Patrick Tranel

A common theme among most of the research projects in my laboratory is variability within weed species. Another uniting feature of my research projects is that nearly all utilize molecular techniques.

Many species owe their success as weeds to their underlying genetic diversity and, consequently, to their ability to adapt to a variety of cropping practices. Probably the best example of weed adaptation to a particular cropping practice is the development of herbicide resistance. For this reason, several past and current projects in my lab focus on the evolution of herbicide resistance. For example, we have characterized several weed populations with resistance to herbicides that inhibit acetolactate synthase (ALS), and identified the underlying mutations in the ALS gene that are responsible for resistance.

More recently, we have been investigating resistance to herbicides that inhibit protoporphyrinogen oxidase (PPO). Waterhemp was the first weed in the world to evolve resistance to this group of herbicides. We determined that an amino acid deletion (rather than a substitution) in a waterhemp PPO gene conferred the resistance. Moreover, this mutation was in a gene that encodes both chloroplast and mitochondria PPOs.

Current efforts in my lab include investigations of glyphosate resistance in waterhemp. In this case, resistance is less "black-and-white" compared to other herbicide resistances, i.e., the level of resistance is not as high and it may be controlled by more than one gene.

Waterhemp is by no means the only weedy Amaranthus (pigweed) species that is commonly found in Illinois. In fact, eight pigweed species (or nine if you distinguish the two waterhemp species) are commonly encountered in our state. Because these species are closely related, they may occasionally hybridize. We are interested in determining if gene exchange among the pigweeds contributes significantly to their weediness. For example, can herbicide resistance move from one pigweed species to another? We are now using various cytogenetic approaches to address questions related to chromosome pairing, gene exchange, and ploidy in the hybrids and backcross progeny. One of the things we have learned is that, although hybrids form relatively easily, genomic incompatibilities may limit subsequent gene introgression.

Herbicide resistance is not the only consequence of genetic variability within weed species. We investigated variability for other traits in common cocklebur. This species is a very widespread and competitive weed, and has a complex taxonomic history. We used molecular fingerprinting to better understand genetic relationships among U.S. cocklebur accessions. These accessions were separated by our molecular markers into two main groups, which followed a north-south distribution. We also determined that different cocklebur accessions varied in their competitive abilities when grown with soybean.

Modern investigations of weeds are limited by the fact that we do not yet have genomic resources for most of these species. In the next few years, we hope to develop such resources. Such resources will help to elucidate the fascinating ways weeds thwart our attempts to control them, and their unique characteristics that make them weeds!

Research papers