Hormonal and Molecular Investigations of Phylloxera Leaf Gall Development
Open Access
- Author:
- Witiak, Sarah Melissa
- Graduate Program:
- Integrative Biosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 21, 2006
- Committee Members:
- Jack C Schultz, Committee Chair/Co-Chair
Hong Ma, Committee Chair/Co-Chair
Michael Craig Saunders, Committee Member
Simon Gilroy, Committee Member - Keywords:
- floral development
MADS-box
phylloxera
insect galling
cytokinin - Abstract:
- Insect galls represent a unique class of plant-insect interactions whereby a chemical stimulus from the insect redirects normal plant development to form a novel organ that provides nutrition and shelter to the insect within. Previous studies show that galls are induced in young plant tissues and suggest that the morphological, hormonal and metabolic characteristics of galls are similar to fruits and seeds, including well-ordered tissues with high levels of cytokinins, proteins and carbohydates. Based on these observations, I tested the hypothesis that galling insects identify existing meristematic cells or dedifferentiated cells as sites for gall induction. In addition, I hypothesized that the insect can redirect the development of meristematic or minimally differentiated cells of normal plant tissue by ectopically inducing suites of developmental genes necessary for fruit or seed development. Using the phylloxera gall (Daktulospharia vitifoliae (Fitch)) on grapevine (Vitis riparia) I characterized gall induction and development using cytological and molecular techniques. I first examined the characterized the cellular structure of the gall throughout development using light and scanning electron microscopy to show that the insect’s stylet is inserted in a radial pattern. I visualized cytokinin distribution, which is critical for meristematic activity and cell division. I show that zeatin, a cytokinin, accumulates in specific gall tissues during gall development. To show whether galling insects achieve gall development through ectopic induction of meristem, fruit and seed genes, I show that transcription of fruit and seed specific MADS-box genes occurs in galls but not the surrounding leaves. I also examine carbohydrate accumulation. By combining cytological observations with gene expression profiles, I developed a model for gall development that is based on atypical expression of normal fruit and seed developmental pathways. Gene expression in a second gall induced by Janetiella brevicauda suggests that ectopic induction of fruit and seed development by galling insects may be a general mechanism of insect gall development.