Analysis of the role of GA and MtSOC1a in Medicago truncatula flowering and plant architecture

Abstract

Legumes are agronomically important food and fodder crops. Flowering time is an important determinant of plant productivity and yield. The mechanism of flowering regulation in Arabidopsis thaliana has been well characterized and is used as a reference for Medicago truncatula, a model legume plant. SUPPRESSOR OF CONSTANS 1 (SOC1) is an important integrator gene of A. thaliana flowering and promotes flowering. There are three M. truncatula homologs, MtSOC1a, SOC1b and SOC1c. MtSOC1a, was shown to have a role in flowering because a delay in flowering time was observed in the Mtsoc1a mutant. The mutant also had a short primary stem, similar in other late flowering time gene mutants of M. truncatula: Mtfta1 and Mtphya-1. Overexpression of MtSOC1a was found to generate plants with taller stems but did not accelerate flowering. The stem height of Mtsoc1a mutants when treated with GA were rescued similar to wild type R108, while 35S:MtSOC1a on GA treatment grew taller than wild type plants. As GA plays a crucial role in controlling stem elongation, the Mtsoc1a mutant with short stem height was proposed to have either a defect in GA response, or low levels of GA production. Therefore, this thesis aims to investigate the relationship between the MtSOC1a gene in flowering time and stem elongation. Foliar spray of GA3 was performed on M. truncatula wild type R108, Mtsoc1a, Mtfta1 and Mtphya-1 mutants under vernalized long day (VLD) conditions. All genotypes showed stem elongation in response to GA indicating they were not defective in their ability to respond to exogenous GA. Flowering time was not accelerated by GA in any genotype. RNA-seq transcriptomic data from wild type R108, Mtsoc1a mutant, and over expression 35S:MtSOC1a plants and differential expression analysis identified candidate Medicago flowering time and floral genes and genes involved in GA pathways. To facilitate the further study of the three MtSOC1 homologs, a CRISPR-Cas9 genome editing system was used to target these genes. Wild type R108 was transformed with the editing binary vector plasmid using Agrobacterium tumefaciens-mediated transformation and regeneration via somatic embryogenesis. The impact of the work is that it has identified candidateMedicago truncatula flowering time and stem length genes thatmight be regulated by MtSOC1a. It also lays the groundwork for studying the function of the other MtSOC1 genes using gene editing and mutant analysis.