Should I fight or should I grow? Cytokinins in defence-growth trade-offs

Constitutive defence activation in plants leads to resistance to a broad-spectrum of pathogens, but also frequently to stunted growth and reduced seed sets; how do plants decide whether to defend against infection or to utilise energy for growth? Plant hormones are important integrators of the physiological responses that influence the outcome of plant-pathogen interactions.

Schematic representation of the role of cytokinins in plant growth and defence against biotrophic pathogens, and in the growth–defence trade-off. Under normal growing conditions, cytokinin promotes shoot growth while inhibiting root growth (yellow arrows). Infection by a biotrophic pathogen stimulates pattern-triggered immunity (PTI) activation, oxidative stress (ROS) and salicylic acid biosynthesis, culminating in salicylic acid-dependent defence responses that suppress biotrophic pathogen growth (blue arrows). Cytokinins can enhance defence activation by salicylic acid-dependent and -independent processes (cytokinin-induced immunity; green arrows). Cytokinins can also help pathogen growth, by mechanisms that include suppression of PTI and ROS (cytokinin-induced susceptibility; red arrows). Increased salicylic acid content/signalling inhibits cytokinin-regulated processes, potentially causing inhibition of plant growth, a likely mechanism by which the growth–defence trade-off may occur. Arrows indicate positive interaction; blunt ends indicate negative interaction (inhibition).

Albrecht and Argueso discuss the mechanisms by which the plant hormone cytokinin regulates both plant growth and response to pathogens, and document the ways in which cytokinins may connect these two processes, ultimately affecting the growth trade-offs observed in plant immunity.