A basic/helix-loop-helix transcription factor controls leaf shape by regulating auxin signaling in apple.

Area of Science:

• Plant biology
• Molecular genetics
• Ecology

Background:

• Climate change causes phenological shifts and leaf shape variation across elevations.
• The molecular mechanisms driving altitude-related leaf narrowing in broadleaf species remain largely unknown.

Purpose of the Study:

• To investigate the molecular basis of altitude-induced leaf shape variation in apple.
• To identify key genes and pathways involved in regulating leaf width in response to environmental gradients.

Main Methods:

• Morphometric analyses and biochemical assays were performed on apple plants.
• Functional identification involved gene overexpression and exogenous auxin application.
• Promoter binding assays were used to confirm transcription factor interactions.

Main Results:

• A basic/helix-loop-helix transcription factor (MdbHLH3) controls apple leaf width reduction with increasing altitude.
• MdbHLH3 modulates transcript levels of MdPIN1 (auxin efflux) and MdGH3-2 (auxin conjugation), lowering free auxin.
• Exogenous auxin partially restored leaf width in transgenic apple lines, confirming auxin's role.

Conclusions:

• MdbHLH3 directly regulates auxin homeostasis and transport by binding to target gene promoters.
• This mechanism alters auxin signaling, leading to changes in leaf shape in response to altitudinal gradients.
• The findings reveal a direct link between a transcription factor, auxin signaling, and adaptive leaf morphology in apple.