# Abiotic factors

Preceding frequent abiotic stress can acclimatize plants by inducing a change in the epigenetic state, and the persistence of the induced state can be plastic. The epigenetic regulation of the abiotic stress response is complex in nature and could be interlinked with genetic networks or can be an independent event. From literature, we could observe a gap in in-depth studies conducted in natural populations. Most studies are done in artificial environments with model plants or cultivars. Reviews by Bej and Basak (2017) and Li et. al (2017) combined the information on abiotic factors and the contribution of different epigenetic mechanisms for different species (Bej & Basak 2017; Li et al. 2017). ‌In the natural environment, plants are exposed to many abiotic stresses such as salinity, drought, temperature, heavy metals. Epigenetic control of stress-responsive mechanisms was observed in several plant species under various abiotic stress conditions, such as extreme temperatures (Ding et al. 2019), drought (Huang et al. 2019), salinity (Yang and Guo 2018), herbivory, and pathogen (Holeski 2007). For example, increased salinity was associated with DNA methylation changes, histone acetylation, methylation, and phosphorylation in species like rice, *Arabidopsis thaliana*, tobacco, and mangrove plants (Kim et al. 2015).&#x20;

In *Arabidopsis thaliana,* histone modifications are involved in the drought stress response (Kim et al. 2015). For heat stress, DNA methylation differed between heat-sensitive and heat-tolerant genotypes in rapeseed (Gao et al.). And in forest trees (Cork oak), an interplay between DNA methylation and H3 acetylation was observed at elevated temperatures (Correia et al. 2013). Also, cold stress response in *A. thaliana* and maize affect DNA methylation and histone acetylation (Steward et al. 2002). An interesting example was recently reported by Song et al. (2015), who found that the alpine subnival plant *Chorispora bungeana* revealed DNA methylation changes that correlated with the exposure to chilling and freezing. Notably, several of the candidate genes were related to physiological chilling and freezing resistance pathways (Song et al. 2015). In summary, induced plant response following abiotic stress seems to be closely related to epigenetic mechanisms that even take an active role in the acclimatization to changing environmental conditions.

> Table 1: List of epigenetic modifications reported for abiotic stress:

| **Components**            | **Stress**                                                                                | **Species**       | **Function**                                                                       | **References**                                                        |
| ------------------------- | ----------------------------------------------------------------------------------------- | ----------------- | ---------------------------------------------------------------------------------- | --------------------------------------------------------------------- |
| **DNA methylation**       |                                                                                           |                   |                                                                                    |                                                                       |
| ZmMI1                     | Cold stress                                                                               | Maize             | Stress-induced non-reversible demethylation                                        | Steward et al. 2000                                                   |
| Ac/Ds                     | Cold stress                                                                               | Maize             | Demethylation of transposon Ac/Ds                                                  | Steward et al. 2002                                                   |
| Tam 3                     | Low temp                                                                                  | Antirrhinum majus | Decrease in methylation                                                            | Hashida et al. 2006                                                   |
| NtGPDL                    | Aluminum, low temp, salt stress                                                           | Tobacco           | Demethylation at coding region of gene                                             | Choi and Sano 2007                                                    |
| HRS60 and GRS             | Salt, osmotic stress                                                                      | Tobacco           | Reversible DNA hypermethylation                                                    | Kovarˇik et al. 1997                                                  |
| **Histone modifications** |                                                                                           |                   |                                                                                    |                                                                       |
| AtGCN5                    | Cold stress                                                                               | A. thaliana       | Affect expression of COR genes                                                     | Stockinger et al. 2001 Vlachonasios et al. 2003                       |
| Ada2b                     | Freezing, salt stress                                                                     | A. thaliana       | Induces COR genes                                                                  | Vlachonasios et al. 2003                                              |
| SKB1                      | Salt stress                                                                               | A. thaliana       | Trimethylation of H4K3                                                             | Zhang et al. 2011                                                     |
| ABO1/ELO1                 | Drought stress                                                                            | A. thaliana       | Drought tolerance                                                                  | Chen et al. 2006                                                      |
| ADH1 and PDC1             | Submergence stress                                                                        | Rice              | Histone modifications of H3                                                        | Tsuji et al. 2006                                                     |
| HD6                       | Freezing stress                                                                           | A. thaliana       | Upregulation confer tolerance                                                      | To et al. 2011                                                        |
| HOS15                     | Cold stress                                                                               | A. thaliana       | Deacetylation of histone H4                                                        | Zhu et al. 2008                                                       |
| HDA6                      | Drought stress,cold                                                                       | A. thaliana       | Deacetylation                                                                      | Kim et al. 2017 Jung et al. 2013                                      |
| HDA9                      | Drought and salinity                                                                      | A. thaliana       | Deacetylation                                                                      | Zheng et al. 2016                                                     |
| HDA15                     | Drought                                                                                   | A. thaliana       | Deacetylation                                                                      | Lee and Seo 2019)                                                     |
| HDA19                     | Drought, heat, salinity                                                                   | A. thaliana       | Deacetylation                                                                      | Ueda et al. 2018a Chen and Wu 2010 Mehdi et al. 2016 Ueda et al. 2017 |
| HDA705                    | Salinity                                                                                  | Rice              | Deacetylation                                                                      | Zhao et al. 2016                                                      |
| BdHD1                     | Drought                                                                                   | Brachypodium      | Deacetylation                                                                      | Song et al. 2019                                                      |
| ATX4/5                    | Drought                                                                                   | A. thaliana       | Methyltransferase                                                                  | Liu et al. 2018)                                                      |
| CAU1/PRMT5/SKB1           | Drought and salinity                                                                      | A. thaliana       | Methyltransferase                                                                  | Fu et al. 2013 Zhang et al. 2011                                      |
| JMJ15                     | Salinity                                                                                  | A. thaliana       | Demethylase                                                                        | Shen et al. 2014                                                      |
| JMJ17                     | Dehydration                                                                               | A. thaliana       | Demethylase                                                                        | Huang et al. 2019                                                     |
| JMJ15                     | Salinity                                                                                  | A. thaliana       | Demethylase                                                                        | Shen et al. 2014                                                      |
| JMJ17                     | Dehydration                                                                               | A. thaliana       | Demethylase                                                                        | Huang et al. 2019                                                     |
| **Small RNA**             |                                                                                           |                   |                                                                                    |                                                                       |
| miR398                    | oxidative stress-causing agents such as high light levels, Cu2+, Fe3+ and methyl viologen | A. thaliana       | posttranscriptional CSD1 and CSD2 mRNA accumulation and oxidative stress tolerance | Sunkar et al. 2007                                                    |
| miR393                    | Cold, dehydration, NaCl, and ABA stress                                                   | A. thaliana       | miR393 is strongly upregulated by mentioned treatments                             | Sunkar and Zhu 2004                                                   |
