Plants are continuously exposed to various environmental stresses and they respond to them in different ways. Ambient temperature is among the most important environmental cues that directly influence plant growth and yield. Research in recent years has revealed that epigenetic mechanisms play a key role in plants' response to temperature stress. Changes in gene expression evoked by stress signals follow post-translational histone modifications, DNA methylation, histone variant incorporation, and the action of chromatin remodeling factors and Polycomb group proteins. The majority of epigenetic modifications induced by temperature stress are reversible in nature; thus, chromatin returns to its previous state after the stress has passed. Some modifications seem stable, however, due presumably to so-called stress memory. Epigenetic modifications can be inherited through mitosis and meiosis. By dint of epigenetic memory, plants can more efficiently respond to future stressful conditions, thereby increasing their potential for environmental adaptation. Recognition of the epigenetic mechanisms that take part in plants' response to changes of ambient temperature will increase our understanding of adaptations to stress conditions.
Meiosis was observed in pollen mother cells in plants of four rye Secale vavilovii Grossh. lines (nos. 52, 109, 116, 225). The behavior of 2R chromosomes with additional heterochromatin, and of other chromosomes, was observed in diakinesis, metaphase I, anaphase I and anaphase II. 2R chromosomes with additional heterochromatin formed ring and rod bivalents without disturbances. Some other chromosomes formed heterobivalents and trivalents as well as chromosome bridges and chromosome fragments, indicating structural modifications.
The distribution of heterochromatin in mitotic chromosomes was studied in five inbred lines of rye Secale vavilovii Grossh. (nos. 52, 109, 116, 121 and 225). An additional heterochromatin band was found on the long arm of one of the chromosomes of pair 2R, at an average distance of 2.08 µm from the centromere. After the plants with an additional band on 2R were reproduced, plants with two chromosomes with an additional band were obtained, as well as those with one chromosome with an additional band and with chromosomes without that band. Distinct differences were observed in 1R in terms of the presence/absence of a band in the vicinity of the NOR constriction, and the size of the telomere bands. Telomere band size also differed in chromosomes 2R, 4R, 6R and 7R. Modifications of the heterochromatin fragment consisted in deletion of a telomeric heterochromatin segment and in the presence of different numbers of interstitial heterochromatin bands. The content of telomeric heterochromatin was the highest in 3R (18.86%) and the lowest in 4R (6.90%). During telophase, daughter nuclei connections in the form of a chromatin bridge were observed in a number of cells.
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