Phenotype analysis and gene mapping of small kernel 7 (<italic>smk7</italic>) mutant in maize

Abstract

<p indent=0mm>In this study, a stable small kernel mutant, named <italic>small kernel 7 </italic>(<italic>smk7</italic>), was isolated from ethylmethane sulfonate (EMS) mutagenesis of maize inbred line B73. Compared with wild type, the <italic>smk7 </italic>mutants showed smaller kernel size, defective embryo and endosperm development and a significant decrease in 100-kernel weight. The <italic>smk7</italic> kernels showed a low level of germination rate at 10% and cannot grow into normal plants. No significant changes were detected in protein, starch and oil content between mature wild type and <italic>smk7</italic> kernels, but the starch grains became significantly smaller and irregular in <italic>smk7</italic> kernels compared with wild type. The <italic>smk7</italic> kernels could be clearly distinguished from the wild type as early as 12 days after pollination (DAP), on the basis of their smaller and emptier phenotype. Microscopic inspection of the paraffin sections revealed that the development of embryo and endosperm were delayed, and the cell wall in growth in basal endosperm transfer layers (BETL) were arrested in <italic>smk7</italic> compared with wild type. The F₂ populations with multiple backgrounds were constructed by crossing heterozygous plants (+/<italic>smk7</italic>) with several other inbred lines. Genetic analysis showed that the mutant phenotype was controlled by a single recessive gene. Based on genotyping by target sequencing (GBTS) strategy, the <italic>SMK7 </italic>was initially mapped on the short arm of chromosome 2. The fine mapping results suggested that <italic>SMK7</italic> was located between markers RM1433917 and RM1535316, with a physical distance of 120 kb. There were eight protein-coding genes in this region. This study laid a foundation for further genes cloning and research of the<italic> SMK7 </italic>function in regulating maize kernel development.