Local root ABA/cytokinin status and aquaporins regulate poplar responses to mild drought stress independently of the ectomycorrhizal fungus Laccaria bicolor

Abstract

The relatively better performance of mycorrhizal plants subjected to drought stress has commonly been linked to improved root water uptake through the fungal regulation of plant aquaporins and hormones. In this study, we examined the role of ectomycorrhizal fungi in plant water relations and plant hormonal balance under mild drought using split-root seedlings of Populus trichocarpa × deltoides either with or without inoculation with Laccaria bicolor. The root compartments where the drought treatment was applied had higher ABA and lower cytokinin tZR contents, and greater expression of the plant aquaporins PtPIP1;1, PtPIP1;2, PtPIP2;5, and PtPIP2;7. On the other hand, the presence of L. bicolor within the roots down-regulated PtPIP1;4, PtPIP2;3, and PtPIP2;10, and reduced the abundance of PIP2 proteins. In addition, expression of the fungal aquaporins JQ585595 and JQ585596 were positively correlated with root ABA content, while tZR content was positively correlated with PtPIP1;4 and negatively correlated with PtPIP2;7. The results demonstrate a coordinated plant-fungal system that regulates the different mechanisms involved in water uptake in ectomycorrhizal poplar plants.

Keywords: Abscisic acid; aquaporins; cytokinins; ectomycorrhiza; mild drought; poplar; root water transport.

Fig. 1.

Effects of drought and inoculation with L. bicolor on (A) leaf stomatal conductance (g_s) and (B) leaf water potential (Ψ _w) in poplar seedlings with roots divided between different treatments in a split-root system. The root compartments were either inoculated with L. bicolor (Lacc) or non-inoculated (NI) and either subjected to well-watered (Ww) or drought (D) conditions in different combinations. Data are means (±SE), n=8 Different letters indicate significant differences among the treatments as determined using ANOVA followed by Tukey’s test (α=0.05).

Fig. 2.

Effects of drought and inoculation with L. bicolor on whole-root hydraulic conductivity (L_pr) in poplar seedlings with roots divided between different treatments in a split-root system. The root compartments were either inoculated with L. bicolor (Lacc) or non-inoculated (NI) and either subjected to well-watered (Ww) or drought (D) conditions in different combinations. Data are means (±SE), n=6 Different letters indicate significant differences among the treatments as determined using ANOVA followed by Tukey’s test (α=0.05).

Fig. 3.

Relative expression of fungal aquaporins in L. bicolor inoculated on roots of poplar seedlings divided between different treatments in a split-root system. The root compartments were either inoculated with L. bicolor (Lacc) or non-inoculated (NI) and either subjected to well-watered (Ww) or drought (D) conditions in different combinations. Data from well-watered and drought compartments are presented in grey and white, respectively. (A) JQ585592, (B) JQ585594, (C) JQ585595, and (D) JQ585596. Data are means (±SE), n=3 Different letters indicate significant differences among the treatments as determined using ANOVA followed by Tukey’s test (α=0.05).

Fig. 4.

Effects of drought and inoculation with L. bicolor on the root abundance of PIP2 of poplar seedlings with roots divided between different treatments in a split-root system. The root compartments were either inoculated with L. bicolor (Lacc) or non-inoculated (NI) and either subjected to well-watered (Ww) or drought (D) conditions in different combinations. Data from well-watered and drought compartments are presented in grey and white, respectively. Data are means (±SE), n=3 Different letters indicate significant differences among the treatments as determined using ANOVA followed by Tukey’s test (α=0.05).

Fig. 5.

Effects of drought and inoculation with L. bicolor on the root contents of (A) tZR (cytokinin) and (B) ABA of poplar seedlings with roots divided between different treatments in a split-root system. The root compartments were either inoculated with L. bicolor (Lacc) or non-inoculated (NI) and either subjected to well-watered (Ww) or drought (D) conditions in different combinations. Data from well-watered and drought compartments are presented in grey and white, respectively. Data are means (±SE), n=4. Different letters indicate significant differences among the treatments as determined using ANOVA followed by Tukey’s test (α=0.05).