Genetic control of branching in Arabidopsis and ... - Directory UMM

54 Growth and development
the base of each side-shoot. This phenotype is due to the
activity of dominant alleles of the genes ART and EAR, one
of which (EAR) can be substituted for by mutant alleles of
other late flowering genes. The aerial rosette phenotype
seems not to be due to a premature initiation of axillary
meristems, but instead they seem to have adopted the
identity of young primary SAMs leading to a prolonged
vegetative phase of the lateral shoots [2].
Mutations in the gene TERMINAL FLOWER 1 (TFL1) convert
the indeterminate Arabidopsis inflorescence into a
determinate one and condition a shorter vegetative phase of
the primary shoot [7,33]. In contrast, overexpression of
TFL1 results in a longer vegetative phase of both primary
and lateral shoots leading to a highly branched inflorescence
phenotype. These observations suggest that TFL1 is
involved in a mechanism regulating the progression through
the different phases of shoot development [34 •• ,35].
In the tomato mutant self pruning (sp), development of the
primary shoot and of the inflorescence are as in the wildtype;
however, the pattern of sympodial shoot
development is abnormal. Whereas in the wild-type three
nodes are formed before the transition to floral development,
in the mutant the numbers of vegetative nodes of
successive sympodial units are progressively reduced, until
the main axis terminates in a sympodial shoot without a
leaf [14,36 •• ]. Recently, the corresponding gene has been
isolated [34 •• ] and proved to be homologous to the TFL1
gene of Arabidopsis [37 • ,38] and the CENTRORADIALIS
(CEN) gene of Antirrhinum [39]. The similarity of TFL1,
CEN, and SP to phosphatidylethanolamin-binding proteins
of animals, known to bind to membrane protein
complexes, suggests that these proteins may play a role in
signalling processes in the apex. Overexpression of SP
results in plants with more than three vegetative nodes per
sympodial unit and a partially leafy inflorescence. These
results are consistent with the assumption that the SP gene
of tomato regulates the progression through different phases
of shoot development in a similar manner to the TFL1
gene of Arabidopsis. Unlike TFL1, however, the SP gene
seems to have no effect on the floral transition of the primary
shoot and on the architecture of the inflorescence.
Conclusions
Recently several factors influencing the process of shoot
branching in plants have been identified. The subtending
leaf seems to play an important role in the process of axillary
meristem initiation influencing the position of
meristems and their growth rate. Molecular evidence indicates
that the plant hormone GA may influence the
formation of axillary meristems.
A comparison of lateral shoot formation in the monopodial
plant Arabidopsis and the sympodial tomato reveal both
differences and similarities, and may uncover factors
responsible for the characteristic shoot architecture
observed in the two species. Whereas in tomato axillary
meristems are initiated early in development, in
Arabidopsis, this happens only after the transition to reproductive
growth. In both species the side-shoots preceding
the inflorescence develop faster than other side-shoots. In
tomato, the sympodial shoot overgrows the comparatively
slowly developing inflorescence resulting in a sympodial
shoot architecture, whereas in Arabidopsis the main inflorescence
develops with a similar growth rate as the
paraclades and remains in a terminal position. In tomato,
the genetic control of the initiation of the sympodial meristem
and of the axillary meristem below it is controlled
differently as compared to other lateral meristems. It
remains to be tested if this is also true for Arabidopsis.
Genes of the TFL1/CEN/Sp family encoding products
related to phosphatidylethanolamin-binding proteins of
animals control the length of all phases of shoot development
in Arabidopsis, but only the length of the vegetative
phase of sympodial shoots in tomato.
Acknowledgements
We thank F Salamini and Z Schwarz-Sommer for critical reading of the
manuscript and members of the laboratory for helpful comments and
discussion. Our work on genes that control branching is supported by the
Deutsche Forschungsgemeinschaft and the European Community.
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