The Physiology and Host Genetics of Quantitative Resistance in Maize to the Fungal Pathogen Cochliobolus heterostrophus.

BELCHER, ARABY RUTH. The Physiology and Host Genetics of Quantitative Resistance in Maize to the Fungal Pathogen Cochliobolus heterostrophus. (Under the direction of Dr. Peter J. Balint-Kurti.) Quantitative disease resistance, despite widespread use, remains poorly understood. A previous project in the NCSU Maize Disease Resistance Genetics lab has generated 253 near-isogenic lines (NILs) in the background of the maize inbred line B73. B73 has excellent agronomic quality but is highly susceptible to a number of diseases. The NILs are genetically differentiated by combinations of 1-5 of 12 total introgressed regions from the multiple disease-resistant parent NC250P. These 12 NC250P introgressions were selected for study as, following an initial B73 x NC250P cross, they had been retained by a program of recurrent backcrossing to B73 and selection for resistance to the fungus Cochliobolus heterostrophus, causal agent of southern corn leaf blight (SLB). Prior research also evaluated the effect of each NC250P introgression in conferring resistance or susceptibility against SLB. Introgressions having an effect can be designated as disease quantitative trait loci, or “dQTLs”. Presented here is a 2-phase study with the ultimate aim of characterizing the physiological basis for the effect on disease severity of these NC250P-derived SLB dQTLs. The first phase attempts to determine more precisely how infection is altered by the two largest-effect introgressions, termed dQTL 3.04 and dQTL 6.01 (or 3B and 6A). To do so, growth chamber juvenile plant trials were used to compare the interactions between C. heterostrophus and 6 select lines B73, the major-gene resistant line B73rhm1 (also a B73-background NIL), and four NILs with varying combinations of dQTLs 3.04 and 6.01 by quantifying spore germination and penetration efficiency, hyphal growth, and host expression of the pathogenesis related genes PR1 and PR5. The second phase investigates dQTL disease specificity by field testing 236 NILs for adult plant resistance to 5 fungal maize pathogens. Based on the results of the first phase, host genotype was not a significant factor for germination or penetration efficiency before 48 hours post-inoculation (hpi) (P≥0.27) nor for hyphal growth at 24 hpi. dQTL 6.01 significantly reduced fungal growth at 48 hpi (P≤0.05). PR1 and PR5 were significantly upregulated at 15 and 24 hpi in all lines (P≤0.01), but were not affected by any dQTLs. Moreover, in a genetic complementation test using mapping population data, it was determined that dQTL 6.01 and rhm1 represent the same resistance gene. In the second phase of experiments, a total of 237 NILs were rated for resistance to SLB, northern leaf blight (causal agent Exserohilum turcicum), gray leaf spot (causal agent Cercospora zeae-maydis), aflatoxin production by Aspergillus flavus, and fusarium ear rot and fumonisin toxin production by Fusarium verticillioides. Correlations for resistance to any given disease pair were low across the NIL population (-0.247 ≤ r ≤ 0.535). However, eight of the twelve NC250P introgressions represented in the NILs were shown to have significant multiple disease resistance effects (P≤0.058 for all significant effects). A dQTL in bin 3.04 had a significant effect on resistance for 4 diseases; and the dQTLs in bins 2.06, 5.07-5.09, and 6.01 had significant effects on 3 diseases. The dQTLs in bins 3.00-3.01, 9.01, 9.02-9.03, and 10.02 had significant effects on resistance for 2 diseases. The combined results of these experiments, by adding to the current knowledge of the timing and specificity of quantitative disease resistance effects within the infection court, can be used to both 1) help plant breeders better deploy quantitative resistance genes through an increased understanding of their effects and 2) provide quantitative resistance and multiple disease resistance gene candidates for future fine-mapping and cloning efforts. The Physiology and Host Genetics of Quantitative Resistance in Maize to the Fungal Pathogen Cochliobolus heterostrophus by Araby Ruth Belcher A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Master of Science