Abstract
Time-lapse imaging of dark-grown Arabidopsis (Arabidopsis thaliana) hypocotyls has revealed new aspects about ethylene signaling. This study expands upon these results by examining ethylene growth response kinetics of seedlings of several plant species. Although the response kinetics varied between the eudicots studied, all had prolonged growth inhibition for as long as ethylene was present. In contrast, with continued application of ethylene, white millet (Panicum miliaceum) seedlings had a rapid and transient growth inhibition response, rice (Oryza sativa 'Nipponbare') seedlings had a slow onset of growth stimulation, and barley (Hordeum vulgare) had a transient growth inhibition response followed, after a delay, by a prolonged inhibition response. Growth stimulation in rice correlated with a decrease in the levels of rice ETHYLENE INSENSTIVE3-LIKE2 (OsEIL2) and an increase in rice F-BOX DOMAIN AND LRR CONTAINING PROTEIN7 transcripts. The gibberellin (GA) biosynthesis inhibitor paclobutrazol caused millet seedlings to have a prolonged growth inhibition response when ethylene was applied. A transient ethylene growth inhibition response has previously been reported for Arabidopsis ethylene insensitive3-1 (ein3-1) eil1-1 double mutants. Paclobutrazol caused these mutants to have a prolonged response to ethylene, whereas constitutive GA signaling in this background eliminated ethylene responses. Sensitivity to paclobutrazol inversely correlated with the levels of EIN3 in Arabidopsis. Wild-type Arabidopsis seedlings treated with paclobutrazol and mutants deficient in GA levels or signaling had a delayed growth recovery after ethylene removal. It is interesting to note that ethylene caused alterations in gene expression that are predicted to increase GA levels in the ein3-1 eil1-1 seedlings. These results indicate that ethylene affects GA levels leading to modulation of ethylene growth inhibition kinetics.
Highlights
Time-lapse imaging of dark-grown Arabidopsis (Arabidopsis thaliana) hypocotyls has revealed new aspects about ethylene signaling
ETHYLENE INSENSITIVE2 (EIN2) acts in part to directly or indirectly reduce ubiquitination of EIN3 and EIL1 by SCFEBF1/2 leading to an increase in EIN3 and EIL1 levels and most ethylene responses
Arabidopsis Columbia (Col) seedling hypocotyls grew in air with a rate of 0.32 6 0.01 mm h21 that diminished to a steady-state rate of 0.04 6 0.01 mm h21 upon application of 10 mL L21 ethylene (Supplemental Fig. S1)
Summary
Time-lapse imaging of dark-grown Arabidopsis (Arabidopsis thaliana) hypocotyls has revealed new aspects about ethylene signaling. It is interesting to note that ethylene caused alterations in gene expression that are predicted to increase GA levels in the ein eil seedlings These results indicate that ethylene affects GA levels leading to modulation of ethylene growth inhibition kinetics. Regulation of EIN3 and EIL1 involves selective proteolysis via the ubiquitin/26S proteasome pathway using SPHASE KINASE-ASSOCIATED1-CULLIN-F-BOX (SCF) E3 ubiquitin ligase complexes containing EIN3-BINDING F-BOX1 and EBF2 F-box proteins (Guo and Ecker, 2003; Potuschak et al, 2003; Yanagisawa et al, 2003; Gagne et al, 2004; Binder et al, 2007; An et al, 2010) This model posits that ethylene binding to the receptors reduces the activity of the receptors, leading to reduced activity of CTR1 and an increase in EIN2 protein levels along with subsequent signaling associated with it (Qiao et al, 2009). We undertook a comparative study examining the growth response kinetics to the application and removal of ethylene in the shoots of several eudicot and monocot plant species
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