ZmSK1 encodes a sucrose synthase essential for starch synthesis and endosperm development in maize

Maize (Zea mays) endosperm is the primary reservoir of nutrients that serve as energy and carbon sources for seed germination and early seedling growth. The characterization of endosperm mutants has provided insights into the mechanism underlying endosperm development and resources for maize breeding. In this study, we identified shriveled kernel1 (zmsk1) as a new maize mutant with shriveled kernels containing an opaque and entirely floury endosperm. ZmSK1 was identified as the gene at the Zm00001d045042 locus using a map-based cloning strategy; a specific nucleotide change resulting in aberrant splice sites is responsible for the phenotype of zmsk1 kernels. Allelism tests verified that ZmSK1 is a novel allele of a previously reported gene (Sh1). ZmSK1 was predicted to encode a sucrose synthase localized in the plasma membrane and cytoplasm. An expression pattern analysis indicated that ZmSK1 is ubiquitously expressed in all examined tissues, but especially in kernels and actively growing tissues. Transcriptome analyses indicated that ABC transporter, ribosome and cell expansion-related pathways were significantly enriched, in the zmsk1 mutant, suggesting that a loss-of-function mutation in ZmSK1 may affect protein synthesis, transmembrane transport and early endosperm cell development and morphogenesis, while also inhibiting normal carbohydrate metabolism. Thus, our study provides insights into the molecular basis of maize kernel endosperm development.