Behaviour of ribosomal genes and nucleolar domains during activation in sugarcane (Saccharum officinarum L.) root primordia: from the unsoaked quiescent state to the steady state of proliferation

Abstract

SUMMARYChanges in the organisation of ribosomal genes and nucleolar protein components were analysed in sugarcane (Saccharum officinarum L. cv Cristalina) from the time the quiescent primordia of the radical bands of nodes were stimulated to proliferate by water imbibition, until the meristematic population reached the steady state of proliferation in the growing roots. The kinetics of proliferation was evaluated by flow cytometry, and by the mitotic indexes, in roots of different lengths. All the quiescent cells were in a pre-replicative state (G 0 ), with a 2C DNA content. During their activation process, they progressively reached the steady state of proliferation (mitotic index 7%), with rather fixed frequencies for cells with 2C (G1), 4C (G2), and values between them corresponding to cells replicating their DNA. Decondensation of the ribosomal genes was followed by FISH with probes for the major 25S and 18S rRNAs, and variations in the numbers of nucleoli were recorded in squashed cells after silver staining. The ultrastructure of nucleoli was analysed by electron microscopy, using the EDTA regressive 143 staining for ribonucleoproteins. Quiescent nucleoli showed a clear segregation of their main components: Fibrillar Centre, Dense Fibrillar Component and Cajal´s bodies while lacked any Granular Component. However the proliferating ones showed them highly intermingled, except for the Cajal´s bodies. Our results revealed a high plasticity of the nucleolar domains in response to cell activation, and allowed to establish a correlation between dispersion of NORs with formation of small fibrillar centers and a nucleolus with all its domains intermingled, and the activation of cell proliferation during root sprouting.

Firing of transcription and compartmentalization of splicing factors in tomato radicle nuclei during germination1

“…After 4 h of soaking, few changes were observed in the nucleoplasmic RNP‐containing structures, which appeared sparsely distributed in the expanding interchromatin domains. The nucleolus presented a dense compact fibrillar organization, typical of inactive states (Figure 4a; Acevedo et al, 2002a). The large perinucleolar CBs appeared sometimes flattened (Figure 4a, inset panel).…”

“…The RNP domains form a compact continuum. The nucleolus depicts typical features of metabolic inactivity: a very dense fibrillar structure, the ribosomal genes condensed in compact NORs instead of forming active FCs, and lack of canonical GCs (Bassy et al, 2000; Acevedo et al, 2002a). The compact interchromatin domains contain a high density of RNP structures and display a reticulated organization in contrast with their loose distribution in active cells (Moreno Díaz de la Espina et al, 1992).…”

“…This is also seen in resting roots of maize and Arabidopsis seeds that have nucleolar CBs; however, the U2B″ protein and the U1/U6 snRNAs are not stored in them (Docquier et al, 2004). The available data of immunofluorescence, flow cytometry (Dolezel and Bartos, 2005; Acevedo et al, 2002a), chromatin organization (Lingua et al, 2002) and taxonomy indicate that the pattern of nuclear compartmentalization of extant splicing components in plants in either CBs or microspeckles, is not related to genome size, condensed chromatin pattern or taxonomic position (Figure 2 and Table 1). The molecular mechanism underlying this compartmentalization remains unknown.…”

Histochemistry: work in progress

“…This opening to plant biology was so successful that, in volume 46, four other good papers have been published (Tagu et al 2002;Acevedo et al 2002;Lanfranco et al 2002;Scialabba et al 2002), which showed the vitality of the interaction between the new histochemistry and basic biology of plants. Over the last years, basic cell biology has been fascinated with the intriguing process of programmed death in animals and plants, which is more and more becoming a challenge for histochemical investigation.…”

Thinking about the nucleus

“…Acevedo et al (2002) have described the behaviour of ribosomal genes toward activation in sugarcane simply by water imbibition of roots.…”