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Study of transcriptional regulators involved in flowering in brassica species

Student Name: Ms Shikha Tyagi
Guide: Prof. Anandita Singh
Year of completion: 2019

Abstract:

B. juncea (Indian mustard) is commercialized primarily as edible oil seeds and is a crucial component of Indian cuisine. Brassicas display enormous morphological diversity largely on account of polyploidy and natural allelic variation. However, high temperature stress at flowering is known to compromise productivity of B. juncea. Since flowering time and seed maturation are impacted by temperature which in turn influences yield and oil profile, one of the strategies for enhancing the productivity of oil-crops involves introgression of the early flowering trait. Developing climate-resilient varieties with reduced flowering time is an effective stress avoidance strategy which can contribute considerably towards mitigation of yield losses. Early flowering also ensures cooler ambient temperatures at the time of seed maturity for optimum oil composition in seeds. Previously, studies in Arabidopsis have established FLOWERING LOCUS T (FT), SUPPRESSOR OF OVER-EXPRESSION OF CONSTANS 1 (SOC1) and FLOWERING LOCUS C (FLC) as central genes regulating flowering time. The core function of FT and SOC1 is floral promotion while FLC is a floral repressor. Since these genes modulate flowering time, characterization of numerous Brassica homologs is important given the complex Brassica genomes. Further, not many studies explored the potential of natural variants in achieving early flowering Brassica varieties when the present doctoral research work was envisaged.

This study was conceptualized to characterize key floral regulators viz., FT, SOC1, FLC from Brassicas at sequence and functional level. The study also investigates performance evaluation of gain-of-function and knock-down Brassica mutants for FT and SOC1 homologs in natural field conditions. Additionally, the applicability of FT and SOC1 in introducing an early flowering trait in B. juncea has also been examined from crop improvement perspective. Another embodiment of doctoral research has been a comparative analysis of genomic regions containing FLC homeologs. Microsynteny analysis was undertaken to understand the implications of polyploidy on gene content and order in the vicinity of FLC locus within Brassica species and few other members of Brassicaceae. Lastly, isolation and characterization of Brassica FLC homologs at sequence level was undertaken to gain insights in context to natural variation, evolutionary relatedness and possible implications of gene sequence on its function.

Findings from this study suggest differential functional potency of distinct natural variants of Brassica FT. Knockdown mutants, each for FT and SOC1 homologs from Brassica, showed functional expansion of these genes in Brassica beyond flowering time control. The varied phenotypic manifestations resulting due to altered expression levels of Brassica FT and Brassica SOC1 suggested trait trade-offs. Study of gene content and order in genomic regions harboring FLC explored distinct evolutionary trajectories of FLC homeologs, identified widespread genomic re-arrangements viz. inversions, duplications, and novel alleles, all adding up to genomic novelty. Isolation and sequence characterization of FLC homologs from Brassica provided interesting variants including intronless copies with distinct phylogeny. In conclusion, the doctoral study provides interesting and relevant insights with respect to the structural and functional evolution of key flowering time regulators which can be exploited in crop improvement programs for Indian mustard.

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