Cyclodipeptides from Pseudomonas aeruginosa modulate the maize (Zea mays L.) root system and promote S6 ribosomal protein kinase activation

BACKGROUND: Pseudomonas aeruginosa is an opportunistic and pathogenic bacterium with the ability to produce cyclodipeptides (CDPs), which belong to a large family of molecules with important biological activities. Excessive amounts of CDPs produced by Pseudomonas strains can activate an auxin response in Arabidopsis thaliana and promote plant growth. Target of rapamycin (TOR) is an evolutionarily conserved eukaryotic protein kinase that coordinates cell growth and metabolic processes in response to environmental and nutritional signals. Target of rapamycin kinase phosphorylates various substrates, of which S6 ribosomal protein kinase (S6K) is particularly well known. The PI3K/Akt/mTOR/S6K signaling pathway has been studied extensively in mammals because of its association with fundamental biological processes including cell differentiation. However, evidences suggest that this pathway also has specific and conserved functions in plants and may thus be conserved, as are several of its components like TOR complex 1 and S6K proteins. In plants, TOR-S6K signaling has been shown to be modulated in response to plant growth promoters or stressors. METHODS: In this study, we evaluated the effects of P. aeruginosa CDPs on the growth and root development of maize plants (Zea mays L.) by adding different CDPs concentrations on culture plant media, as well as the effect on the phosphorylation of the maize S6K protein (ZmS6K) by protein electrophoresis and western blot. RESULTS: Our results showed that P. aeruginosa CDPs promoted maize growth and development, including modifications in the root system architecture, correlating with the increased ZmS6K phosphorylation and changes induced in electrophoretic mobility, suggesting post-translational modifications on ZmS6K. These findings suggest that the plant growth-promoting effect of the Pseudomonas genus, associated with the CDPs production, involves the TOR/S6K signaling pathway as a mechanism of plant growth and root development in plant–microorganism interaction.