How Do Plant Roots Find Their Way Downward? Protein 'Hand-Holding' Determines Direction [Reading Science]

Direct Binding Between Auxin Transporters PIN Reveals Mechanism of Localization and Movement

Disrupting "Hand-Holding" Weakens Root Gravitropic Response...Published in PNAS

How do immobile plants extend their roots downward and grow their stems toward the light? It has been discovered that plant hormone transporter proteins, which determine the direction of growth, regulate their positioning and movement by alternately "joining and releasing hands" with each other.

On September 29, the research team announced that they have identified a new mechanism by which 'PIN-FORMED (PIN)' proteins, which determine the direction of auxin hormone (auxin) movement, directly bind to each other to regulate their localization and movement within the cell membrane. The results of this study were published online in the Proceedings of the National Academy of Sciences (PNAS) on September 28.

Handshake between PIN proteins of auxin transporters through β-sheet. Provided by the research team

Auxin is a key hormone that enables plants to change their direction of growth in response to environmental factors such as light, gravity, and water. When auxin accumulates more on one side of a tissue, differences in cell growth rates cause roots or stems to bend in a particular direction.

PIN proteins determine the direction in which auxin moves. They do not distribute evenly across the cell membrane but instead cluster at one side, directing auxin to move in a specific direction. This is similar to how the placement of a door on a specific wall affects the direction in which people move inside a building.

Breaking the PIN-to-PIN binding even altered the direction of root growth

The research team focused on the 'hydrophilic loop (HL)' located on the inside of PIN proteins. While HL was previously thought to be a region where other regulatory proteins attach, this study identified that HL regions of different PIN proteins directly bind to each other.

In particular, the 'beta-sheet (β-sheet)' structure within the HL played a crucial role in PIN-to-PIN binding. When this structure was removed, the binding between PIN proteins was disrupted and the proteins tightly aggregated at certain areas of the cell membrane. This also caused abnormalities in their internal movement and degradation within the cell, thereby diminishing the ability of roots to bend and grow properly in response to gravity.

Photo of the research team. Professor Hyung-Taek Cho (left) and postdoctoral researcher Kwangho Maeng. Provided by Seoul National University

When the researchers used a compound to reassemble PIN proteins lacking the beta-sheet, the abnormal aggregation, localization, internal movement, and even the gravity response of roots, were mostly restored to normal. This demonstrated the importance of direct binding between PIN proteins for their proper function.

The team also confirmed that 'phosphorylation'—the addition of a phosphate group to the HL of PIN proteins—weakens the binding between PIN proteins. This suggests that cells can regulate the binding, movement, and arrangement of PIN proteins by altering their phosphorylation status.

This study presents a new principle: that PIN proteins themselves can directly interact with each other to regulate their position and movement, moving beyond the conventional view that other regulatory proteins solely determine the localization and function of PINs.

The team expects that these findings will provide a foundation for better understanding auxin transport and the mechanisms of directional growth in plants, and, in the long term, be instrumental in research on crop root development and environmental adaptation.

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