Arabidopsis: at 1 DAP, all ovule types (WT,gcs1,Atbg_ppap, andOEAtBG_ppap) contain a PE primordium with branched phloem tubes; however, mature PE is formed at approximately 2 DAP. After fertilizing the central cell in WT, the cells (early endosperm) send fertilization completion signals to the PE. Subsequently, enzymes for callose degradation, including AtBG_ppap, degrade the callose in the PE, thus allowing the transport of nutrients to the fertilized ovule. Ingcs1, as the fertilization of the central cell fails, the unfertilized cell does not send fertilization completion signals to the PE, and the callose degradation enzymes are not synthesized; thus, the PE continues to deposit callose without transporting nutrients, which results in the abortion of the ovule. InAtbg_ppap, the central cell is fertilized, and the early endosperm sends fertilization completion signals to the PE. However, as the AtBG_ppap protein is not functional, callose degradation remains incomplete. The partial callose deposition interferes with the PE nutrient transport system, resulting in the formation of smaller seeds. By contrast, theOEAtBG_ppapline constitutively expresses the AtBG_ppap enzyme, which degrades callose in the PE and facilitates nutrient flow from the funiculus to the ovule, producing larger seeds than those produced by WT.
