Hormonal and genetic control of leaf and vein development in maize
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Grasses (Poaceae) have successfully colonized diverse terrestrial ecosystems and impacted human civilization by producing the bulk of world food supply. The grass leaf is distinct for having a sheathing base and strap-like blade -- an evolutionary modification from the petioled leaf of eudicots. A further innovation in the grass leaf is the venation pattern composed of parallel longitudinal veins (long veins) interlinked by transverse veins. The long veins are formed as the leaf grows wide early in the development. The small indolic plant hormone, auxin, controls leaf and vein development in plants. However, how it controls leaf and vein development in grasses is not wellunderstood. In Chapter 1, I review the literature on leaf vein development and highlight the knowledge gap on the role of auxin in the regulation of vein formation in grasses. In Chapters 2 and 3, I address two important aspects of grass leaf and vein research using Zea mays as the model system: 1) to develop a tool that will enable high-throughput analysis of vein number and morphology; and 2) to investigate the roles of auxin and two other plant growth hormones, gibberellic acid (GA) and cytokinin (CK), in regulating the leaf width and long vein number in grasses. Through collaboration, I helped develop the first image analysis framework dedicated for the parallel venation in grasses which can quantify vein traits and detect vein patterning defects. In Chapter 2, I describe the first detailed spatial-temporal map of auxin, CK, and GA during early leaf development in a grass species. I propose a conceptual model for hormonal regulation of medial-lateral growth and vein proliferation. This model can be used to develop testable predictions about hormonal regulation of leaf phenotypes altered by mutation or environmental response in maize and other grass species. Finally, in Chapter 4, I offer future perspectives about decoupling leaf growth and vein formation by focusing on spatialtemporal auxin response, which will be important for the targeted improvement of vein density to increase productivity in grass crops. Therefore, this thesis contributes technical innovations, advances the understanding of plant development, and provides new insights for future work.
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Ph. D.