ARABIDOPSIS Bsister and SEEDSTICK MADS-box transcription factors modulate maternal nutrient flow for seed development in Arabidopsis.

Successful seed development in Angiosperms depends on the coordinated transport and allocation of sugars from maternal tissues to the developing embryo and endosperm. In Arabidopsis thaliana, ovules function as carbohydrate sink organs, accumulating starch in both gametophytic and sporophytic domains prior to fertilization. During early embryogenesis, the stored starch is then mobilized. Despite extensive knowledge of starch metabolism in photosynthetic tissues, the regulatory mechanisms governing sugar transport in reproductive organs remain poorly understood. Recent studies have identified fertilization-dependent changes in nutrient flow, including callose-mediated modulation of symplastic transport at the phloem unloading site of developing seeds. However, the molecular players orchestrating these transitions are still largely unknown. Here, we show that the MADS-box transcription factors ABS/TT16 and STK play critical roles in regulating maternal nutrient flow during ovule maturation and seed development. We dissect their functional roles using omics and genetic approaches, underscoring the importance of different ovule tissues in coordinating sugar transport pathways for post-fertilization development. Our findings reveal a previously underappreciated layer of genetic control over nutrient allocation in reproductive tissues and provide new insights into the metabolic reprogramming required for successful seed formation.