Plant Development

Plant Development (Prof.dr. Ronald Koes)
Genetic control of flower development

Research theme:
Higher plants and animals have a complex bodyplan that consists of numerous different organs and tissues, all placed in well defined positions. Although we tend to take this for granted, it is actually a small miracle, that a single fertilised egg cell develops (reproducibly !) into such a complex organism. Two separable processes underly development

Determination of developmental patterns, i.e. the earliest process in which cells in an apparently homogeneous population become different from one another and a pattern appears. At this stage the cells are "programmed" to develop in a particular direction,
but no visible alterations are yet evident.

Realisation of the pattern, the process following pattern formation, in which "programmed" cells proceed their development in the chosen direction and become visibly different from one another (differentiation). This involves the activation of 100s to 1000s of cell-type specific genes. To study this phenomenon we study genes involved in the synthesis of anthocyanins as a model.
voor stage projecten zie Student projects

Transposon mutagenesis
For many of the genes involved in pattern formation on differentiation we have little or no idea about the nature of the gene products, thus making it impossible to identify them by biochemical methods. As an alternative we use petunia lines, in which the petunia transposon dTph1 jumps very frequently, for random and directed mutagenesis experiments. This way we isolated collect mutants with alterations in leaf- or flower-pigmentation, growth characteristics and pattern formation etc. etc.

Genetic control inflorescence and flower architecture
The development of the plant body depends on the activity of sets of undifferentiated cells at
the apex (the shoot apical meristem or SAM) and the root tip (the root meristem). During plant development, these meristems generate new secondary meristems and primordia (the initials of organs) in well defined positions which subsequently adopt a certain fate or identity.
The development of a petunia flower, for instance, requires the formation of four concentric whorls, each consisting of multiple primordia in well defined relative positions, that develop -from outside to inside- into sepals, petals, stamens and carpels respectively.

Although considerable progress has been made in elucidating the mechanisms that determine organ or meristem identity, we still know very little about the mechanisms which determine that primordia arise in defined positions. The transposon mutagenesis experiments yielded several mutants in which specific organs develop in the wrong numbers and/or the wrong places.
One such a mutant, no apical meristem (nam), fails to generate a an apical meristem during
embryogenesis, and during flower development generates too many floral organ primordia. Using
a transposon-tagged allele we isolated the nam gene and showed it is expressed at the boundary
of all newly arising meristems and primordia, suggesting that nam is necessary to define the borders of a new meristem or primordium, presumably by regulating cell division or elongation patterns.
NAM represents a new family of plant specific proteins (presumably transcription factors) that all share a well conserved 150 aminoacid domain, now called the NAC domain. The petunia genome contains a family of >20 NAC-domain genes, and co-supression experiments suggest that other members of the NAC- domain gene family are also involved in the formation of axillary meristems
and floral organ primordia.

Wildtype petunia has a so-called cymose inflorescence that contains an indeterminate number of flowers. Detailed microscopical studies showed that this architecture is generated by bifurcations of the apical inflorescence meristem, after which one half develops as an (determinate) floral meristem that will subsequently generate floral organ primordia, while the other half remains an inflorescence meristem and will undergo a new bifurcation to generate the next flower. By mutational analysis we identified several genes that control patterning of the inflorescence.

The genes aberrant leaf and flower (alf),double-top (dot) and evergreen (evg) are all required to specify the identity of floral meristems; in the absence of alf, dot or evg flowers are replaced (transformed) by inflorescence shoots. Interestingly, alf and dot encode homologs of leafy and unusual floral organs (ufo) of Arabidopsis and floricaula and fimbriata of Antirrhinum, two plant species with a fundamentally different (racemose) inflorescence architecture.

The extrapetals (exp) locus is required for bifurcation (branching) of the inflorescence meristem; exp- mutants have an tulip-type inflorescence that consists of a single flower. The floozy locus (fzy)is required for the positioning of bracts (leaf-like organs that subtend flowers) and floral organs.
We recently isolated the exp and fzy locus via transposon-tagged alleles.This will help us to understand how these factors control the correct positioning of meristems and organ primordia in the inflorescence. Moreover it will shed light on the evolution of different inflorescence architectures, which may ultimately open the way to alter inflorescence architecture in specific species (a tulip with multiple flowers ?)

Selected publications:
Souer, E., F. Quattrocchio, N. de Vetten, J. Mol, and R. Koes (1995) A general method to isolate genes tagged by a high copy number transposable element. Plant J.7, 677-685.
Koes, R., E. Souer, A. van Houwelingen, L. Mur, C. Spelt, F. Quattrocchio, J. Wing, B. Oppedijk,
S. Ahmed, T. Maes, T. Gerats, P. Hoogeveen, M. Meesters, D. Kloos, and J. Mol. (1995) Targeted gene inactivation in petunia by PCR-based selection of transposon insertion mutants. Proc. Natl. Acad. Sci. USA81, 8149-8153.
Souer, E., A. van Houwelingen, D. Kloos, J.N.M. Mol, and R.E. Koes. (1996). The no apical meristem gene of petunia is required for pattern formation in embryos and flowers and is expressed at meristem and primordia boundaries. Cell 85: 159-170.
Souer, E., A.R. van der Krol, D. Kloos, C. Spelt, M. Bliek, J. Mol, and R. Koes. (1998). Genetic control of branching pattern and floral identity during Petunia inflorescence development. Development 125: 733-742.