APOMIXIS INVOLVEMENT IN INHERITANCE OF GLYPHOSATE RESISTANCE IN AMARANTHUS PALMERI FROM MISSISSIPPI. D. N. Ribeiro*1, Z. Pan2, F. E. Dayan3, S. O. Duke4, V. K. Nandula5, D. R. Shaw6, B. S. Baldwin1; 1Mississippi State University, Starkville, MS, 2University of Mississippi, Oxford, MS, 3USDA-ARS, University, MS, 4USDA-ARS, Oxford, MS, 5USDA-ARS, Stoneville, MS, 6Mississippi State University, Mississippi State, MS (438)


Glyphosate is considered by many as the most important herbicide ever developed. Repeated glyphosate applications causes selective genetic pressure toward evolution of resistant weeds. Glyphosate-resistant (R) Palmer amaranth populations (R1 and R2) were identified in Mississippi. The inheritance of glyphosate resistance was examined by reciprocally crossing R maternal parents with susceptible (S) paternal parents (R/S) and crossing S maternal parent with R paternal parents (S/R) to generate F1. Individuals from the F1 populations were submitted to glyphosate dose-response assays resulting in a range of phenotypes from R to S. The response to glyphosate was more similar to the R than S parent when the female parent was R. Conversely, when R was used as pollinator the response to glyphosate was more similar to the S parent. Thus, the level of resistance was strongly influenced by the direction of the cross.  This led us to three hypotheses: (1) maternal effect of the R trait, (2) facultative apomictic reproduction, and (3) differences in transmission rates between male and female gametes. Sequence comparisons of the predicted 5-Enolpyruvylshikimate-3-phosphate synthase (EPSPS) mature protein from R1, R2, and S did not identify a target site mutation known to confer resistance in R populations. EPSPS activity was lower in S and S/R plants than in R and R/S plants in the absence of glyphosate; all were equally inhibited by the presence of glyphosate. Genomic estimation of EPSPS gene copy number relative to acetolactate synthase (ALS) using quantitative PCR showed that R and R/S contain more copies of EPSPS than S and S/R. Western Blot analysis confirmed that increased EPSPS protein levels were correlated with EPSPS copy number. Quantitative RT-PCR on cDNA revealed that EPSPS was highly expressed in R1 and R/S, but was  poorly expressed in S, S/R, and R2. The apomixis involvement in glyphosate resistance inheritance was studied using 44 S, 36 R1 and 38 R2 reproductively isolated female individuals. In all cases seeds were produced, with the exception of one R1 plant. We found 60 to 100% (depending on the population) produced 1 to 1,000 seeds, but some individuals produced up to 6,000 seeds. These results strongly suggest that A. palmeri can produce seed both apomictically (facultative apomixis), and sexually, with apomixis the determinant of low copy number inheritance in S/R population. Moreover, facultative apomixis would guarantee the glyphosate resistant trait stability in R populations. dnr34@pss.msstate.edu