Fig. 1 and S1 showed several parameters of lateral root formation, namely lateral root (LR) number, density, length, and number of lateral root primordia (LRP), in tomato seedlings upon the indicated concentrations of naphthalene acetic acid (NAA, a synthetic auxin), N-1-naphthylphthalamic acid (NPA; IAA depletion) and indole-3-acetic acid (IAA, a kind of nature auxin). In comparison with control samples, we observed that the pretreatment of 100 nM NAA or IAA could significantly induce lateral root formation without severely impairing the growth of primary root in tomato seedlings, which could be caused by 1000 nM NAA or IAA. By contrast, 200 nM NPA was able to significantly prevent lateral root formation. Further results showed that exogenously applied sodium hydrosulfide (NaHS, concentrations between 0.01 and 5 mM), an H2S donor, was able to dose-dependently rescued NPA-achieved inhibition of lateral root formation, with a maximal response at 1 mM (Fig. 1). Dose-dependent induction of lateral root formation was also observed when NaHS was applied alone, although its inducing effects were weaker than the action of 100 nM NAA or IAA regarding the changes of LR density. Additionally, no additive effects were observed when NAA was co-treated with NaHS at concentrations lower than 10 mM. Similar results were also obtained when IAA treatment was used instead of NAA (Fig. S2), clearly indicating a possible interrelationship between auxin and H2S during lateral root formation.