Endosperm cellularization failure induces a dehydration stress response leading to embryo arrest

The endosperm is a nutritive tissue supporting embryo growth in flowering plants. Most commonly, the endosperm initially develops as a coenocyte (multinucleate cell) and then cellularizes. This process of cellularization is frequently disrupted in hybrid seeds generated by crosses between different flowering plant species or plants that differ in ploidy, resulting in embryo arrest and seed lethality. The reason for embryo arrest upon cellularization failure remains unclear. In this study, we show that triploid Arabidopsis thaliana embryos surrounded by uncellularized endosperm mount an osmotic stress response that is connected to increased levels of abscisic acid (ABA) and enhanced ABA responses. Impairing ABA biosynthesis and signalling aggravated triploid seed abortion, while increasing endogenous ABA levels as well as the exogenous application of ABA induced endosperm cellularization and suppressed embryo growth arrest. Taking these results together, we propose that endosperm cellularization is required to establish dehydration tolerance in the developing embryo, ensuring its survival during seed maturation. Embryos were dissected from developing seeds of Arabidopsis thaliana and their transcriptome profiles were analyzed. Firstly, by comparing three replicates of the triploid wild-type embryos derived from crosses (2x wt × 4x wt) against three replicates of the diploid wild-type embryos derived from crosses (2x wt × 2x wt). Secondly, by comparing three replicates of the triploid nrpd1 embryos derived from crosses (2x wt × 4x nrpd1) against three replicates of the triploid wild-type embryos.