Dissecting the TOR-S6K signal transduction pathway in maize seedlings: relevance on cell growth regulation

Abstract

Insulin and 'insulin-like' growth factors (IGFs) are known to regulate cell growth in eukaryotes by stimulating a signal transduction pathway that exerts translational control. Intermediate kinases of this pathway, target of rapamycin (TOR) and ribosomal protein S6 kinase (S6K), have been reported in Arabidospsis thaliana and Zea mays. However, upstream signal inducers and downstream targets of the pathway are not well known in plants. The objective of this work is to inquire whether plant growth is regulated by a signal transduction pathway similar to the insulin/IGF-stimulated pathway in other metazoans. Insulin as well as Zea mays insulin-related peptide (ZmIGF), which is a maize, 20-kDa peptide fraction recognized by insulin antibody, were used as effectors to stimulate maize axes growth from germinating seeds. ZmIGF expression was identified in axes from germinating maize seeds and immunolocalized in the meristems of these tissues. Significant enhancement of specific de novo protein synthesis of the translational apparatus components was found in the stimulated axes. Reverse-transcription-polymerasechain reaction analysis of total and polysomal RNA pools in ZmIGF- or insulin-stimulated axes confirmed these data by revealing specific mRNA recruitment into polysomes. In addition, the same stimuli induced activation of S6 ribosomal protein kinase (ZmS6K) in germinating maize axes. All the above effects were inhibited by rapamycin, indicating that they depend on TOR activity. We conclude that a TOR-S6K signal transduction pathway is functional in maize germination, as that found for non-photosynthetic eukaryotes. The evolutionary implications of these findings are discussed.