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

Changes 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 (G0), 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 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.