Discovering the role of the Medicago truncatula SOC1 gene family in control of flowering time

Abstract

Flowering in the legume Medicago truncatula (Medicago) is promoted in spring by long day (LD) photoperiod and vernalisation, exposure to winter cold, by the transcription factor MtFTa1. In Arabidopsis thaliana, flowering is promoted by the same cues, by AtFT, to activate AtSOC1 at the shoot apex. However, in Medicago, the role of the three MtSOC1 genes in control of flowering time remains unknown. It was hypothesised MtFTa1 activates at least one MtSOC1 gene to induce flowering. This thesis aimed to discover the function and transcriptional regulation of MtSOC1 by inductive floral cues, to determine a role in the control of Medicago flowering time. The ability of each MtSOC1 gene to complement the late-flowering Arabidopsis soc1-2 mutant was examined in stable transgenic plants. 35S::MtSOC1a lines complemented soc1-2, 35S::MtSOC1b lines partially complemented, whilst 35S::MtSOC1c lines did not. Flowering time phenotypes positively associated with transgene mRNA levels in selected lines, confirmed by re-introducing AtSOC1, with 1 of 7 lines flowering at the same time as wild type (WT). MtSOC1 genes are differentially expressed in various aerial tissues under inductive conditions and all three are expressed in nodes (the location of the shoot apex), consistent with a role in flowering time. MtSOC1 expression is also observed in leaves, where MtFT genes are expressed under LD. A photoperiodic shift experiment tested MtSOC1 responsiveness to changes in photoperiod. Following a shift to 3 LD, all three genes were up-regulated in nodes and remained expressed upon return to short days (SD). As 3 LD is sufficient to commit vernalised WT plants to flowering, it is likely MtSOC1 activation is important for heralding the Medicago floral transition. MtFTa1-independent mechanisms also act to up-regulate MtSOC1 in the LD photoperiod pathway, possibly via MtFTb genes which respond to LD. During a developmental time course, MtSOC1b and MtSOC1c were strongly induced in WT plants under inductive conditions, prior to flowering. In late-flowering, vernalisation-insensitive fta1-1 mutants, expression was greatly attenuated and not up-regulated by vernalisation. In nodes of 15 day old WT plants, MtSOC1 genes were activated by vernalisation. In stable transgenic 35S::MtFTa1 Medicago, the levels of MtSOC1b and MtSOC1c were dramatically higher. Nodes of non-vernalised WT plants mimicked that of fta1-1 mutants, indicating MtFTa1 expression is important for MtSOC1 regulation. Transient over-expression of MtFTb1 + MtFD, but not MtFTa1+ MtFD, in fta1-1 leaves by Agrobacterium infiltration resulted in expression of MtPIM and a weak expression of MtSOC1a. This result may provide an insight into the LD photoperiod regulation of candidate flowering time genes downstream of MtFT genes. Under non-inductive SD photoperiod, exogenous GA3 promotes A. thaliana and tobacco flowering via a gibberellin-responsive cis-element in SOC1, when FT is low. In contrast with Arabidopsis and tobacco studies, exogenous did not promote Medicago flowering under SD photoperiod. A detailed transcriptional characterisation of MtSOC1 regulation by inductive floral cues suggests MtSOC1b and MtSOC1c are primary targets of MtFTa1 for promoting the floral transition in Medicago.