Loss of microtubular orientation and impaired development of prophase bands upon inhibition of RNA synthesis in root meristem cells

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Abstract
Synchronous cells in Allium cepa L. root meristems were treated with 3′deoxyadenosine (cordycepin, an inhibitor of RNA synthesis) when roughly half of the population had reached G2 phase (fast cells) while the other half was still in late S (slow cells). Since this drug also blocks DNA replication, slow cells remained in S while the treatment was continued. Indirect immunodetection of tubulin showed that a band typical of prophase formed in some cells with a portion of their nuclear DNA unreplicated. Most of the cells arrested in late S which had not developed a prophase band had a diminished number of cortical microtubules. Changes in their orientation (from transversal to oblique or longitudinal) occurred in roughly a cycle time (28 h). In the presence of 3′deoxyadenosine, fast cells which had reached G2 at the start of the treatment proceeded to prophase and remained there. Mature prophase bands were also formed in these cells but eventually they disoriented and, finally, disappeared. The data suggest that microtubular orientation in the meristematic cells depends on long-lived RNA species.