Nitrate-Regulated Glutaredoxins Control Arabidopsis Primary Root Growth.

Nitrogen is an essential soil nutrient for plants, and lack of nitrogen commonly limits plant growth. Soil nitrogen is typically available to plants in two inorganic forms: nitrate and ammonium. To better understand how nitrate and ammonium differentially affect plant metabolism and development, we performed transcriptional profiling of the shoots of ammonium-supplied and nitrate-supplied Arabidopsis (Arabidopsis thaliana) plants. Seven genes encoding class III glutaredoxins were found to be strongly and specifically induced by nitrate. RNA silencing of four of these glutaredoxin genes (AtGRXS3/4/5/8) resulted in plants with increased primary root length (approximately 25% longer than the wild type) and decreased sensitivity to nitrate-mediated inhibition of primary root growth. Increased primary root growth is also a well-characterized phenotype of many cytokinin-deficient plant lines. We determined that nitrate induction of glutaredoxin gene expression was dependent upon cytokinin signaling and that cytokinins could activate glutaredoxin gene expression independent of plant nitrate status. In addition, crosses between "long-root" cytokinin-deficient plants and "long-root" glutaredoxin-silenced plants generated hybrids that displayed no further increase in primary root length (i.e. epistasis). Collectively, these findings suggest that AtGRXS3/4/5/8 operate downstream of cytokinins in a signal transduction pathway that negatively regulates plant primary root growth in response to nitrate. This pathway could allow Arabidopsis to actively discriminate between different nitrogen sources in the soil, with the preferred nitrogen source, nitrate, acting to suppress primary root growth (vertical dimension) in concert with its well-characterized stimulatory effect on lateral root growth (horizontal dimension).

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.