Plants contain discrete populations of self-renewing stem cells that give rise to the diverse differentiated cell types found throughout the plant. Stem cell function is therefore ultimately responsible for the aesthetic and economic benefits plants provide us. My lab uses multiple approaches to dissect plant stem cell signaling networks including genetics, genomics, live tissue imaging, and cell biological and biochemical methods.
Faculty Research Area: Plant Biology
I am a plant systematist, plant community ecologist, biogeographer, and conservation biologist focused on the species and systems of the Southeastern United States. Students in my lab focus on the systematics and biogeography of the Southeastern United States, community classification developing the U.S. National Vegetation Classification, and land management, conservation planning, and environmental policy questions involving the conservation of Southeastern United States ecosystems and species. Prior to coming to UNC in 2002, I had an extensive career in applied conservation biology, working with the North Carolina Natural Heritage Program, The Nature Conservancy, and NatureServe (the Association for Biodiversity Information). My conservation interests and activities continue, with my service as Trustee of the N.C. Natural Heritage Trust Fund (http://www.ncnhtf.org/) from 2008-2013 (which has provided $328 million through 518 grants to support the conservation of more than 298,000 acres of natural areas in North Carolina), Chair of the N.C. Plant Conservation Program’s Scientific Advisory Committee (http://www.ncagr.gov/plantindustry/plant/plantconserve/index.htm), and Chair of the N.C. Natural Heritage Program Advisory Committee (http://www.ncnhp.org/). I am the author of Flora of the Southern & Mid-Atlantic States (http://www.herbarium.unc.edu/flora.htm), a taxonomic manual covering about 7000 vascular plant taxa, now the standard in use across much of the Southeastern United States. With J. Chris Ludwig and Johnny Townsend, I am co-author of the Flora of Virginia (http://www.floraofvirginia.org/), published in 2012 and awarded the Thomas Jefferson Award for Conservation, and am also an active author, editor, reviewer, and director of the Flora of North America project (http://fna.huh.harvard.edu/). I was a co-founder of the Carolina Vegetation Survey (http://cvs.bio.unc.edu/), and continue as one of its four organizers.
Hormones influence virtually every aspect of plant growth and development. The elucidation of the molecular mechanisms controlling the biosynthesis and perception of these hormonal signals, and how these signals are integrated with each other and with other developmental and environmental signals remain fundamental questions in plant biology. There are three main areas of focus to the research in my laboratory: cytokinin signaling, the regulation of ethylene biosynthesis, and the regulation of cell elongation. Using a combination of genetic, molecular and biochemical approaches we are attempting to elucidate elements involved in these processes and ultimately to understand how these elements contribute to control plant growth and development.
Cytokinin Signal Transduction
Cytokinins were discovered by their property of promoting cell division. These N6-substituted adenine-based molecules have been associated with various developmental roles including germination, shoot and root development and leaf senescence. A model for cytokinin signal transduction has emerged that is similar to bacterial two-component systems (Fig. 1). Two-component elements in Arabidopsis are encoded by multi-gene families and similar families have been identified in the monocots maize and rice.
We seek to understand how these signaling elements interact with each other, how this pathway outputs to regulate the many processes that cytokinin regulates and to use mutants in various signaling components to further define the role of cytokinin in plant growth and development.
Regulation of Ethylene Biosynthesis
The simple gas ethylene has been recognized as a plant hormone since the turn of the century. It has been shown to influence a diverse array of plant processes, such as leaf and flower senescence and abscission, fruit ripening and the response to a wide variety of stresses. In order to understand how ethylene or any signaling molecule affects development, one must consider how its biosynthesis is controlled and the molecular mechanisms underlying its perception. Almost all plant tissues have the capacity to make ethylene, although in most cases the amount of ethylene produced is very low. There is a diverse group of factors that increase the level of ethylene biosynthesis, including other hormones (auxin, cytokinin, ethylene), numerous stresses, as well as various developmental events. We have taken a genetic approach to understanding how ethylene biosynthesis is regulated. Two classes of mutants have been isolated: those that overproduce ethylene (Eto mutants) and mutants that fail to increase ethylene biosynthesis in response to a specific inducer, cytokinin (Cin mutants). Together, these mutants identify elements involved in regulating ethylene biosynthesis. Studies from our lab have provided compelling evidence that ACS protein stability is regulated and have converged on a common mechanism. Research in our lab focuses on unraveling this mechanism regulating ACS protein stability. We are exploring the role of protein phosphorylation in this process. We are examining if the stability of ACS protein increases during developmental events that are associated with a rise in ethylene biosynthesis or in response to exogenous cues such as light. We are using various genetic screens to identify novel elements involved in controlling ACS protein stability. These studies will shed light on the mechanism regulating the regulation of the stability of ACC synthase proteins and how this contributes to the control of the biosynthesis of ethylene.
