Fig. 1.
Role of the transcription factor TGA1 in coordinating proportional transcriptional, metabolic, and growth responses to N availability in roots of A. thaliana. (A) TGA1 target genes fit the Michaelis–Menten model (wild type; black line) as a function of N availability. TGA1 overexpression (TGA1 OX; green line) increases the rate of change of N-responsive gene mRNAs across N concentrations but also raises Km. Plant growth rates also fit the Michaelis–Menten model, with the growth kinetic changes of TGA1 overexpressing plants mirroring the changes observed for transcripts of TGA1 target genes. (B) Schematic of regulation of N uptake, N reduction, N assimilation, and translation of mRNAs produced by the TGA1 transcriptional network, which is putatively regulated by the nitrate transceptor NRT1.1. TGA1 directly regulates 92 transcription factors in a network that regulates thousands of downstream genes. Enzyme classes are underlined, whereas TGA1-regulated genes are shown in italics and are color coded according to up-regulation (magenta) or down-regulation (green) in the network. Genes shown in bold are direct targets of TGA1. Abbreviations for enzymes: AS, asparagine synthetase; AspAT, aspartate aminotransferase; GDH, glutamate dehydrogenase; GOGAT, glutamine synthase; GS, glutamine synthetase; NIR, nitrite reductase; NR, nitrate reductase. Abbreviations for molecules/metabolites: Asn, asparagine; Asp, aspartate; Gln, glutamine; Glu, glutamate; NH4+, ammonium; NO2−, nitrite; NO3−, nitrate; OAA, oxaloacetate; 2-OG, 2-oxoglutarate.