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Abstract
Wild peanut relatives (Arachis spp.) are genetically diverse and were selected throughout evolution to a range of environments constituting, therefore, an important source of allelic diversity for abiotic stress tolerance. In particular, A. duranensis and A. stenosperma, the parents of the reference Arachis A-genome genetic map, show contrasting transpiration behavior under limited water conditions. This study aimed to build a comprehensive gene expression profile of these two wild species under dehydration stress caused by the withdrawal of hydroponic nutrient solution. For this purpose, roots of both genotypes were collected at seven time-points during the early stages of dehydration and used to construct cDNA paired-end libraries. Physiological analyses indicated initial differences in gas exchange parameters between the drought-tolerant genotype of A. duranensis and the drought-sensitive genotype of A. stenosperma. High-quality Illumina reads were mapped against the A. duranensis reference genome and resulted in the identification of 1,235 and 799 Differentially Expressed Genes (DEGs) that responded to the stress treatment in roots of A. duranensis and A. stenosperma, respectively. Further analysis, including functional annotation and identification of biological pathways represented by these DEGs confirmed the distinct gene expression behavior of the two contrasting Arachis species genotypes under dehydration stress. Some species-exclusive and common DEGs were then selected for qRT-PCR analysis, which corroborated the in silico expression profiling. These included genes coding for regulators and effectors involved in drought tolerance responses, such as activation of osmosensing molecular cascades, control of hormone and osmolyte content, and protection of macromolecules. This dataset of transcripts induced during the dehydration process in two wild Arachis genotypes constitute new tools for the understanding of the distinct gene regulation processes in these closely related species but with contrasting drought responsiveness. In addition, our findings provide insights into the nature of drought tolerance in wild germoplasm, which might be explored as novel sources of diversity and useful wild alleles to develop climate-resilient crop varieties.
Highlights
Climate change models suggest a trend towards increasing risks in the agricultural sector worldwide, leading to the emergence of less productive or even inappropriate areas for cultivation in the tropics [1]
Three signaling cascades are known to act following the onset of the drought stress [5,6,7]: (1) ABAdependent signaling, mediated by the abscisic acid (ABA)-responsive element (ABRE) present in the promoter of various abiotic stress-related genes; (2) ABA-independent signaling towards the dehydration-responsive element (DRE); and (3) ethylene-signaling pathways that are associated with ERE sequences in promoters
The intercellular CO2 concentration was the only physiological parameter for which no statistically significant difference between the two species was observed (Fig 1F). Any changes in these parameters were detected over the course of the experiment, most likely due to the fact that these measurements were made at the beginning of the stress treatment which, while a long enough period to introduce a pronounced stress in roots of these species, as demonstrated by the alterations in gene expression presented below, was not long enough to affect leaf gas exchange [27]
Summary
Climate change models suggest a trend towards increasing risks in the agricultural sector worldwide, leading to the emergence of less productive or even inappropriate areas for cultivation in the tropics [1]. Upon perception of a reduction in water availability, well adapted plants rapidly react to minimize water loss, maintain cellular osmotic adjustment, control water flux, and protect cells against oxidative stress and the damaging effects of dehydration. As part of this response, orchestrated molecular networks are activated, and interconnected responses at physiological, morphological, and biochemical levels are triggered to direct the plant to cope with water scarcity [4]. Three signaling cascades are known to act following the onset of the drought stress [5,6,7]: (1) ABAdependent signaling, mediated by the ABA-responsive element (ABRE) present in the promoter of various abiotic stress-related genes; (2) ABA-independent signaling towards the dehydration-responsive element (DRE); and (3) ethylene-signaling pathways that are associated with ERE (ethylene-responsive element) sequences in promoters
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