Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

Document Type



Leaf scald is an important disease of sugarcane caused by Xanthomonas albilineans (Xa). Leaf scald is controlled by the development and planting of resistant cultivars. However, erratic symptom expression makes conventional screening for resistance difficult. In addition, the mechanisms of resistance to leaf scald are not completely understood. Real-time, quantitative polymerase-chain-reaction (qPCR) assays were developed utilizing SYBR Green for a highly sensitive detection method or a TaqMan probe to quantify Xa populations in infected plants. Xa populations detected by qPCR followed similar trends to disease severity ratings and vascular infection results for two resistant and two susceptible cultivars under greenhouse and field conditions. Low bacterial populations were found in newly emerged, systemically infected leaves of resistant cultivars. The results suggest that Xa population quantification by qPCR has the potential to be used as an alternative method for leaf scald resistance screening. A comparative proteomic analysis identified differentially expressed proteins that suggested mechanisms for the sugarcane resistance response to Xa infection. Protein expression was compared for inoculated and mock-inoculated plants of two resistant and two susceptible cultivars during a time-course encompassing the responses to initial and systemic infection. Differential expression also was compared across cultivars with and without Xa infection. The number of up- and down-regulated proteins increased in the resistant cultivars during systemic infection. Identified, differentially expressed proteins were mostly in the chloroplast (67%), and 48% were involved in photosynthesis. Identified proteins were homologous to cyclophilin, translationally controlled tumor protein (TCTP), thylakoid ascorbate peroxidase (tAPOD), germin-like protein (GLP), and thioredoxins. All are proteins that have been associated with induced defense responses. Down-regulation of APOD and the thioredoxins and up-regulation of GLP could result in accumulation of reactive oxygen species, particularly H2O2, in the cytoplasm and the apoplast. In addition, proteins involved in ethylene biosynthesis, another key signaling molecule in induced systemic resistance (ISR), were differentially expressed in resistant cultivars. Differences found between the two resistant cultivars indicate that resistance mechanisms can vary between genotypes, but the suggested resistance mechanisms for both were inducible responses. Finally, some differentially expressed proteins were involved in primary metabolism that could represent a plant fitness mechanism to provide energy needed for ISR. The differences in Xa populations detected by qPCR and comparative proteomic analysis both suggest the existence of an induced mechanism of resistance against Xa during systemic infection. Multiple lines of evidence from the proteomic analysis suggest a triggering of ISR that would result in the limited Xa colonization of the new xylem developing above the apical meristem and lack of symptom development that is evident in leaf scald resistant sugarcane genotypes.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Hoy, Jeffrey W.