Transcripts related with ammonium use and effects of gibberellin on expressions of the transcripts responding to ammonium in two invasive and native Xanthium species

Abstract

Soil nitrogen forms are important for exotic plant invasions. However, little effort has been made to study the molecular mechanisms underlying the utilization of different N forms in co-occurring invasive and native plants. The invasive plant Xanthium strumarium prefers nitrate relative to ammonium, and mainly invades nitrate-dominated environments, while it co-occurring native congener X. sibiricum prefers ammonium. Here, we addressed the genetic bases for the interspecific difference in ammonium use and the effects of gibberellin (GA). Twenty-six transcripts related with GA biosynthesis and ammonium utilization were induced by ammonium in X. sibiricum, while only ten in X. strumarium and none for ammonium uptake. XsiAMT1.1a, XsiGLN1.1 and XsiGLT1b may be crucial for the strong ability to absorb and assimilate ammonium in X. sibiricum. All tested transcripts were significantly up-regulated by GA1 and GA4 in X. sibiricum. XsiGA3OX1a, which was also induced by ammonium, may be involved in this regulation. Consistently, glutamine synthetase activity increased significantly with increasing ammonium-N/nitrate-N ratio for X. sibiricum, while decreased for X. strumarium. Our study is the first to determine the molecular mechanisms with which invasive and native plants use ammonium differently, contributing to understanding the invasion mechanisms of X. strumarium and its invasion habitat selection.

Keywords: ammonium; assimilation; gibberellin; transcriptome analysis; uptake.

Figure 1

The numbers of differently expressed transcripts (DETs) between ammonium and nitrate treatments in Xanthium sibiricum and X. strumarium, respectively. Closed columns indicate the DETs induced by ammonium, i.e., expressed more highly under amonium relative to nitrate. Open columns indicate the DETs induced by nitrate.

Figure 2

Pathways enrichment analysis for differently expressed transcripts (DETs) in Xanthium sibiricum (A) and X. strumarium (B). Circle size indicates the number of DETs enriched in each pathway. Q value indicates significance of the enrichment, increasing from red to green. Rich factor represents the ratio of the enriched DETs to total transcripts in this pathway. The 12 unique pathways for each species are marked with *, and the 18 common pathways are not.

Figure 3

Differently expressed transcripts (DETs) of Xanthium sibiricum involved in GA biosynthesis and metabolism. Blue and red words represent that this kind of DETs were induced by ammonium and nitrate, respectively.

Figure 4

Expression patterns of the differently expressed transcripts (DETs) related with gibberellin synthesis and metabolism (A), ammonium transporter (B), glutamine synthetase (C) and glutamate synthase (D) in Xanthium sibiricum and X. strumarium. For X. strumarium, there were no DETs related with ammonium transporter. Different colors represent different expression levels (increasing from green to red). A and N indicate ammonium and nitrate treatments, respectively. The names of the transcripts were gased on our phylogenetic analyses ( Figures S2-5 ).

Figure 5

Differences in expression levels of XsiGA3OX1a (A), XsiAMT1.1a (B), XsiGLN1.1 (C) and XsiGLT1b (D) between nitrate (N; open bars) and ammonium (A; closed bars) treatments based on our quantitative real-time PCR. XsiActin1 was used as internal controls. Means + SD (n = 3). Asterisk indicates significant difference between ammonium and nitrate treatments (P< 0.05; independent sample t-test). The names of the transcripts were based on our phylogenetic analyses ( Figures S2-5 ).

Figure 6

Effects of gibberellin (GA1 and GA4) on expression levels of XsiAMT1.1a (A), XsiGLN1.1 (B) and XsiGLT1b (C). White bars, grown in nutrient solution including 5 mmol L^-1 ammonium and without GA1 and GA4 addition; gray bars, grown in nutrient solution including 5 mmol L^-1 ammonium and with 1 mmol L^-1 GA1 addition; black bars, grown in nutrient solution including 5 mmol L^-1 ammonium and with 1 mmol L^-1 GA4 addition. XsiActin1 was used as internal controls. Means + SD (n = 3). Different letters indicate significant difference between treatments (P< 0.05; one-way ANOVA). The names of the transcripts were based on our phylogenetic analyses ( Figures S3-5 ).

Figure 7

Total biomass of Xanthium sibiricum (striped bars) and X. strumarium (closed bars) in different nitrogen treatments. 0.5 N and 1 N indicate 50% and 100% nitrogen concentration of the Hoagland nutrient solution, respectively. Means + SE (n = 6). Different upper- and lowercase letters indicate significant dfferences among nitrogen treatments for X. strumarium and X. sibiricum, respectively (P< 0.05; one-way ANOVAs); * indicates significant difference between the invasive and native species in the same treatment (P< 0.05; independent sample t-test).

Figure 8

Glutamine synthetase (GS) activities in leaves (A) and roots (B) of Xanthium sibiricum (striped bars) and X. strumarium (closed bars) in different nitrogen treatments. 0.5 N and 1 N indicate 50% and 100% nitrogen concentration of the Hoagland nutrient solution, respectively. Means + SE (n = 6). Different upper- and lowercase letters indicate significant dfferences among nitrogen treatments for X. strumarium and X. sibiricum, respectively (P< 0.05; one-way ANOVAs); * indicates significant difference between the invasive and native species in the same treatment (P< 0.05; independent sample t-test).