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.