Uncovering shoot to root transport of the transcription factor HY5: A splice variant confers specificity to shoot-root signalling

• Applicant: Dr. Alena Patnaik
• Subject Area: Plant Cell and Developmental Biology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 557313743

Plants in dense vegetation compete for resources and recognize competitors through far-red (FR) light reflected by neighbouring plants. Thus, Red (R) and FR light act as competitive signals and are detected by phytochromes. A reduced R:FR ratio causes a variety of plant responses, including shoot elongation to avoid shade and decreased root growth, which is likely to conserve resources. Despite the well-documented effects of light perception on shoot-on-root architecture, the latter aspect has received little attention. Recently, it has been described that HY5, a phloem-mobile transcription factor, transmits the low R:FR light signal from the shoot to the root by suppressing auxin signalling and modulating lateral root growth. The preliminary data from the lab of Dr. van Gelderen shows that HY5 is transported through the phloem and delivered to the protophloem pericycle cells at the root tip. In this proposed study, I aim to elucidate the molecular mechanism underlying HY5 shoot-to-root transport and investigate the specificity of a HY5 splice variant in regulating root development in Arabidopsis thaliana. A splice variant of HY5 has been identified (HY5.2), which features a functional nuclear export signal (NES) that potentially confers increased mobility to the HY5.2 protein. This could be very useful for the shoot-to-root transport of HY5. This proposal aims to study the regulation of the HY5.2 splice variant and its role in shoot-to-root signalling. I will monitor HY5 transport dynamics in real-time using a 90°tilted confocal microscopy and observe associated developmental changes. Laser ablation will also be employed to block phloem transport and confirm phloem unloading of HY5 proteins. I will map the differences between HY5 and its splice variant HY5.2 with regards to expression pattern, protein function and interaction. This comprehensive investigation will illuminate the involvement of HY5.2 in shoot-to-root signalling, phloem transport and root development, providing fundamental insight into long-distance plant signalling. With my academic background, practical experience in studying light signalling in plants, adept analytical skills, and collaborative mindset, I am well-positioned to undertake this research.

• DFG Programme: WBP Position