Cytokinin regulates energy utilization in Botrytis cinerea
Authors: Anand Gautam , Gupta Rupali , Bar Maya 
Published: September 2021

The plant hormone cytokinin (CK) is an important developmental regulator, promoting morphogenesis and delaying senescence. Previous work by us and others has demonstrated that CKs also mediate plant immunity and disease resistance. Some phytopathogens have been reported to secrete CKs, and may manipulate CK signaling to regulate the host cell cycle and nutrient allocation, to improve their pathogenic abilities. In a recent work, we demonstrated that CK directly inhibits the growth, development, and virulence of fungal phytopathogens, by down regulating the cell cycle and reducing cytoskeleton organization and cellular trafficking in the fungus. Here, focusing on Botrytis cinerea (Bc), we report that the effect of CK on Bc is tied to nutrient availability; CK strongly inhibits Bc growth and de-regulated cytoskeleton organization in a nutrient rich environment, but has a diminished effect when nutrients are scarce. Using biochemical assays and transgenic redox sensitive botrytis lines, we examined the effect of CK on energy consumption in the fungus, and demonstrate that CK promotes glycolysis and energy consumption in Bc, both in vitro and in planta. Here, glycolysis and increased oxidation were stronger with waning nutrient availability. Transcriptomic data further supports our findings, demonstrating significant upregulation to glycolysis, oxidative phosphorylation, and sucrose metabolism, upon CK treatment. The metabolic effects of CK on the fungus likely reflect the role of plant CK during early infection by necrotrophic pathogens, which are known to have an initial, short biotrophic phase. In addition to the plant producing CK during its interaction with the pathogen for defense priming and pathogen inhibition, the pathogen may take advantage of this increased CK to boost its metabolism and energy production, in preparation for the necrotrophic phase of the infection. Thus, the role of CK in controlling senescence can be exploited by diverse phytopathogens to their advantage.

Author summary

Cytokinin (CK) is one of the primary plant developmental hormones, regulating many developmental processes. Several works have highlighted the involvement of CK in plant defense. We recently reported that CK can directly inhibit fungal plant pathogens. CK inhibits Botrytis cinerea growth by arresting the cell cycle and de-regulating cytoskeleton organization and cellular trafficking. Here, we report that CK positively regulates B. cinerea energy consumption, causing an increase in glycolytic rates and energy consumption. The effect of CK on B. cinerea was dependent on nutrient availability, with CK causing stronger increases in glycolysis and lower growth inhibition when nutrient availably was low, and weaker glycolytic increases coupled with stronger growth inhibition in a high nutrient environment. We propose that CK can be viewed as a bidirectional signaling molecule in plant pathogen interactions: CK acts as a signal to the fungus that plant tissue is present, causing it to activate sugar and energy metabolism pathways to take advantage of the available food source, while at the same time, CK is employed by the plant to inhibit the attacking pathogen.