Beyond Structure: The Role of the Outer Integument in Embryo, Endosperm, and Seed Development in Annona

In most angiosperms, the embryo develops within maternal tissues and is enclosed by two integuments, which play a crucial role in embryo and seed development. However, while this role has been extensively studied in model eudicots like Arabidopsis thaliana, it remains understudied in non-model plants and largely unexplored in basal and early-divergent angiosperms. Here, we examine the role of the outer integument in embryo development and seed formation using the unique spontaneous mutant "Thai Seedless" of the early-divergent angiosperm Annona squamosa (Annonaceae). This mutant lacks the outer integument due to a deletion in the INNER NO OUTER (INO) gene, leading to seed abortion. Comparative histological and transcriptomic analyses between the Annona ino mutant and wild-type Annona show that, unlike the polar growth observed in the nucellus of the wild-type genotype, Annona ino exhibited lateral nucellus expansion. Gene ontology (GO) enrichment revealed increased expression of reproductive development genes in wild-type seeds, while Annona ino showed altered gene expression patterns associated with cell death. These findings highlight both morphological and transcriptomic differences between Annona ino and wild-type genotypes, demonstrating that the absence of the outer integument disrupts early seed morphology and gene regulation, providing new insights into the genetic and developmental roles of integuments in seed development.