa-miRf12029-akr | AUGUUGGCCGCCGCCGCUGCCAU | + | 5’ | 86 | 23 | 1 | 69.767 | 41.7 | 48.488 | 0.695 |
ptc-miRf10053-akr | CUGUAGUAGUUGCUGCUGCUGC | + | 3’ | 80 | 22 | 1 | 53.750 | 29.8 | 37.250 | 0.693 |
Table 3
The distribution of A, U, G, and C in the identified pre-miRNAs of Indica rice.
miRNAs | miRNA Family | A% | U% | G% | C% | A/U ratio | G/C ratio | (A + U) % |
ath-miR5021 | miR5021 | 26.316 | 37.427 | 26.901 | 9.357 | 0.703 | 2.875 | 63.743 |
csi-miR3948 | miR3948 | 32.026 | 38.562 | 22.222 | 7.190 | 0.831 | 3.091 | 70.588 |
osa-miRf11773-akr | miRf11773-akr | 16.867 | 22.892 | 32.530 | 27.711 | 0.737 | 1.174 | 39.759 |
osa-miRf12029-akr | miRf12029-akr | 11.628 | 18.605 | 41.860 | 27.907 | 0.625 | 1.500 | 30.233 |
ptc-miRf10053-akr | miRf10053-akr | 5 | 41.250 | 33.750 | 20 | 0.121 | 1.688 | 46.250 |
Morphological response rice seedlings to drought stress treatments
Rice seedlings subjected to 15% PEG showed chlorosis (loss of chlorophyll) of the shoot region, curling of shoot regions, necrosis of shoot tissue, necrosis of root tissue and stunted growth of both shoot and root (Fig. 5).
Quantitative RT- PCR analysis of selected OsABI3s genes expression in response to drought stress
The response of selected OsABI3 genes under drought stress was studied using RT-qPCR. Expression analysis showed up-regulation of all the seven selected ABI3 genes under drought stress treatment (Fig. 6). Genes with accession number BGIOSGA020315, BGIOSGA006943 and BGIOSGA013136 showed significant up-regulation in their expression. Under drought stress treatment BGIOSGA013136 showed more than 7-fold increase in the expression. There was not much change in the expression of remaining ABI3 genes in response to drought stress treatment