Topic 25.12

A Gene That Regulates the Floral Stimulus in Maize

Maize plants flower after producing a fixed number of leaves regardless of photoperiod—a characteristic of autonomous regulation. If the shoot apical meristem from a maize plant is excised, and the “explant” is cultured in vitro, it is possible to regenerate a complete plant that will eventually flower. As long as the explant includes 4-6 leaf primordia, the meristem “remembers” its developmental stage at the time of excision and produces only the additional number of leaves needed to make up the fixed amount before flowering. However, if the explant contains fewer than 4 leaf primordia, the apex “forgets” its developmental stage and produces a complete set of new leaves before flowering.

The indeterminate 1 (id1) mutant of maize is unable to undergo a normal transition to flowering, and produces many more leaves than the wild type. Moreover, the flowers produced by id1 mutants are abnormal, having vegetative characteristics. The ID1 gene encodes a zinc-finger protein, suggesting that it functions as a transcriptional regulator. The expression pattern of ID1 differs from that of many other flowering time control genes isolated from Arabidopsis in that it is expressed in young leaves rather than in the shoot apical meristem (Colasanti et al. 1998).

Are the effects of the ID1 gene “cell autonomous”; that is, restricted to those cells in which the gene is expressed, or does the ID1 gene regulate the production of a transmissible signal that can affect cells beyond the site of gene expression? Traditionally, grafting experiments between the mutant and the wild type have been carried out to answer such questions, but maize (a monocot) lacks a vascular cambium and is therefore not suitable for grafting experiments.

In a second set of experiments, Colasanti et al. (1998) did the next best thing to grafting by analyzing plants that were genetic chimeras (that is, containing cells with different genotypes) for ID1 function. The mutant allele of id1 analyzed was caused by the insertion of a transposable element, and the spontaneous excision of this element during development generated clonal sectors of wild-type ID1-expressing cells against an overall mutant id1 background. It was found that chimeric id1 plants containing sectors of restored ID1 function flowered earlier than completely mutant id1 plants, even though the shoot apical meristems of these chimeric plants remained mutant. This experiment is comparable to grafting a wild type scion onto an id1 mutant stock, and finding that the id1 stock flowered earlier as a result. This observation in combination with the expression data provides compelling evidence for the involvement of idt1 in the production in leaves of a transmissable signal from immature leaves that induces flowering in the apex.

The Arabidopsis genome contains several potential homologs of the maize ID1 gene, but no id1-like mutants have been identified with a phenotype similar to that in maize. All of the genes involved in the autonomous pathway of Arabidopsis thus far seem to be expressed exclusively in the apical meristem.