Phd in GENETICS AND MOLECULAR BIOLOGY

Thesis title: Unraveling the molecular circuit governing the Lateral Root Cap Programmed Cell Death events

In the model organism Arabidopsis thaliana, the lateral root cap (LRC) represents the outermost tissue of the root, involved in sensing environmental stimuli and translating them into a developmental response controlling root growth and architecture, aiding soil penetration, and protecting root meristematic cells. The uppermost lateral root cap cells (ULRCs) are characterized by a recurrent Programmed Cell Death (PCD) event, as a result of which ULRCs are sloughed off the root into the soil. This cellular turnover ensures the correct development of the LRC and guarantees its functions. Evidence has shown a correlation between the activity of the phytohormone auxin and PCD events, including those occurring in the ULRCs. However, the molecular mechanism that triggers ULRC PCD remains unclear. Using a cell biology approach, I demonstrated that auxin accumulation in the ULRCs triggered PCD. Furthermore, I established that auxin efflux transporter PIN-FORMED2 (PIN2), by modulating auxin levels, affects the timing of ULRC PCD occurrence. Thus, we propose a ULRC-specific molecular mechanism that provides positional cues to coordinate LRC cellular turnover. Indeed, auxin turned out to be the crucial signal to instruct ULRCs on their developmental stage and thus ensure PCD progression. Moreover, I designed the experimental protocol to identify the molecular players involved in ULRC PCD by using a transcriptomic analysis. Specifically, I generated inducible lines that enable alterations in auxin biosynthesis and PIN2 expression solely in the ULRCs. I successfully identified the optimal time point for sorting ULRCs after PIN2 removal and auxin accumulation. These analyses will help unravel the regulators of this process downstream of PIN2 activity and auxin signalling. Overall, this research advances our understanding of how auxin regulates PCD and offers a foundation for further investigations into auxin-controlled PCD events, providing a basis forconsidering auxin-regulated PCD as a key mechanism for future agricultural applications.