Iron-response regulator helps Arabidopsis roots withstand salt stress

A second role for an iron-response regulator

Peijun Zhou and colleagues report that bHLH121, a protein involved in regulating iron nutrition, helps Arabidopsis thaliana tolerate salt stress through its control of CAT2, a gene encoding the antioxidant enzyme catalase. Their study was posted on bioRxiv on November 4, 2026, and has not been certified by peer review.

Salt can damage plants through both disrupted ion balance and excessive accumulation of reactive oxygen species, or ROS. These chemically reactive molecules include hydrogen peroxide. Their distribution also helps regulate root development, making control of their location and abundance important—not simply their removal.

The researchers focused on bHLH121 because it is a transcription factor: a protein that helps regulate gene expression. It already has an established role in the network controlling responses to iron deficiency.

Removing bHLH121 makes seedlings salt-sensitive

The team tested two Arabidopsis mutants in which bHLH121 had been knocked out. On growth medium containing 75, 100 or 150 millimolar sodium chloride, both showed stronger inhibition of primary root elongation than wild-type plants. Subsequent experiments mainly used one mutant, bhlh121-7.

Restoring bHLH121 under its native regulatory sequence restored root growth and biomass under salt stress, supporting the conclusion that loss of the gene caused the increased sensitivity. However, producing extra bHLH121 in otherwise wild-type plants did not significantly improve root growth or biomass beyond wild-type levels.

The mutant also showed higher levels of a marker of oxidative damage. Its shoots had greater disruption of sodium and potassium balance under salt treatment, whereas the corresponding root measurements did not differ significantly from wild type.

Hydrogen peroxide accumulates in root tips

Staining experiments revealed stronger hydrogen peroxide signals in mutant root tips after six hours of salt exposure. The researchers examined the first two millimetres of the root, capturing changes that measurements across larger tissue samples can obscure.

This distinction matters: a separate assay after seven days of salt treatment found no significant difference in hydrogen peroxide content among whole-root samples from wild-type plants, the mutant and CAT2-overexpressing mutant lines. The evidence therefore supports a localized root-tip disturbance rather than a consistently detectable increase across the entire root.

Salt also reduced the root apical meristem—the growing region containing dividing cells—more strongly in the mutant. The authors report a reduction in meristem length of about 64%, compared with about 30% in wild type. Treatments with glutathione or potassium iodide, used as ROS scavengers, alleviated mutant root-growth inhibition and improved meristem measurements under salt stress. These treatments also inhibited growth in some conditions without added salt.

CAT2 provides a direct connection

Gene-expression analyses found lower CAT2 expression in the bhlh121 mutant. Catalase breaks hydrogen peroxide down into water and oxygen.

Using purified protein and DNA fragments, the researchers showed that bHLH121 bound directly to the CAT2 promoter, a regulatory region of DNA. Competition experiments supported binding through specific DNA sequences called E-boxes. The authors also cite earlier evidence of this association inside plants.

They then engineered three independent mutant lines to produce extra CAT2. After ten days on medium containing 100 millimolar sodium chloride, these plants had improved growth and greener leaves. Root length and fresh weight largely recovered toward wild-type levels. Catalase activity returned to wild-type levels, and root-tip hydrogen peroxide staining was also comparable to wild type.

Although the abstract describes a full rescue, the detailed results describe growth measurements as approaching wild-type levels. Extra CAT2 did not rescue the mutant’s growth inhibition or leaf yellowing under iron deficiency.

A contribution alongside the SOS pathway

The established Salt Overly Sensitive, or SOS, pathway helps plants regulate sodium transport. To investigate its relationship with bHLH121, the team crossed the bhlh121 mutant with a mutant lacking functional SOS2.

Under salt treatment, the double mutant had shorter roots, lower fresh weight and less chlorophyll than either single mutant. The authors interpret this additive sensitivity as evidence that bHLH121 and SOS2 act in parallel genetic pathways. They nevertheless acknowledge that interactions between SOS2 and downstream targets of bHLH121 cannot be excluded.

What the experiments establish—and what they do not

Hormone-related gene-expression changes and chemical treatments also implicated jasmonic acid and auxin responses, but the precise coordination with bHLH121 remains unresolved. The authors likewise leave open whether additional proteins are required for CAT2 regulation and whether iron-dependent processes contribute to salt tolerance.

Related genes from poplar, wild soybean, cucumber and clementine substantially restored salt-stressed growth when expressed in mutant Arabidopsis. Those experiments support shared gene function, but were not tests of salt tolerance or yield in the donor species. The study establishes a laboratory-supported regulatory connection in Arabidopsis seedlings, not a demonstrated method for improving crops in saline fields.