Exogenous application of 5-aminolevulinic acid increases the transcript levels of sulfur transport and assimilatory genes, sulfate uptake, and cysteine and glutathione contents in Arabidopsis thaliana

5-Aminolevulinic acid (ALA), a key precursor of porphyrin biosynthesis, promotes plant growth and crop yields. Although ALA is known to promote carbon fixation and nitrogen assimilation in plants, the effects of ALA on sulfur assimilation have not been determined. In the present study, we analyzed the effect of ALA on sulfur assimilation. We used a fusion gene construct consisting of a promoter region of the high-affinity sulfate transporter SULTR1;2 from Arabidopsis and green fluorescent protein ([GFP] P_SULTR1;2-GFP) to determine whether ALA treatment influences the expression of the sulfur transport gene. The GFP levels in P_SULTR1;2-GFP plants were significantly increased by 0.3 and 1 mmol L^-1 ALA under both sulfur-sufficient and sulfur-deficient conditions. Real-time reverse transcription-polymerase chain reaction experiments revealed that these concentrations of ALA also increased the mRNA levels of other key sulfur transport and assimilatory genes, such as SULTR, adenosine 5′-phosphosulfate reductases and serine acetyl transferase. Sulfate uptake was enhanced by ALA treatment under sulfur-sufficient conditions. In addition, ALA treatment increased the accumulation of cysteine and glutathione, particularly in the shoot. Our data demonstrated that exogenously applied ALA increases the transcript levels of some sulfur assimilatory genes, sulfate uptake, and the contents of cysteine and glutathione. We propose a new role for ALA in regulating the sulfur assimilatory pathway.