Аннотация:A.thaliana mutation na causes premature termination of shoot apical meristem (SAM) function. SAM size in plants heterozygous for na is 2–3 fold smaller than in wild type plants, which indicates that na gene participates in the regulation of stem cell pool size or SAM cells proliferative activity. In order to understand whether na gene exerts the influence on these processes by affecting cell divisions we studied the effect of mutation on the expression level of transgene CycB1;1::GUS (reporter gene beta-glucuronidase uidA, fused to promoter of the cyclin gene CycB1;1), which is widely used as a cell division marker. Study of the CycB1;1::GUS expression in seedlings and juvenile plants revealed some differences in its expression pattern in na mutant in comparison to wild type. However, we found great variation in the activity of the reporter in different samples of mutant plants. Such variation may be due to the fact that the time of termination of the SAM proliferation in different plants may differ slightly, as seen from the variation of mutant phenotype. To avoid adverse effects on cell division associated with abnormalities of morphogenesis we continued our studies on callus culture that is characterized by a simple level of organization. We analyzed callus formation on Gamborg’s B5 medium using wild type and na leaf explants. We found significant reduction in the proliferative activity of mutant cells compared to wild type. Already after 2 weeks of cultivation all wild type explants were transformed into callus and actively expressed the reporter gene. Explants from mutant formed only a few small foci of cell proliferation and much weaker expressed a reporter gene. The obtained data suggest a link between the breach of the functioning of SAM and anomalies in cell proliferative activity of the mutant.