Two blue-light photoreceptors (cry1 and phot1) function differently in hypocotyl light responses

Plants have several types of light receptors that control various responses to light, such as leaf expansion, time-of-flowering, and hypocotyl elongation. When seeds are germinated in the dark, their hypocotyl elongates rapidly because darkness is perceived as being deep underground; the elongation is an effort to reach light. Hypocotyl elongation is quickly arrested in the presence of blue light. In this new work, Bustamante et al. developed a sophisticated approach to monitor hypocotyl responses to blue light in wild type plants as well as loss-of-function mutants of two blue light receptors, cry1 and phot1. Previous studies showed that although blue-light slows hypocotyl elongation in cry1 mutants, that effect is transient, and the mutant hypocotyls resume elongation after a short period. The authors set out to identify why the cry1 hypocotyls transiently pause elongation, and how different cells in the hypocotyl respond to light signals. The authors discuss several of the challenges in precisely determinig the relative elemental growth rate (REGR) in various parts of the Arabidopsis hypocotyl. Because measuring growth over time requires a non-invasive procedure, cell length must be determined in intact hypocotyls, which is complicated by the fact that the hypocotyl twists and bends. The authors therefore used machine learning to develop a pipeline for analyzing the images, which included identifying a hypocotyl midline for reference (see the paper for details). The results show that phot1 is required for blue light-induced elongation suppression, and that cry1-suppressed elongation occurs in a more apical hypocotyl region that was not previously known to be involved in the elongation response. This paper is an elegant example of how machine learning can be employed for high-resolution data analysis that can reveal novel mechanisms of plant development. (Summary by Mary Williams @PlantTeaching.bsky.social) Curr. Biol. 10.1016/j.cub.2024.11.021