TOR coordinates Cytokinin and Gibberellin signals mediating development and defense

Abstract

Target of Rapamycin (TOR) kinase is a conserved energy sensor that regulates plant growth and development in response to nutritional and environmental inputs. TOR is also involved in the regulation of plant immunity. TOR downregulation, for example, results in enhanced plant defense responses and plant resistance to several pathogens in Arabidopsis, rice, and tomato. Similarly, the plant hormone cytokinin (CK) has also been demonstrated to mediate both development and defense processes. Although TOR is positioned at the interface between development and defense, little is known about the mechanisms in which TOR may potentially regulate the switch between these two modalities. Here, we investigated the interplay between TOR-mediated development and TOR-mediated defense. We show that TOR-silencing or inhibition led to enhanced defense responses and disease resistance in WT or CK-deficient backgrounds. However, in a high-CK background, which is already immuno-activated and disease resistant, TOR-inhibition reduced defense responses and disease resistance. TOR-silencing resulted in the normalization of developmental phenotypes associated with high or low CK levels, as well as of other classical developmental mutants, demonstrating that TOR is required for the execution of developmental cues. We found that CK represses TOR activity, suggesting the existence of a cross talk mechanism between the two pathways. Our results demonstrate that TOR likely acts downstream to CK signaling and hormonal signaling in general, executing signaling cues resulting from both high and low CK, in both defense and development. Thus, differential regulation of TOR or TOR-mediated processes could underlie a development-defense switch.