Apical Hook Development and Hormonal Regulation in Seedlings

Angiosperm seedlings form an apical hook to shield the shoot apex and cotyledons as they emerge through soil. This structure arises from asymmetric cell elongation in the upper hypocotyl and is orchestrated by a tightly integrated hormonal network. Auxin accumulates asymmetrically across the hook, driving curvature, while gibberellins and ethylene fine-tune this gradient via transcriptional regulators such as DELLA proteins and ETHYLENE INSENSITIVE 3. Light perception mediates hook opening through photoreceptors that attenuate PHYTOCHROME INTERACTING FACTORS and modulate hormone levels, marking the transition from etiolated growth to photomorphogenesis. Key developmental genes, notably the acetyltransferase-like protein HOOKLESS1 and MADS-box factors, govern initiation and opening phases. Understanding this interplay illuminates seedling vigour under varied soil conditions and offers prospects for crop improvement through targeted manipulation of hormone pathways.

Research from Nature Portfolio

Recent studies have traced the evolutionary origin and functional diversification of HOOKLESS1, revealing that this acetyltransferase-like protein emerged in early land plants and acquired specialised roles in angiosperm seedling development. Comparative analyses across bryophytes, lycophytes and eudicots show that while HLS1 orthologues can modulate thermomorphogenesis, only the angiosperm version fully rescues apical hook formation and flowering time regulation. Mechanistic work has uncovered an interaction between HOOKLESS1 and the CONSTANS transcription factor, linking hook development to photoperiodic control. These findings provide a molecular framework for how HLS1 evolved to coordinate seedling emergence with environmental cues.

Apical Hook Development and Hormonal Regulation in Seedlings publication trend

The graph below shows the total number of articles in apical hook development and hormonal regulation in seedlings across all publications each year (not limited to Nature Index journals).

Technical terms

Apical hook: A curved structure formed at the top of the hypocotyl in etiolated seedlings, protecting the shoot apex during soil emergence.

Hypocotyl: The stem-like region below the seed leaves and above the root in a germinating seedling.

Auxin: A class of plant hormones that regulate cell elongation and pattern formation, notably by forming concentration gradients.

Gibberellins: A group of diterpenoid hormones that promote stem elongation, seed germination and developmental transitions.

Ethylene: A gaseous phytohormone involved in stress responses and developmental processes, including hook formation and opening.

HOOKLESS1 (HLS1): An acetyltransferase-like protein critical for apical hook formation, regulating transcriptional networks in seedlings.

MADS-box transcription factor: A family of DNA-binding proteins that control diverse developmental processes by regulating gene expression.

Chemical genetics: An approach using small molecules to perturb biological pathways and identify functional components.

References

1. The origin, evolution and functional divergence of HOOKLESS1 in plants. Communications Biology (2023).
2. Differential growth at the apical hook: all roads lead to auxin. Frontiers in Plant Science (2013).
3. Hormonal networks involved in apical hook development in darkness and their response to light. Frontiers in Plant Science (2014).
4. FRUITFULL Is a Repressor of Apical Hook Opening in Arabidopsis thaliana. International Journal of Molecular Sciences (2020).
5. HYPONASTIC LEAVES 1 is required for proper establishment of auxin gradient in apical hooks. Plant Physiology (2021).
6. New Wine in an Old Bottle: Utilizing Chemical Genetics to Dissect Apical Hook Development. Life (2022).