Transcriptional responses to 2,4-D herbicide treatment of a Eurasian (Myriophyllum spicatum) and a hybrid (M. spicatum × M. sibiricum) genotype of watermilfoil that differ in their sensitivity to 2,4-D

Aquatic plant managers frequently treat Eurasian watermilfoil (Myriophyllum spicatum L.; EWM) and hybrid watermilfoil (Myriophyllum spicatum L. x Myriophyllum sibiricum Komarov) with 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide. However, watermilfoil genotypes can differ in their response to 2,4-D. In this study, we compared facultative and constitutive gene expression differences for two watermilfoil genotypes (one Eurasian and one hybrid) that differ in their sensitivity to 2,4-D. To do this, we compared between control and 0.5 mg L-1 2,4-D treated plants at four time points after treatment. We also assembled the first de novo watermilfoil transcriptome. We found that while qualitatively similar, the facultative transcriptional response of the EWM genotype to 2,4-D treatment was much stronger than the hybrid genotype, indicated by a greater number and logfold-change of differentially expressed genes at all time points after treatment. Further, we found that the EWM and hybrid genotype differed in their 9-cis-epoxycarotenoid dioxygenase (NCED) and abscisic acid (ABA) gene response, and that there was a greater amount of photosynthesis gene downregulation (both in number and logfold-change) in the EWM than the hybrid genotype. At the constitutive level, overall, the hybrid expressed genes at a higher level than the EWM genotype, but not the genes of the 2,4-D response pathway. These differences in gene expression match with the degree of phenotypic difference in growth observed between these genotypes when exposed to 2,4-D. The hybrid genotype used here mitigates the effects of 2,4-D treatment better than the EWM genotype at both the molecular and phenotypic level. More study is needed to understand the mechanism (s) of mitigation and whether this is a cause of hybridity, or the specific genotypic backgrounds used here.