Molecular and physiological responses in roots of two full-sib poplars uncover mechanisms that contribute to differences in partial submergence tolerance

Abstract

Poplar is a major afforestation tree species in flood-prone areas. Here, we compared molecular and physiological responses in the roots of two full-sib poplar clones, LS1 (flood-tolerant) and LS2 (flood-susceptive), subjected to stagnant flooding using transcript and metabolite profiling. LS1 displayed less phenotypic damage and superior leaf gas exchange and plant growth compared with those of LS2. We concluded that three characteristics might contribute to the differences in flood tolerance between LS1 and LS2. First, fermentation was initiated through lactic dehydrogenation in LS1 roots under flooding and subsequently dominated by alcohol fermentation. However, lactic dehydrogenase was persistently active in flooded LS2. Second, 13 differentially expressed genes associated with energy and O₂ consumption processes under soil flooding had lower transcript levels in LS1 than those in LS2, which might contribute to better energy-/O₂-saving abilities and behaviours in flood-tolerant LS1 than those in flood-susceptible LS2 under hypoxic stress. Third, LS1 possessed increased reactive oxygen species scavenging abilities compared with those of LS2 under edaphic flooding. Our data are a valuable contribution to understanding the mechanisms involved in the flood tolerance of poplar.

Figure 1

Effects of flooding on phenotype (a), growth (b), as well as gas exchange and chlorophyll fluorescence (c) in LS1 and LS2. The growth parameters were the increase in height, root-collar diameter and leaf number for 18 days during the experiment (from day zero to day 18), respectively. Different letters indicated significant difference at p < 0.05 level (Duncan).

Figure 2

Venn diagram (a) and number of DEGs (b) between treatment and clones. Venn diagram presented the numbers of treatment- or clone-specific expression genes. DEGs were genes differentially expressed between treatments or clones. Numbers on bars were DEG numbers.

Figure 3

Most enriched GO terms (a) and KEGG pathways (b) in LS1_FL and LS2_FL compared with CK. Venn diagram of significant enriched GO terms (corrected p-value < 0.01) among four conditions were presented. GO terms were classified into three main categories: MF, molecular function; CC, cellular component; BP, biological process. Only top 20 enriched KEGG pathways were showed.

Figure 4

Contents of carbohydrates (a), soluble protein (b) and amino acid (c) in LS1 and LS2 roots under hypoxia. Carbohydrates and amino acid contents were displayed by relative contents normalized to internal standard (ribitol). Soluble protein content was displayed by % of the control. Statistically significant difference between treatments and controls were indicated by black asterisks, statistically significant difference between LS1 and LS2 were indicated by red asterisks. One asterisks and double asterisks indicated significant difference at p < 0.05 and p < 0.01 levels, respectively.

Figure 5

Effects of hypoxia on gene expressions, enzyme activities and substance contents involved in fermentation (a) and ROS generation/scavenging system (b) in LS1 and LS2 roots. Transcript levels obtained from RNA-seq data were displayed by log2-fold changes compared to the control. Enzyme activities and substance contents were showed by percentage of the control. Statistically significant difference between treatments and controls were indicated by black asterisks, statistically significant difference between LS1 and LS2 were indicated by red asterisks. One asterisks and double asterisks indicated significant difference at p < 0.05 and p < 0.01 levels, respectively.

Figure 6

Heatmap of genes involved in ATP or O₂ consumption (a) and transcription factors (b) differently expressed between LS1 and LS2 under flooding stress. Log2-fold change values of gene RPKM values were normalized to means before heatmap construction.

Figure 7

Experiment design drawing of the study.