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Summary Root hair (RH) cells can elongate to several hundred times their initial size, and are an ideal model system for investigating cell size control. Their development is influenced by both endogenous and external signals, which are combined to form an integrative response. Surprisingly, a low‐temperature condition of 10°C causes increased RH growth inArabidopsisand in several monocots, even when the development of the rest of the plant is halted.Previously, we demonstrated a strong correlation between RH growth response and a significant decrease in nutrient availability in the growth medium under low‐temperature conditions. However, the molecular basis responsible for receiving and transmitting signals related to the availability of nutrients in the soil, and their relation to plant development, remain largely unknown.We have discovered two antagonic gene regulatory networks (GRNs) controlling RH early transcriptome responses to low temperature. One GNR enhances RH growth and it is commanded by the transcription factors (TFs)ROOT HAIR DEFECTIVE 6(RHD6),HAIR DEFECTIVE 6‐LIKE 2 and 4(RSL2‐RSL4) and a member of the homeodomain leucine zipper (HD‐Zip I) group I 16 (AtHB16). On the other hand, a second GRN was identified as a negative regulator of RH growth at low temperature and it is composed by the trihelix TFGT2‐LIKE1(GTL1) and the associated DF1, a previously unidentified MYB‐like TF (AT2G01060) and several members of HD‐Zip I group (AtHB3, AtHB13, AtHB20, AtHB23).Functional analysis of both GRNs highlights a complex regulation of RH growth response to low temperature, and more importantly, these discoveries enhance our comprehension of how plants synchronize RH growth in response to variations in temperature at the cellular level. 

Abstract Glutaredoxins (GRXs) are small oxidoreductase enzymes that can reduce disulfide bonds in target proteins. The class III GRX gene family is unique to land plants, andArabidopsis thalianahas 21 class III GRXs, which remain largely uncharacterized. About 80% ofA. thalianaclass III GRXs are transcriptionally regulated by nitrate, and several recent studies have suggested roles for these GRXs in nitrogen signaling. Our objective was to functionally characterize two nitrate‐induced GRX genes,AtGRXS5andAtGRXS8, defining their roles in signaling and development in theA. thalianaroot. We demonstrated thatAtGRXS5andAtGRXS8are primarily expressed in root and shoot vasculature (phloem), and that the corresponding GRX proteins display nucleo‐cytosolic subcellular localization. Ectopic expression ofAtGRXS8in transgenic plants caused major alterations in root system architecture: Normal primary root development, but a near absence of lateral roots. RNA sequencing demonstrated that the roots ofAtGRXS8‐overexpressing plants show strongly reduced transcript abundance for many primary nitrate response genes, including the major high‐affinity nitrate transporters. Correspondingly, high‐affinity nitrate uptake and the transport of nitrate from roots to shoots are compromised inAtGRXS8‐overexpressing plants. Finally, we demonstrated that the AtGRXS8 protein can physically interact with the TGA1 and TGA4 transcription factors, which are central regulators of early transcriptional responses to nitrate inA. thalianaroots. Overall, these results suggest thatAtGRXS8acts to quench both transcriptional and developmental aspects of primary nitrate response, potentially by interfering with the activity of the TGA1 and TGA4 transcription factors.