Elucidating cytokinin response mechanisms using mutants

Cytokinins (CKs) have powerful effects on many elements of plant development, but little is known about the cellular and molecular mechanisms of CK action. This review describes recent progress in identification and characterization of mutants of flowering plants that may permit elucidation of CK response mechanisms. Several Nicotiana plumbaginifolia mutants that are resistant to high levels of exogenous CK have been isolated. Characterization of these mutants has led to information about relationships between CKs and other hormones, and CKs and nutrient metabolism. Two Arabidopsis thaliana mutants that are specifically resistant to CKs in a root elongation assay, cyrl and stpl, have been described, and may represent lesions in the CK signal transduction pathway. A mutant that produces elevated levels of CKs, ampl, has provided surprising information about the role of CKs in cotyledon formation. A set of tagged mutations that result in CK independent growth in culture has been identified, and the affected gene, CKII, cloned. The possibility that this gene encodes a CK receptor is discussed. Continued molecular/genetic analysis of CK responses is predicted to result in rapid progress in the next few years in understanding how CKs act.