Insights into the TOR-S6K Signaling Pathway in Maize (Zea mays L.). Pathway Activation by Effector–Receptor Interaction

Abstract

The primordial TOR pathway, known to control growth and cell proliferation, has still not been fully described for plants. Nevertheless, in maize, an insulin-like growth factor (ZmIGF) peptide has been reported to stimulate this pathway. This research provides further insight into the TOR pathway in maize, using a biochemical approach in cultures of fast-growing (FG) and slow-growing (SG) calli, as a model system. Our results revealed that addition of either ZmIGF or insulin to SG calli stimulated DNA synthesis and increased the growth rate through cell proliferation and increased the rate of ribosomal protein (RP) synthesis by the selective mobilization of RP mRNAs into polysomes. Furthermore, analysis of the phosphorylation status of the main TOR and S6K kinases from the TOR pathway revealed stimulation by ZmIGF or insulin, whereas rapamycin inhibited its activation. Remarkably, a putative maize insulin-like receptor was recognized by a human insulin receptor antibody, as demonstrated by immunoprecipitation from membrane protein extracts of maize callus. Furthermore, competition experiments between ZmIGF and insulin for the receptor site on maize protoplasts suggested structural recognition of the putative receptor by either effector. These data were confirmed by confocal immunolocalization within the cell membrane of callus cells. Taken together, these data indicate that cell growth and cell proliferation in maize depend on the activation of the TOR-S6K pathway through the interaction of an insulin-like growth factor and its receptor. This evidence suggests that higher plants as well as metazoans have conserved this biochemical pathway to regulate their growth, supporting the conclusion that it is a highly evolved conserved pathway.

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“…Thus, growth stimulation induced by zmIGF and insulin in maize is due to the activation of TOR pathway involving phosphorylation of TOR and S6K. In addition, it was shown that zmIGF interact with a receptor located in the plasma membrane of maize cells (Garrocho-Villegas et al, 2013). Therefore, it is possible that OC kappa may interact with a receptor located in the plasma membrane leading to the production inositol 1,4,5 triphosphate (IP3) which induces realease of calcium from ER activating CDPKs na the latter may may activate enzymes that produced G6P and T6P which inhibit SnRK1 leading to TOR activation explaining, at least in part, the stimulation of growth observed in E. glopulus trees (see model in Fig.…”

“…OC kappa-induced increase in the ievei of TOR-P and tor transcripts but not s6k transcripts OC kappa increases in the level of TOR-P from week 1 until the end of the experiment week 21with peaks at weeks 1, 2, 6, 12 and 16 compare to control plants. In this sense, it has been shown that an insulin-like growth factor (zmIGF) a well as bovine insulin induced an increase in cell division and growth in maize callus, phosphorylation of TOR in ser2448 and S6K in thr389 (Garrocho-Villegas et al, 2013). In addition, it was shown that rapamycin inhibited TOR and S6K phosphorylations.…”

“…In plants, TOR is a protein of around 250 kDa, 39% identical in its amino acid sequence to human TOR (Dobrenel et al, 2016) and TOR is phosphorylated in Ser2448 since antibodies antihuman TOR-P Ser2448 recognize phosphorylated TOR (TOR-P) in cells of maize callus (Garrocho-Villegas et al, 2013). In addition, it has been shown that insulin-like growth factors (IGF) and bovine insulin promote growth in maize cells which correlates with the increase in the level of TOR-P. On the other hand, rapamycin inhibits IGF-induced growth stimulation as well as TOR phosphorylation in Ser 2448 (Garrocho-Villegas et al, 2013). In plants, TOR kinase interacts with RAPTOR, LST8 and rapamycin-FKBP12 (Dobrenel et al, 2016;Mahfouz et al, 2006;Garrocho-Villegas et al, 2013).…”

“…In addition, it has been shown that insulin-like growth factors (IGF) and bovine insulin promote growth in maize cells which correlates with the increase in the level of TOR-P. On the other hand, rapamycin inhibits IGF-induced growth stimulation as well as TOR phosphorylation in Ser 2448 (Garrocho-Villegas et al, 2013). In plants, TOR kinase interacts with RAPTOR, LST8 and rapamycin-FKBP12 (Dobrenel et al, 2016;Mahfouz et al, 2006;Garrocho-Villegas et al, 2013). TOR pathway in plants is activated by glucose, sucrose and amino acids, among others (Dobrenel et al, 2016;Ren et al, 2012).…”

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“…shown that zmIGF interact with a receptor located in the plasma membrane of maize cells (Garrocho-Villegas et al, 2013). Therefore, it is possible that OC kappa may interact with a receptor located in the plasma membrane leading to the production inositol 1,4,5 triphosphate (IP3) which induces realease of calcium from ER activating CDPKs na the latter may may activate enzymes that produced G6P and T6P which inhibit SnRK1 leading to TOR activation explaining, at least in part, the stimulation of growth observed in E. glopulus trees (see model in Fig.…”