Cytokinin production and sensing in fungi

Plant hormones act as chemical messengers, transducing cellular and organ-level cues, executing plant growth, development, reproduction, metabolism, and response to environmental stress. In addition to the production of hormones by plants, fungi can also produce compounds that are similar to phytohormones, and may modulate growth, physiology, and immunity in both plants and fungi. The "classical" plant growth hormone, cytokinin (CK) is known to have roles in plant-fungi interactions. In plants, CKs are involved in various processes including plant growth and development, seed germination, apical dominance, balance between shoot and root tissue, leaf senescence, and plant-pathogen-interactions. We recently reported that CK can also affect fungal development. CK is not produced solely by plants, as can be synthesized by plant-associated microorganisms such as bacteria and fungi. Fungal phytopathogens may also activate plant CK signalling/sensing via secretion of effector molecules. Fungal CKs secreted by (hemi)biotrophic pathogens can serve as virulence factors, however, most necrotrophic fungal plant pathogens have not been reported to secrete CKs during plant infection. Though a lifestyle-dependent role for CK signalling/perception was suggested for fungal plant pathogens, little is known about CK perception, sensing, and signalling in fungal organisms. In this review, we focus on the production of fungal CKs and their role in development and virulence, as well as the possibilities for CK perception and signalling in the fungal kingdom, where CHASE-domain containing proteins are largely absent.