Stringency at four regions of the plant cell cycle where proteins regulating its progression are synthesized

Abstract

Different responses were detected when inhibiting protein synthesis (by 1μgml−1 anisomycin) for similar times (3 h) in each of the four regions of the plant cell cycle where synthesis of specific proteins seems to control further cycle progression. Synchronous cells, labelled as binucleate by 1-h, 5 mM-caffeine treatment, were used for this study in Allium cepa L. root meristems proliferating under steady-state kinetics. In the experimental conditions used, G1 lasts 8 h in the synchronous control populations. G1 houses two such regions. The first region is located in early G1. Inhibition of protein synthesis in this region produced 8.2 h of delay in reaching the S period. Moreover, the treatment halved the proliferative fraction of the population, behaving as a restriction area where cells should be rendered competent to go on cycling by newly synthesized proteins. The second region, located in late G1 (before the 6th hour of the cycle), produced, when inhibiting protein synthesis in the population, a delay of 2.4 h (shorter than the treatment) in reaching the S period and no decrease of its proliferative fraction. A similar inhibition of protein synthesis in the third region mapped in early G2 (before the 20th hour of the cell cycle) produced 4.4 h of delay in reaching mitosis and a 20 % decrease in the proliferative fraction. Finally, there is a fourth region in early prophase (before the 25th hour of the cell cycle) where all the cells seemed to abandon the cycle during inhibition of protein synthesis. Note: This article is dedicated to Dr F. A. L. Clowes, Oxford, on the occasion of his retirement.