Fertilization Initiates Seed Nutrition via Phloem End by a Callose Degradation Enzyme

Why plants need fertilization to produce seeds has long been discussed. We recently identified a new specialized tissue required for seed formation at the ovule's chalazal end, showing the final form of the phloem end and supporting its transport function; however, it is blocked by callose deposition. Callose is removed after central cell fertilization (open state), allowing nutrients to be transported to the seed. However, if fertilization fails, callose deposition persists (closed state), preventing the tissue from transporting nutrients. A β-1,3-glucanase gene, AtBG_ppap, was identified, and the AtBG_ppap mutant showed the closed state, producing smaller seeds due to incomplete callose degradation. Contrarily, the AtBG_ppap overexpression line produced larger seeds due to continuous callose degradation, showing that the tissue is the "gateway" for the seed nutrients.