Developmental studies on leafy spurge (Euphorbia esula). Histochemical and autoradiographic studies of the adventitious shoot apices

Abstract

Analysis of apices at four different developmental stages (progressively older from I to IV) of adventitious shoots of leafy spurge (Euphorbia esula L.) was carried out. In this study, some selected histochemical and autoradiographic techniques were used to study DNA, histone, and RNA changes during vegetative shoot growth. In the early vegetative phases (stages I to III), the DNA distribution and DNA synthesis based on Feulgen stain and thymidine-H3 incorporation respectively were found to be greater in the peripheral sectors (S1 and S3) than in the midsector (S2) and consequently the apex showed a characteristic cytozonation. The central zone (S2) showed less DNA staining and the rate of DNA synthesis was slower than that in the peripheral sectors. This central zone, which is often considered as méristème d'attente, cannot be interpreted similarly for there was slow DNA synthesis as indicated by the low incorporation of thymidine-H3. It is suggested that the nuclei in S2 in stages II and III had a longer mitotic cycle than those in S1 and S3. The zonal pattern seen in the early stages disappeared in stage IV. This final stage may be considered as the "intermediate phase" during the transformation of the vegetative apex to the floral apex. The distribution of nucleohistones based on results of alkaline fast-green staining procedure was similar to that of Feulgen stain for DNA at all four stages. However, when lysine groups in the histone were blocked by acetylation method, the apices showed that the arginine-rich histone fraction was dominant and uniform in all four stages. This means that in stages II and III, S2 had less lysine-rich histone fraction than S1 and S3. The nuclei in S2 in stages II and III had longer mitotic cycle and this was probably due to less lysine-rich histone content. The cytoplasmic RNA observed by following pyronin Y – methyl green staining method and RNA synthesis based on uridine-H3 incorporation showed no sharp differences among the three sectors of the apex in all four developmental stages. It is interpreted, therefore, that the central zone (S2) in the apex was, although slow, quite active metabolically during the development of the adventitious shoot.