Distinctive expression and functional regulation of the maize (Zea mays L.) TOR kinase ortholog

TOR (Target of rapamycin) kinase is a central component of a signal transduction pathway that regulates cellular growth in response to nutrients, mitogens and growth factors in eukaryotes. Knowledge of the TOR pathway in plants is scarce, and reports in agronomical relevant plants are lacking. Previous studies indicate that Arabidopsis thaliana TOR (AtTOR) activity is resistant to rapamycin whereas maize TOR (ZmTOR) is not, suggesting that plants might have different regulation mechanisms for this signal transduction pathway. In the present work maize ZmTOR cDNA was identified and its expression regulation was analyzed during germination on different tissues at various stages of differentiation and by the main ZmTOR regulators. Our results show that ZmTOR contains all functional domains characteristic of metazoan TOR kinase. ZmTOR expression is highly regulated during germination, a critical plant development period, but not on other tissues of contrasting physiological characteristics. Bioinformatic analyses indicated that maize FKBP12 and rapamycin form a functional structure capable of targeting the ZmTOR protein, similar to other non-plant eukaryotes, further supporting its regulation by rapamycin ( in contrast with the rapamycin insensitivity of Arabidopsis thaliana) and the conservation of rapamycin regulation through plant evolution.