PHYTOCHROME AND CYTOKININ RESPONSES

Cytokinins (CKs) and light can elicit similar morphogenic and biochemi cal responses in a wide range of plant species. Contradictory reports have been presented that CKs and phytochrome may have independent or i dentical mechanisms of action in photomorphogenic processes. These rep orts, relating to seed dormancy and germination, seedling development and growth efficiency, pigment production, and the photoperiodic contr ol of flowering are reviewed. Based on historical data and recent gene tic 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 tow ards entrainment of circadian rhythms and thus participate in photoper iodic signalling. Both light and CKs apparently also participate in nu trient assessment pathways. Current models propose that light and CKs might act independently or sequentially through common signal transduc tion intermediates to control the same downstream responses. We presen tly have a poor understanding of the mechanism(s) whereby these signal s are integrated at the molecular level and the physiological signific ance of the apparent overlap between the actions of phytochrome and CK cannot yet be fully appreciated.