Auxin and cytokinin interactions links named stimuli, coordinating structures and effectors so body systems or plant tissues produce an integrated biological response.
Auxin and cytokinin act as interacting long-distance signals that integrate shoot and root growth.
Shoot tips produce auxin that is transported toward roots, while root tips produce cytokinin that is transported toward shoots. Each signal carries information about growth at one end of the plant to tissues at the other end.
Shoot tip → auxin → root responses; root tip → cytokinin → shoot responses. Their interaction helps balance resource acquisition below ground with photosynthetic growth above ground.
A growing root system can send more cytokinin toward shoots, while growing shoot tips send auxin toward roots; target-tissue responses coordinate development rather than allowing either system to grow independently.
This objective is about root-tip and shoot-tip production, transport and interaction. A tissue-culture hormone-ratio rule is not a substitute for the whole-plant integration mechanism.
Plant signalling integrates directional growth, phytohormones, and responses to stimuli. Tropisms are directional growth responses in seedlings; experiments compare shoot or root growth under lateral light or gravity stimuli. Shoots show positive phototropism toward lateral light; auxin redistribution causes greater elongation on the shaded side. Phytohormones are plant signalling chemicals controlling growth and development; auxin, cytokinin, and ethylene coordinate responses to stimuli and internal state. PIN auxin efflux carriers actively move auxin between plant cells; asymmetric carrier placement maintains auxin gradients during tropic responses. Auxin activates proton pumping into the apoplast; acidified cell walls loosen, allowing water uptake, elongation, and bending. Auxin generally promotes root formation; cytokinin promotes shoot formation and division; their ratio coordinates root-shoot growth and plant tissue culture outcomes. Ethylene is a gaseous phytohormone that promotes fruit ripening; ripening fruit releases more ethylene, creating positive feedback in nearby fruit.