Coordination of cell growth and cell division in maize (Zea mays L.) relevance of the conserved TOR signal transduction pathway

Abstract

Mechanisms that bring about coordination of cell growth and cell division in different organisms are biological events not yet clearly revealed. In maize, insulin effector of the phosphatidylinositol 3-kinase (PI3K)–target of rapamycin (TOR) signal transduction pathway in metazoan or an intrinsic maize growth factor similar to insulin has shown to regulate cell growth. This research has been undertaken to analyze the role of PI3K–TOR signal transduction pathway in maintaining coordinated regulation of cell growth and cell division in maize tissues. Results indicate that DNA synthesis as well as mitotic index increased in maize callus in vitro cultures after insulin or maize factor stimulation. Biomass and ribosomal protein synthesis also showed significant increment after this stimulation, and the cell morphology composition of the cultures drastically changed. Two proteins related to cell cycle, D-type cyclins and proliferating cell nuclear antigen, were selectively synthesized under these conditions. Reverse transcription-polymerase chain reaction analysis of total and polysomal RNAs revealed that this effect was mainly due to specific mobilization of the correspondent mRNAs into polysomes rather than to transcriptional activation. All these events were sensitive to rapamycin inhibition, indicating that the stimulatory effect was mediated by TOR kinase activation. It is concluded that the evolutionary conserved PI3K–TOR pathway might coordinately regulate cell growth and cell division in maize.