Comparative transcriptome analysis reveals K+ transporter gene contributing to salt tolerance in eggplant

Abstract

Background: Soil salinization is one of the most crucial abiotic stresses that limit the growth and production of eggplant. The existing researches in eggplant were mostly focused on salt-induced morphological, biochemical and physiological changes, with only limited works centered on salt-response genes in eggplant at the transcriptomic level.

Results: Our preliminary work found that Zhusiqie (No.118) is salt-tolerant and Hongqie (No.30) is salt-sensitive. Consequently, they were re-named as ST118 and SS30, respectively. ST118 showed less damaged on growth and higher K⁺/Na⁺ ratios in leaves than SS30. Comparative-transcriptome analysis was used as a powerful approach to understand the salt-response mechanisms in the leaves and roots of SS30 and ST118. And it revealed that genotype-specific and organ-specific manners exist in eggplant in response to salt stress. Strikingly, the genotype-specific differentially expressed genes (DEGs) in ST118 were considered crucial to its higher salt-tolerance, because the expression patterns of common DEGs in the leaves/roots of the two eggplant genotypes were almost the same. Among them, some transcription factors have been reported to be in response to elevated external salinity, including the members of C2C2-CO-like, WRKY, MYB and NAC family. In addition, the AKT1, KAT1 and SOS1 were up-regulated only in the leaves of ST118. Furthermore, the complementation assays demonstrated that the salt-tolerances of both yeast and Arabidopsis akt1 mutants were enhanced by heterologous expression of SmAKT1.

Conclusion: The comparative-transcriptome analysis indicated that the salt-tolerance can be increased by higher transcript level of some genotype-specific genes. This work revealed that eggplants seem to be more inclined to absorb K⁺ rather than to exclude Na⁺ under salt stress conditions because seven K⁺ transporters were significantly up-regulated, while only one Na⁺ transporter was similarly regulated. Finally, the complementation assays of SmAKT1, which is genotype-specific up-regulated in ST118, suggest that the other TFs and K⁺ transport genes were worthy of future investigation for their functions in salinity tolerance.

Keywords: Comparative-transcriptome; Eggplant (Solanum melongena L.); Genotype-specific expression; Salt stress; SmAKT1.

Fig. 1

Morphological trait performance of two eggplant genotypes measured under control and salt conditions. The cross-cut length and width represent stem development. Bars represent means ± SD of three biological replicates. Columns with different letters indicate significant differences at P < 0.05 (Duncan’s test)

Fig. 2

The distribution of Na⁺ and K⁺ and the change of K⁺/Na⁺ ratio in leaves/roots of two eggplant genotypes along with salt treatment. a The distribution of Na⁺ and K⁺ in leaves and roots. b The change of K⁺/Na⁺ ratio in leaves and roots. Three replicates were used in each time point, with three seedlings per replicate. Bars represent means ± SD of three biological replicates. Duncan’s Multiple Range test (*P < 0.05 and **P < 0.01) was used to analyze statistical significance

Fig. 3

Overview and Venn diagrams of up- or down-regulated genes by salt stress in leaves and roots of both two eggplant materials at a level of ≥2-fold and adjusted P-value ≤0.001. a The total number of differentially expressed genes found in the leaves (L) and roots (R) of SS30 and ST118 in the comparison between salt-stressed (12 h) and non-stress treatments (0 h). b Four-way Venn diagram indicating that the DEGs were genotype-specific. The number of salt-up-regulated (c) and -down-regulated (d) genes found exclusively in the roots and leaves of two eggplant genotypes were analyzed

Fig. 4

GO classification of the genotype-specific DEGs in the leaves/roots of SS30 or ST118. The left y-axis shows the percentages of genes identified, and the right y-axis shows the gene number. The genes were categorized according to the annotation of GO, and the number of every category is displayed based on biological process, cellular components, and molecular functions. The enriched GO terms were identified using a threshold of P-value < 0.05

Fig. 5

Functional characterization of SmAKT1 in yeast and Arabidopsis under low K⁺ condition. a SmAKT1 and AKT1 complement the K⁺ uptake-deficient yeast mutant R5421 on AP medium containing different K⁺ concentrations. b Real-time PCR verification of SmAKT1 and AKT1 expression in different plant materials. c Phenotype comparison of wild-type Arabidopsis (Col), akt1 mutant and two complementary lines (akt1/SmAKT1–1 and akt1/SmAKT1–2) grown on MS and LK (100 mM K⁺) medium for 7 d. d Average Fv/Fm values of the whole plant. Bars represent means ± SD of three biological replicates. Columns with different letters indicate significant differences at P < 0.05 (Duncan’s test)

Fig. 6

SmAKT1 is involved in response to salt stress in yeast and Arabidopsis under salt condition. a Expression of SmAKT1 and AKT1in yeast mutant strain R5421. Yeast cells were plated on AP medium containing various concentrations of Na⁺ (100, 200 and 300 mM) with different K⁺ concentrations (1, 5 and 10 mM). b Phenotype comparison of the four Arabidopsis lines after 200 mM NaCl treatment for 0 and 7 days. c Real-time quantitative PCR analysis of the expression pattern of SOS1, HKT1 and NHX in the four Arabidopsis lines treated without (control) or with 200 mM NaCl for 12 h and 7 days. Bars represent means ± SD of three biological replicates. Columns with different letters indicate significant differences at P < 0.05 (Duncan’s test)