Effect of drought acclimation on sugar metabolism in millet

Abstract

Drought stress triggers sugar accumulation in plants, providing energy and aiding in protection against oxidative damage. Plant hardening under mild stress conditions has been shown to enhance plant resistance to severe stress conditions. While sugar accumulation and metabolism under drought stress have been well-documented in crop plants, the effect of drought acclimation treatment on sugar accumulation and metabolism has not yet been explored. In this study, we investigated the impact of drought stress acclimation on sugar accumulation and metabolism in the leaves and root tissues of two commonly cultivated foxtail millet (Setaria italica L.) genotypes, 'PI 689680' and 'PI 662292'. Quantification of total sugars (soluble sugar, fructose, glucose, and sucrose), their related enzymes (SPS, SuSy, NI, and AI), and the regulation of their related transcripts (SiSPS1, SiSuSy1, SiSWEET6, SiA-INV, and SiC-INV) revealed that drought-acclimated (DA) plants exhibited levels of these indicators comparable to those of control plants. However, under subsequent drought stress conditions, both the leaves and roots of non-acclimated plants accumulated higher levels of total sugars, displayed increased activity of sugar metabolism enzymes, and showed elevated expression of sugar metabolism-related transcripts compared to drought-acclimated plants. Thus, acclimation-induced restriction of sugar accumulation, transport, and metabolism could be one of the metabolic processes contributing to enhanced drought tolerance in millet. This study advocates for the use of acclimation as an effective strategy to mitigate the negative impacts of drought-induced metabolic disturbances in millet, thereby enhancing global food security and promoting sustainable agricultural systems.

Keywords: Drought acclimation; Metabolism; Millet; Osmotic adjustment; Total sugar transport; Transcriptional regulation.

Fig.1

The diagram illustrates the experimental framework. Seedlings of millet genotypes ‘PI 662292’ and ‘PI 689680’ were divided into three groups: the control group (CK, unstressed), the drought acclimation group (DA; exposed to two stress episodes, S1 followed by S2, with a recovery ® period), and the non-acclimation group (NA, subjected to a single stress episode, S2, synchronized with DA without any recovery). The hardening stress (S1) began at 0 days after transplanting the seedlings (DAT), continued until 10 DAT, and was followed by the sampling of shoot and root tissues for both CK and drought stress (DS) plants. Afterward, the seedlings were watered for 5 days to facilitate recovery (R) and then subjected to another stress episode (S2) for 10 days (from 15 to 25 DAT). Sampling was conducted at 25 DAT for CK, DA, and NA plants. CK, DS, DA, NA, and DAT represent control, drought stress or hardening, drought acclimation, non-acclimation, and days after seedling transfer, respectively

Fig. 2

Expression of sugar-related transcripts in the leaves (A-E) and roots (F-J) of millet genotypes after drought hardening (S1) treatment. The relative expression levels of (A) SiSPS1, (B) SiSuSy1, (C) SiSWEET6, (D) SiA-INV, and (E) SiC-INV in the leaves, and (F) SiSPS1, (G) SiSuSy1, (H) SiSWEET6, (I) SiA-INV, and (J) SiC-INV in the roots of millet genotypes. CK represents the control group and DS represents the drought stress (hardening) group. The data represent the mean (± S.E.) from biological triplicates. Significant differences are indicated by distinct letters on the error bars, with significance set at a probability level of (p ≤ 0.05)

Fig. 3

(A) Soluble sugar content, (B) sucrose, (C) fructose, and (D) glucose content in the leaves, and (E) soluble sugar content, (F) sucrose, (G) fructose, and (H) glucose content in the roots of control (CK), drought-acclimated (DA), and non-acclimated (NA) plants after the second stress (S2) treatment. The data represent the mean (± S.E.) from biological triplicates. Significant differences are indicated by distinct letters on the error bars, with significance set at a probability level of (p ≤ 0.05). CK, DA, NA represents control, drought acclimation, and non-acclimation, respectively

Fig. 4

Effect of drought acclimation on the activities of sugar metabolism-related enzymes. (A) sucrose phosphate synthase, (B) sucrose synthase, (C) neutral invertase, and (D) soluble acid invertase activity in the leaves, and (E) sucrose phosphate synthase, (F) sucrose synthase, (G) neutral invertase e, and (H) soluble acid invertase activity in the roots of control (CK), drought-acclimated (DA), and non-acclimated (NA) plants after the second stress (S2) treatment. The data represent the mean (± S.E.) from biological triplicates. Significant differences are indicated by distinct letters on the error bars, with significance set at a probability level of (p ≤ 0.05). CK, DA, NA, and DAT represent control, drought acclimation, non-acclimation, and days after seedling transfer, respectively

Fig. 5

Expression of sugar-related transcripts in the leaves and roots of millet genotypes after subsequent stress (S2) treatment. The relative expression levels of (A) SiSPS1, (B) SiSuSy1, (C) SiSWEET6, (D) SiA-INV, and (E) SiC-INV in the leaves, and (F) SiSPS1, (G) SiSuSy1, (H) SiSWEET6, (I) SiA-INV, and (J) SiC-INV in the roots of millet genotypes. The data represent the mean (± S.E.) from biological triplicates. Significant differences are indicated by distinct letters on the error bars, with significance set at a probability level of (p ≤ 0.05). CK represents the control group and DA represents the drought-acclimated, and NA denotes non-acclimated plant groups

Fig. 6

Correlation plot of physiological and molecular indicators studied in the leaves (A) and roots (B) of control (CK) and drought stress (DS) plants of millet genotypes after drought hardening (S1) treatment

Fig. 7

Correlation plot of physiological and molecular indicators studied in leaf (A) of drought acclimated plants, leaf (B) of non-acclimated plants (B), root (C) of drought acclimated plants, and root (D) of non-acclimated plants after drought stress (S2) treatment

Fig. 8

A model of sugar metabolism in drought-acclimated (DA) millet seedlings. Drought hardening (acclimation) causes a sugar-mediated tandem response, which enhances drought tolerance in millets. Drought acclimation altered the expression levels of key regulatory metabolic genes and the activities of sugar metabolism enzymes, modulating sugar accumulation and activating the transcription of sugar transporters to regulate sugar allocation in response to drought stress. Down-regulated items are indicated by downward red arrows