Transcriptomic regulatory network underlying morphological and physiological acclimation to nitrogen starvation and excess in poplar roots and leaves.

Nitrogen (N) is an essential macronutrient that plays a pivotal role in plant growth and development. It is a component of many biomolecules, including amino acids, chlorophylls and nucleic acids. High N availability is a prerequisite for plants to achieve high productivity. This is particularly important for fast growing woody plants, such as Populus species, which often grow on N poor soils (Rennenberg et al. 2010). For instance, the aboveg-round biomass production of eastern cottonwood (Populus deltoides Bartr.) in the southeastern USA was significantly elevated when N fertilizers were applied (Coyle et al. 2013). During the last few decades, with the development of short-rotation forest plantations, N fertilizers have often been applied to the soil to achieve higher productivity (Coleman et al. 2006, Paris et al. 2011). However, excess N may not only increase the costs of growers but may also lead to serious environmental pollution. Nitrogen availability markedly affects the morphological and physiological characteristics of woody plants (Rennenberg et al. 2009, 2010, Zhao et al. 2011, Zhang et al. 2014). Nitrogen fertilization stimulates photosynthetic rates and leaf area of most woody plants, while low N availability has the opposite effects (Hyvönen et al. 2007, Chen et al. 2011, Li et al. 2011, Luo et al. 2013a). In comparison with an appropriate N supply, a high N supply shifts allocation away from roots, whereas a low N level increases root length and biomass (Ingestad and Ågren 1988, Li et al. 2012, Luo et al. 2013a). Although the morphological and physiological responses of trees to changes in N availability have been studied intensively , the molecular regulatory mechanisms underlying the morphological and physiological acclimation to different N levels remain largely unknown. Understanding these mechanisms is of great importance not only for the scientific community of tree biology but also for practical tree breeders, as knowledge of trees' responses to N availability enables breeding trees with a high N use efficiency. To shed light on this knowledge gap, Luo et al. (2015) in this issue employed a comprehensive approach to dissect the transcriptomic regulatory network underpinning morphological and physiological acclimation to N starvation and excess in poplar roots and leaves. In the study reported by Luo et al. (2015), the authors treated cuttings of Populus simonii Carr. with contrasting levels of N supplies , i.e., 0 (N starvation), 2 (control) or 10 (N excess) mM NH 4 NO 3 , for 4 …