Regulation of Cell Expansion
The regulation of cell expansion is a primary determinant in the size and shape of plant organs. Understanding how cells regulate this process is crucial in understanding the development of plant form. The Arabidopsis root is an excellent model system to dissect this process as it has a relatively simple, well defined architecture and mutants that alter cell elongation can easily be identified by their effect on root length. In the root, two distinct regions of cell expansion can be distinguished. Both longitudinal and radial cell expansion occurs in the root apical meristem, defining the root diameter and moving cells into the elongation zone of the root. In the elongation zone, just above the meristem, elongation rates are also uniform, but are much higher and occur almost exclusively in the longitudinal direction. This polar cell expansion is known as anisotropy. Both the extent and orientation of cell expansion is regulated in plants. Cell expansion can be a dynamic process, and the orientation of expansion can change in response to various stimuli, such as wounding and hormonal treatments. Expansion of cells is driven by turgor pressure and the re
lative alignment and composition of cell wall material, which determines both the extent and orientation of elongation.
The plant cell wall is comprised of cellulose microfibrils that are crosslinked with glycans and are associated with a pectin matrix and various extracellular proteins. The primary load bearing elements of the cell wall are the cellulose microfibrils, and thus their orientation and crosslinking are key factors in both the direction and extent of cell expansion.In anisotropically elongating cells, the cellulose microfibrils are wound in a helical spiral transversely around the cell, in a manner that has been likened to hoops around a barrel. This arrangement allows expansion of the cell specifically in the longitudinal direction by stretching of this cellulose “spring”, but restricts expansion in the transverse or radial direction. Cellulose microfibrils are synthesized at the plasma membrane by a hexameric protein complex called the terminal complex or rosette, and the polysaccharides made by this complex are extruded into the extracellular space through some type of a pore in the plasma membrane, where they then associate with other cellulose chains to form microfibrils.
A major question regarding cell expansion is how is the deposition of the cellulose microfibrils is regulated to give, for example, primarily transverse microfibrils in root cells. Early studies revealed that the cytoplasmic microtubules were aligned transversely in some plant cells, correlated to the orientation of the cellulose microfibrils in those cells, and this and other data led to the idea that the microtubules act as a template for the synthesis of the cellulose microfibrils.
Our lab seeks to explore the links between ethylene, cell expansion and a pair of receptor-like kinase gene that we have evidence may link these processes. We identified a number a member of the leucine-rich receptor like Ser/Thr protein kinase (LRR-RLK) family in Arabidopsis (AIK1) that interacts with ACC synthase. Disruption of AIK results in Arabidopsis results in a plant in which the root cells lose their ability to elongation anisotropically. Further analysis suggests that ACC may act as a novel signal in this process. Molecular and genetic analysis has identified additional components in what now defines a novel regulatory circuit that controls the extend and orientation of cell expansion. These studies will shed light on how plants regulate both the extent and orientation of cell expansion and will help understand how the plant cell wall is constructed.
At a Glance
- How do plants recognize pathogens?
- How do intracellular innate immune receptors of the NLR family function?
- How is the plant associated microbiome condition plant growth and environmental response?
- And how do these commensal microbes navigate or evade the plant immune system?
Many interactions between plants and microbes begin with specific recognition. The nature of this recognition, and the interpretation of subsequent signal transduction by both plant and microbe have profound impact on the outcome of the interaction. Plants have evolved effective mechanisms to recognize pathogenic microbes and halt their biotrophic or necrotrophic growth in the plant. Active plant defense mechanisms obviously force the selection of microbe variants which can evade the plant’s recognition capabilities. This evolutionary tug of war has led to a complex set of both plant and microbe genes, whose interactions lead to a successful plant resistance reaction. As well as a potentially large array of cognitive gene functions, a number of subsequent signal transduction steps must be necessary to generate a completely effective resistant phenotype. Plant-microbe interactions can also benefit the plant and plant’s select a small and taxonomically constrained set of microbes from the very rich microbiome of the soil. These commensals help the plant access minerals and can protect against pathogens. We study both the molecular mechanisms of the plant immune system and the intricacies of how that immune system sculpts the well organized and functional root microbiome. Our ultimate aim is to use knowledge, genetics and microbes from nature to enhance plant performance and soil sustainability across the globe by defining fundamental rules of microbiome assembly and function.
