Agriculture Reference
In-Depth Information
Fig. 4.4  The revised ABC model of flower development. A, B, C , and E are four activities that are
present in adjacent whorls of the flower. These four activities are postulated to function combina-
torially to specify the identity of the four organs in the flower: sepals, petals, stamens, and carpels.
A second major tenet of the ABC model is that A and C activities are mutually repressive. The
specific molecular interactions between A and C class genes as well as their regulators are shown
at right . The majority of ABC genes encode MADS domain transcription factors. Recent evidence
suggests that MADS proteins function together in complexes larger than a dimer. The “quartet”
model postulates that tetramers of MADS proteins specify floral organ identity (shown as colored
ovals). Interactions demonstrated to be direct are indicated in red . (Jack 2004 )
and to the type of shoot carrying reproductive structures. In species carrying in-
florescences, differentiation leads primarily to the formation of inflorescence axis,
which is controlled by TFL (  Terminal FLower ). The expression of TFL appears
to be incompatible with that of LFY and AP1 , which are flower meristem identity
genes.
Flower organ differentiation proceeds rapidly after transition phase, so that all
the flower parts are distinctly formed in many species by the time the tree enters
into endo-dormancy. During the rest period, the differentiation process is blocked
or proceeds very slowly up to the pollen mother cell formation, which is the most
advanced stage of differentiation. When endo-dormancy is overcome, microspo-
rogenesis occurs while, at the ovary level, differentiation is resumed and progres-
sively continues up to ovule differentiation and embryo sac development, event
that occurs in correspondence or immediately before anthesis. In the case of grape,
the reproductive development is clearly time regulated. During the first growing
season, the transition phase is followed by the differentiation of the inflorescence
axis. Flower formation occurs only in the second growing season and starts when
the endo-dormancy is overcome and the heat threshold achieved the critical level
required for differentiation.
Search WWH ::




Custom Search