Description of Research
Our growing society faces new and dynamic challenges, such as global climate change, the scarcity of arable land and the need for sustainable energy. Maximizing the utility of plants in each of these areas is key to meeting these challenges. Overall growth rate and biomass is largely regulated by the temporal and spatial control of stem cell self-renewal and differentiation of their progeny. When a stem cell divides it produces a copy of itself, and it produces a daughter cell which can develop into different types of cells. The means and mechanisms, by which this occurs, are poorly understood.
The Sozzani Lab research focuses on understanding how stem cells are organized and maintained in the root of the model plant Arabidopsis thaliana. Our goal is to gain a coherent qualitative and quantitative understanding of stem cell maintenance at the systems-level. Our research leverage techniques derived from molecular, developmental and cell biology, mathematics, physics, chemistry, computer science and engineering. In plant systems, stem cell regulation has clear implications for increasing the production of crops used for food, fiber and fuel. Our research will reveal a specific molecular pathway of plant stem cells, and provide broader insight into the fundamental properties of stem cell across the plant and animal kingdom.
Unraveling the genetic programs and molecular mechanisms of the stem cell niche in the Arabidopsis root
Overview: Development in multicellular organisms requires production of an increasing number of specialized cell types and sophisticated mechanisms of coordination among them. Stem cells are the source of different cell types and the balance between stem cell self-renewal and differentiation of their progeny regulates the overall organ growth. Disruption of this balance can lead to unchecked growth, producing tumors or deformed organs. There is increasing evidence that maintenance of stem cells, in both plants and animals, is controlled by signals from the local microenvironments, commonly known as the stem cell niche. However, the molecular programs involved in stem cell identity and maintenance of stem cell niches have yet to be unraveled.
Growth and development in plants require proliferative tissues called meristems, which contain stem cell pools that are found at the shoot and root tips (Carles et al, 2003). In the root apical meristem, four sets of stem cells -vascular, cortex/endodermal (CEI), epidermal/lateral root cap (Epi/LRC) and columella (COL) initials- surround the quiescent center (QC), which are less mitotically active cells required for stem cell maintenance [Fig.1]. Stem cells continuously undergo asymmetric divisions to produce daughter cells that are displaced from the stem cell niche and start to differentiate. Because plant cells do not move and stem cells divide in a stereotypical manner, the root is organized into cell layers where entire cell lineages, from stem cells to differentiated progeny, are spatially restricted. Therefore, the Arabidopsis root provides an excellent system in which to address questions of stem cell maintenance and identity.
Plant development shares many similarities with developmental processes in animals, however, unlike animals, development of plant organs primarily occur post-embryonically. In the root, the reservoir of stem cells is considered to be the source of all post-embryonic root development. Fundamental questions, then, are how stem cell self-renewal is regulated and how cell fate specification and differentiation are controlled in the stem cell progeny to generate distinct root tissues. Recent progress has been made in characterizing the genetic programs of the various cell types in the root, however this information is largely absent for stem cells. Elucidating the regulatory networks, as well as the molecular mechanisms underlying root stem cell biology, will provide insight into how a multicellular organism initiates and maintains growth of its tissues and organs.
Characterization of the genetic programs and molecular mechanisms functioning in the stem cell niche is particularly tractable in Arabidopsis roots, due to their continuous development. In plants, key events such as asymmetric cell divisions of initial cells occur repeatedly, and these cells are accessible to study due to their location at the tip. A rich molecular toolbox, a large collection of knockout mutant lines available in Arabidopsis, together with forward genetic screens, allow for rapid and targeted genetic manipulation of regulators of the root stem cell niche. Furthermore, florescence-activated cell sorting (FACS) coupled with expression profiling has been successfully employed to examine the transcriptional profiles of individual root cell-types. One of the main challenges in stem cell biology is how to manipulate individual stem cells and the surrounding stem cell niche. In mammalian systems, the lack of definitive stem cell markers, the inaccessibility of these cells and cell movement confounds analysis on these cells. These limitations can be overcome by using the Arabidopsis root as an experimental system. Direct transcriptional profiling of the root stem cells and the use of stem cell specific markers, ultimately has the potential to reveal the genetic components of the niche cells. Our long term goals are to identify genes involved in stem cell maintenance and determine how these genes are organized in unique regulatory circuits that maintain stem cell fate.
