Figure 1. GRN involved in the response of poplar roots to LN. Yellow nodes in the center represent genes from Stage 1 and green nodes at the right side contain genes from Stage 3. All other nodes in various colors contain genes from Stage 2. Genes from Stage 2 were classified into different functional types indicated in different colors as follows: cell growth and division (orange), cell wall formation (magenta), transcription factors from the NAC family (deep sky blue), nitrogen metabolism (tan), and root specific growth (red), and dark-violet represents the unclassified genes. PtaNAC1 and PtaRAS connection edges, which are discussed in the text, are presented in bold and green and red color respectively.