Over-Expression of Glutamine Synthetase in Poplar Results in Transcriptomic Rewiring: Implications for Drought Stress Tolerance

Glutamine synthetase (GS) plays the central role in assimilation of ammonium into amino acids and other reduced nitrogen compounds in plants.  Hybrid poplar (Populus tremula X P. alba, INRA 717-1-B4) expressing ectopically the pine glutamine synthetase (GS1a) gene display pleiotropic phenotypes, including increased growth, increased nitrogen use efficiency, and enhanced tolerance to drought.  This prompted us to profile transcriptomic changes associated with GS overexpression during pre-drought, drought, and recovery conditions in poplar tissues using whole-genome microarrays.  Under pre-drought: aerial tissues showed more than double the number of significantly differentially expressed genes compared with roots; among up-regulated genes in transgenics were genes associated with photosynthesis, protein turnover, and stress; many transcription factors were down-regulated.  Drought conditions: a shift was seen in the percentage of differentially expressed genes in transgenics with regard to controls; a higher percentage of genes was down-regulated in aerial tissues in transgenics whereas roots showed the opposite trend; among up-regulated genes, the stress group was one of the most significant.  During recovery: the number of genes differentially expressed in roots was highest among all tissues and conditions, included in this group were genes associated with cell wall metabolism and secondary metabolism. A union set of genes exhibiting differential expression between transgenics and wild type across all tissues and conditions was subjected to weighted correlation network analysis. Considerable transcriptome rewiring was evident in transgenics, with overall lower connectivity affecting stress and protein related genes. The transgenic network showed fewer, but interesting highly-connected hubs (e.g. hormones and signaling genes) than the wild-type network.