Given the fact that the tripeptide thiol glutathione (GSH) might play an important role in directing root morphogenesis (Sánchez-Fernández et al., 1997), we further detected the distribution of GSH in vivo ( Fig. S3). Interestingly, the fluorescent intensity in the potential lateral root-inducible segments was stronger in NaHS- and NAA-treated tomato seedling roots. Comparatively, a weaker response was observed in NPA- or HT-treated samples, implying the possible interaction between the increased GSH content and the induction of lateral root formation triggered by NaHS and NAA. This interplay should be further investigated.