ANNOUNCEMENTS
MicroRNAs (miRNAs) are an interesting class of small non-coding RNAs (~21-nt) playing a crucial role in regulation of genes involved in key developmental processes including flower development. Among miRNAs, MIR172 is implicated in floral transition in model plant Arabidopsis. Through a fascinating cascade of molecular interactions, the 21-nt miRNA172 seeks and down-regulates the entire spectrum of target genes including AP2. At the time of conceptualization of my doctoral research theme, the sequence and functional variation among MIR172 and its target AP2 was not characterized in economically important Brassica species that constitute an important source of oilseed, vegetable and condiments. Further, the dynamics of interaction and role of MIR172:AP2 regulatory module in context to floral transition was poorly understood. One possible reason for existing lacunae in this area may be ascribed to challenges associated while unraveling complex Brassica genomes. Large scale whole genome duplication events, marking evolutionary history of Brassicas, are known to result in expansion in gene copies of MIR172:AP2. This was hypothesized to result in complex combinatorial interactions influencing floral transition. With this premise, my doctoral research study “Characterization of MIRNA172 genes in Brassica” was designed. My study revealed extensive natural variation in MIR172 family members and its target AP2 in Brassicas. The collection of sequence accessions of MIR172 and AP2 genes, isolated from diverse cultivars of six Brassica species, constitutes a valuable genetic resource that may be exploited in future. MIR172 displayed high conservation in predicted precursors relative to flanking region. SNPs were detected in miRNA and miRNA*. SNPs and indels in precursors led to varied stem-loop structures with differing stabilities (ΔG). AP2 sequence variants showed a conserved miR172 binding site while considerable polymorphism was reported in the flanking sequences. My results suggested a strong evolutionary constraint imposed on mature miR172 and its binding site in AP2 which are required for maintaining interaction dynamics. In-silico and in-vivo analyses to study MIR172:AP2 interaction revealed potency of miR172 in mediating cleavage of AP2 transcripts. Comparison of gain-of-function mutant phenotypes in Arabidopsis established relevance of Brassica MIR172 in mediating floral transition. Introgression of MIR172e into B. juncea resulted in earliness in flowering time in transgenics. My doctoral thesis thus expands current understanding on the role of MIR172:AP2 regulatory module in control of flowering time. In an inevitable scenario of climate change, such insights are valuable as they would ultimately help translate fundamental knowledge base to crop improvement
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