HERBICIDE RESISTANCE IN GRAIN SORGHUM by KELLAN SCOTT KERSHNER B.S, Purdue University, 2005 AN ABSTRACT OF A DISSERTATION submitted in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY Department of Agronomy College of Agriculture KANSAS STATE UNIVERSITY Manhattan, Kansas 2010 Abstract Sorghum acreage is declining throughout the United States because management options and yield have not maintained pace with maize improvements. The most extreme difference has been the absence of herbicide technology development for sorghum over the past twenty years. The objectives of this study were to evaluate the level of resistance, type of inheritance, and causal mutation of wild sorghums that are resistant to either acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides or acetohydroxyacid synthase (AHAS)-inhibiting herbicides. ACCase-inhibiting herbicides used in this study were aryloxyphenoxypropionate (APP) family members fluazifop-P and quizalofop-P along with cyclohexanedione (CHD) family members clethodim and sethoxydim. The level of resistance was very high for APP herbicides but low to nonexistent to CHD herbicides. With genetic resistance to APP herbicides, the resistance factors, the ratio of resistance to susceptible, were greater than 54 to 64 for homozygous individuals and greater than 9 to 20 for heterozygous individuals. Resistance to CHD herbicides was very low with resistance factors ranging from one to about five. Genetic segregation studies indicate a single gene is the cause of resistance to APP herbicides. Sequencing identified a single mutation that results in cysteine replacing tryptophan (Trp-2027-Cys). Trp-2027-Cys has previously been reported to provide resistance to APP but not CHD herbicides. The other wild sorghum evaluated in this study was resistant to AHAS-inhibiting herbicides including imidazolinone (IM) family member, imazapyr, and sulfonylurea (SU) family member, nicosulfuron. Resistance factors in this genotype were very high, greater than 770 for the IM herbicide and greater than 500 for the SU herbicide, for both herbicide chemical families. Genetic segregation studies demonstrate that resistance was controlled by one major locus and two modifier loci. DNA sequencing of the AHAS gene identified two mutations, Val-560-Ile and Trp-574-Leu. Val-560-Ile is of unknown importance, but valine and isoleucine are similar and residue 560 is not conserved. Trp-574 is a conserved residue and Leu-574 is a known mutation that provides strong cross resistance to IM and SU herbicides. The results of these studies suggest that these sources of APP, SU, and IM resistance may provide useful herbicide resistance traits for use in sorghum. HERBICIDE RESISTANCE IN GRAIN SORGHUM by KELLAN SCOTT KERSHNER B.S., Purdue University, 2005 A DISSERTATION submitted in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY Department of Agronomy College of Agriculture KANSAS STATE UNIVERSITY Manhattan, Kansas 2010 Approved by: Approved by: Co-Major Professor Co-Major Professor Mitchell R. Tuinstra Kassim Al-Khatib Copyright KELLAN SCOTT KERSHNER 2010 Abstract Sorghum acreage is declining throughout the United States because management options and yield have not maintained pace with maize improvements. The most extreme difference has been the absence of herbicide technology development for sorghum over the past twenty years. The objectives of this study were to evaluate the level of resistance, type of inheritance, and causal mutation of wild sorghums that are resistant to either acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides or acetohydroxyacid synthase (AHAS)-inhibiting herbicides. ACCase-inhibiting herbicides used in this study were aryloxyphenoxypropionate (APP) family members fluazifop-P and quizalofop-P along with cyclohexanedione (CHD) family members clethodim and sethoxydim. The level of resistance was very high for APP herbicides but low to nonexistent to CHD herbicides. With genetic resistance to APP herbicides, the resistance factors, the ratio of resistance to susceptible, were greater than 54 to 64 for homozygous individuals and greater than 9 to 20 for heterozygous individuals. Resistance to CHD herbicides was very low with resistance factors ranging from one to about five. Genetic segregation studies indicate a single gene is the cause of resistance to APP herbicides. Sequencing identified a single mutation that results in cysteine replacing tryptophan (Trp-2027-Cys). Trp-2027-Cys has previously been reported to provide resistance to APP but not CHD herbicides. The other wild sorghum evaluated in this study was resistant to AHAS-inhibiting herbicides including imidazolinone (IM) family member, imazapyr, and sulfonylurea (SU) family member, nicosulfuron. Resistance factors in this genotype were very high, greater than 770 for the IM herbicide and greater than 500 for the SU herbicide, for both herbicide chemical families. Genetic segregation studies demonstrate that resistance was controlled by one major locus and two modifier loci. DNA sequencing of the AHAS gene identified two mutations, Val-560-Ile and Trp-574-Leu. Val-560-Ile is of unknown importance, but valine and isoleucine are similar and residue 560 is not conserved. Trp-574 is a conserved residue and Leu-574 is a known mutation that provides strong cross resistance to IM and SU herbicides. The results of these studies suggest that these sources of APP, SU, and IM resistance may provide useful herbicide resistance traits for use in sorghum. Table of Contents List of Figures ................................................................................................................................ ix List of Tables .................................................................................................................................. x Acknowledgements ........................................................................................................................ xi Dedication .................................................................................................................................... xiv Preface ........................................................................................................................................... xv CHAPTER 1 - Sorghum Production and Putative Herbicide Targets ............................................ 1 Sorghum Production in the United States ................................................................................... 2 History of Sorghum Production in the United States .............................................................. 2 Breeding Efforts ...................................................................................................................... 3 Changing Management and Production Methods ................................................................... 4 Impact of Weeds on Sorghum Yield and Importance of Herbicide Use ................................ 5 Acetyl-Coenzyme A Carboxylase as a Protein and as a Herbicide Target ................................. 6 Discovery of the ACCase Protein ........................................................................................... 6 ACCase Structure, Function, Regulation, and Conserved Residues ...................................... 7 ACCase-Inhibiting Herbicides ................................................................................................ 8 Resistance to ACCase-Inhibiting Herbicides .......................................................................... 9 Measuring Herbicide Affect and Resistance Factors .......................................................... 9 Types of Herbicide Resistance.......................................................................................... 10 Target Site Resistance to APP and CHD Herbicides ........................................................ 10 Metabolism Based Resistance to APP and CHD Herbicides ............................................ 11 Fitness Cost of Mutations Associated with Herbicide Resistance .................................... 12 Frequency of TSR Compared to NTSR ............................................................................ 12 Was Quick Development of Resistance Related to the Specific Chemicals Used? ......... 13 Is Resistance Caused by Other Mode-of-Action Herbicides? .......................................... 14 APP and CHD Herbicide Resistance Found in Sorghum ................................................. 14 APP and CHD Herbicide Resistance Developed in Maize Tissue Culture ...................... 15 Acetohydroxyacid Synthase as a Protein and as a Herbicide Target ........................................ 16 Discovery of AHAS Protein and Naming Conflict ............................................................... 16 vi AHAS Structure and Function .............................................................................................. 17 AHAS-Inhibiting Herbicides ................................................................................................ 18 Resistance to AHAS Inhibiting Herbicides .......................................................................... 18 Crops Resistant to AHAS-inhibiting Herbicides .............................................................. 19 Gene Flow to Wild Sorghum and Integrated Weed Management ............................................ 20 Comparing ACCase and AHAS as Herbicide Targets in Sorghum .......................................... 21 Figures and Tables