Abstract
Cassava is an important tropical and sub-tropical root crop that is adapted to drought environment. However, severe drought stress significantly influences biomass accumulation and starchy root production. The mechanism underlying drought-tolerance remains obscure in cassava. In this study, changes of physiological characters and gene transcriptome profiles were investigated under dehydration stress simulated by polyethylene glycol (PEG) treatments. Five traits, including peroxidase (POD) activity, proline content, malondialdehyde (MDA), soluble sugar and soluble protein, were all dramatically induced in response to PEG treatment. RNA-seq analysis revealed a gradient decrease of differentially expressed (DE) gene number in tissues from bottom to top of a plant, suggesting that cassava root has a quicker response and more induced/depressed DE genes than leaves in response to drought. Overall, dynamic changes of gene expression profiles in cassava root and leaves were uncovered: genes related to glycolysis, abscisic acid and ethylene biosynthesis, lipid metabolism, protein degradation, and second metabolism of flavonoids were significantly induced, while genes associated with cell cycle/organization, cell wall synthesis and degradation, DNA synthesis and chromatin structure, protein synthesis, light reaction of photosynthesis, gibberelin pathways and abiotic stress were greatly depressed. Finally, novel pathways in ABA-dependent and ABA-independent regulatory networks underlying PEG-induced dehydration response in cassava were detected, and the RNA-Seq results of a subset of fifteen genes were confirmed by real-time PCR. The findings will improve our understanding of the mechanism related to dehydration stress-tolerance in cassava and will provide useful candidate genes for breeding of cassava varieties better adapted to drought environment.
Highlights
Cassava (Manihot esculenta Crantz) is an important cash crop for many poor farmers in marginal areas of tropics and sub-tropics regions [1,2]
In order to investigate the dynamic changes of cassava in response to different polyethylene glycol (PEG) treatments, five physiological traits, including proline, MDA, soluble sugar and soluble protein content, and POD activity that play important roles under drought stress to either maintain water content or reduce oxidative damage in plant cells, were measured under five PEG concentrations (0%, 20%, 30%, 40% and 50%) across eight time points (0, 2, 4, 6, 8, 12, 24 and 48 h) in leaves (Figure 1 and Figure S1 (Supplementary Material))
Proline content, POD activity, and soluble sugar content consistently showed strict gradient changes: steady linear increases in contents and/or activities were observed under both 20% and 30% PEG treatments over time, while there was a sudden and sharp increase under 40% PEG but decrease under 50% PEG treatment, respectively, at the time point from 24 to 48 h (Figure 1A–C)
Summary
Cassava (Manihot esculenta Crantz) is an important cash crop for many poor farmers in marginal areas of tropics and sub-tropics regions [1,2]. Besides AREB/ABFs, TFs such as WRKY [23], NAC [24], AP2 [25], and MYB [26] were involved in plant drought stress response. These results were mainly derived from model species (e.g., Arabidopsis), far less was known about tropical species such as cassava. We report our findings on physiological changes of cassava leaves in response to different levels of dehydration stress simulated by polyethylene glycol (PEG), which is commonly used when evaluating plant drought tolerance. The results will provide new insights in the regulation networks of dehydration stress in cassava and the genetic improvement of cassava varieties better adapted to drought environment
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