Morphological Physiological and Transcriptional Response to Low Nitrogen Stress in Populus Deltoides Marsh. Clones With Contrasting Nitrogen Use Efficiency

Abstract

\n Background: Nitrogen (N) is one of the main factors limiting the wood yield in poplar cultivation. Understanding the molecular mechanism of N utilization could play a guiding role in improving the nitrogen use efficiency (NUE). Results: In this study, three N-efficient genotypes (A) and three N-inefficient genotypes (C) of Populus deltoides were cultured under low N stress (5 μM NH4NO3) and normal N supply (750 μM NH4NO3). The dry matter mass, leaf morphology, and chlorophyll content of both genotypes decreased under N starvation. Interestingly, N starvation induced fine root growth in A, but not in C. Next, a detailed time-course analysis of enzyme activities and gene expression in leaves identified 2,062 differentially expressed genes (DEGs) in A and 1,118 in C, most of which were up-regulated. Moreover, the sensitivity to N starvation of A was weak, and DEGs related to hormone signal transduction played an important role in the low N response in A. The weighted gene co-expression network analysis identified genes related to membrane, catalytic activity, enzymatic activity, and response to stresses might be critical for poplar’s adaption to N starvation and these genes participated in the negative regulation of various biological processes. Finally, ten influential hub genes and twelve transcription factors were identified in the response to N starvation, among them Podel.19G001200, Podel.19G035300, Podel.02G021400, and Podel.04G076900 were related to programmed cell death, and the defense response, and PodelWRKY41, PodelWRKY75, PodelWRKY18, PodelBHLH25, PodelBHLH30, PodelBHLH, and PodelHY5 were involved in plant signal transduction.Conclusions: Under the condition of N starvation, A showed stronger adaptability and a better NUE than C in morphology and physiology. The discovery of hub genes and TFs provided a new information for the analysis of the molecular mechanism of N efficient utilization and the improvement of NUE of poplar.