Phytochrome and cytokinin responses

Cytokinins (CKs) and light can elicit similar morphogenic and biochemical responses in a wide range of plant species. Contradictory reports have been presented that CKs and phytochrome may have independent or identical mechanisms of action in photomorphogenic processes. These reports, relating to seed dormancy and germination, seedling development and growth efficiency, pigment production, and the photoperiodic control of flowering are reviewed. Based on historical data and recent genetic approaches using Arabidopsis mutants, the possible role of CKs in physiological and biochemical pathways affected by light are discussed briefly. Together with the phytochrome system, CKs may contribute towards entrainment of circadian rhythms and thus participate in photoperiodic signalling. Both light and CKs apparently also participate in nutrient assessment pathways. Current models propose that light and CKs might act independently or sequentially through common signal transduction intermediates to control the same downstream responses. We presently have a poor understanding of the mechanism(s) whereby these signals are integrated at the molecular level and the physiological significance of the apparent overlap between the actions of phytochrome and CK cannot yet be fully appreciated.