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A new study has revealed the 3D structure of a barley root protein that protects plants from toxic aluminum in acidic soils. Unlike most transporters, this protein exports citrate—an anion that binds to harmful aluminum ions—thereby shielding the roots. The findings offer fresh insights into how plants adapt to hostile soils and could help guide the breeding of crop varieties capable of thriving on acidic farmland worldwide.

For billions of people worldwide, soil health is a deciding factor in food security. Nearly 40% of the world's arable land is acidic, creating a hostile environment for crops. In such soils, aluminum ions are released in large amounts, poisoning plant roots, damaging nutrient uptake, and dramatically reducing yields. Farmers often attempt to tackle this problem with soil modifications, but these solutions are costly, temporary, and often out of reach for smallholder farmers in developing regions.

To survive in these challenging conditions, some plants have evolved natural defenses against aluminum stress. One common strategy is the release of organic acids—such as citrate, malate, or oxalate—from their roots. These acids bind to aluminum ions in the soil, neutralizing their toxicity and protecting root growth.

Barley, one of the world's most important cereals for food, feed, and brewing, is generally susceptible to acidic soils and the toxic effects of aluminum. However, some barley cultivars stand out for their remarkable resilience: they possess a specialized root protein that actively pumps citrate into the soil, thereby neutralizing aluminum before it can damage the plant.

This adaptation enables these select cultivars to thrive in challenging environments where most other barleys—and many crops—struggle to grow. Until now, however, the detailed structure of this protective protein—and the molecular mechanism behind its function—has remained unknown.

To explore this mechanism, a new study published in the on August 5, 2025, was led by Professor Michihiro Suga from the Research Institute for Interdisciplinary Science at Okayama University, Japan. The team also included Tran Nguyen Thao, Dr. Namiki Mitani-Ueno, and Professor Jian Feng Ma, all from Okayama University. Together, they uncovered the first detailed structure of HvAACT1, the barley root protein that enables the plant to tolerate aluminum-rich acidic soils. This provides the first structural basis for citrate efflux in plants, filling a long-standing knowledge gap.

HvAACT1 belongs to the multidrug and toxic compound extrusion (MATE) family of protein transporters, which are widely found across plants, animals, and microbes. "HvAACT1 is unlike most structurally characterized MATE proteins," explains Prof. Suga. "While many MATE transporters move positively charged molecules, this one specializes in exporting negatively charged citrate molecules. Once released, citrate binds toxic aluminum outside the root, making the soil safer for the plant."

To capture the protein in action, the researchers used powerful tools of structural biology. They determined its structure using X-ray crystallography at a synchrotron facility, combined with molecular dynamics simulation and mutational analysis, creating high-resolution images that reveal the protein's design at near-atomic detail. These images showed that HvAACT1 contains two separate but coordinated sites—one that recognizes citrate and another that binds protons (hydrogen ions). The interaction between these sites enables the protein to pump citrate efficiently into the soil.

This breakthrough not only explains how barley handles aluminum stress but also highlights a new kind of transporter biology. Unlike other proteins in the same family, which usually move positively charged or aromatic molecules, HvAACT1 transports negatively charged compounds. This unusual ability expands scientific understanding of plant resilience and protein versatility.

"Understanding the exact structure of HvAACT1 gives us a blueprint of how plants handle aluminum stress," says Prof. Suga. "It's the first clear evidence of how this type of transporter moves negatively charged molecules at the molecular level."

This discovery builds on earlier research that first identified the barley transporter responsible for aluminum tolerance. The current study provides the long-awaited structural explanation of how the protein works, unlocking possibilities for practical applications in agriculture and beyond.

"As scientists, we are always inspired by how nature solves problems," adds Prof. Suga. "By revealing the structure of this protein, we now have a foundation to design or breed crops that can withstand acidic soils, ensuring stable harvests even under difficult conditions."

Overall, the study highlights how understanding the hidden strategies of plants can help address one of agriculture's greatest challenges. As acidic soils continue to limit food production worldwide, insights from molecular biology may pave the way for resilient farming practices and innovative biotechnological solutions—offering hope for a more secure and sustainable global food supply.