Cold acclimation-induced freezing tolerance ofMedicago truncatulaseedlings is negatively regulated by ethylene

Abstract

To evaluate the role of ethylene in cold acclimation and cold stress, freezing tolerance and characteristics associated with cold acclimation were investigated using legume model plant Medicago truncatula Gaertn Jemalong A17. There was a rapid suppression of ethylene production during cold acclimation in A17 plants. Ethylene level was negatively correlated with freezing tolerance as inhibition of ethylene biosynthesis by inhibitors of ethylene biosynthesis enhanced freezing tolerance, while exogenous application of ethylene reduced cold acclimation-induced freezing tolerance. The involvement of ethylene signaling in modulation of freezing tolerance and cold acclimation was further studied using ethylene-insensitive mutant sickle skl. Although skl mutant was more tolerant to freezing than its wild-type counterpart A17 plants, cold acclimation enhanced freezing tolerance in 17 plants, but not in skl mutant. Expression of several ethylene response genes including EIN3, EIN3/EIL and ERFs was suppressed in skl mutant compared to A17 plants under non-cold-acclimated conditions. Cold acclimation downregulated expression of EIN3, EIN3/EIL and ERFs in A17 plants, while expression patterns of these genes were relatively constant in skl mutant during cold acclimation. Cold acclimation-induced increases in transcription of MtCBFs and MtCAS15 were suppressed in skl mutant compared with A17 plants. These results suggest that MtSKL1 is required for perception of the change of ethylene level in M. truncatula plants for the full development of the cold acclimation response by suppressing expression of MtEIN3 and MtEIN3/EIL1, which in turn downregulates expression of MtERFs, leading to the enhanced tolerance of M. truncatula to freezing by upregulating MtCBFs and MtCAS15.

Mechanistic Insights in Ethylene Perception and Signal Transduction

“…In freezing tolerance, which is negatively regulated by ethylene (e.g. Zhao et al, 2014), EIN3 binds to the promoters of cold-induced C-repeat Binding Factor/ Dehydration-Responsive Element (DRE) Binding Factor (CBF) genes and type-A ARABIDOPSIS RESPONSE REGULATOR (ARR) genes to prevent their transcription (Shi et al, 2012;Fig. 3A).…”

Expression of an exogenous 1-aminocyclopropane-1-carboxylate deaminase gene in psychrotolerant bacteria modulates ethylene metabolism and cold induced genes in tomato under chilling stress

“…Shi et al (2012) showed that, increase in ethylene levels led to reduced tolerance to freezing temperatures and blocking of ethylene biosynthesis as well as its signaling resulted in enhanced freezing tolerance. Addition of Ethephon caused a reduction in the expression of cold regulated genes MtCBF1 and MtCBF3 within hours of exposure to 4 C and their regulator MtCBF2 expression, found to increase (Zhao et al, 2014). This implies that ethylene reduces the expression of cold-induced genes in Medicago trunculata plants.…”

“…As mentioned earlier ethylene and its precursor compounds, have been reported to cause reduced expression of cold regulated genes MtCBF1 and MtCBF3 (Zhao et al, 2014). Moreover, the tripartite interactions between ethylene, protein RCI1A and cold acclimation in Arabidopsis thaliana has established an indirect relationship of ethylene in regulating cold acclimation genes (Catala et al, 2014).…”

“…Chilling induced ethylene emission starts as early as 12 h on exposure in case of rye plants (Yu et al, 2001). In case of M. trunculata, ethylene levels increase in non-acclimated plants in response to cold at 4 C (Zhao et al, 2014). Ethylene production in rye plants due to cold (at 5 C) remained high from 12 h up to 7 days (Yu et al, 2001).…”

“…While the ACCD oxidase transcripts in uninoculated, OR306-wt and OS211-wt inoculated plants remained at 0.99, 1.0 and 1.1 (normalized fold expression) respectively. In a recent report, the process of cold acclimation reduced ethylene emission in M. trunculata seedlings and down regulated expression of ethylene transcriptions family members and ethylene response factors (ERFs) (Zhao et al, 2014). Also, chilling effect on avocado fruits have been reported to induce ethylene biosynthesis and its receptor gene, PaETR expression (Hershkovitz et al, 2009).…”