Abstract
Metabolism of molecular hydrogen (H&sub2;) in bacteria and algae has been widely studied, and it has attracted increasing attention in the context of animals and plants. However, the role of endogenous H&sub2; in lateral root (LR) formation is still unclear. Here, our results showed that H&sub2;-induced lateral root formation is a universal event. Naphthalene-1-acetic acid (NAA; the auxin analog) was able to trigger endogenous H&sub2; production in tomato seedlings, and a contrasting response was observed in the presence of N-1-naphthyphthalamic acid (NPA), an auxin transport inhibitor. NPA-triggered the inhibition of H&sub2; production and thereafter lateral root development was rescued by exogenously applied H&sub2;. Detection of endogenous nitric oxide (NO) by the specific probe 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM DA) and electron paramagnetic resonance (EPR) analyses revealed that the NO level was increased in both NAA- and H&sub2;-treated tomato seedlings. Furthermore, NO production and thereafter LR formation induced by auxin and H&sub2; were prevented by 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO; a specific scavenger of NO) and the inhibitor of nitrate reductase (NR; an important NO synthetic enzyme). Molecular evidence confirmed that some representative NO-targeted cell cycle regulatory genes were also induced by H&sub2;, but was impaired by the removal of endogenous NO. Genetic evidence suggested that in the presence of H&sub2;, Arabidopsis mutants nia2 (in particular) and nia1 (two nitrate reductases (NR)-defective mutants) exhibited defects in lateral root length. Together, these results demonstrated that auxin-induced H&sub2; production was associated with lateral root formation, at least partially via a NR-dependent NO synthesis.