Identification of the “Final form of the phloem end” for seed formation

Abstract

In flowering plants, the mechanism linking fertilization to nutrient supply has long remained unclear. Here, we report the discovery of a novel structure in the chalazal region of the ovule, termed “the final form of the phloem end.” Using aniline blue staining, we found that unfertilized ovules accumulate callose in a ring-shaped pattern that blocks nutrient influx. Live imaging revealed that fertilization of the central cell triggers callose degradation, thereby opening the “gate” for nutrient flow from maternal tissues. Transcriptome analysis identified a fertilization-dependent β-1,3-glucanase gene, AtBG_ppap. Loss of AtBG_ppap impaired callose degradation and nutrient flow, whereas its overexpression enlarged seed size by approximately 16%. A conserved mechanism was also confirmed in rice. These findings represent the first discovery of a new plant tissue in 160 years since the Casparian strip and establish a new principle of fertilization-coupled nutrient regulation. This review summarizes the discovery and discusses its broad implications for understanding seed development.