Cell wall integrity modulates HOOKLESS1 and PHYTOCHROME INTERACTING FACTOR4 expression controlling apical hook...

Area of Science:

• Plant Biology
• Developmental Biology
• Cell Biology

Background:

• Apical hook formation in etiolated dicot seedlings is essential for photoprotection and involves differential growth regulated by hormones like auxin and gibberellins (GAs).
• The role of cell wall feedback mechanisms in regulating apical hook development remains largely unexplored.
• Cell expansion is fundamentally controlled by the cell wall, but its contribution to hook formation is unclear.

Purpose of the Study:

• To investigate the role of cell wall integrity and pectin biosynthesis in Arabidopsis apical hook development.
• To elucidate the signaling pathways linking cell wall status to hormone-mediated differential growth during hook formation.
• To identify key regulators and environmental factors influencing hook development in response to cell wall perturbations.

Main Methods:

• Analysis of Arabidopsis quasimodo2-1 (qua2) mutants defective in pectin biosynthesis.
• Treatment of wild-type seedlings with the cellulose inhibitor isoxaben (isx).
• Genetic manipulation (DELLA protein mutants, HLS1 overexpression) and hormone treatments (GAs).
• Measurement of auxin distribution, gene expression (HLS1, PIF4), and GA accumulation using a Förster resonance energy transfer (FRET)-based sensor.
• Investigating the role of the cell wall integrity sensor THESEUS 1 (THE1).

Main Results:

• The qua2 mutant and isoxaben-treated seedlings exhibit severe defects in apical hook formation, loss of auxin maxima, and reduced differential cell expansion.
• Expression of positive hook regulators HOOKLESS1 (HLS1) and PHYTOCHROME INTERACTING FACTOR4 (PIF4) is decreased in qua2 and isx-treated seedlings.
• Exogenous GAs, loss of DELLA proteins, or HLS1 overexpression partially rescue hook development in these mutants.
• Increased agar concentration restores hook formation, auxin maxima, and HLS1/PIF4 expression in both qua2 and isx-treated seedlings.
• Isoxaben treatment reduces GA accumulation in the apical hook in a turgor-dependent manner.
• Loss of THESEUS 1 restores hook formation in qua2 and isx-treated seedlings, indicating a role for turgor regulation.

Conclusions:

• Cell wall integrity, specifically pectin biosynthesis and cellulose integrity, is essential for proper apical hook formation in Arabidopsis seedlings.
• Turgor-dependent signals originating from the cell wall act upstream of the PIF4-HLS1 module to regulate differential cell elongation.
• The findings reveal a novel feedback mechanism where cell wall status influences hormonal signaling (GAs) and gene expression to control plant development.