The definitive proof of understanding the underpinnings of stem cell maintenance and identity will be the newly gained ability to engineer or reprogram plant stem cells. These engineered plant stem cells may ultimately be used to improve crop yield and/or biomass production. Identifying the essential features that govern the regulatory networks is a necessary step towards the experimental manipulation of developmental potency. Engineering these gene regulatory networks using a synthetic biology approach will help us to gain knowledge of the design rules which govern the stem cell wiring diagram. Our long-term goal is to build a simple synthetic genetic circuit from the mathematical models. Taken as a whole, the construction of larger and more complex genetic circuits will ultimately drive the potential outcomes for translational/synthetic plant biology such as improved biofuels, more efficient and hardier crops, and the use of roots as carbon sinks.
I. Determine the molecular signatures that make stem cells unique.
II. Determine if there is a common genetic program that operates within different stem cells.
III. Determine whether a common QC-derived signal promotes “stemness”.
IV. Characterize the emergent behaviors of stem cell networks.
V. Model the network dynamics of stem cells.
Stem cell divisions, patterning and differentiation are all fundamental aspects of developmental biology. The timing and location of stem cell regeneration, as well as the production of other types of cells, directly allow building new organs. There are several excellent models for studying these processes in animals and plants. The Arabidopsis root, due to the continuous nature of its development, has emerged as a leading system to address these questions. Stem cell esearch is an example of the sometimes difficult ethical cost-benefit analysis, which affects scientists. Even though many issues regarding the ethics of stem cell research have now been solved, there are still controversies on this issue. By proposing a study on stem cell programs in plants, ethical issues are not as weighted, thus providing an excellent system in which to address questions of stem cell maintenance and identity. This may parallel with animal stem cells and therefore, one day, have some impact on human stem cell research.
Approximately two-thirds of human calories are derived from plants, either directly or indirectly through animal feed. Additionally, plants are used as raw materials for numerous industrial products including generation of cellulosic ethanol. Therefore, plant growth is an area of considerable research interest to the scientific community, commercial enterprise and society as a whole. Adapting plants to address ever changing human needs is a necessary strategy to create sustainable energy solutions and to sustain the nutritional and energy requirements of a growing global population. Advances in plant science will help the international community to meet the demand for an increase in global food production by half by 2030. In plant systems, stem cell regulation is an area of considerable research interest because it has clear implications for increasing the production of crops used for food, fiber and fuel. Similarly, plant breeding combined with biotechnology plays a major role in agricultural improvement. However, such efforts focus on optimizing observable above-ground characters of plants, while root systems are ignored. Plant roots play a vital role in plant growth, besides providing anchorage, roots are the primary site of nutrient and water acquisition. Consequently, we have to extend our view into the soil if we have to confront such fundamental issues as nutrient and water availability. Therefore, as a long term project the results from this research in the Arabidopsis root will have practical relevance given the central role of plant roots to human life.
The Sozzani Lab is currently founded by the National Science Foundation and by North Carolina State University
- Modelling emergent behaviour of gene networks controlling plant stem cells (NSF MCB- Systems and Synthetic Biology) (CAREER award)
- Modeling the effects of intrinsic and extrinsic signaling on cellular differentiation in plants (NSF-BBSCR)
- High-resolution in vivo imaging of stem cell divisions with light sheet microscopy (NCSU Research and Innovation Seed Funding Program (RISF))
- Fluorescent Correlation Measurements of Biomolecules in Differentiating Tissues (NCSU Research and Innovation Seed Funding Program (RISF))
- Identification of quantitative molecular markers of sweetpotato internal necrosis (SPIN) (NCSU College of Agricultural and Life Science (CALS))