Cytokinin conjugation: recent advances and patterns in plant evolution

Cytokinin (CK) conjugates are important in plant development because they regulate active CK concentrations, CK transport, storage, and irreversible inactivation. While numerous CK conjugates have been identified in higher plants, the biological functions of these compounds, their location within cells and tissues, and the enzymes and genes involved in their regulation are not clearly understood. In this paper, recent advances are reported which have occurred through the study of transgenic plants containing the ipt or rolC genes, the identification of new regulatory enzymes affecting CKs, and the characterization of new CK conjugates. In addition, a survey of the literature is presented which examines the pattern of CK conjugates found in different plant taxa. Based on current knowledge, it appears that green algae, mosses, and ferns contain relatively few CK conjugates of isopentenyl adenine (iP) and zeatin (Z). In contrast, higher land plants, such as gymnosperms and angiosperms, contain a more complex set of CKs, primarily conjugates of Z and dihydrozeatin (DHZ). This suggests that the pattern of CK conjugation has become more complex in parallel with the increasing complexity of higher plants.