| dc.description.abstract | Cotton fleahoppers (Pseudatomoscelis seriatus) are an early season pest of upland cotton. Feeding damage is characterized by death and abscission of developing cotton floral buds, called squares, and is thought to result from infection of the plant tissue with the bacterial pathogen, Pantoea ananatis, which is vectored by the insect during feeding. Heavy infestations cause delayed maturity and can result in substantial yield loss. Cotton fleahoppers are primarily controlled by chemical insecticides, and thus there exists a strong need to identify resistance in the available upland germplasm for resistance breeding purposes. To that end, three integrated projects were designed to identify and characterize host plant resistance in the available upland germplasm: (1) field evaluation of candidate germplasm to identify resistance and introgression of the resistance trait through backcross breeding, (2) characterization of resistance identified in the first objective through assays of feeding behavior and morphological analysis of the plants and cotton fleahoppers, and (3) RNA-seq transcriptome analysis of plant response to herbivory in one susceptible and three resistant genotypes identified in the first objective.
Germplasm obtained from a previous cotton fleahopper breeding effort at Texas A&M and from the Texas A&M AgriLife Research Cotton Improvement Lab at College Station was screened for resistance by estimating percent square loss in three years of field tests in College Station and Corpus Christi, TX and included two high-yielding breeding lines and 18 lines derived from crosses of Pilose (a densely pubescent cultigen resistant to cotton fleahopper) with ‘Deltapine50,’ ‘All-Tex Atlas,’ and ‘TAM 96 WD- 69s’. Field evaluations identified resistance to cotton fleahoppers in lines derived from crosses with Pilose. Field evaluations of backcross progeny lines identified one line, 12525, with high resistance to cotton fleahoppers in both College Station and Corpus Christi and good yield and fiber traits. Behavioral assays examined the interactions of adult cotton fleahoppers with excised cotton squares. Behavior was categorized as walking, resting, probing, feeding or cleaning. Analysis revealed significant differences among parental and backcross progeny lines in time cotton fleahoppers spent feeding, indicating non-preference as a mechanism of resistance. Morphological analysis of square structure, in which square width and length and depth of the developing ovary were measured, indicated variation in depth of the developing ovary may contribute to resistance to cotton fleahoppers; squares with greater ovary depth may escape direct penetration by the proboscis of a feeding cotton fleahopper. RNA-seq transcriptome profilining examined the effects of cotton fleahopper herbivory on gene expression. Analysis revealed differential expression of transcripts associated with three regulators of the hypersensitive response (HR)—myb transcription factor, alternative oxidase (AOX), and BAX inhibitor-1— and indicated the difference between susceptible types (plants that shed squares) and resistant types (plants that retain squares) may lie in regulation of HR-associated lesion formation. Together, the projects presented in this dissertation indicate that the relationship between cotton fleahopper and upland cotton is complex and involves several host plant resistance mechanisms that can be exploited in future efforts to breed for resistance to this insect in cotton. | en |
