What is plant memory?

Before diving in, I want to quickly mention that a general knowledge of DNA modifications and chromatin structure is recommended to fully understand the memory processes I will be discussing in this post. If you are not fully comfortable with these topics, I highly recommend watching either of these two lessons offered by Khan Academy:

1. DNA and chromatin regulation (5 min)

2. Chromosomes, chromatids, and chromatin (18 min)

Unlike us, plants do not have a brain to associate ‘memories’ with it; hence, when referring to plant memory we are often describing how cells and their molecules respond to stresses. Stress can be defined as anything out of the ordinary that affects the plant, and it can be abiotic (e.g. heat, frost, drought, etc.) or biotic (e.g. insects, pathogens, bacteria, etc.). Battling different stressors is normal for plants, but climate change makes this battle more unpredictable and severe each year. For instance, the 2019 Global Climate Report from NOAA National Centers for Environmental Information [1] reported that since the 1980s the combined ocean and land temperature increased at an approximate annual rate of 0.15°C. This temperature rise has a domino effect that ultimately can cause extreme growth conditions that inhibit normal plant development.

There is a growing number of studies [2-4] suggesting that plants keep a memory of past events using epigenetic modifications to help them prepare for reoccurring ones, such as temperature increase. Epigenetics has two general definitions:

• One refers to genetic changes that are mitotic and/or meiotically inheritable without altering the DNA sequences. These types of changes can alter the transcription levels by remodeling the chromatin structure to either activate or deactivate a response. For example, DNA demethylation leads to an open chromatin conformation which activates gene expression, while methylation does the exact opposite.

• The other one (a more practical definition in my opinion!) refers to a phenomenon in which genetically identical cells express their genomes differently when exposed to different biotic or abiotic stressors, leading to phenotypic differences.

Figure 1. The priming response of a plant after exposure to dehydration. The plant initially wilts under dehydration but recovers after a period of rain (rehydration). During the second drought stress, the plant 'remembers' the past experience which helps it to achieve better resistance and enhance its survival. Adapted from Kimoshita and Seki [7].

Plants can adjust their responses to a triggering stress cue through priming [4,5]. Priming describes the phenomenon by which a short environmental stimulus modifies a plant for future stress exposure (a ‘triggering stress cue’). This may lead to faster, stronger, sensitized, or altered responses in order to deal with the stress cue [4]. For instance, cold priming helps the common grapevine to suffer less cellular damage when exposed to heat stress [6], which is becoming a common occurrence due to climate change. The cold priming process benefits the plants because it triggers the synthesis of important plant hormones and proteins such as salicylic acid and heath-shock protein which stabilizes the plant processes under stress [6]. Perhaps the next time you are enjoying a glass of wine, you can share this cool (or hot) fact with your friends!

If the plants are successful at adapting/surviving the stress, then they will be primed or have a memory (Figure 1). This means that the DNA alterations may produce enhanced fitness traits for the organism, and it would only make sense that the plant would want to remember how to do that. However, there are costs and benefits to everything, and plant memory is no exception!

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References

1. NOAA National Centers for Environmental Information. 2020. State of the Climate: Global Climate Report for Annual 2019. https://www.ncdc.noaa.gov/sotc/global/201913.

2. Chinnusamy, V., Z. Gong, and J. Zhu. 2008. Abscisic acid-mediated epigenetic processes in plant development and stress responses. Journal of integrative plant biology 50:1187–1195.

3. Thiebaut, F., A. Hemerly, P. Cavalcanti, and G. Ferreira. 2019. A role for epigenetic regulation in the adaptation and stress responses of non-model plants. Frontiers in Plant Science 10:1-7.

4. Lamke, J., and I. Baurle. 2017. Epigenetic and chromatin-based mechanisms in environmental stress adaptation and stress memory in plants. Genome Biology 18:124. https://doi.org/10.1186/s13059-017-1263-6

5. Hossain, M., Z. Li, T. S. Hoque, D. Burritt, M. Fujita, and S. Munne-Bosch. 2018. Heat or cold priming-induced cross-tolerance to abiotic stresses

in plants: key regulators and possible mechanisms. Protoplasma 255:399-412.

6. Wan, S. B., L. Tian, R. Tian, Q. Pan, J. Zhan, P. Wen, J. Chen, P. Zhang, W. Wang, and W. Huang. 2009. Involvement of phospholipase D in the low temperature acclimation-induced thermotolerance in grape berry. Plant Physiol Biochem 47:504–510.

7. Kinoshita, T., and M. Seki. 2014. Epigenetic memory for stress response and adaptation in plants. Plant Cell Physiol 55:1859–1863.