Abstract
BackgroundNitrogen (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) of poplar.ResultsIn this study, three N-efficient genotypes (A1-A3) and three N-inefficient genotypes (C1-C3) 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. The low nitrogen adaptation coefficients of the leaves and stems biomass of group A were significantly higher than those of group C (p < 0.05). Interestingly, N starvation induced fine root growth in group A, but not in group C. Next, a detailed time-course analysis of enzyme activities and gene expression in leaves identified 2062 specifically differentially expressed genes (DEGs) in group A and 1118 in group C. Moreover, the sensitivity to N starvation of group A was weak, and DEGs related to hormone signal transduction and stimulus response played an important role in the low N response this group. Weighted gene co-expression network analysis identified genes related to membranes, catalytic activity, enzymatic activity, and response to stresses that 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, four hub genes were related to programmed cell death and the defense response, and PodelWRKY18, with high connectivity, was involved in plant signal transduction. The expression of hub genes increased gradually with the extension of low N stress time, and the expression changes in group A were more obvious than those in group C.ConclusionsUnder N starvation, group A showed stronger adaptability and better NUE than group C in terms of morphology and physiology. The discovery of hub genes and transcription factors might provide new information for the analysis of the molecular mechanism of NUE and its improvement in poplar.
Highlights
Nitrogen (N) is one of the main factors limiting the wood yield in poplar cultivation
We found that the ‘magenta’ module, including 171 genes that are mainly involved in the response to stress and negative regulation of many biological processes, was negatively related to the changing trend of glutamine synthetase activities (GS), glutamine oxoglutarate aminotransferase (GOGAT), and amino acid contents (AAs) in the leaves of poplar during the response to low N stress (Figs. 7 and 8)
In the present study, plant growth, chlorophyll synthesis, and enzyme activities related to N metabolism of P. deltoides were inhibited under low N stress, and the Nefficient genotypes showed stronger adaptability and a better nitrogen use efficiency (NUE) than the N-inefficient genotypes
Summary
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) of poplar. N is the main limiting factor of plant productivity and crop yield [4]. It is not feasible to increase the timber yield of perennial trees by fertilization; it is important to improve the nitrogen use efficiency (NUE) of plants for crops, especially for trees. NUE is a comprehensive characteristic of the interaction between the available N content in the plant growth environment and various biological processes, including absorption, transport, assimilation, signal transduction, and regulation [8]. Gene expression and physiological activity in the leaves, especially photosynthesis, play an important role in NUtE [18]. Key genes involved in the response to low N stress in leaves were ignored
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