Maternal auxin supply contributes to early embryo patterning in Arabidopsis

Hélène S. Robert, Chulmin Park, Carla Loreto Gutièrrez, Barbara Wójcikowska, Aleš Pěnčík, Ondřej Novák, Junyi Chen, Wim Grunewald, Thomas Dresselhaus, Jiří Friml, Thomas Laux

The angiosperm seed is composed of three genetically distinct tissues: the diploid embryo that originates from the fertilized egg cell, the triploid endosperm that is produced from the fertilized central cell, and the maternal sporophytic integuments that develop into the seed coat1. At the onset of embryo development in Arabidopsis thaliana, the zygote divides asymmetrically, producing a small apical embryonic cell and a larger basal cell that connects the embryo to the maternal tissue2. The coordinated and synchronous development of the embryo and the surrounding integuments, and the alignment of their growth axes, suggest communication between maternal tissues and the embryo. In contrast to animals, however, where a network of maternal factors that direct embryo patterning have been identified3,4, only a few maternal mutations have been described to affect embryo development in plants5-7. Early embryo patterning in Arabidopsis requires accumulation of the phytohormone auxin in the apical cell by directed transport from the suspensor8-10. However, the origin of this auxin has remained obscure. Here we investigate the source of auxin for early embryogenesis and provide evidence that the mother plant coordinates seed development by supplying auxin to the early embryo from the integuments of the ovule. We show that auxin response increases in ovules after fertilization, due to upregulated auxin biosynthesis in the integuments, and this maternally produced auxin is required for correct embryo development.