My lab has studied the genetics of plant-pathogen interactions since 1989. Our two main interests are the control of pathogen recognition by plants via their two tiered immune system consisting of extracellular pattern recognition receptors and intracellular nucleotide-binding, leucine-rich repeat (NLR) receptors, and the formation and function of the root microbiome. In the immune system, we study NLR activation and its outcomes, activation of transcriptional re-programming to result in pathogen growth restriction and, ultimately, hypersensitive cell death. We also study the structure, function and evolutionary genomics of bacterial pathogen type III effectors and oomycete effectors. These pathogen proteins suppress host defense by manipulating host protein machinery. We are dedicated to mapping that set of host machines using the evolved tools of the pathogen to identify them. We study these interactions at the molecular and structural level. In the root microbiome, we are defining the community structure of plant rhizoplane and endophytic microbiomes and trying to define design rules for small bacterial consortia that will enhance plant health and productivity. This project relies on genomics, ecological modeling, metabolic modeling and both forward and reverse genetics. We have expertise and momentum in these research arenas and we collaborate widely to extend our technical and intellectual reach. My lab is a diverse set of individuals from several countries who each bring different expertise to their projects, and to the other projects in the group. We stress small team approaches to the problems introduced above. My students and post-docs thus have access to a variety of inputs about their work. We work on biological scales from nanometers to small mesocosms. Our lab alumnae have been successful in job placement in a variety of careers.
We use the reference model plant species, Arabidopsis thaliana, in our research.
Most plant development occurs post-embryonically, and is tied closely to environmental signals. We use techniques of genetics, molecular biology, microscopy, physiology, and biochemistry to study how environmental and endogenous signals regulate plant development. For these studies we use the model plant Arabidopsis thaliana, which has numerous technical advantages that facilitate experimental progress. We hope in the long run to reconstruct how endogenous developmental programs and exogenous signals cooperate to determine plant form and the partitioning of growth among different organs.
The hormone auxin regulates multiple developmental processes in plants, including embryo and meristem patterning, organ growth, and flower maturation. Auxin induces gene expression through a family of transcription factors called ARFs (Auxin Response Factors), whose activity is regulated by Aux/IAA proteins. Auxin switches ARF proteins between gene repressing and gene activating states by promoting Aux/IAA protein turnover. We are currently studying how this regulatory system controls seedling growth, ovule development, and flower opening and fertilization.
Among genes regulated by auxin are some (including those encoding Aux/IAA proteins) that feed back negatively on auxin response; and others encoding proteins that affect intercellular auxin transport. We are interested in how feedback controls affect the dynamics of auxin response, and in how regulated intercellular auxin movement coordinates growth and differentiation of different cells during development.
Prospective students: Near retirement, I am no longer taking students but explore Biology and the Environment, Ecology, and Energy Program (including Geography Department faculty affiliated with this Program) websites for ecologists who may help you! Though retiring, I am always glad to give advice to prospective and current graduate students through email.
Peter White is a plant ecologist with interests in communities, floristics, biogeography, species richness, the distance decay of similarity and beta diversity, conservation biology, and disturbance and patch dynamics. In vegetation science he is interested in the composition and dynamics of plant communities, the relationship between vegetation and landscape, and role of disturbance, and the ecology of individual species in a dynamic setting. In conservation biology he is interested in the distribution and biology of rare species, the design and management of nature reserves, alien species invasions, and conservation ethics.
From 1986 to 2014, Peter White directed the University’s North Carolina Botanical Garden through a period of exciting changes and growth. In this role, he and the staff have sought to redefine the scope of botanical gardens to focus on conservation, sustainability, and gardens as the healing interface with and gateway to nature. The Garden became one of the first gardens to enact policies aimed at diminishing the risk of release of exotic pest organisms in 1998 and was presented with a Program Excellence Award in 2004 by the American Association of Botanical Gardens and Arboreta. In 2009, the Garden opened the Education Center, a 29,000 sq ft facility that became the first LEED Platinum building on any of the 17 University of North Carolina campuses.
The Vision lab studies genome evolution and the architecture of complex traits, with a (non-exclusive) focus on the flowering plants. Among the questions we ask are:
- What is the genetic basis for ecologically important differences between species, and what evolutionary forces generate that variation?
- What mutational and evolutionary processes are responsible for the structural rearrangement of chromosomes among taxa?
