Flowering in plants is a consequence of the transition of the shoot apex from vegetative to reproductive growth in response to environmental and internal signals. The indeterminate1 gene (id1) controls the transition to flowering in maize. We show by cloning the id1 gene that it encodes a protein with zinc finger motifs, suggesting that the id1 gene product functions as a transcriptional regulator of the floral transition. id1 mRNA expression studies and analyses of transposon-induced chimeric plants indicate that id1 acts non-cell-autonomously to regulate the production of a transmissible signal in the leaf that elicits the transformation of the shoot apex to reproductive development. These results provide molecular and genetic data consistent with the florigen hypothesis derived from classical plant physiology studies.

Higher plants use multiple perceptive measures to coordinate flowering time with environmental and endogenous cues. Physiological studies show that florigen is a mobile factor that transmits floral inductive signals from the leaf to the shoot apex. Arabidopsis FT protein is widely regarded as the archetype florigen found in diverse plant species, particularly in plants that use inductive photoperiods to flower. Recently, a large family of FT homologues in maize, the Zea CENTRORADIALIS (ZCN) genes, was described, suggesting that maize also contains FT-related proteins that act as a florigen. The product of one member of this large family, ZCN8, has several attributes that make it a good candidate as a maize florigen. Mechanisms underlying the floral transition in maize are less well understood than those of other species, partly because flowering in temperate maize is dependent largely on endogenous signals. The maize indeterminate1 (id1) gene is an important regulator of maize autonomous flowering that acts in leaves to mediate the transmission or production of florigenic signals. This study finds that id1 acts upstream of ZCN8 to control its expression, suggesting a possible new link to flowering in day-neutral maize. Moreover, in teosinte, a tropical progenitor of maize that requires short-day photoperiods to induce flowering, ZCN8 is highly up-regulated in leaves under inductive photoperiods. Finally, vascular-specific expression of ZCN8 in Arabidopsis complements the ft-1 mutation, demonstrating that leaf-specific expression of ZCN8 can induce flowering. These results suggest that ZCN8 may encode a florigen that integrates both endogenous and environmental signals in maize.

We describe the isolation of cDNA clones encoding a p34ak2 homologue from a higher plant, Zea mays (maize). A full-length cDNA clone, cdc2ZmA, was isolated, sequenced, and shown to complement a cdc28 mutation in Saccharomyces cerevisiae. Comparison of the deduced amino acid sequence of the maize p342 protein with other homologues showed that it was 64% identical to human p34'dc2 and 63% identical to Schizosaccharomyces pombe and S. cerevisiae p34cdc2 proteins. Studies of expression of the maize cdc2 gene(s)by Northern blot analysis indicated a correlation between the abundance of cdc2 mRNA and the proliferative state of the tissue. Southern blot analysis, as well as isolation of another cDNA clone, cdc2ZmB, which is 96% identical to cdc2ZmA, indicates that maize has multiple cdc2 genes.The p34cdc2 protein kinase has been found in a wide variety of yeast and animal species and is believed to be a central component of the mechanism controlling cell division in eukaryotes (1, 2). The p34cdc2 protein kinase was first identified as the product of the CDC28 gene of Saccharomyces cerevisiae (3), and later as the product of the cdc2 gene of Schizosaccharomyces pombe (4). In both S. cerevisiae and Sch. pombe the product of this gene is required for progression through the G1/S and G2/M transitions of the cell cycle (5-8). The central role played by cdc2 in the eukaryotic cell cycle became apparent when a cdc2 homologue was identified as part of M-phase-promoting factor (MPF), the multiprotein complex required to stimulate Xenopus and starfish oocytes to undergo meiosis (9-11). The p34 cdc2 protein kinase is activated at M phase, possibly by dephosphorylation of a tyrosine residue at its ATP binding site (12), and is thought to phosphorylate key proteins that lead to changes associated with M-phase-specific events, including chromosome condensation, spindle assembly and reorganization of the cytoskeleton, breakdown of the nuclear envelope, and changes in cell shape (2,13