Date of Award
Master of Science
Genetics, Development and Cell Biology
Michael G. Muszynski
Phillip W. Becraft
Four distinct segments define the maize leaf along the proximal-distal axis: the proximal sheath and the distal blade, which are separated by the ligule and auricle. The process of maize leaf morphogenesis, patterning and development requires the coordination of a number of proteins and hormones. The semi-dominant gain-of-function mutant Hairy sheath frayed1 (Hsf1) disrupts a number of leaf growth parameters: (i) leaves are shorter and more narrow, (ii) an increased density of macrohairs are apparent on the abaxial sheath and adaxial blade and (iii) the normal proximal-distal leaf pattern is disrupted, resulting in outgrowths, or prongs, of ectopic sheath, auricle and ligule from the margins of the normal leaf blade. The underlying cause for the Hsf1 phenotype is a missense mutation near the binding pocket of Zea mays Histidine Kinase1, a cytokinin (CK) receptor, which results in CK hypersignaling. To investigate the role of cytokinins, a class of plant growth regulatory hormones, in leaf morphogenesis and development, we conducted exogenous hormone germination assays and double mutant analysis of genes acting downstream of CK signaling. Our results confirm the Hsf1 phenotype is due to an increase in CK signaling and is more sensitive to CK. CK treatment affects leaf growth, cell identity and proximal distal patterning in a dose-dependent manner at specific developmental stages. Genetic diversity underlies CK responses in a diverse selection of CK-treated maize inbred lines, as well as in other species. Hsf1 alters proximal-distal leaf patterning in a specific manner and double mutant analysis revealed genetic interactions with genes downstream of CK signaling. Hsf1 prong formation is enhanced by dominant knotted1-like homeobox (knox) mutants and low GA levels, although knotted1 itself does not appear to be required for prong formation. Double mutants with aberrant phyllotaxy1 and tassels replace upper ears1 reveal dosage-dependent phyllotaxy effects.
James Francis Cahill
Cahill, James Francis, "Analysis of cytokinin-induced maize leaf developmental changes and interacting genetic modifiers" (2015). Graduate Theses and Dissertations. 14788.