Highlight:Under physiological conditions, it was shown that the same cell model may establish two distinct spatial manners of cytoplasmic snRNP assembly.

Gene regulation ensures that the appropriate genes are expressed at the proper times. Nuclear retention of incompletely spliced or mature mRNAs emerges as a novel, previously underappreciated layer of post-transcriptional gene regulation. Studies on this phenomenon indicated that it exerted significant impact on the regulation of gene expression by regulating export and translation delay, which allows synthesis of specific proteins in response to a stimulus, e.g. under stress conditions or at strictly controlled time points, e.g. during cell differentiation or development. Here, we found that transcription in microsporocytes, during prophase of the first meiotic division, occurs in pulsatile manner. After each pulse, the transcriptional activity is silenced, but the transcripts synthesized at this time are not exported immediately to the cytoplasm, but are retained in the nucleoplasm and Cajal bodies (CBs). In contrast to nucleoplasm, mature transcripts were not found in CBs. Only non-fully-spliced transcripts with retained introns were stored in the CBs. Retained introns are spliced at precisely defined times, and fully mature mRNAs are released into the cytoplasm, where the proteins are produced. These proteins are necessary for further cell development during meiotic prophase. Our findings provide new insight into the regulatory mechanisms of gene expression based on mRNA retention in the nucleus during the development of generative cells in plants. Similar processes were observed during spermatogenesis in animals. This indicates the existence of an evolutionarily conserved mechanism of gene expression regulation during generative cells development in Eukaryota.

Gene regulation ensures that the appropriate genes are expressed at the proper time. Nuclear retention of incompletely spliced or mature mRNAs is emerging as a novel, previously underappreciated layer of post-transcriptional regulation. Studies on this phenomenon indicated that it exerts a significant influence on the regulation of gene expression by regulating export and translation delay, which allows the synthesis of specific proteins in response to a stimulus or at strictly controlled time points, e.g. during cell differentiation or development. Here, we show that transcription in microsporocytes of European larch (Larix decidua) occurs in a pulsatile manner during prophase of the first meiotic division. Transcriptional activity was then silenced after each pulse. However, the transcripts synthesized were not exported immediately to the cytoplasm but were retained in the nucleoplasm and Cajal bodies (CBs). In contrast to the nucleoplasm, we did not detect mature transcripts in CBs, which only stored non-fully-spliced transcripts with retained introns. Notably, the retained introns were spliced at precisely defined times, and fully mature mRNAs were released into the cytoplasm for translation. As similar processes have been observed during spermatogenesis in animals, our results illustrate an evolutionarily conserved mechanism of gene expression regulation during generative cells development in Eukaryota.

Recent studies show a crucial role of post-transcriptional processes in the regulation of gene expression. Our research has shown that mRNA retention in the nucleus plays a significant role in such regulation. We studied larch microsporocytes during meiotic prophase, characterized by pulsatile transcriptional activity. After each pulse, the transcriptional activity is silenced, but the transcripts synthesized at this time are not exported immediately to the cytoplasm but are retained in the cell nucleus and especially in Cajal bodies, where non-fully-spliced transcripts with retained introns are accumulated. Analysis of the transcriptome of these cells and detailed analysis of the nuclear retention and transport dynamics of several mRNAs revealed two main patterns of nuclear accumulation and transport. The majority of studied transcripts followed the first one, consisting of a more extended retention period and slow release to the cytoplasm. We have shown this in detail for the pre-mRNA and mRNA encoding RNA pol II subunit 10. In this pre-mRNA, a second (retained) intron is posttranscriptionally spliced at a precisely defined time. Fully mature mRNA is then released into the cytoplasm, where the RNA pol II complexes are produced. These proteins are necessary for transcription in the next pulse to occur.mRNAs encoding translation factors and SERRATE followed the second pattern, in which the retention period was shorter and transcripts were rapidly transferred to the cytoplasm. The presence of such a mechanism in various cell types from a diverse range of organisms suggests that it is an evolutionarily conserved mechanism of gene regulation.