Modulation of polar auxin transport identifies the molecular determinants of source-sink carbon relationships and sink strength in poplar

Abstract

Source-to-sink carbon (C) allocation driven by the sink strength, i.e., the ability of a sink organ to import C, plays a central role in tissue growth and biomass productivity. However, molecular drivers of sink strength have not been thoroughly characterized in trees. Auxin, as a major plant phytohormone, regulates the mobilization of photoassimilates in source tissues and elevates the translocation of carbohydrates toward sink organs, including roots. In this study, we used an 'auxin-stimulated carbon sink' approach to understand the molecular processes involved in the long-distance source-sink C allocation in poplar. Poplar cuttings were foliar sprayed with polar auxin transport modulators, including auxin enhancers (AE) (i.e., IBA and IAA) and auxin inhibitor (AI) (i.e., NPA), followed by a comprehensive analysis of leaf, stem and root tissues using biomass evaluation, phenotyping, C isotope labeling, metabolomics and transcriptomics approaches. Auxin modulators altered root dry weight and branching pattern, and AE increased photosynthetically fixed C allocation from leaf to root tissues. The transcriptome analysis identified highly expressed genes in root tissue under AE condition including transcripts encoding polygalacturonase and β-amylase that could increase the sink size and activity. Metabolic analyses showed a shift in overall metabolism including an altered relative abundance levels of galactinol, and an opposite trend in citrate levels in root tissue under AE and AI conditions. In conclusion, we postulate a model suggesting that the source-sink C relationships in poplar could be fueled by mobile sugar alcohols, starch metabolism-derived sugars and TCA-cycle intermediates as key molecular drivers of sink strength.

Keywords: auxin; carbon; carbon isotope labeling; metabolomics; poplar; root; source–sink communication.

Figure 1

Experimental design and workflow for foliar spraying of auxin modulators, ¹³CO₂ enrichment and sample harvesting for omics analyses.

Figure 2

Foliar administration of auxin enhancers (AE) and inhibitor (AI) modulates root dry weight. Representative images from control (n = 8), AE (n = 6) and AI (n = 8) treated biological replicates are shown. Average dry weight measurements calculated from leaf, stem and root tissues are presented using bar charts. A t-test was used for statistical analysis and error bars represent standard error.

Figure 3

Foliar administration of AE and AI altered RSA. The plant root image analyses showed significant differences in root parameters, including lateral root branching frequency (LR Bra. Freq.), root area and maximum root width (width max). Data averaged from control (n = 6), AE (n = 7) and AI (n = 8) treated biological replicates. A t-test was used for statistical analysis and error bars represent standard error. AE: auxin enhancer; AI: auxin inhibitor.

Figure 4

Transcriptomic analysis of AE-root tissue. Heatmap showing the log2 foldchange values of 42 differentially expressed genes (DEGs) in root tissue of AE-treated plants compared with controls. Green color represents upregulated genes, and red color represents down-regulated genes. Arrowheads highlight key upregulated genes involved in plant metabolism that play roles in lateral root growth (blue arrows) and starch catabolism (orange arrow). DESEQ was used for statistical analysis (Padj < 0.05), and data were averaged from control (n = 6), AE (n = 3) and AI (n = 4) treated biological replicates. AE: auxin enhancer; AI: auxin inhibitor.

Figure 5

Metabolomic profiling of selected metabolites in leaf, stem and root tissues of AE and AI foliar-sprayed plants compared with their controls. Relative abundance levels of (A) sugars, (B) sugar alcohols, (C) carbohydrate metabolism (glycolysis, pentose phosphate pathway and other pathways) and (D) TCA/glyoxylate cycle intermediates are presented. The percentage change in relative abundance levels of significantly altered metabolites in AE vs C and AI vs C are presented in % values above the bars. Significantly altered metabolites between treated plants (AE or AI) and their respective controls are shown with ^** for P < 0.05 and with ^* for 0.05 < P ≤ 0.1. One-way ANOVA was used for statistical analysis, and data averaged from n = 3 to 6 biological replicates with three technical replicates. AE: auxin enhancer; AI: auxin inhibitor.

Figure 6

Foliar administration of AE and AI altered the allocation of source leaf fixed ¹³CO₂ to the sink tissues. Eleven days post-hormone spraying, the plants were moved inside a Plexiglass chamber enriched with ¹³CO₂ for 4 days. The isotope enrichment levels in different tissues and root segments measured by IRMS are presented. Data averaged from control (n = 4), AE (n = 4) and AI (n = 4) treated biological replicates with three technical replicates. A t-test was used for statistical analysis, and error bars represent standard error. AE: auxin enhancer; AI: auxin inhibitor.

Figure 7

A model depicting source–sink carbon relationships including molecular drivers of sink strength in poplar. We suggest that the source–sink carbon relationships in poplar could be fueled by mobile sugar alcohols and starch metabolism-derived sugars as key molecular drivers of sink strength. We also postulate a very important role of TCA cycle intermediates in providing the energy demands during sink (root) growth. Black solid arrows indicate direct relationships between intermediates derived from the KEGG pathway, whereas black dashed arrows represent indirect relationships. Metabolites in green and red colors denote an increase and decrease in abundance, respectively (under AE condition). Transcripts in blue color are increased in expression levels under the influence of auxin enhancers. Green dashed arrows highlight our hypotheses on the possibility of galactinol transport from stem to root and the possible role of TCA cycle intermediates in providing energy for lateral root branching and improving sink strength. (^** denotes P < 0.05 and ^* denotes 0.05 < P ≤ 0.1). PP pathway, pentose phosphate pathway.