Genetic background effects on morphological and molecular expression of vegetative phase change in wild type and Corngrass1 (Cg1) maize (Zea mays L.)

Vegetative phase change, or the transition from juvenile vegetative to adult vegetative tissues, occurs at the shoot apex and is essential to the lifecycle of higher plants. Maturation of the shoot apex appears to be governed by a series of sequential, overlapping processes that have a strong genetic basis. Analyses of phase change mutants glossy15 and Corngrass1 (Cg1) have revealed two microRNAs, miR156 and miR172, that appear to be the main factors determining the timing of vegetative phase change.;The Cg1 mutation of maize is a caused by ectopic expression of miR156 and prolongs the juvenile-vegetative phase. Mo17 suppresses phenotypic expression of Cg1 relative to other maize inbreds including B73. Effects of B73 and Mo17 genetic backgrounds on Cg1 phenotypic expression were assessed by measuring typical Cg1 traits among B73, B73Cg1, Mo17, and Mo17Cg1 crosses in a nested mating design. miR156 and miR172 expression were also assessed using the same nested mating design. Both experiments revealed significant genetic background effects. B73 had significantly greater expression of seven of the eight measured Cg1 phenotypic traits relative to Mo 17. B73 also had significantly greater expression of miR156 relative to Mo17 in the presence of Cg1. The degree of Cg1 expression across and within inbred backgrounds was heritable. When combined, results of the two experiments show a relationship between the level of miRNA expression and severity of the Cg1 phenotype.;Agronomic and developmental traits in the maize population Minn11 were significantly altered by eight cycles of divergent recurrent selection for timing of vegetative phase change. Allele frequency changes at miR156 and miR172 transcript regions and expression levels of miR156, miR172, and 188 other miRNAs were examined over cycles of selection. Allele frequencies differed between the eighth cycles of selection in the early and late phase change directions at both transcript regions. miRNA expression analyses, however, failed to find significant differences among cycles of selection in expression for any miRNAs extracted from shoot apices 23 days after planting. miRNA expression may be altered at another time point, or the developmental changes may be caused by another mechanism such as miRNA target site variation.