Date palm diverts organic solutes for root osmotic adjustment and protects leaves from oxidative damage in early drought acclimation

Abstract

Date palm (Phoenix dactylifera L.) is an important crop in arid regions and it is well adapted to desert ecosystems. To understand its remarkable ability to grow and yield in water-limited environments, we conducted experiments in which water was withheld for up to 4 weeks. In response to drought, root, rather than leaf, osmotic strength increased, with organic solutes such as sugars and amino acids contributing more to the osmolyte increase than minerals. Consistently, carbon and amino acid metabolism was acclimated toward biosynthesis at both the transcriptional and translational levels. In leaves, a remodeling of membrane systems was observed, suggesting changes in thylakoid lipid composition which, together with the restructuring of the photosynthetic apparatus, indicated an acclimation preventing oxidative damage. Thus, xerophilic date palm avoids oxidative damage under drought by combined prevention and rapid detoxification of oxygen radicals. Although minerals were expected to serve as cheap key osmotics, date palm also relies on organic osmolytes for osmotic adjustment in the roots during early drought acclimation. The diversion of these resources away from growth is consistent with the date palm strategy of generally slow growth in harsh environments and clearly indicates a trade-off between growth and stress-related physiological responses.

Keywords: Phoenix dactylifera L; Antioxidant; halophyte; lipid metabolism; membrane remodeling; osmolyte; oxidative stress; reactive oxygen species; water deficit.

Fig. 1.

Phoenix dactylifera cv. Khalas accumulates abscisic acid in roots and leaves and jasmonoyl-isoleucine mainly in roots under drought. Concentrations of abscisic acid, salicylic acid, and jasmonoyl-isoleucine in (A) roots and (B) leaves of date palms grown in phytotrons simulating Saudi Arabia summer climate conditions after 3, 10, and 31 d under well-watered or drought regimes. Data are presented as box plots featuring the maxima, 75 quartiles, medians, 25 quartiles, and minima. Points shown represent raw data; n=10–5; Tukey’s, P≤0.05; different letters indicate significant differences of comparisons between water regime and time point.

Fig. 2.

Anti-oxidative activity is stimulated in both roots and leaves of Phoenix dactylifera cv. Khalas under drought, yet foliar oxygen radicals accumulate. Response of metabolites and enzymes assigned to the Foyer–Halliwell–Asada cycle in leaves and roots of date palms grown in phytotrons simulating Saudi Arabia summer climate conditions after 3, 10, and 31 d under well-watered or drought regimes. APX, ascorbate peroxidase activity; AsA, reduced ascorbate; DHA, dehydroascorbate; DHAR, dehydroascorbic acid reductase activity; Gly, glycine; GR, glutathione reductase activity; GSH, total glutathione; GSSG, glutathione disulfide; myo-Ino, myo-inositol; γ-EC, gamma-glutamylcysteine. Means of the fold-change_log2 are indicated by a color code (n=5; t-test; *P≤0.05; **P≤0.01; ***P≤0.001).

Fig. 3.

Changes in protein levels in roots and leaves of Phoenix dactylifera cv. Khalas in response to a 4 week drought exposure. Networks based on combined transcriptomic and proteomic data reveal strong responses of (A) roots and (B) leaves to 31 d of drought exposure. Transcriptome data were integrated with proteome data to match detected protein and underlying mRNA. Over-represented Gene Ontology (GO) terms were identified based on drought-responsive proteins. GO terms with respect to the three domains ‘Molecular function’, ‘Biological process’, and ‘Cellular component’ are shown in blue network hubs. Node labels on blue edges show individual gene annotations. Each of the rectangular-shaped boxes in the outer layer represents a protein feature annotated to the respective gene. Turquoise coloring of the outermost node indicates a significant change at the protein level (n=4; P≤0.05), which could be either an increase or a decrease as indicated by a fold-change_log2.

Fig. 4.

Root and leaf growth of Phoenix dactylifera cv. Khalas stagnates under drought stress, while tissue hydration remains unaffected with a parallel increase in osmolyte concentration. Biomass of (A) roots and (B) leaves, tissue hydration of (C) roots and (D) leaves, and osmolyte concentration of (E) roots and (F) leaves of date palms grown in phytotrons simulating Saudi Arabia summer climate conditions after 3, 10, and 31 d under well-watered (control) or drought conditions. Data are presented as box plots featuring the maxima, 75 quartiles, medians, 25 quartiles, and minima. Points shown represent raw data; n_Biomass=5, n_Hydration=5; n_Osmolytes=5–4; Tukey’s, P≤0.05; different letters indicate significant differences of water regime comparison within a time point; no letters, P>0.05. Color code as shown in (A) for all panels.

