Heritability of Resistance to 4-Hydroxyphenylpyruvate Dioxygenase (HPPD) Inhibitor Herbicides.

Herbicide resistance has become more prevalent across agriculture since the increased use and reliance of herbicides since the 1960’s. This anthropogenic selection pressure has resulted in weed species evolving and adapting to herbicides at a faster rate due to the application frequency and the effectiveness of herbicides. The most recently discovered herbicide site of action is p-hydroxyphenylpyruvate dioxygenase (HPPD) which joined the growing list of herbicide resistance when in 2010, populations of waterhemp (Amaranthus tuberculatus (Moq.) Sauer.) were reported in Illinois and Iowa.  In Iowa we are currently evaluating the trend of waterhemp resistance to Group 27 herbicides. The data predicts the percentage of fields in Iowa containing HPPD-inhibitor resistance is now up to 24-27%.Because of this burgeoning problem, we will determine the heritability of the HPPD-inhibiting herbicide resistance trait in waterhemp to better understand how the herbicide resistance will spread. The genetic factors we are examining are the number of genes containing genetic information for the resistance trait and if the expression of the alleles that code for HPPD resistance are dominant or recessive. We have made reciprocal crosses with two homogeneous lines of a susceptible waterhemp population and a HPPD-resistant waterhemp population. The progeny will be “selfed” and the two generations (F₁ and F₂) will be evaluated in the greenhouse to determine the phenotype.  The ratios of HPPD resistant:susceptible will be based on plant survival to the HPPD herbicide screen. Tissue samples will be taken from each generation. RNA will be extracted from the tissue samples and then used to sequence the genomes using next generation sequencing.  In order to better manage and prolong the efficacy of the limited available herbicide technology it is essential to have a full understanding of the flow of the herbicide resistance traits in problematic weed species.