Using a single transgenic event to infer fitness effects in crop-weed hybrids: a reply to the Letter by Grunewald & Bury (2014) Grunewald & Bury (2014; in this issue of New Phytologist, pp. 367-369) criticize our recent peer-reviewed paper (Wang et al., 2014; in this issue of New Phytologist, pp. 679-683), stating that we 'unnecessarily harm the sensitive debate on GM crops.' We will not focus on this politically charged topic here, but we do want to address scientific questions about our study of transgenic cropweed (Oryza sativa and O. sativa f. spontanea) hybrids of rice. Grunewald and Bury propose that an insertion effect that stimulated tiller formation in the EP3 crop parent of these hybrids offers a more convincing explanation of our results than direct effects of a transgene for over-expressing 5-enolpyruvoylshikimate-3-phosphate synthase (epsps) (our central hypothesis). In our study, the genetically engineered (GE) segregants had significantly greater expression of epsps and produced significantly greater amounts of the enzyme of EPSPS than the non-GE segregants, as expected (Wang et al., 2014). As discussed later, we doubt that an insertion effect could account for the clear and significant increases in our transgenic lines in terms of enhanced fecundity, greater tryptophan concentrations in leaves, and other traits, which are more parsimoniously explained by transgenic over-expression of epsps and its key role in the shikimic acid pathway (Wang et al., 2014). Tryptophan is produced by this pathway and is a precursor of growth hormones (auxin) and secondary metabolites that play a role in plant defense (e.g. Maeda & Dudareva, 2012). Tryptophan is just one of many products of the shikimic acid pathway, which can account for as much as 35% of a plant's biomass (e.g. Franz et al., 1997).To review, our two crop parental rice lines (Fig. 1) were the inbred line Minghui-86 and a transgenic rice line (EP3), which was obtained by transforming Minghui-86 (Su et al., 2008; Lu et al., 2014 (this issue of New Phytologist, pp. 363-366); Wang et al., 2014). Thus, these lines differed only in the absence or presence of a single-copy insertion of the transgenic construct and possible unknown side-effects of transformation. Su et al. (2008) and our Supporting Information Table S1 (Wang et al., 2014) showed that EP3 produced significantly more tillers and panicles per plant than Minghui-86. This suggests a direct effect of the transgene on plant growth and reproduction in EP3. We do not understand why Grunewald and Bury do not even acknowledge this explanation under 'option (2)' of their letter. Instead, they assume that the superior performance of EP3 was due to a linked sequence that