Plant development depends on the activity of two main meristems, the root meristem and the shoot apical meristem, which serve as a source of pluripotent stem cells and provide new cells for repetitive organ initiation. Thus, plant meristems are the stem cell niches that allow stem cells to remain undifferentiated and to proliferate. Meristems are dynamic structures that can be generated de novo, for example during flower formation. Furthermore, new meristems are continuously formed when plants produce new branches or lateral roots. Understanding meristem function is therefore central to understand how plants can establish so many different growth types, ranging from tiny herbs to huge trees. Furthermore, the sizes and numbers of meristems that are initiated during later development control the size and number of fruits and the generation of seeds.
We are investigating how plant stem cells are initiated and maintained, and how they function.
The shoot meristem generates the above-ground plant organs. In the model plant Arabidopsis thaliana, stem cells reside at the meristem tip. Their fate is controlled by cells in a deeper region, the organizing centre. Stem cells secrete the short peptide CLAVATA3 (CLV3) which is perceived by the receptor kinase CLAVATA1, and a heteromeric complex formed by the receptor protein CLAVATA2 (CLV2) together with the kinase CORYNE (CRN). This signal is transmitted from the cell-surface receptors to the nucleus to alter the expression levels of transcription factors, such as WUSCHEL (WUS). Because WUS promotes stem cell fate at the meristem tip, a feedback circuitry is established that regulates stem cell homeostasis. Other receptors contributing to these signalling pathways are the BAM receptors, and RPK2.
In recent years, we have studied how these receptor proteins interact at the plasmamembrane and signal to the nucleus, and how functional receptor complexes assemble in the ER. We are using a range of different methods to unravel these receptor interactions in vivo. Very important tools are fluorescence imaging technologies and confocal microscopy .
Funding: SFB590 (DFG)
From an evolutionary perspective, root meristems represent late inventions that enabled higher plants to grow in dry habitats. The root stem cell system is structurally very different from the shoot system. Here, a small group of mitotically quiescent cells (the quiescent centre, or QC) maintain stem cell fate in immediately abutting cells. Roots consist of specialized cell types arranged in regular files, that are generated from the dedicated stem cells at the root tip.
Surprisingly, the stem cell control pathways in the root resemble those in the shoot and employ related molecules. In the QC, the transcription factor WOX5 (related to the WUS gene of the shoot) maintains stem cells nearby. A feedback signal is generated from distal columella cells that secrete the CLE40 peptide (related to CLV3). However, the receptor kinase transmitting the signal is ACR4, which is also involved in epidermal integrity and the initiation of lateral root meristems.
Funding: iGRAD-PLANT (DFG)
Mechanisms of reproductive isolation are a central topic of evolutionary research because they determine the frequency of genetic exchange between populations and ultimately contribute to the origin of new species. The genetic basis of reproductive isolation in plants is poorly understood. We have started to study mechanisms of reproductive isolation by investigating the genetic parameters controlling success or failure of reproduction itself. We hypothesized that evolutionary divergence of reproductive processes after ecological and geographic separation leads to reduced reproductive success and gene flow between populations or closely related species. Therefore, reproductive divergence constitutes an important step in speciation.
In this coordinated research programme, we concentrate on components that act post-mating, but pre- and immediately post-zygotic, such as genes controlling the formation and number of gametophytes, their communication with each other preceding fertilization, and factors governing fertilization success. Starting from established knowledge about the underlying developmental processes and evolutionary genomics, we will then identify and investigate the function of candidate genes controlling reproduction, ask how evolution has shaped gene products and their interplay, and study the fitness effects of gene variants in different ecological and genetic backgrounds. Our European collaborative and cross-disciplinary project will analyse the genetic and molecular basis of plant reproductive barriers in a systematic and rigorous fashion, using state-of-the-art tools of ecological and evolutionary functional genomics.
Via genome wide association mapping and QTL approaches, our lab is exploiting natural variation of seed number between ecotypes of Arabidopsis thaliana to isolate the genes that control patterns of ovule initiation. We also use laser-capture microdissection for cell-type specific transcriptomics with next-generation RNA sequencing to identify genes and enable further functional studies.
Funding: EVOREP (DFG)
Plant growth behaviour and architectures, such as different branching patterns, stem compactness or leaf angles, can strongly affect overall plant fitness and productivity. This joint research consortium addresses the entire value added chain, from plant architecture to compund processing, in order to produce high-value oils, polyphenols and raw products for polymers and plastics. Together with the labs of U. Schurr (FZJ), G. Coupland (MPIPZ), Saaten-Union and our project cooordinators at Phytowelt GreenTechnologies GmbH, we are attempting to improve the architecture of rapeseed for higher performance and adapted growth styles.
Funding: SynRG (BMELV)
Organs such as leaves or flower organs are initiated at the flanks of shoot or floral meristems in a regular pattern. A key factor that positions organ founder cells is the phytohormone auxin, which is dynamically redistributed during plant growth via the activities of local auxin influx and efflux carriers. Organs generally initiate where auxin accumulates; at these sites, the expression of meristem specific genes is repressed. Several genes belonging to the LATERAL ORGAN BOUNDARY DOMAIN (LBD) family of transcription factors are required to maintain the boundary between organs and the remainder of the meristem, and thereby restrict cells with stem cell fate and differentiating cells to separate, but adjacent domains.
The gene JAGGED LATERAL ORGANS (JLO) belongs to the LBD family and controls both meristem specific gene expression patterns, but also auxin transport and signalling in root and shoot development. We are studying JLO function and interactions to learn how differentiation of meristem cells is controlled, and how organ initiation patterns are determined.