Indole acetic acid (IAA) is an important regulator of adventitious rooting via the activation of complex signaling cascades. In animals, carbon monoxide (CO), mainly generated by heme oxygenases (HOs), is a significant modulator of inflammatory reactions, affecting cell proliferation and the production of growth factors. In this report, we show that treatment with the auxin transport inhibitor naphthylphthalamic acid prevented auxin-mediated induction of adventitious rooting and also decreased the activity of HO and its by-product CO content. The application of IAA, HO-1 activator/CO donor hematin, or CO aqueous solution was able to alleviate the IAA depletion-induced inhibition of adventitious root formation. Meanwhile, IAA or hematin treatment rapidly activated HO activity or HO-1 protein expression, and CO content was also enhanced. The application of the HO-1-specific inhibitor zinc protoporphyrin IX (ZnPPIX) could inhibit the above IAA and hematin responses. CO aqueous solution treatment was able to ameliorate the ZnPPIX-induced inhibition of adventitious rooting. Molecular evidence further showed that ZnPPIX mimicked the effects of naphthylphthalamic acid on the inhibition of adventitious rooting, the down-regulation of one DnaJ-like gene (CSDNAJ-1), and two calcium-dependent protein kinase genes (CSCDPK1 and CSCDPK5). Application of CO aqueous solution not only dose-dependently blocked IAA depletion-induced inhibition of adventitious rooting but also enhanced endogenous CO content and up-regulated CSDNAJ-1 and CSCDPK1/5 transcripts. Together, we provided pharmacological, physiological, and molecular evidence that auxin rapidly activates HO activity and that the product of HO action, CO, then triggers the signal transduction events that lead to the auxin responses of adventitious root formation in cucumber (Cucumis sativus).

We are grateful for the grant obtained from the program for new century excellent talents in university, and the 111 project of China (grant no. B07030). We also thank Dr Meixue Zhou and Dr Evan Evans from the University of Tasmania, Australia, for their kind help in writing the manuscript.

Salt stress induced an increase in endogenous carbon monoxide(CO) production and the activity of the CO syntheticenzyme haem oxygenase (HO) in wheat seedling roots. Inaddition, a 50% CO aqueous solution, applied daily, notonly resulted in the enhancement of CO release, but led toa significant reversal in dry weight (DW) and water losscaused by 150 mM NaCl treatment, which was mimicked bythe application of two nitric oxide (NO) donors sodiumnitroprusside (SNP) and diethylenetriamine NO adduct(DETA/NO). Further analyses showed that CO, as well asSNP, apparently up-regulated H+-pump and antioxidantenzyme activities or related transcripts, thus resulting inthe increase of K/Na ratio and the alleviation of oxidativedamage. Whereas, the CO/NO scavenger haemoglobin(Hb), NO scavenger or synthetic inhibitor methylene blue(MB) or NG-nitro-L-arginine methyl ester hydrochloride(L-NAME) differentially blocked these effects. Furthermore,CO was able to mimic the effect of SNP by stronglyincreasing NO release in the root tips, whereas theCO-induced NO signal was quenched by the addition ofL-NAME or cPTIO, the specific scavenger of NO. Theresults suggested that CO might confer an increased toleranceto salinity stress by maintaining ion homeostasis andenhancing antioxidant system parameters in wheat seedlingroots, both of which were partially mediated by NO signal.