We are characterizing a suite
of Pisum sativum mutants that alter inflorescence architecture to construct a model for the genetic regulation of inflorescence development
in a plant with a compound raceme. Such a model, when compared with those created for
Antirrhinum majus and
Arabidopsis thaliana, both of which have simple racemes, should provide insight into the evolution of the development of inflorescence architecture.
The highly conserved nature of cloned genes that regulate reproductive development in plants and the morphological similarities
among our mutants and those identified in
A. majus and
A. thaliana enhance the probability that a developmental genetics approach will be fruitful. Here we describe six
P. sativum mutants that affect morphologically and architecturally distinct aspects of the inflorescence, and we analyze interactions
among these genes. Both vegetative and inflorescence growth of the primary axis is affected by
UNIFOLIA TA, which is necessary for the function of
DETERMINATE (
DET).
DET maintains indeterminacy in the first-order axis. In its absence, the meristem differentiates as a stub covered with epidermal
hairs.
DET interacts with
VEGETATIVE1 (
VEG1).
VEG1 appears essential for second-order inflorescence (I
2) development.
veg1 mutants fail to flower or differentiate the I
2 meristem into a rudimentary stub,
det veg1 double mutants produce true terminal flowers with no stubs, indicating that two genes must be eliminated for terminal flower
formation in
P. sativum, whereas elimination of a single gene accomplishes this in
A. thaliana and
A. majus. NEPTUNE also affects I
2 development by limiting to two the number of flowers produced prior to stub formation. Its role is independent of
DET, as indicated by the additive nature of the double mutant
det nep. UNI, BROC, and PIM all play roles in assigning floral meristem identity to the third-order branch.
pim mutants continue to produce inflorescence branches, resulting in a highly complex architecture and aberrant flowers.
uni mutants initiate a whorl of sepals, but floral organogenesis is aberrant beyond that developmental point, and the double
mutant
uni pim lacks identifiable floral organs. A wild-type phenotype is observed in
broc plants, but
broc enhances
the pim phenotype in the double mutant, producing inflorescences that resemble broccoli. Collectively these genes ensure that only
the third-order meristem, not higher- or lower-order meristems, generates floral organs, thus precisely regulating the overall
architecture of the plant.
Gene symbols used in this article: For clarity a common symbolization is used for genes of all species discussed in this article.
Genes are symbolized with italicized capital letters. Mutant alleles are represented by lowercase, italicized letters. In
both cases, the number immediately following the gene symbol differentiates among genes with the same symbol. If there are
multiple alleles, a hyphen followed by a number is used to distinguish alleles. Protein products are represented by capital
letters without italics.
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