Sobol M. "Structural-functional organization of the plant cell nucleolus under the conditions of altered gravity"

Complex investigation of the influence of altered gravity on the structural-functional organization of the plant cell nucleolus was carried out. Complex model of ultrastructural organization and functions of root apical cell nucleoli in cress seedlings was created on the base of ultrastructural localization of both DNA and the major functional nucleolar proteins - fibrillarin, NhL90 and NhL68. The principal stages of ribosome biogenesis, namely ribosomal gene activation, ribosomal DNA transcription and ribosomal RNA processing, were reflected in this model. The zones of processing pre-rRNA molecules associated with fibrillarin as the element of RNP-complexes - processomes were shown. Fibrillarin in a nucleolus was determined can be the part of processomes not bound with rDNA molecule or to be bound with pre-rRNA synthesizing on rDNA, or to belong to the pool of free fibrillarin. Cress fibrillarin was shown to have molecular weight of 41 kDa. The major specific argentophile proteins in cress are nucleolin homologues named by us NhL90 and NhL68. In cress analogue of Allium cepa NopA100 - NhL90 fulfils the functions mainly concerned with r-chromatin condensation regulation and rDNA transcription. Analogue of Allium cepa NopA64 - NhL68 realizes the functions predominantly concerned with rRNA processing. Re-localization of rDNA from dense fibrillar component and transition zone FC-DFC to fibrillar centers was demonstrated under the conditions of altered gravity. The re-localization correlates with NhL90 quantity alterations in nucleolar subcomponents that indicates lowering rDNA transcription level and turning rDNA in potentially-active state under the clinorotation. The increase in both the total quantity of and the average quantity on the nucleolus of geterogenous fibrillar centers was shown to correlate with the decrease in NhL68 quantity in fibrillar centers under the conditions of clinorotation. It testifies about the condensation of ribosomal DNA certain part to the formation of heterochromatin blocks, consequently about lowering the quantity of not only transcribed r-genes but rDNA potentially-active clusters too. The decrease in the quantity of NhL68 and fibrillarin in transition zone FC-DFC was determined under the conditions of altered gravity that points out lowering the level of rRNA early processing. The decrease in the quantity of fibrillarin, NhL90 and NhL68 in dense fibrillar component testifies about lowering the level of rRNA processing on the following stages that results in the decrease in dense fibrillar component relative volume. Essential peculiarities of nucleolar subcomponents ultrastructural organization connected with the alterations of both rDNA localization and the quantity of the most important functional nucleolar proteins - fibrillarin, NhL90 and NhL68 were revealed under the conditions of clinorotation. They testify about the decrease in the level of ribosomal DNA transcription and ribosomal RNA processing. It allows to establish lowering the level of nucleolus functional activity under the influence of altered gravity. The results concerning to the differences in rRNA synthesis dynamics, the association of fibrillar centers with nucleolar vacuoles, the delay of protein/RNP pool exchange between a nucleolus and cytoplasm, the decrease in the cell volume in elongation zone, the decrease in the content of argentophile proteins in a nucleolus indicate, in the first place, the inhibition of different stages of rRNA processing and the accumulation of rRNA as a part of immature precursors of ribosomal subunits in a nucleolus under clinorotation. Obtained data about the ultrastructural characteristics of the nucleolus and the alteration dynamics of the major functional proteins - NhL90, NhL68 and fibrillarin can be used for monitoring of the influence of real microgravity in space flight experiments and other unfavorable factors on the plants.