Career:

Research Interests:

My lab is interested in plant growth control.  Plants grow throughout their life cycle, mediated by stem cell collections called meristems.  Meristems are very responsive to the environment and graded changes in activity occur in response to many cues.  However, it is still poorly understood how the perception of environmental change, or developmental programmes are translated into altered plant growth behaviour.  We are tackling plant growth control from several angles.

Coupling cell growth and cell division.

We are interested to understand how cell growth and cell division are coordinated.  In most cell division cycles, cell growth is a prerequisite for division.  Cell production rates, and therefore cell growth rates, vary strongly as cells pass through different zones of the meristem.  We have recently identified a transcription factor involved in the coordination of cell cycle gene expression and ribosome biosynthesis.  We are currently testing the hypothesis that this factor is involved in conferring a meristematic, as opposed to a differentiation, state on cells expressing it.  Current work is targeted towards understanding how this gene is regulated and what its function is.

Cell division activity is not uniformly distributed in the Arabidopsis root apex

The highest level of mitotic activity is observed in a zone just distal to the position of the precursor cells for the different cell types in the Arabidopsis root (indicated by the black bar). It follows that this is also the zone of highest rates of cell growth. We are interested to determine how cell growth and division are co-regulated. (QC: quiescent center)

Model for the control of meristem proliferating cell population size

We propose that root apical growth rates are controlled by regulating the size of the proliferating cell population. The most parsimonious way to do this is by regulating exit from cell division and transition to differentiation at the distal edge of the zone of high-level proloferation (indicated by the yellow arrow). The red zone defines the zone of competence for cell division in the root apex, and is where the CDKA1 gene is expressed.

Cyclins: specialized or redundant?

In evolution, the genes for cell cycle regulators in plants were highly amplified.  The model plant Arabidopsis has more cyclin genes than humans, but also more than budding yeast that has a similar lifestyle to plants.  This raises the question whether cyclin function has diversified or whether these genes are largely redundant.  Strong conservation of individual cyclins and cyclin sub-groups in different plant lineages argues for important, but yet unknown functions and against redundancy.  We are pursuing a functional analysis of B-type cyclins, by characterizing loss-of-function mutants and analyzing their expression.  In collaboration with Dietlind Gerloff, we are using computational approaches to determine the likelihood of interaction between different cyclin and CDK proteins.

Mitoses in the root meristem

Roots from Arabidopsis plants transformed with a construct containing the cyclin B1;1promoter, cyclinB1;1 gene fused to GFP. Cells fluorescing in green are in mitosis

Distinct expression patterns of closely-related cyclins

Phylogenetic tree of some Arabidopsis cyclins (left) and root expression patterns of some cyclins. Even closely-related genes can have distinct expression patterns arguing for specific biological functions.

Nutritional signals:  

Phosphate and nitrogen are two plant macro-nutrients that also strongly affect plant growth rates and patterns when absent or present in surplus.  We are interested to understand the mechanisms how such mineral nutrients are perceived by plants and how this information is processed to regulate growth and growth patterns.  We have found that nitrogen and carbon metabolism, which controls relative shoot-root growth ratios, control plant responses to phosphate starvation by setting different levels of demand.  Current projects in the laboratory include genetic screens and molecular-physiology experiments targeted towards early steps in nitrogen and phosphate signal perception and transduction.

At4::LUC reporter responds to phosphate supply in phosphate-starved plants

False colour images of phosphate-starved plants responding to phosphate perception. The activity of the At4::LUC reporter diminishes when the plants perceive phosphate. Similar plants are used in genetic screens to identify signalling components.

Shoot branching.

Branching – the post-embryonic initiation of a new growth axis – is a developmental process unique to plants.  Flowers are ontogenetically branches and hence branch or axillary meristems are subjected to changes in their developmental identity just as the primary shoot meristems produced during embryogenesis.  Moreover, the control of branching has a profound impact on competitive success, particularly in ruderals such as Arabidopsis, because it strongly impacts on the number of seed produced.  Therefore, we are interested in branching from a fundamental, ‘plant-architectural’ as well as growth control point of view. 

Reduced branching in gain- and loss-of-function alleles of a myb transcription factor

Gain-of-function (left) alleles were identified in an activation-tagged collection, while loss-of-function alleles (right) were found in a T-DNA collection. Both have less branches when compared to the wild type (middle)

We have identified gain and loss-of-function mutants, which have branching, flowering time and fecundity phenotypes.  The phenotypes are caused by defects in transcription factor genes homologous to the MYB oncogene.  The detailed morphological analysis of these mutants have shown that one of the affected genes is required for the establishment and maintenance of stem cells in leaf axils and therefore defines a gene required for stem cell niches.  We have characterised the expression pattern of this factor and have identified potential target genes.  These experiments have revealed a novel connection between the plant hormone gibberellic acid involved in flowering time control and branching.  Biochemical analysis supports our model that this gene promotes vegetative identity (and hence axillary meristem formation) and suppresses flowering.  Our models are supported by the analysis of double mutant phenotypes.  Future work will concentrate on two aspects: What are the mechanisms involved in specifying the stem cell niche micro-environment, and what constitutes this environment; How do these genes function in other plant families.