Light is a major regulator of plant growth and development by antagonizing gibberellins (GA), and we provide evidence for a role of light perception and GA in seed coat formation and seed tolerance to deterioration. We have identified two activation‐tagging mutants of Arabidopsis thaliana, cog1‐2D and cdf4‐1D, with improved seed tolerance to deterioration linked to increased expression of COG1/DOF1.5 and CDF4/DOF2.3, respectively. These encode two homologous DOF transcription factors, with COG1 most highly expressed in seeds. Improved tolerance to seed deterioration was reproduced in transgenic plants overexpressing these genes, and loss of function from RNA interference resulted in opposite phenotypes. Overexpressions of COG1 and CDF4 have been described to attenuate various light responses mediated by phytochromes. Accordingly, we found that phyA and phyB mutants exhibit increased seed tolerance to deterioration. The phenotype of tolerance to deterioration conferred by gain of function of COG1 and by loss of function of phytochromes is of maternal origin, is also observed under natural aging conditions and correlates with a seed coat with increased suberin and reduced permeability. In developing siliques of the cog1‐2D mutant the expression of the GA biosynthetic gene GA3OX3 and levels of GA₁ are higher than in the wild type. These results explain the antagonism between phytochromes and COG1 in terms of the inhibition and the activation, respectively, of GA action.