QTL mapping of drought-related traits in the hybrids of Populus deltoides 'Danhong'×Populus simonii 'Tongliao1'

Abstract

Background: Poplar trees provide a large amount of wood material, but many parts of the world are arid or semi-arid areas because of insufficient annual precipitation, which seriously affects the growth of poplar trees. Populus simonii 'Tongliao1' shows strong tolerance to stress environments, and Populus deltoides 'Danhong' shows a stronger growth rate in a suitable environment. To identify drought tolerance-related QTLs and genes, an F₁ population derived from the cross between the 'Danhong' and 'Tongliao 1' Populus was assessed under drought stress.

Results: We measured drought-related traits such as the relative height growth, relative diameter growth, leaf senescence number, specific leaf area, and leaf relative water content in the population under control and drought environments. The results showed that drought stress reduced the plant height relative growth, ground diameter relative growth, specific leaf area and leaf relative water content and increased the number of leaf drops. A total of 208 QTLs were identified by QTL mapping analysis, and they consisted of 92, 63 and 53 QTLs under control, drought stress treatment and drought index conditions, respectively. A molecular identification marker for drought tolerance, np2841, which was associated with a QTL (qDLRWC-LG10-1) for relative leaf water content, was initially developed. We mined 187 candidate genes for QTL regions of five traits under a drought environment. The reference genome annotation for Populus trichocarpa and a homologous gene analysis of Arabidopsis thaliana identified two candidate genes, Potri.003G171300 and Potri.012G123900, with significant functions in response to drought stress. We identified five key regulatory genes (Potri.006G273500, Potri.007G111500, Potri.007G111600, Potri.007G111700, and Potri.007G111800) related to drought tolerance through the poplar coexpression network.

Conclusion: In this study, our results indicate that the QTLs can effectively enhance the drought tolerance of poplar. It is a step closer towards unravelling the genetic basis of poplar drought tolerance-related traits, and to providing validated candidate genes and molecular markers for future genetic improvement.

Keywords: Coexpression network; Crossbreeding; Drought stress; Drought-related traits; Populus; QTL mapping.

Fig. 1

Phenotypic analysis of F₁ populations under drought and control environments. CK: control environment, red; DS: drought environment, light blue; frequency distribution histogram (on the below); comparative analysis of phenotype data under drought and control environments by boxplots (on the right); density dissolution curve (on the diagonal); correlation analysis (above diagonal), CK was control, DS was drought condition, corr was integrated control and drought treatment data; and scatter plot for correlation analysis (below the diagonal); Asterisks show the different degrees of significant positive or negative correlation, respectively. *, **, and *** indicate significant difference at the P <0.01, 0.05, and 0.001 levels, respectively

Fig. 2

Heat map of the cluster analysis on the drought tolerance index

Fig. 3

Principal component analysis of population phenotypes under control and drought environments. A Scree plot, the variance contribution rate of each principal component; B representative quality plot, the representative quality of each variable to each principal component; C PCA biplot, showing the correlation between the principal component scores of the sample points and the principal components; CK:control environment; DS: drought environment

Fig. 4

Circle map of the linkage group positions of QTLs for drought-related traits. The first circle represents 19 linkage groups, scale for cM; the 2nd, 3rd and 4th circles are the positions of the QTLs in the 19 linkage groups under the control environment, drought environment, and drought tolerance index, respectively. The relative height growth, relative diameter growth, leaf senescence, specific leaf area and leaf relative water content QTLs are represented by red, blue, yellow, purple and green lines, respectively

Fig. 5

GO enrichment analysis of candidate genes for QTLs of drought tolerance related traits

Fig. 6

Coexpression network analysis of candidate genes for the drought-related traits QTL. Red: Candidate genes for relative height growth (RH) QTLs; Yellow: Candidate genes for relative diameter growth (RD) QTLs; Pink: Candidate genes for leaf senescence number (LS) QTLs; Green: Candidate genes for specific leaf area (SLA) QTLs; Blue: Candidate genes for leaf relative water content (LRW) QTLs