Fig. 5.

Mineral osmolyte concentration in roots of Phoenix dactylifera cv. Khalas remains unaffected during 4 weeks of drought exposure. Total concentration of the major mineral osmolytes of halophytes [potassium (K⁺), chloride (Cl^–), and sodium (Na⁺)] in (A) root and (B) leaf, and individual mineral concentrations in roots and leaves, respectively, of K⁺ (C) and (D), of Cl^– (E) and (F), and of Na⁺ (G) and (H) of date palms grown in phytotrons simulating Saudi Arabia summer climate conditions after 3, 10, and 31 d under well-watered (control) or drought conditions. (A, B) Means ±SE; (C–H) Data are presented as box plots featuring the maxima, 75 quartiles, medians, 25 quartiles, and minima. Points shown represent raw data; n=10–5; Tukey’s, P≤0.05; different letters indicate significant differences of water regime comparison within a time point; no letters, P>0.05. Color code as shown in (A) or (C) for all panels.

Fig. 6.

Sugar concentration increases in roots of Phoenix dactylifera cv. Khalas after 4 weeks of drought exposure while that of leaves remains unaffected. Concentrations in roots and leaves, respectively, of soluble sugars (A) and (B), starch (C) and (D), and (E) individual changes in sugar (alcohols and derivates) levels in roots and leaves of date palms grown in phytotrons simulating Saudi Arabia summer climate conditions after 3, 10, and 31 d under well-watered (control) or drought conditions. (A–D) Means ±SE; points shown represent raw data; Tukey’s, P≤0.05; different letters indicate significant differences of water regime comparison within a time point; no letters, P>0.05. (E) Means of the fold-change_log2 of the relative metabolite contents are indicated by a color code. Asterisks indicate the level of significance (*P≤0.05; **P≤0.01; ***P≤0.001); n=4–5.

Fig. 7.

Amino acid concentration increases in roots of Phoenix dactylifera cv. Khalas already after 10 d of drought exposure. Total concentration of amino acids in (A) root and (B) leaf and the top five individual amino acids with highest contributions to total amino acid accumulation under drought exposure in (C) roots and (D) leaves of date palms grown in phytotrons simulating Saudi Arabia summer climate conditions after 3, 10, and 31 d under well-watered (control) or drought conditions. Means ±SE. Points shown represent raw data; n=5; Tukey’s, P≤0.05; different letters indicate significant differences in water regime comparisons within a time point; no letters, P>0.05. Individual amino acids were selected according to the highest positive delta to the well-watered controls at the peak of accumulation (Supplementary Fig. S2); a full list of all amino acids detected is provided in Supplementary Fig. S3. Color code as in (A) for all panels.

Fig. 8.

Total osmolyte concentration in roots of Phoenix dactylifera cv. Khalas increases with minerals, soluble sugars, and amino acids. Correlation of the sum of measured osmolyte concentrations with concentrations of minerals (A) and (B), sugars (C) and (D), and amino acids (E) and (F) in roots and leaves, respectively, of date palms grown in phytotrons simulating Saudi Arabia summer climate conditions after 3, 10, and 31 d under well-watered (control) or drought conditions. Measured osmolyte concentrations were related to cellular hydration of individual date palms. Pearson’s regression based on the raw data shown as open symbols; filled symbols represent means ±SE; n=5; Tukey’s, P≤0.05; different letters in parentheses indicate significant differences of individual osmolyte concentrations with respect to comparison of water regimes within a time point that is indicated by the shape code; no letters, P>0.05. Shape and color codes as for (A) for all panels.

Fig. 9.

Early acclimation of date palm (Phoenix dactylifera L.) to desert drought. Leaf acclimation includes a remodeling of thylakoid lipid composition together with a restructuring of the photosynthetic apparatus (i.e. a reduction of antennae), both of which prevent the formation of reactive oxygen species (ROS). In leaves and roots, oxidative damage resulting from increased ROS formation is counteracted by increased activity of the anti-oxidant machinery. Roots adapt osmotically to water deficit by accumulating both mineral and organic osmolytes, with organic osmolytes accounting for ~75% of the increase in osmotic strength. Oligosaccharide and amino acid biosynthesis are stimulated, fueling the accumulation of organic osmolytes. Significant increases in osmolytes after 31 d of drought, shown as a difference from the well-watered controls, are highlighted with a white background (Tukey’s, P≤0.05).