Plant root development: is the classical theory for auxin-regulated root growth false?

Area of Science:

• Plant Biology
• Plant Physiology
• Developmental Biology

Background:

• The traditional model of plant root development emphasizes auxin (indolylacetic acid, IAA) as the primary inhibitor of root cell elongation.
• This auxin-centric view has been foundational in understanding root growth regulation for decades.

Purpose of the Study:

• To investigate the role of auxin in root growth regulation under different environmental conditions.
• To propose a revised model for plant root development based on experimental findings.

Main Methods:

• Exogenous application of IAA to maize roots grown in solution versus humid air.
• Measurement of root cell elongation and observation of ethylene emission and lateral rooting.

Main Results:

• Exogenous IAA inhibits root cell elongation only when roots are submerged in liquid, not when grown in humid air.
• IAA treatment consistently induces ethylene emission and lateral rooting, irrespective of the growth medium.
• Ethylene accumulation within the root, dependent on diffusion, appears critical for growth inhibition in solution-grown roots.

Conclusions:

• The cardinal role in root growth regulation is played by ethylene, triggered by IAA, rather than IAA directly inhibiting cell elongation.
• A new model is proposed where IAA-induced ethylene's regulatory effect is contingent on environmental factors, specifically diffusion conditions.
• Ethylene's impact on root cell elongation is modulated by its ability to accumulate within the root tissue, which is dependent on whether roots are in liquid or air.