Our research focuses on understanding how plants respond to their environment and how their growth processes are regulated at the tissular, cellular and molecular levels. We are particularly interested in early seed development in the model plant Arabidopsis thaliana and its relative Brassica napus (oilseed rape) and how high temperature affects seed production and quality. We observed that high temperatures above the optimum growth temperature resulted in the production of seeds with ruptured seed coats in Brassica napus cv Topas. This phenotype is associated with accelerated embryo development. However, the relationships between the three events – high temperature, embryo growth pace, and seed coat rupture – remain unclear. To investigate the occurrence of temperature-induced seed coat rupture, we combined detailed phenotyping approaches of oilseed rape seeds with transcriptomics, histology, immunolabelling, hormone and cell wall profiling. Our data suggest that high temperatures accelerate embryo growth, resulting in larger embryos but not larger seeds. Such large embryos exert a putative mechanical pressure on the seed coat cells, for which we observed a reduced cell layer thickness. The seed coat began to mature prematurely with the accumulation of demethylesterified pectin, possibly making the cell wall stiffer, which eventually ruptured. Our data present novel observations on the impact of high temperatures on seed development, tackling issues linked to seed biomechanic features.