Genetic and Biochemical Regulation of Maize Architecture

Norman B Best, Purdue University

Abstract

Forward genetics is the most effective method to unequivocally assign gene function to phenotypes of interest. Forward genetics utilizes mutations that perturb gene function and result in a change in phenotype. The techniques to identify the molecular identity of the causative mutation are often difficult and costly. This dissertation utilizes a suite of forward genetic approaches to uncover genes affecting the architecture of maize plants and the underlying physiological mechanisms responsible for these mutant phenotypes. Modulation of phytohormone concentration by application of chemical inhibitors of hormone biosynthetic enzymes is often utilized to identify the physiological functions of phytohormones. Interaction between growth substrate and chemical inhibitor can complicate these studies. The triazole compounds propiconazole and uniconazole inhibit cytochrome P450 enzymes in various hormone biosynthetic pathways. I found that the efficacy of these inhibitors and the phytohormone brassinolide, varied depending on the soilless media substrate when applied as a media drench in the greenhouse. Using Fourier Transform Infrared Spectroscopy and sorption isotherms it was determined that hydrophobic interactions between calcined clays and these molecules caused the loss of efficacy in some soilless media. Phytohormones regulate the pattern of plant form that determines the capacity to compete for water, nutrients, and light. Gibberellins (GA) and brassinosteroids (BRs) control plant height, branching, and reproductive development in maize. I identified the molecular nature of the classical dwarf mutant nana plant2 (na2) as a defect in a delta-24 sterol reductase required for conversion of 24-methylene cholesterol to campesterol in the BR biosynthesis pathway. In addition to a dwarf phenotype, na2 mutants exhibit decreased axillary branches (tillers) and persistence of pistils in the terminal staminate flowers of the tassel (POPIT). Previously identified GA deficient mutants are also dwarfed, but tiller profusely and exhibit persistence of stamens in the axillary pistillate flower of the ear (anther-ear). Double mutants between BR and GA deficient mutants identify developmentally specific interactions between these pathways. BR and GA mutants additively increase plant height, BR mutants were epistatic to GA mutants for increased tiller development, GA mutants were epistatic to BR mutants for persistence of pistils in tassel florets, and there was no interaction between BR and GA mutants for persistence of stamens in the ear. Lastly, I identified the aladin1 mutant in an EMS-mutagenized population of maize with shortened upper internodes, upright leaves, altered tassel architecture, and abnormal asymmetric division of stomatal subsidiary cells. Using high throughput sequencing and bulked segregant analysis, I identified a nonsense mutation in the last exon and truncation of the last 16 amino acids of the nuclear pore complex subunit, aladin1, as the cause of these phenotypes. A targeted mutagenesis screen identified a second allele, aladin1-2, and confirmed the identity of the gene. The nuclear pore complex (NPC) regulates the movement of macromolecules between the nucleus and cytoplasm and defects in this complex have negative outcomes in all eukaryotes studied. In humans, this gene is responsible for the genetic disease Triple A syndrome. Analysis of differential mRNA accumulation in the aladin1-1 mutant identified up-regulation of other NPC components in the mutant. These findings demonstrate that the aladin gene is required for cell division and shoot architecture in maize.

Degree

Ph.D.

Advisors

Johal, Purdue University.

Subject Area

Genetics|Plant sciences|Biochemistry

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