Research

Our research is elucidating the relationships between the plant hormones auxin, abscisic acid (ABA), and ethylene, determining the role of the auxin precursor indole-3-butyric acid (IBA) in plant development, and understanding the establishment of the Outer Lateral Domain of plant cells.  We use Arabidopsis mutants to identify genes that contribute to regulating auxin levels and responses and the establishment of the Outer Lateral Domain using a variety of molecular and biochemical techniques to understand the contributions of these genes.

The auxin indole-3-acetic acid (IAA) directs cell division, elongation, and differentiation, making it a critical regulator of all aspects of plant growth.  Because the levels and distribution of auxin controls plant growth and morphogenesis, modulation of auxin levels and responsiveness by interaction with other hormones, controlling biosynthesis, regulating transport, and managing storage forms is critical to plant survival.  

 

THE ROLE OF IBR5 IN AUXIN, ABA, AND ETHYLENE RESPONSES

Auxins, abscisic acid (ABA), and ethylene are plant hormones controlling multiple important aspects of growth and development.  For example, auxin controls cell division and elongation to modulate leaf expansion, embryo development, vascular development, and tropic responses; ABA controls stomatal closure, storage protein synthesis, seed dormancy, and shoot and root growth in response to stresses; and ethylene regulates fruit ripening, senescence, seed germination, abscission, and stress responses.  Although connections have been made between auxin and ABA signaling, auxin and ethylene signaling, and ethylene and ABA signaling, the molecular nature of the relationships among these three phytohormones remains largely undefined.  We are using the Arabidopsis iba response5 (ibr5) mutant, defective in a MAP kinase phosphatase and resistant to auxin, ABA, and ethylene, as an exciting tool with which to study the interactions among these three hormones and the roles of MAP kinase signaling in development.

Our goal is to define the molecular interactions between auxin, ABA, and ethylene signaling in the model plant Arabidopsis thaliana by exploiting IBR5 as an entry point into the network.  IBR5 is unique among auxin signaling components in that it appears to modulate auxin response independently of the TIR1 pathway of destabilizing Aux/IAA transcriptional repressors.  Elucidating additional players in the IBR5 pathway and determining this pathway’s mechanism of contributing to auxin response will increase our understanding of signaling mechanisms and crosstalk between hormone response pathways.

We gratefully acknowledge funding from the NIH to study the role of IBR5 in auxin, ABA, and ethylene responses.

 

THE ROLE OF IBA-DERIVED AUXIN IN PLANT DEVELOPMENT

In addition to modulating auxin responsiveness, plants regulate auxin biosynthesis, transport, and storage forms.  The auxin precursor indole-3-butyric acid (IBA) is converted into active auxin (IAA) by a peroxisomal process similar to fatty acid β-oxidation.  Several peroxisomal enzymes appear to be necessary for IBA-to-IAA conversion and mutants in these enzymes have revealed that IBA-derived auxin has diverse roles for in plant development and a greater input into the pool of auxin than anticipated.  Understanding the role of IBA-derived auxin and will allow us to deepen our knowledge of how the plant uses auxin and auxin precursors to regulate growth and development.

Similar to the active auxin IAA, the auxin precursor IBA is transported long-distance through the plant.  However, examined IAA transporters do not transport IBA.  In addition, transporters that move IBA out of the plant are unable to transport IAA.  Thus, transporters necessary to move IBA long-distance through the plant are unidentified.  We are using forward genetics in Arabidopsis to identify additional factors required for the transport of IBA.  Identifying these factors and understanding their roles in IBA transport and metabolism will allow us to gain new insights into the complex mechanisms used by plants to control auxin homeostasis.

 

THE OUTER LATERAL DOMAIN

Two ABC transporters necessary for IBA efflux, ABCG36 and ABCG37, are enriched at the Outer Lateral Domain (OLD), an outward, polarized face of plant cells.  The OLD serves as the dynamic boundary between the interior and exterior of the plant.  Understanding how the OLD is specified, established, and maintained is crucial because this domain is required for plant – environment interactions and ultimately for plant survival.  Although establishing cell polarity to provide the boundary between the plant and its environment is critical, little is known about how plants create and maintain this outermost membrane.

We are using forward genetics in Arabidopsis to identify and characterize factors required for establishment of the OLD plant – environment interface with the goal of gaining insight into how this domain is specified, established, and maintained.  The OLD plays many important roles, including uptake of nutrients, extrusion of toxic compounds, and interaction with microbes, thus insight into OLD establishment is critical to our understanding of how plants, and perhaps other organisms, interact with their environment.