Analysis and reconstitution of regulatory networks governing epidermal differentiation and polar root hair growth

• Applicant: Professor Dr. Jörg Kudla
• Subject Area: Plant Cell and Developmental Biology
• Term: since 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 264387517

Tip-focused growth represents an important mechanism to establish cellular polarity and is a fundamental process that underlies the growth of neuronal axon, fungal hyphae and plant root hairs. Root hairs (RHs) are specialized protuberances of the root epidermis that facilitate uptake of water and nutrients as well as anchoring in the soil. RHs provide an excellent model system to study fundamental aspects of cell differentiation and growth because they exhibit fast and oscillatory polarized tip growth (TG), are readily accessible for microscopic analyses and due to their non-essential function are very well amenable for genetic analyses through mutant characterization. TG of RHs is brought about by a complex interplay of hormones, second messengers, and pH homeostasis. After an initial RH bulge growth phase, further RH expansion through TG displays pulsatory nature in which phases of fast polar growth alternate with periods of slower expansion. This discontinuous growth behavior is accompanied by phase-shifted oscillations of second messengers like Ca2+, reactive oxygen species (ROS) and cellular/apoplastic pH. The ROS generating NOX RBOHC is an established central regulator of RH growth and its expression and polar localization can be detected already from the onset of RH bulging. Although Ca2+ binding to RBOHC is known to be essential for its activity, fundamental aspects of its regulation remain enigmatic. It is, for example unclear how RBOHC is activated during early RH growth (before Ca2+ oscillations build up), how the oscillatory RBOHC activity during fast TG is mechanistically caused and how the counteracting deactivation of this NOX during fast TG and TG cessation is brought about. In addition to the Ca2+ activated CBL1-CIPK26/CPK4/CPK10 module, we have identified FER/RIPK and ERU/MARIS RLK/RLCK modules that can confer Ca2+ independent RBOHC activation and defined the phosphatases AUN1/2 as de-activators of RBOHC. This defines the major objectives of this project, which aim to define the molecular components and mechanisms that govern RH differentiation and growth. To this end, we are using the NOX RBOHC as an experimental focal point since this enzyme represents a central regulatory hub of RH biology on which multiple pathways converge and from which multiple responses are controlled. Consequently, we propose to focus our studies on the following three specific objectives: (i) Defining the mechanisms and function of Ca2+-activated phosphorylation and potentially pH-activated de-phosphorylation for the regulation of RBOHC function (ii) Mechanistic and functional characterization of the FER-RIPK-RBOHC module and the potential pH modulation of RH peptide signaling (iii) Functional and mechanistic characterization of ERU-MARIS-ROBHC module. Collectively, our approaches should reveal detailed and fundamental insights of the complex interplay of these signaling components in growing RH.

• DFG Programme: Research Grants