Control of embryo size by inositol phosphate signaling revealed by <i>big embryo</i> mutants of maize

Inositol phosphates (InsP) play diverse signaling roles in regulating development, phosphate sensing, and energy metabolism. Here, we identify 4 maize (Zea mays) mutants, big embryo 2 (bige2), big embryo 3 (bige3), big embryo 4 (bige4), and low phytic acid 1 (lpa1), that show enlargement of the embryo at the expense of endosperm. Bige2 (identical to Lpa2), Bige3 (identical to Lpa3), and Bige4 genes encode inositol phosphate triphosphokinase (ITPK) and mono-inositol phosphate kinase (MIK), both of which catalyze lipid-independent InsP biosynthesis, and inositol polyphosphate kinase (IPK2) in the lipid-dependent InsP pathway, respectively. Lpa1 encodes a tonoplast InsP6 transporter. InsP pathway mutants primarily affect scutellum growth, with each mutant exhibiting a distinct spatial pattern of cell enlargement and/or cell number. Genetic epistasis and transcriptome analyses reveal overlapping and nonredundant roles of lipid-independent and -dependent pathways in regulation of embryo development. Strikingly, ectopic expression of endosperm-specific genes in lpa2-bige2 and bige4 embryos reveals a shift toward endosperm organ identity. We identify a network of NAC transcription factors implicated in shaping lpa2-bige2 and bige4 transcriptomes. Disruption of lipid-independent InsP biosynthesis in lpa2-bige2 is associated with upregulation of a subnetwork of SOG1-related NAC proteins linked to DNA damage repair and endoreduplication. The lpa2-bige2 phenotype is fully suppressed by lpa1, suggesting that a block in InsP6 uptake into the vacuole restores signaling by cytosolic InsP intermediates. Together, these results establish a genetic framework for dissecting complex roles of InsP signaling in seed development.