Post-embryonic hypocotyl growth is coordinated by the expansion of a few cells, which is modulated by hormones and responses to exogenous cues. The crosstalk between gibberellin (GA) and brassinosteroid (BR) is fundamental for hypocotyl growth and it has been proposed at the molecular level by the physical interaction between DELLA and BZR1. Although previous observation indicated that the miR156/SPL module may be associated with cell expansion regulation in Arabidopsis, the mechanism is unknown. The miR156/SPL module has been recognized as fundamental for controlling several agronomical important traits. This control is partially mediated by the miR156/SPL module association with hormone response in many aspects of plant development. This project aims to provide evidence that miR156/SPL module acts as a regulatory "hub" in the gibberellin and brassinosteroid crosstalk in Arabidopsis and tomato during early hypocotyl growth. To test this hypothesis, Arabidopsis and tomato mutants overexpressing or silencing miR156 and SPL genes will be evaluated for hypocotyl length. It will be also analyzed gibberellin and brassinosteroid treatment responses, as well as, cell size in hypocotyl tissue. The spatiotemporal expression of miR156 and four SPL genes will be evaluated in hypocotyl at three, five, and ten days post-germination seedlings using reporter genes and transcript quantification. The molecular mechanism will be evaluated by gene expression profiles and by protein-protein interaction analysis between SPLs and main proteins from GA and BR signaling pathways. We expected to provide better knowledge regarding how the miR156/SPL module impacts early hypocotyl growth and mediate the crosstalk between GA and BR.
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