GPI-anchored SKU5/SKS are maternally required for integument development in Arabidopsis

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) play crucial roles in various processes in eukaryotes. In Arabidopsis, SKS1, SKS2, SKS3 and SKU5 from SKU5/SKS gene family could encode GPI-anchored proteins, and they were recently reported to regulate cell polar expansion and cell wall synthesis redundantly in roots. Here, we report that, they are also redundantly crucial for seed production and seed morphogenesis in Arabidopsis through regulating maternal integument development. Their loss-of-functions resulted in disrupted development of integuments that failed to protect embryo sacs from exposure to external space due to physical restriction, and lead to female gametophytic abortion. Interestingly, those less defective ovules could be fertilized and develop into seeds normally, however, their seed morphogenesis was largely affected. Our research made SKS1, SKS2, SKS3 and SKU5 be not only the first class of GPI-anchored proteins that could regulate maternal integument development, but also the first class of proteins that could regulate cell polar expansion in both root and integument cells besides several MAPK cascade components. Our study also underlined the regulation by integument development in reproductive processes.