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 well-understood.

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 a 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 also 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 spatial-temporal 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.

Publications

Robil JM, Gao K, Neighbors CM, Boeding M, Carland FM, Bunyak F, McSteen P. grasviq: an image analysis framework for automatically quantifying vein number and morphology in grass leaves. Plant J. 2021 Jul;107(2):629-648. doi: 10.1111/tpj.15299. Epub 2021 May 27. PMID: 33914380.

Matthes MS, Darnell Z, Best NB, Guthrie K, Robil JM, Amstutz J, Durbak A, McSteen P. Defects in meristem maintenance, cell division, and cytokinin signaling are early responses in the boron deficient maize mutant tassel-less1. Physiol Plant. 2022 Mar;174(2):e13670. doi: 10.1111/ppl.13670. PMID: 35292977.

Matthes MS, Robil JM, McSteen P. From element to development: the power of the essential micronutrient boron to shape morphological processes in plants. J Exp Bot. 2020 Mar 12;71(5):1681-1693. doi: 10.1093/jxb/eraa042. PMID: 31985801; PMCID: PMC7067301.

Matthes MS, Best NB, Robil JM, Malcomber S, Gallavotti A, McSteen P. Auxin EvoDevo: Conservation and Diversification of Genes Regulating Auxin Biosynthesis, Transport, and Signaling. Mol Plant. 2019 Mar 4;12(3):298-320. doi: 10.1016/j.molp.2018.12.012. Epub 2018 Dec 25. PMID: 30590136.

Matthes MS, Robil JM, Tran T, Kimble A, McSteen P. Increased transpiration is correlated with reduced boron deficiency symptoms in the maize tassel-less1 mutant. Physiol Plant. 2018 Mar 26. doi: 10.1111/ppl.12717. Epub ahead of print. PMID: 29577325.

Committee

• Dr. Paula McSteen, Chair
• Dr. Antje Heese
• Dr. Mannie Liscum
• Dr. David Braun