Phytohormonal control of differential growth regulation

The phytohormone auxin (Gk: auxein - to grow) is a master regulator of plant growth. However, our global understanding of how small molecules, such as auxins, can carry the required information for shape regulation in plants is largely unknown. To obtain truly mechanistic insights into differential growth regulation in plants, we are addressing upstream and downstream components that execute or instruct auxin-dependent differential growth regulation on subcellular, cellular, tissue, and organ level. To achieve that, we focus on three main research questions: 1.) How does a plant coordinate differential growth on an organ level to establish its architecture? Complex phytohormonal signal integration (crosstalk) instructs plant development; however, mechanistic insight is largely lacking. We are using lateral roots to address non-cell autonomous signals that affect the overall root architecture. Our research group focuses on asymmetric growth regulation in lateral roots in response to gravity that substantially differs from responses in the main root. While the main root displays positive gravitropic response, the directional growth of lateral roots points strictly away from the main root. We investigate non-cell autonomous signals derived from the main root that modulate auxin/gravitropic responses in lateral roots. These comparative studies will reveal novel insights into hormonal crosstalk on an organ level. 2.) How is growth coordinated between neighboring tissue types? Cellular growth within a tissue needs to be coordinated for shape generation. In plants, this has outstanding importance due to the surrounding cell wall that is shared by neighboring cells. We are investigating neighboring root epidermal cell files (tricho- and atrichoblasts) that display different growth and division rates. Atrichoblasts display higher elongation, whereas trichoblasts higher division rates. We use these cell files, which functionally facilitate the root-soil interface, as a novel model to monitor how hormonal signals are integrated to execute coordinated growth in a tissue context. 3.) How does a single cell monitor active auxin levels and regulate the desired levels for defined cellular development? Subcellular auxin compartmentalization and its potential role in auxin homeostasis have just recently (Mravec et al., 2009) emerged. We particularly address subcellular mechanisms that affect cellular auxin homeostasis. We focus on PILS proteins, a novel protein family of putative intracellular auxin carriers, to address auxin homeostasis on a sub- and cellular resolution. This will broaden our understanding of how single cells, in a tissue context, regulate auxin levels in a cell autonomous and non-autonomous manner. We investigate auxin-dependent growth regulation at different levels to obtain a comprehensive understanding of plant growth regulation. Our research efforts are directly linked to the actual function of roots (e.g. epidermal surface enlargement and tropistic growth for water and nutrient uptake) and, hence, our research will provide not only important insights into basic mechanisms of how plants regulate, monitor, and trigger growth, but will have additionally importance for applied, agronomic research fields.