Date of Degree
PhD (Doctor of Philosophy)
Erin E. Irish
As maize plants undergo vegetative phase change, they both exhibit heteroblasty, an abrupt change in pattern of leaf morphogenesis, and gain the ability to produce flowers. Both processes are under the control of microRNA 156, whose levels decline at the end of the juvenile phase. Gain of ability to flower is conferred by expression of miR156 targets that encode Squamosa Promoter-Binding (SBP) transcription factors, which in turn induce the expression of MADS-box transcription factors that promote maturation and flowering. What gene expression differences underlie heteroblasty, as well as what causes the reduction in miR156 levels, remain open questions. Here, we compare the gene expression in primordia that will develop into juvenile or adult leaves to identify genes that define these two developmental states and may influence vegetative phase change. In comparisons among successive leaves at the same developmental stage of plastochron 6, three-fourths of approximately 1,100 differentially expressed genes were more highly expressed in juvenile primordia. This juvenile set was enriched in photosynthetic genes, particularly those associated with cyclic electron flow at photosystem I, and genes involved in oxidative stress and retrograde redox signaling. Pathogen responsive pathways including jasmonic acid, salicylic acid, and benzoxazinoids were also up-regulated in juvenile primordia and indeed, we found that exogenous application of jasmonic acid, and hydrogen peroxide delays vegetative phase change in maize seedlings. These results suggest that the timing of vegetative phase change in maize is coordinated in part downstream of photo-oxidative stress signaling. Photo-oxidative stress during greening likely amplifies heterotrophic energy insufficiency. The successful amelioration of these stress signals may ultimately determine the duration of miR156-mediated juvenility.
The life cycles of flowering plants are important for maintenance of the human population because of our direct dependence on plant products for food. The cereal grains, mainly maize, rice, and wheat, make up about half of world food consumption on a caloric basis. Floral induction, the process resulting in flowering and seed (food) production, is well understood – however, for floral induction to occur, a plant must first undergo a developmental transition called vegetative phase change (loosely analogous to human puberty). This process is less well-understood than flowering and involves a transition from a nonflowering juvenile vegetative state to an adult vegetative state in which flowering may occur. Here we investigate the genetic basis of vegetative phase in maize by comparing gene expression between juvenile and adult leaves. We find that the duration of the juvenile state is likely determined by stress signaling which inhibits growth, and demonstrate that certain factors involved in stress signaling – jasmonic acid and hydrogen peroxide – can delay vegetative phase change. Determining factors underlying the duration of juvenility is desirable for use both in shortening it (causing trees to bear fruit quickly) and lengthening it (increasing plant mass for biofuel production).
development, jasmonate, juvenility, maize, microRNA, photosynthesis
viii, 63 pages
Includes bibliographical references (pages 56-65).
Copyright 2014 Benjamin Dupree Beydler