Date of Completion

5-11-2018

Embargo Period

5-10-2018

Major Advisor

Yi Li

Associate Advisor

Richard McAvoy

Associate Advisor

John Inguagiato

Field of Study

Plant Science

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Turfgrass is cultivated across the globe as an aesthetically pleasing and functional groundcover. One of the most popular turfgrass species is perennial ryegrass (Lolium perenne L.), which is valued for its fast establishment, dark green color, and adaptability to different soil types. In 2011, the Li lab initiated a mutation breeding program focused on developing new perennial ryegrass cultivars with desirable traits for the turfgrass industry. This project first centered around identifying dwarf mutant plants and later evolved into an exploration of beneficial secondary phenotypes associated with dwarfism, such as shade tolerance. One mutant, called shadow-1, was analyzed in detail over the course of multiple years of greenhouse and field studies. These studies have determined that shadow-1 plants possess leaves which elongate slowly compared to wild type and are significantly resistant to the impact of shade stress. These traits were found to be stably inherited in shadow-1 progeny and segregate together. Both the dwarfism and shade tolerance exhibited by shadow-1 plants could be abolished through the exogenous application of the phytohormone gibberellin. Hormone analysis revealed that endogenous gibberellin levels were decreased in shade-stressed shadow-1, but increased in light-grown shadow-1, compared to wild type under each respective condition. Through transcriptome analysis it was determined that shade stress altered the expression of a greater number of genes than those altered by the mutation(s) found in shadow-1. Transcriptome analysis also uncovered downregulation of gibberellin biosynthesis genes in shadow-1 plants, although this was more severe in shade-stressed shadow-1. Additionally, the gibberellin degradation gene GA2ox was downregulated in shadow-1 kept under light conditions while DELLA, the negative regulator of gibberellin response was upregulated. Taken together, these data provide evidence that dwarfism in shadow-1 is caused by partial gibberellin insensitivity while shade tolerance is caused by gibberellin deficiency and both of these phenotypes are caused by a single mutation which can impact both pathways. These findings provide valuable information to geneticists and breeders who are interested in developing dwarf and/or shade tolerant plant cultivars

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