| Summary: | Seed size is the result of complex molecular networks controlling the development of the seed coat (of maternal origin) and the two fertilization products, the embryo and the endosperm. In this study we characterized the role of <i>Arabidopsis thaliana</i> MADS-domain transcription factor SEEDSTICK (STK) in seed size control. STK is known to regulate the differentiation of the seed coat as well as the structural and mechanical properties of cell walls in developing seeds. In particular, we further characterized <i>stk</i> mutant seeds. Genetic evidence (reciprocal crosses) of the inheritance of the small-seed phenotype, together with the provided analysis of cell division activity (flow cytometry), demonstrate that STK acts in the earlier phases of seed development as a maternal activator of growth. Moreover, we describe a molecular mechanism underlying this activity by reporting how STK positively regulates cell cycle progression via directly activating the expression of <i>E2Fa</i>, a key regulator of the cell cycle. Altogether, our results unveil a new genetic network active in the maternal control of seed size in <i>Arabidopsis</i>.
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