Ethylene and rooting of mung bean cuttings. The role of auxin induced ethylene synthesis and phase-dependent effects

Abstract

We have re-examined the role of ethylene during rooting of mung bean cuttings. Cuttings were treated for 5 days with a low or a high concentration of NAA (naphthaleneacetic acid). During this 5 days period, we also applied STS (silverthiosulfate, an inhibitor of ethylene action) or ACC (1-aminocyclopropane-l-carboxylic acid, a direct precursor of ethylene). At high NAA concentration, STS promoted and ACC inhibited rooting, respectively. At low NAA concentration, the effects were opposite, STS being inhibitory and ACC promotive. AVG (aminoethoxyvinylglycine, an inhibitor of ethylene synthesis) gave similar results as STS. Together, these data suggest supraoptimal and suboptimal ethylene levels in the tissue at high and low NAA concentration, respectively. We also examined whether the effect of ethylene varied during the successive phases of the rooting process. Thus, we gave 24 h pulses with either STS or ACC during the rooting treatment. During the first two days (0-48 h), ACC-pulses were promotive and STS-pulses inhibitory. Later on (48-168 h), the ACC-pulses were inhibitory and the STS-pulses promotive. Whether this effect was observed or not was dependent on the NAA concentration. These data indicate that ethylene promotes or inhibits rooting depending on the stage in the rooting process. When ACC was added only during the initial period, rooting was increased at all NAA concentrations in a NAA dose-response curve and the optimal NAA concentration remained the same. This suggests that ethylene renders more cells responsive to NAA.

Ethylene Controls Adventitious Root Initiation Sites in Arabidopsis Hypocotyls Independently of Strigolactones

“…For example, in tomato ACC enhanced adventitious root formation at 1 and 10 µM (Negi and others 2010), whereas in tobacco 2.1 µL L −1 ethylene treatments inhibited adventitious root formation (McDonald and Visser 2003). In mung bean, ethylene both promoted and inhibited adventitious rooting depending on the auxin status of the seedlings (De Klerk and Hanecakova 2008). By studying the distribution of adventitious roots, interactions between ethylene and other hormones may become clearer, such as demonstrated here for strigolactones.…”

“…2a). In addition, it has been shown in mung bean hypocotyls that ethylene inhibits adventitious root formation at higher auxin levels but promotes adventitious root formation at lower auxin levels (De Klerk and Hanecakova 2008). Since the strigolactone mutants have been reported to have enhanced auxin transport (Bennett and others 2006;Crawford and others 2010;Shen and others 2012), this may explain the relatively larger inhibition of adventitious rooting seen in strigolactone mutants.…”

“…To determine the order of action of ethylene and strigolactone in regulating adventitious root formation, the effect of increased concentrations of ACC on strigolactone-insensitive (max2) and strigolactone-deficient (max4) mutants was analyzed in parallel. One and 10 μM are commonly used concentrations of ACC (Ruzicka and others 2007;Swarup and others 2007;De Klerk and Hanecakova 2008; Negi and others 2010; Ellison and others 2011) so we chose a dose-response assay which included these concentrations. As a stress hormone, ethylene can accumulate to very high levels.…”

“…In apple (Malus domestica Borkh), ethylene appeared to have no effect on adventitious rooting (Harbage and Stimart 1996). Adding even more complexity, at high auxin levels ethylene is inhibitory in mung bean and eucalyptus hybrids (De Klerk and Hanecakova 2008;Kilkenny and others 2012), whereas at low auxin levels ethylene promoted adventitious rooting in mung bean (De Klerk and Hanecakova 2008).…”

Strigolactones fine-tune the root system

“…So far, there is no direct evidence for a link between strigolactone and ethylene in regulating adventitious rooting; however, ethylene is known to regulate adventitious roots in different ways depending on the conditions. For instance, the activity is strongly dependent on the concentration because at high auxin levels ethylene is inhibitory (De Klerk and Hanecakova 2008;Kilkenny et al 2012), whereas at low auxin levels ethylene promotes adventitious rooting (De Klerk and Hanecakova 2008). More research is required to determine whether ethylene, strigolactones and auxin interact to regulate adventitious rooting.…”