- What effect do genome structural changes have on organismal phenotypes?
To address these questions, we use the tools of both molecular and computational biology, and are actively involved in the development of new computational methods.
My research activities are diverse and span aspects of vegetation science from plant interactions to global patterns. However, the four projects described below serve to illustrate my current primary research interests. Please note that I am retired to the rank of Research Professor and no longer accept graduate and postdoctoral students, but I am continuing my research projects, continue to collaborate with colleagues, and continue to advise undergraduate research projects.
Community dynamics. Much of my early research at UNC focused on plant community dynamics, a topic that I have continued to investigate throughout my career. With students and collaborators I have made extensive use of permanent plots to address these kinds of questions. We recently completed an additional resurvey of forest demography plots (some dating back to 1934) and are using these data to address a broad range of questions including urban impact, evolving successional dynamics reflecting local and global change, and changes in productivity resulting from factors such as successional dynamics and changes in atmospheric CO2.
Ecoinformatics. Ecoinformatics arrived as a subdiscipline of ecology only around the start of the 21st century. I have been active during this period in developing the necessary cyber-infrastructure and addressing science questions in the area of ecoinformatics that draws on the increasing availability of data that document attributes of places, attributes of biological taxa (species), and records of occurrence and co-occurrence of species in specific places. In the past, studies of ecological communities were largely local case studies and no one knew how generalizable they were; many simply reflected the idiosyncrasies of a particular combination of time and space. We are now in a position to analyze community patterns over very large scales and assess their generality and the impacts of local contingencies.
Vegetation Classification. Formal, widely-adopted vegetation classifications are important for many purposes ranging from inventory to mapping to management prescription to simply documenting the context within which research has been conducted. In 1994 I established a collaboration consisting of the Ecological Society of American, the Nature Conservancy, the USGS and the US Federal Geographic Data Committee (with the US Forest Service as lead agency) to develop an open and scientifically credible US National Vegetation Classification (USNVC). We proposed national standards and in 2008 the Federal Geographic Data Committee adopted the key components as the US national standard. My research group and collaborators built the USNVC data archive in the form of VegBank.org, developed a peer-review system, and have completed the first formal revision and documentation of a significant set of Associations for the National Vegetation Classification.
Vegetation of the Carolinas. I have always been fascinated by the patterns of vegetation and biodiversity across landscapes. The Carolinas are remarkably diverse and the factors responsible for the vegetation of the region are poorly understood. In 1988, I established a collaboration to systematically document the natural vegetation of the Carolinas. Subsequently we have acquired and databased over 10,000 vegetation plots covering most of the over 500 USNVC vegetation types of the Carolinas. The resulting data are summarized on our website (cvs.bio.unc.edu) for use by applied scientists and the general public. In addition, we provide digital tools for predicting the natural vegetation of sites to guide restoration efforts.
Professional Service. I have and continue to contribute to the scientific community in numerous ways. I have served the International Association for Vegetation Science as President (2007-2011) and Publications Officer (2011-2015), I co-founded the Journal of Vegetation Science and served as one of the original Coeditors-in-Chief (1990-1995), I organized of the North American Section, and am active in efforts to establish international standards for vegetation data. I have served the Ecological Society of America as Secretary (1992-1995), Editor-in-Chief of Ecology and Ecological Monographs (1995-2000), and co-organizer of the Vegetation Section and the Southeastern Chapter, in addition to participating in numerous other roles.
Links to websites maintained by Prof. Peet and his collaborators:
Research in my laboratory is centered on the initial interactions of the plant pathogenic bacterium, Agrobacterium tumefaciens, with potential host cells. A. tumefaciens is a soil bacterium. Infections of wound sites of plants with this bacterium result in the formation of crown gall tumors. We are particularly interested in the initial attachment of the bacteria to plant host cells. This initial loose binding is required for bacterial virulence and T-DNA transfer to plant cells. Bacterial mutants which are unable to carry out this attachment are avirulent. We are currently engaged in cloning and sequencing the genes required for this attachment. Their expression is regulated by signals from the wounded plant tissue. We are characterizing the compounds which are involved in this signaling and the genes which are regulated. Our results suggest that there is an initial exchange of signals between the bacterium and the plant before the bacteria bind irreversibly to the plant and the transfer of T-DNA can begin. We are also interested in the plant receptor to which the bacteria bind. We have identified mutants of the plant Arabidopsis which appear to be unable to bind the bacteria and are in the process of identifying the plant genes and proteins involved in bacterial binding.