Abstract
Stress can be remembered by plants in a form of ‘stress memory’ that can
alter future phenotypes of previously stressed plants and even
phenotypes of their offspring. It was shown that DNA methylation is
among the mechanisms mediating the memory. It is not known for how long
the memory is kept by plants. If the memory is long lasting, it can
become maladaptive in situations when parental-offspring environment
differ. We investigated for how long can a parental plant “remember”
that it experienced a stress and pass the memory to its clonal
offspring. We grew parental plants of three genotypes of Trifolium
repens for five months either in control conditions or in control
conditions that were interrupted with drought pulses applied for two
months in four different time-slots. We also treated half of the
parental plants with 5-azacytidine (5-azaC) to test for the potential
role of DNA methylation in the stress memory. Then, we transplanted
parental cuttings (ramets) individually to control environment and
allowed them to produce offspring ramets for two months. The drought
stress experienced by parents affected phenotypes of offspring ramets.
Such a memory resulted in enhanced number of offspring side branches
originating from plants that experienced drought stress maximally 6
weeks before their transplantation to control environment. We did not
find any transgenerational memory in offspring of plants that
experienced drought stress later than 6 weeks before their
transplantation. 5-azaC also reduced the effect of transgenerational
memory on offspring ramets. We confirmed that drought stress can trigger
transgenerational memory in T. repens that is very likely mediated by
DNA methylation. Most importantly, the memory was time limited and was
gradually erased. We conclude that the time limited memory on
environmental stress can be adaptive as climate tends to be variable and
parental-offspring environmental conditions often do not match.