Drought Stress Priming Improved the Drought Tolerance of Soybean

Abstract

The capability of a plant to protect itself from stress-related damages is termed "adaptability" and the phenomenon of showing better performance in subsequent stress is termed "stress memory". While drought is one of the most serious disasters to result from climate change, the current understanding of drought stress priming in soybean is still inadequate for effective crop improvement. To fill this gap, in this study, the drought memory response was evaluated in cultivated soybean (Glycine max). To determine if a priming stress prior to a drought stress would be beneficial to the survival of soybean, plants were divided into three treatment groups: the unprimed group receiving one cycle of stress (1S), the primed group receiving two cycles of stress (2S), and the unstressed control group not subjected to any stress (US). When compared with the unprimed plants, priming led to a reduction of drought stress index (DSI) by 3, resulting in more than 14% increase in surviving leaves, more than 13% increase in leaf water content, slight increase in shoot water content and a slower rate of loss of water from the detached leaves. Primed plants had less than 60% the transpiration rate and stomatal conductance compared to the unprimed plants, accompanied by a slight drop in photosynthesis rate, and about a 30% increase in water usage efficiency (WUE). Priming also increased the root-to-shoot ratio, potentially improving water uptake. Selected genes encoding late embryogenesis abundant (LEA) proteins and MYB, NAC and PP2C domain-containing transcription factors were shown to be highly induced in primed plants compared to the unprimed group. In conclusion, priming significantly improved the drought stress response in soybean during recurrent drought, partially through the maintenance of water status and stronger expression of stress related genes. In sum, we have identified key physiological parameters for soybean which may be used as indicators for future genetic study to identify the genetic element controlling the drought stress priming.

Keywords: drought memory; gene expression; photosynthesis; physiology; transpiration; water content.

Figure 1

Phenotypes of the drought-sensitive soybean C08 plants after drought treatment. (a) Left: plants after 2 cycles of drought treatment (2S: 7 days without irrigation until the first sign of wilting, followed by 5 days of recovery and then 10 days without irrigation). Right: plants receiving only 1 cycle of drought treatment (1S: 10 days without irrigation with no priming). (b) Phenotype of unstressed plants (US) well irrigated throughout the experiment. (c) The 2S plants laid out individually on a flat surface. (d) The 1S plants laid out individually on a flat surface.

Figure 2

Performance of primed (2S) and unprimed (1S) soybean C08 plants under drought treatment compared to the untreated control (US). (a) Drought stress index, n = 28–30 plants. (b) Percentage of surviving leaves, n = 20–30 plants. (c) Relative water content, n = 19–29 plants. (d) Shoot water retention, n = 26–30 plants. Error bars indicate standard deviation. Wilcoxon rank-sum test was used to compare between the mean values of each treatment following one-way ANOVA. Different letters above the bars indicate significant differences between groups at p < 0.05. Each experiment was performed twice (First and Second Experiment), with similar results.

Figure 3

Rates of fresh weight loss over an hour from a detached leaflet of the top trifoliate leaves of primed (2S), unprimed (1S) and unstressed control (US) soybean C08 plants relative to the initial fresh weight immediately after detachment. n = 12 plants. Error bars indicate standard deviation. The experiment was performed twice (Experiments 1 and 2), with similar results.

Figure 4

Changes in the photosynthesis-related parameters of primed (2S) and unprimed (1S) soybean C08 plants under drought treatment compared to the unstressed control (US). (a) Rate of transpiration, n = 12 plants. (b) Stomatal conductance, n = 12 plants. (c) Rate of photosynthesis, n = 12 plants. (d) Water usage efficiency, n = 12 plants. Error bars indicate standard deviation. Wilcoxon rank-sum test was used to compare between the mean values of each treatment following one-way ANOVA. Different letters above the bars indicate significant differences between groups at p < 0.05.

Figure 5

Box-and-whisker plots of the chlorophyll contents of primed (2S) and unprimed (1S) soybean C08 plants under drought treatment compared to the unstressed control (US). The whiskers represent the maximum and minimum values in the sample. Wilcoxon rank-sum test following one-way ANOVA was used to compare between the mean values of each treatment. Different letters indicate significant differences between groups at p < 0.05. n = 19–30.

Figure 6

The relative expression levels of selected genes in primed (2S) and unprimed (1S) soybean C08 plants under drought treatment and the unstressed control (US) were analyzed by RT-qPCR. (a) Expression of Glyma.06G248900, (b) Glyma.05G234600, (c) Glyma.14G195200, and (d) Glyma.19G147200 was calculated by 2^−ΔΔCT method. The data are presented as the mean of three technical replicates ± SD. Tukey’s honest significance test was used to compare between the mean values of each treatment following one-way ANOVA. Different letters above the bars indicate significant differences between groups at p < 0.05. act11 and elf1b were used as the reference genes. The scale on the left y-axis refers to the data from the first experiments and that on the right y-axis refers to those from the second experiment.