Putting a STOP to acid stress

Plant growth can be badly stunted by excess ions in the soil. This effect, called acid soil syndrome, can cause severe agricultural yield losses, especially in areas prone to drought. For this reason, a team of researchers from RIKEN and two Japanese universities are working to identify genes that regulate a plant’s tolerance of ions¹.

Much work has been done on aluminum toxicity in plants, but little is known about the genes that control direct tolerance to acid in the form of hydrogen ions, or protons. The researchers prepared thale cress, Arabidopsis thaliana, from seeds treated with ethyl methanesulfonate to introduce random point mutations in their genome. The seeds were cultivated in an acidic (proton-rich) environment, and the researchers looked for seedlings that failed to grow roots.

“We carried out screening using 25,000 seedlings,” says project leader Satoshi Iuchi from the RIKEN BioResources Center in Tsukuba. “Finally we obtained one mutant that had an acid sensitive phenotype.”

The mutant plant, named stop1 (Sensitive TO Proton), was cloned and subjected to DNA sequencing. The sequencing revealed mutations in a part of the genome that encodes a protein called STOP1, consisting of 499 amino acids. The protein contains four ‘zinc-finger’ domains that regulate DNA transcription in the cell nucleus.

The researchers next investigated whether the stop1 mutant strain was sensitive to other toxic ions. It showed no particular sensitivity to cadmium, copper, sodium, lanthanum or manganese, but was extra sensitive to aluminum ions—stop1 plants showed 80–90% reduced root growth when exposed to aluminum, compared to only 30% in control plants.

Arabidopsis is known to tolerate aluminum by excreting malate, an ionized form of malic acid that is regulated by a gene called AtALMT1. This new study confirmed the link—stop1 mutants failed to express AtALMT1 in the toxic aluminum, and did not excrete any malate.

However, when AtALMT1 was deliberately disrupted in the control plants, the proton sensitivity was not affected. Therefore STOP1 must regulate different genes related to proton sensitivity (Fig. 1).

This work puts STOP1 on the expanding list of transcriptional factors that respond to stress and regulate genes to ensure a plant’s survival. Iuchi believes genetic modification of proteins such as STOP1 is the best way to improve farming efficiency. “Large amounts of chemical fertilizer are used in agriculture, which causes problems,” he says. “If enhanced-tolerance plants can be used, chemical fertilizer usage can be reduced.”