Epigenetic mechanisms of abiotic stress response and memory in plants

Being sessile organisms, plants are exposed to multiple stimuli without possibility for escape. Therefore, plants have evolved to be able to adapt their developmental and physiological responses to the surrounding environment. Some environmental stresses will rarely occur during the life of the plant, but others, such as seasonal drought or heat, can be recurrent. Therefore, plant responses to these stresses can be transient to provide plants with the required tools to acclimate and survive, whereas others may promote a state that we will refer to as "memory" throughout the chapter, which predisposes the plant for a more efficient stress response upon next encounter of stress. The possibility of transferring this memory to the next generation has been also proposed, which implies a lack of resetting of the priming memory during sexual reproduction. Different epigenetic and chromatin-related modifications such as DNA methylation, histone modifications, and chromatin remodeling have been associated with the memory to both biotic and abiotic stresses. This chapter reviews how and which epigenetic processes are involved in remembering a past abiotic stress event and also forgetting it. Contradictory arguments concerning transgenerational memory and its implications in phenotypic variation are critically discussed. In addition, the stability of epigenetic modifications during asexual propagation and its impact on clonally propagated plants is addressed. Finally, we mention possible agricultural implications of the epigenetic mechanisms involved in plant memory and propose future applications for breeding of epigenetically modified crops considering new challenges arising from climate change.