Auxin homeostasis for stomatal function and stress response

The life of a plant is a permanent response to environmental stimuli. Plants monitor and constantly integrate the environmental fluctuations in order to adjust their growth and development. Plant hormones are central to these adaptive growth responses. Auxin is a major plant hormone that mediates a plethora of developmental responses in a concentration-dependent manner. Thus, auxin action requires a tight control of cellular free (active) auxin. Besides auxin metabolism, also subcellular compartmentalization of auxin contributes to defined cellular auxin responses. PILS proteins have been identified as putative auxin carriers that mediate the intracellular auxin transport at the endoplasmic reticulum (ER). PILS activity at the ER affects free auxin levels presumably via the auxin conjugation to amino acids. Subsequently, PILS-mediated auxin homeostasis negatively affects the nuclear auxin signaling and impacts on various plant growth and development aspects. Currently, I am studying mainly evolutionary aspects of PILS proteins in the lab of Dr. Kleine-Vehn and I have found interesting aspects of PILS-mediated subcellular auxin homeostasis and its importance for plant growth and development. Moreover, I have revealed that PILS activity affects plant performance to stressful environments, such as drought. These promising results prompted me to use PILS as genetic tools to investigate the role of auxin homeostasis for stress adaptational responses. My preliminary data suggests that PILS proteins affect stomatal function for adaptive stress responses. I revealed that three (PILS2, 3 and 6) out of seven PILS genes are expressed in cotyledons and leaves of Arabidopsis. Among these, PILS6 showed most restricted expression in the stomatal guard cells. Overexpression of PILS6 led to conjugation-based inactivation of free auxin and impaired auxin signaling in the cotyledon guard and epidermal cells. Furthermore, plants overexpressing PILS6 had more closed stomata and showed enhanced resistance to drought. These findings indicate that PILS- mediated auxin homeostasis plays role in stomatal function and regulates drought stress adaptive responses in Arabidopsis. Auxin has been suggested to affect stomatal function, but its physiological role remains largely enigmatic. My results suggest that subcellularly defined auxin homeostasis plays a role in environmentally controlled stomatal movement. Based on this initial insight, I propose to investigate the role of auxin in stomata-dependent stress adaptation. I will further address the functional redundancy of stomata expressed PILSes, will determine the cellular function of PILS proteins in the guard cell, and unravel intrinsic and extrinsic signals for PILS-dependent stomatal function. Altogether, in this research project, I aim to investigate the role of (PILS-mediated) auxin homeostasis in stomatal function. My research will reveal how auxin homeostasis-mediated stomatal function contributes to stress adaptive responses, such as drought. This line of research has importance for plant productivity and survival. Hertha Firnberg_Elena Feraru

The major outcome of Hertha Firnberg project is that the PIN-LIKES (PILS) proteins - known as putative auxin carriers at the endoplasmic reticulum (ER) - regulate nuclear availability of auxin and, subsequently, plant growth in response to environmental stresses such as high temperature. The life of a plant is a permanent response to environmental stimuli. Plants monitor and constantly integrate the environmental fluctuations in order to adjust their growth and development. Plant hormones are central to these adaptive growth responses. Auxin is a tightly controlled plant hormone that mediates a plethora of developmental responses in a concentration-dependent manner. PILS proteins have been identified as putative auxin carriers that mediate the intracellular auxin accumulation at the ER. PILS activity at the ER presumably compartmentalizes auxin into the ER and thereby prevents the auxin diffusion into the nucleus. In agreement with this assumption, our work illustrates that PILS proteins negatively affect the auxin nuclear availability as well as signalling and, thereby, impact on various plant growth and developmental aspects. In contrast to the well-established role of auxin in promoting elongation of above ground organs (hypocotyl, petioles), the role of auxin in mediating root response to high temperature is mechanistically under-investigated and controversial. The Hertha Firnberg research illustrated that PILS6 regulates nuclear auxin signalling rates and root growth under high temperature (29oC). PILS6, similarly to other PILS proteins, localizes to the ER, where it gates nuclear auxin accumulation and perception. High temperature decreases the abundance of PILS6 proteins, consequently increasing nuclear auxin signalling and root organ growth. Thus, this project revealed a novel auxin-based mechanism, implementing root response to high temperature sensing. Altogether, the Hertha Firnberg project advances our understanding of how plant organs or developmental programs respond to high temperature, revealing a novel subcellular mechanism in roots, which links PILS6-dependent control of cellular auxin sensitivity with high temperature-induced organ growth.