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How cells stop their growth once an organ has reached a certain size is a key question in developmental Biology. In animals, it has been proposed that mechanical signals could act, together with biochemical factors, in the control of organ growth and size. In plants, mechanical signals regulate a variety of cellular processes such as growth, division or gene expression; but their contribution to the regulation of organ size remains to be demonstrated. Using the seed of Arabidopsis thaliana as a model system, we are studying the mechanical regulation of organ growth in plants. By combining experimental approaches with modelling, we propose that seed size is regulated through an incoherent mechanosensitive feedforward loop where the turgor pressure of the inner tissues (endosperm) induces growth directly but represses it indirectly by promoting the stiffening of the cell walls of the outer tissues (seed coat) in a tension-dependent manner. Our work sheds new light on the mechanisms of organ size control in plants and allows us to redefine the contribution of turgor pressure to plant organ growth.