The BIG gene is involved in regulation of sulfur deficiency-responsive genes in Arabidopsis thaliana

The Arabidopsis thaliana mutants altered sulfur response 1-1 (asr1-1) and asr1-2 were isolated using the green fluorescent protein gene (GFP), as a marker, driven by a sulfur deficiency-responsive promoter containing the beta(SR) fragment, which is responsible for the induction of gene expression under sulfur deficiency. In the asr1 mutants, the expression of three sulfur deficiency-responsive genes beta(SR)-driven GFP, sulfate transporter 2;2 (SULTR2;2) and adenosine-5'-phosphosulfate reductase 1 (APR1) were induced in medium containing a normal sulfate concentration. The ASR1 locus was mapped to a 53-kb region on the upper arm of chromosome III; this is also the region of the BIG gene, which encodes a calossin-like protein necessary for the polar transport of auxin. The morphology of the asr1 mutants, i.e. reduced leaf size and inflorescence elongation, resembled that of big mutants. Using nucleotide sequence analysis of the BIG gene, we identified independent nonsense mutations in asr1-1 and asr1-2. To confirm that ASR1 was BIG, we established lines of transgenic A. thaliana carrying a transfer DNA (T-DNA) insertion in the BIG gene. In these T-DNA insertion mutants, mRNA levels of beta(SR)-driven GFP and APR1 were upregulated in normal sulfate medium. The F-1 plants from crosses between asr1-1 and T-DNA insertion lines exhibited reduced leaf size and inflorescence length, indicating that ASR1 was indeed BIG. Taken together, the present results established that BIG is involved in the regulation of beta(SR)-driven GFP and APR1 mRNA level gene expression. Indole-3-acetic acid also upregulated beta(SR)-driven GFP and APR1 together with SULTR2;2 mRNA level, suggesting that the big effect on beta(SR)-driven GFP and APR1 is a pleiotropic aspect of the BIG gene.