Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 181

level flowering occurs (Weller et al. , 1997b). Further progress in understanding the
identity of these signals has been delayed by a lack of molecular information, since
none of the genes associated with production of the floral stimulus or the inhibitor
of flowering have been cloned.
Analysis of the INDETERMINATE ( ID ) gene of maize provided the first molecu-
lar information on a gene that regulates the floral stimulus. Mutations in the ID gene
dramatically delay the transition to flowering, so that many more leaves are formed
than in wild-type plants (Colasanti et al. , 1998). Eventually id mutants do flower, but
the reproductive structures develop abnormally and show vegetative characteristics.
The male tassel formed at the apex of a wild-type plant forms vegetative plantlets on
its flanks, while the female inflorescence, which is generated from axillary meristems
in the wild-type plant, either does not form or is converted into vegetative branches.
The ID gene was cloned and shown to encode a protein containing predicted zinc
fingers that may act as a transcriptional regulator (Colasanti et al. , 1998). Analysis
of the expression of ID detected the mRNA in young, immature leaves, but not in
the SAM or in mature leaves. The expression of ID in the leaves, but not the SAM,
indicated that it acts to regulate long-distance signals that influence the transition
to flowering of the meristem. The expression of ID appears to occur in sink leaves,
which receive nutrients from photosynthetically active source tissues, and not to be
expressed in source leaves (Colasanti & Sundaresan, 2000). This observation led to
the suggestion that ID may not promote the production of the floral stimulus, but
rather act in the developing leaves to regulate its flow. However, the mechanism by
which ID regulates flowering requires further knowledge of the identity and function
of the genes whose expression it regulates.
7.2.3 Molecular genetic analysis of flowering-time control in Arabidopsis
places the long-distance signal within a regulatory hierarchy
7.2.3.1 A network of pathways controls flowering of Arabidopsis
The genetic control of flowering has been most extensively studied in Arabidopsis .
The behavior of mutants exhibiting a severe delay in flowering was first described
in detail by Redei (Redei, 1962), and this analysis was later broadened and extended
by Koornneef (Koornneef et al. , 1991, 1998). More recently, a large number of
mutants showing either later or earlier flowering have been described (Mouradov
et al. , 2002). Furthermore, study of natural genetic variation between Arabidopsis
accessions identified loci that were not detected by extensive mutagenesis of standard
laboratory accessions (Alonso-Blanco & Koornneef, 2000).
Environmental conditions influence flowering-time of Arabidopsis . Flowering is
promoted by exposure to long days and delayed under short days, whereas vernal-
ization treatments promote flowering (Martinez-Zapater et al. , 1994). In addition
to these seasonal cues, less dramatic changes in ambient conditions also strongly
influence flowering-time. Exposure to lower temperatures (16 ◦ C) delays flowering
compared to the effect of growing plants at typical growth temperatures of 20-24 ◦ C,
and exposure to the high ratios of far-red to red light associated with shading