Control of embryo size by inositol phosphate signaling revealed by big embryo 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.