An early-morning gene network controlled by phytochromes and cryptochromes regulates photomorphogenesis pathways in Arabidopsis.

Light perception at dawn plays a key role in coordinating multiple molecular processes and in entraining the plant circadian clock. The Arabidopsis mutant lacking the main photoreceptors, however, still shows clock entrainment, indicating that the integration of light into the morning transcriptome is not well understood. In this study, we performed a high-resolution RNA-sequencing time-series experiment, sampling every 2 min beginning at dawn. In parallel experiments, we perturbed temperature, the circadian clock, photoreceptor signaling, and chloroplast-derived light signaling. We used these data to infer a gene network that describes the gene expression dynamics after light stimulus in the morning, and then validated key edges. By sampling time points at high density, we are able to identify three light- and temperature-sensitive bursts of transcription factor activity, one of which lasts for only about 8 min. Phytochrome and cryptochrome mutants cause a delay in the transcriptional bursts at dawn, and completely remove a burst of expression in key photomorphogenesis genes (HY5 and BBX family). Our complete network is available online (http://www-users.york.ac.uk/∼de656/dawnBurst/dawnBurst.html). Taken together, our results show that phytochrome and cryptochrome signaling is required for fine-tuning the dawn transcriptional response to light, but separate pathways can robustly activate much of the program in their absence.

Breakout-lateral root emergence in Arabidopsis thaliana.

Lateral roots are determinants of plant root system architecture. Besides providing anchorage, they are a plant's means to explore the soil environment for water and nutrients. Lateral roots form post-embryonically and initiate deep within the root. On its way to the surface, the newly formed organ needs to grow through three overlying cell layers; the endodermis, cortex and epidermis. A picture is emerging that a tight integration of chemical and mechanical signalling between the lateral root and the surrounding tissue is essential for proper organogenesis. Here we review the latest progress made towards our understanding of the fascinating biology underlying lateral root emergence in Arabidopsis.