Auxin regulates endosperm cellularization in Arabidopsis

The endosperm is an ephemeral tissue that nourishes the developing embryo, similar to the placenta in mammals. In most angiosperms, endosperm development starts as a syncytium, in which nuclear divisions are not followed by cytokinesis. The timing of endosperm cellularization largely varies between species, and the event triggering this transition remains unknown. Here we show that increased auxin biosynthesis in the endosperm prevents its cellularization, leading to seed arrest. Auxin-overproducing seeds phenocopy paternal-excess triploid seeds derived from hybridizations of diploid maternal plants with tetraploid fathers. Concurrently, auxin-related genes are strongly overexpressed in triploid seeds, correlating with increased auxin activity. Reducing auxin biosynthesis and signaling reestablishes endosperm cellularization in triploid seeds and restores their viability, highlighting a causal role of increased auxin in preventing endosperm cellularization. We propose that auxin determines the time of endosperm cellularization, and thereby uncovered a central role of auxin in establishing hybridization barriers in plants. We extracted total RNA from 6 DAP WT and DD25::IaaH seeds, using the MagJET Plant RNA Purification Kit (Thermo Fisher Scientific). We then purified messenger RNA using the NEBNext Poly(A) mRNA Magnetic Isolation Module and used this RNA to prepare sequencing libraries with the NEBNext Ultra II Library Prep Kit for Illumina. Three biological replicates for each sample were submitted for sequencing at the National Genomic Infrastructure (NGI) from SciLife Laboratory (Uppsala, Sweden) on an Illumina HiSeq2500 in paired-end 125bp read length.