Effect of microgravity on the cell cycle in the lentil root

Characteristics of the cell cycle in cortical regions (0-0.6 mm from the ro ot-cap junction) of the primary root of lentil (Lens culinaris L.) during g ermination in the vertical position on earth were determined by iododeoxyur idine labelling and image analysis. All cells were in the G1 phase at the b eginning of germination and the duration of the first cell cycle was about 25 h, At 29 h, around 14% of the cortical nuclei were still in the G2 or M phases of the first cell cycle, whereas 53 and 33% of the nuclei were respe ctively in the G1 or S phase of the second cell cycle. In parallel, the cel l cycle was analysed in root tips of lentil seedlings grown in space during the IML 2 mission (1994), (1) on the 1-g centrifuge for 29 h, (2) on the 1 -g centrifuge for 25 h and placed in microgravity for 4 h, (3) in micrograv ity for 29 h, (4) in microgravity for 25 h and placed on the 1-g centrifuge for 4 h, The densitometric analysis of nuclear DNA content showed that in microgravity there mere less cells in DNA synthesis and more cells in G1 th an in the controls on the 1-g centrifuge (flight and ground). The compariso n of the sample grown continuously on the 1-g centrifuge in space and of th e sample grown first in 1-g and then in microgravity indicated that 4 h of microgravity modified cell cycle, increasing the percentage of cells in the G1 phase. On the contrary, the transfer from microgravity to the 1-g centr ifuge (for 4 h) did not provoke any significant change in the distribution of the nuclear DNA content. Thus the effect of microgravity could not be re versed by a 4 h centrifugation, As the duration of the first cell cycle in the lentil root meristem is about 25 h, the results obtained are in agreeme nt with the hypothesis that the first cell cycle and/or the second G1 phase was lengthened in absence of gravity. The difference observed in the distr ibution of the nuclear DNA content in the two controls could be due to the fact that the Ig control on board was subjected to a period of 15 min of mi crogravity for photography 25 h after the hydration of the seeds, which ind icated an effect of short exposure to weightlessness. The mitotic index of cortical cells was greater on the 1-g centrifuge in space than in any other sample (flight and ground) which could show an effect of the centrifugatio n on the mitosis.