Under the influence of abiotic and biotic stress factors, gene expression

can be altered with or without the involvement of stress hormones (a);

changes in gene transcription may be made (b); stress factors can directly

affect chromatin by methylation of DNA, modification of histones terminal

parts and influence the condensation and recondensation of chromatin (c).

These changes are largely reversible, but can alter the metabolic or morphological

characteristics of plants under stress conditions. These processes

can contribute substantially to variations in plant growth, influencing morphology

and plasticity, especially under stress conditions. Usually, new phenotypes

are not transmitted to the offspring, although the uniformity of the

characteristics of the new combinations of epigenetic diversity is observed.

The epigenetic inheritance which presupposes the transmission of information

from one generation of an organism to the next that affects the traits of

offspring without alteration of the primary structure of DNA is very rare. In

most researches on epigenetic hereditary phenomena it is not excluded the

involvement of genetic mechanisms (such as quantitative traits, segregation

distortion, and cytoplasmic inheritance) or effects that require the ongoing

presence of the stimulus that can lead to non-Mendelian patterns of inheritance.

Actually, the term epigenetic heredity is often include the transmission

of the acquired in ontogenesis information not only through mitosis, but

also to the next generation through meiosis.

Our researches aimed to investigate the possible implication the epigenetic

phenomena in determining the resistance to heat and cold stress of different

genotypes of hexaploid wheat. In researches of the primary resistance

to high temperatures and frost were involved germinated seeds that represented

12 wheat genotypes with different resistance at extreme temperatures.

One set of seeds was reproduced in the Kharkov region (Ukraine), and

another - in the central area of the Republic of Moldova. Our studies aimed

to determine whether conditions of seed reproduction influenced genotype

primary resistance to high temperatures and frost. Obtained results have

shown that conditions of seeds reproduction substantially influenced the

distribution of wheat genotypes according their resistance to both high temperatures

and frost. At the same time, the distribution of genotypes by their

resistance to high temperatures or frost was different, in both location of

seeds multiplication. Only one genotype from twelve demonstrated high resistance

to both factors, regardless of the zone of seed reproduction. These

data demonstrate that epigenetic phenomena influence the primary resistance

of wheat to extreme temperatures. In our previous researches we have

shown that after the second phase of winter hardening the plants resistance

to frost was different, depending of genotype. These results support the vision

that the processes characteristic for adaptation to extreme temperatures

might involve the epigenetic inheritance. Of particular interest are the

data about kinetics of wheat seeds germination during their incubation at

1 or 4°C. They have demonstrated that the seeds of genotypes with higher

primary resistance to frost germinates slower in mentioned conditions. Surprisingly,

these results has common connotations with the law of Bergonié

and Tribondeau regarding the resistance of biological systems to ionizing

radiation. According to this law the high proliferation rate for cells and high

growth rate for tissue result in increased radiosensitivity: Cells are most radiosensitive

when actively proliferating, highly metabolic, undifferentiated,

and well nourished.

Phenomena related with epigenetics inheritance are observed after treatment

of plants with compounds that induce the establishment of a unique

primed state of defense or resistance. Primed plants show enhanced defense

reactions upon further challenge with biotic or abiotic stress. Our results

demonstrated that primed state, induced in winter wheat genotypes

after treatment seeds before sowing with biostimulator Reglalg, is triggering

wheat plants resistance to heat and cold stress. The influence tends to

remain functional during ontogenesis and in the next generation, without

supplementary treatment. The progeny of primed plants has a higher basal

level of resistance to cold stress and an enhanced capacity to react to additional

priming treatments. When transgenerationally primed plants were

subjected to an additional priming treatment, their descendants displayed a

stronger primed phenotype, suggesting that they can inherit a sensitization

for the priming phenomenon. This is evidence that plants have a memory

of encountered stress situations that allow them to better adapt to changing

conditions. These results confirm the information of scientific literature

that demonstrates the implication of the epigenetic mechanisms underlying

plant defense responses to biotic and abiotic stresses.

Good examples of epigenetic mitotic memory of environmental conditions

represent vernalization of autumn wheat. Vernalization is the exposure to

long-term cold that occurs during this overwintering period and renders

plants competent to flower early in the spring. Seeds of this specie germinate

in the fall; plants overwinter in vegetative state, and subsequently, develop

transition to generative development in the spring season when the

days lengthen. We show that the autumn wheat transition to generative state

during vernalization is accompanied with induction of the transcription of at

least 200 structural genes. Plants with this lifecycle can take advantage of

an ecological niche that enables successful development in the early spring,

when many other plant varieties have just begun to germinate. Although

the specific mechanisms of vernalization vary between species, clear evidence

shows an epigenetic basis of vernalization in Arabidopsis. Before the

extended period of cold, the floral repressor is expressed, in part, by changes

to chromatin, including histone modifications. This altered chromatin and

expression state is then stably transmitted mitotically and renders plants

competent to flower, even in the absence of cold temperature. Vernalization

is not meiotically heritable, because it resets every generation. Failure to reset

the requirement for vernalization could be detrimental, because it would

lead plants to flower rapidly before the onset of winter, reducing overall reproductive

success.

The description of a phenomenon as epigenetic becomes particularly dif-

ficult in organisms that are not tractable to genetic studies. Epigenetics inheritance

suppose the stable transmission of information through mitosis

or meiosis in the absence of the original inducing signal, that also are not the

result of underlying genetic changes. These requirements were only rarely

assured in real experiments and were misapplied with the label epigenetic

without showing the lack of primary sequence differences driving the phenomenon.

By implying different signals, such as small RNAs, are created selfperpetuating

signals that can also trigger RNA-directed DNA methylation,

thereby providing signals that are later translated into potentially heritable

modifications of chromatin. In most cases, the results of the epigenetic legacy

after a complex analysis were based on the transmission of the traits acquired

through mitosis or various purely genetic mechanisms. For example;

transposon insertions can cause unstable phenotypes that behave in unexpected

fashions. Studies on heritable changes of some flax (Linum usitatissimum)

varieties in response to environmental stress also point to genetic

rather than epigenetic changes.

The role of epigenetics in plant development is most likely limited to mitotic

transmission of gene expression states. If the epigenetic memory of developmental

decisions was inherited through meiosis, it would likely interfere

with development of the subsequent generation. Conceptually, mitotically

transmissible memory that programs responses to environmental cues

may provide part of the mechanism that plants use to alter gene expression

in response to the environment. They also can imply the ongoing presence

of the stimulus that is provided from the surrounding cells and tissues and

thus assuring the meiotic inheritance. The influence of such mechanisms

is confirmed by the maternal effects which describe the situation when an

organism shows the phenotype expected from the genotype of the mother,

irrespective of its own genotype, often due to the mother supplying messenger

RNA or proteins to the embryo. Maternal inherence is important for the

evolution of adaptive responses to environmental heterogeneity.

The presented results have obvious implications for natural and agronomical

ecosystems. Inheritance of the primed state as observed in transgenerational

priming is expected to contribute to improved adaptation of

the progeny to environments. The influence of the conditions seed reproduction

on the primary resistance of the plants obtained from them suggests the

need to test the resistance of new selected genotypes in different areas. Generally,

they support the view that the processes of acclimatization of plants

in different areas may involve the induction of different epigenetic mechanisms

for increasing the primary resistance of plants to excessive temperatures.

Long time cultivation and breeding processes in new conditions may

of harmonious combinations between epigenetic mechanisms and the new

genetic modifications. Of particular interest is the induction of epigenetic

adaptations under the influence of biostimulants, maintaining these specific

states during the entire period of ontogenesis.