The AtGRXS3/4/5/7/8 glutaredoxin gene cluster on Arabidopsis thaliana chromosome 4 is coordinately regulated by nitrate and appears to control primary root growth.

Nitrate and ammonium are the 2 most common forms of inorganic nitrogen available to plants in the soil. We previously identified a group of class III glutaredoxin genes whose expression is strongly upregulated by nitrate, but not ammonium, in Arabidopsis thaliana shoots and roots. A reverse genetics approach was used to functionally characterize a subset of these nitrate-regulated glutaredoxins, and we found that the AtGRXS3,4,5, and 8 genes function as negative regulators of primary root growth. AtGRXS3/4/5/8 are arranged in a tandem array on Arabidopsis chromosome 4, and these genes show very high levels of sequence similarity. Interestingly, there is one additional glutaredoxin, AtGRXS7, in this same gene cluster, but this gene was not identified as nitrate-responsive in our previous studies. We show here that AtGRXS7 is upregulated by nitrate and shows strong co-expression with the other glutaredoxins in this gene cluster. Further, AtGRXS7 was effectively silenced by the RNAi construct used to target AtGRXS3/4/5/8 for previous functional analyses. Overall, it appears that the 5 genes in the AtGRX3/4/5/7/8 cluster share virtually identical sequences, regulatory patterns, and functions, collectively acting to regulate primary root growth in response to soil nitrate.