Auxin homeostasis in plant organ responses to high temperature

One of the major negative consequences of human impact on the environment is the climate change. The last 30 years were likely the warmest of the last millennium in the Northers Hemisphere and, unfortunately, the global mean surface temperature is predicted to further rise over the 21st century. The increase in the global surface temperature will subsequently affect the soil temperature and, thus, the global warming is becoming a general problem for crop productivity. 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. PIN-LIKES (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 presumably compartmentalizes auxin into the ER and thereby prevents the auxin diffusion into the nucleus. Accordingly, PILS proteins negatively affect the nuclear auxin signaling and thereby impact on various plant growth and developmental aspects. In contrast to the well-established role of auxin in promoting hypocotyl or petioles elongation, the role of auxin in mediating root response to high temperature is mechanistically under-investigated and controversial. Similarly, the role of auxin in regulating female floral organ development under high temperature has not yet been thoroughly investigated. Facing the consequences of global warming, plant growth response to high temperature is a timely, fundamental research topic with additional potential for applied research fields. In my project, I present preliminary results and propose further experiments that aim to reveal the molecular mechanisms that regulate PILS6-dependent nuclear auxin signalling during root and female floral organ development under high temperature. This research will firstly identify new genes involved in plant organ adaptive responses to high temperature. Secondly, it will clarify some controversy in the literature and will enable me to dissect whether different plant organs use distinct auxin-dependent mechanisms. Thirdly, this research will advance the mechanistic understanding of female floral organ and fruit development and the potential role of auxin to regulate these developmental aspects under high temperature condition. By performing this research, I aim to fill some knowledge gaps and solve some controversy in our understanding of auxin-mediated thermo-responses. This project will contribute to several research fields, connecting temperature sensing, intracellular auxin transport and plant adaptive growth under high temperature. Overall, my results will further our understanding of how plant organs or developmental programs respond to high temperature.

The plant hormone auxin is probably the most fascinating molecule, whose concentration and signaling output coordinate multiple aspects of plant growth and development. In our research, we mainly used the PILS putative auxin transporters, whose intracellular activity at the endoplasmic reticulum negatively affects the auxin availability for nuclear signaling, which has consequences on plant growth and development. One major output of the Elise Richter research is that auxin signaling affects the abundance of PILS proteins in Arabidopsis thaliana. More auxin increases PILS proteins level, while less auxin signaling reduces PILS proteins levels, suggesting that this way auxin regulates its own abundance. We uncovered a feedback mechanism on auxin signaling output, which will help us understand auxin signaling regulation during plant growth and development. Another line of research revealed that moderately high temperature reduces PILS levels, thus increasing auxin signaling in root tip and promoting total root growth. Moreover, ongoing research shows that short term high temperature triggers non-optimal auxin responses that affect ovules fertilization and subsequent seed and fruit development. Together, these discoveries will help scientists to understand auxin signaling regulation during plant growth and development and they will advance our knowledge on how different plant organs respond and adapt to high temperature.