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What Do Plants Remember

We all know that memories are actively handed down in humans through storytelling, rituals and pictures to mention a few, and this occurs from one generation to the next. But there is a different type of transgenerational memory involving epigenetics, and this is completely different.

Epigenetics  is the study of the process by which genetic information is translated into the substance and behavior of an organism; specifically, the study of the way in which the expression of heritable traits is modified by environmental influences or other mechanisms without a change to the DNA sequence. Hence, it is a study of the chemical modification of specific genes or gene-associated proteins of  an organism.  Researchers have uncovered a range of possible chemical modifications to DNA, and to proteins called histones that associate tightly with DNA in the nucleus.  These modifications can determine when, or even if, a given gene is expressed in a cell or organism.

This type of memory usually involves information regarding some form of stress, either environmental or physical, that is passed down from parents to offspring.  Barbara Hohn’s lab in Basel, Switzerland was the first to provide evidence for such transgenerational memory in plants. She and her colleagues knew the conditions that create stress on a plant, such as ultraviolet light or pathogen attack, would lead to changes in the plant genome that result in modification of its DNA.

These stress induced changes make sense, because plants like any other organism need to find ways to survive under such conditions.  Hohn’s study showed that not only do the stressed plants make modifications to their DNA, but their offspring also make the new modifications, even though they themselves had not been exposed to the stressors.

The stress in the parents caused a stable heritable change that was passed on to all their offspring, and they remembered that their parents had been through this stress and reacted similarly.  The use of the word remembered  may seem strange when referring to plants, but let’s look at it in this way:  the parents had a memory of a stressor, retained it and then passed it on to their offspring, who then recalled that information and reacted in kind, and in this case with genomic changes.  The implications of Hohn’s study are enormous!  Hohn’s plants post stressor exposure, had increased genetic variation and passed it on to all of their offspring ( in this case the arabidopsis plant which produces thousands of seeds!)  This can not be explained by mutations in the DNA sequence of the stressed plants, because this could only be passed on to a very small percentage of the offspring.  On the other hand, if the stressors produce epigenetic changes, this would happen in ALL the cells at once, including pollen and egg cells, and be passed down to the entire next generation as well as future ones!!!

Scientists are still uncertain as to what exactly is the nature of the epigenetic change involved, and Igor Kovalchuk created a flow-up study using other stressors on genetic variation in plants and their offspring, including heat and salt.  Kovalchuk’s results showed that not only did the second generation of plants show genetic variation, confirming Hohn’s results, but that they were also more tolerant to the various stressors.  So in other words, the stressed parents gave rise to offspring that grew better under harsh conditions compared with regular plants.

Kovalchuk was certain that the stressors induced epigenetic changes in chromatin structure in the parents, which were then passed on, because if the offspring were treated with a chemical that wiped out the epigenetic information, these same plants lost their ability to thrive under environmental stress.  The idea of stress leading to memories that are passed down from one generation to the next is supported by increasing numbers of studies, not only in plants, but in animals as well.  In all cases, this memory is based on some form of epigenetic heredity.

The oaks and the pines, and their brethren of the wood, have seen so many suns rise and set, so many seasons come and go, and so many generations pass into silence, that we may well wonder what “the story of the trees” would be to us if they had tongues to tell it, or we ears fine enough to understand.   – Maud van Burns, Quotations for Special Occasions

 

 

The research for this blog post was found in multiple sources including:  What a Plant Knows, Daniel Chamovitz

 

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