Under field conditions crops are routinely subjected to a number of different abiotic stress factors simultaneously. Recent studies revealed that the response of plants to a combination of different abiotic stresses is unique and cannot be directly extrapolated from simply studying each of the different stresses applied individually. These studies have also identified specific regulatory transcripts, combinations of metabolites and proteins, and physiological responses that are unique to specific stress combinations, highlighting the importance of studying abiotic stress combination in plants. Here we describe the interactions between drought and other abiotic stresses with emphasis on drought and heat stress. We compile new data about the different molecular, physiological and metabolic adaptations of different plants and crops to this stress combination and we highlight the importance of reactive oxygen species (ROS) metabolism and stomatal responses for plant acclimation to drought and heat stress combination. We further emphasize the need for developing crops with enhanced tolerance to drought and heat stress combination in order to mitigate the negative impacts of predicted global climatic changes on agricultural production worldwide.

Plant adaptations to the combination of drought and high temperatures

In field conditions, plants are often simultaneously exposed to multiple biotic and abiotic stresses resulting in substantial yield loss. Plants have evolved various physiological and molecular adaptations to protect themselves under stress combinations. Emerging evidences suggest that plant responses to a combination of stresses are unique from individual stress responses. In addition, plants exhibit shared responses which are common to individual stresses and stress combination. In this review, we provide an update on the current understanding of both unique and shared responses. Specific focus of this review is on heat-drought stress as a major abiotic stress combination and, drought-pathogen and heat-pathogen as examples of abiotic-biotic stress combinations. We also comprehend the current understanding of molecular mechanisms of cross talk in relation to shared and unique molecular responses for plant survival under stress combinations. Thus, the knowledge of shared responses of plants from individual stress studies and stress combinations can be utilized to develop varieties with broad spectrum stress tolerance.

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Shared and unique responses of plants to multiple individual stresses and stress combinations: physiological and molecular mechanisms.

In field conditions, crops are adversely affected by a wide range of abiotic stresses including drought, cold, salt, and heat, as well as biotic stresses including pests and pathogens. These stresses can have a marked effect on crop yield. The present and future effects of climate change necessitate the improvement of crop stress tolerance. Plants have evolved sophisticated stress response strategies, and genes that encode transcription factors (TFs) that are master regulators of stress-responsive genes are excellent candidates for crop improvement. Related examples in recent studies include TF gene modulation and overexpression approaches in crop species to enhance stress tolerance. However, much remains to be discovered about the diverse plant TFs. Of the >80 TF families, only a few, such as NAC, MYB, WRKY, bZIP, and ERF/DREB, with vital roles in abiotic and biotic stress responses have been intensively studied. Moreover, although significant progress has been made in deciphering the roles of TFs in important cereal crops, fewer TF genes have been elucidated in sorghum. As a model drought-tolerant crop, sorghum research warrants further focus. This review summarizes recent progress on major TF families associated with abiotic and biotic stress tolerance and their potential for crop improvement, particularly in sorghum. Other TF families and non-coding RNAs that regulate gene expression are discussed briefly. Despite the emphasis on sorghum, numerous examples from wheat, rice, maize, and barley are included. Collectively, the aim of this review is to illustrate the potential application of TF genes for stress tolerance improvement and the engineering of resistant crops, with an emphasis on sorghum.

Transcription Factors Associated with Abiotic and Biotic Stress Tolerance and Their Potential for Crops Improvement

Hexaploid wheat (Triticum aestivum) is a globally important crop. Heat, drought and their combination dramatically reduce wheat yield and quality, but the molecular mechanisms underlying wheat tolerance to extreme environments, especially stress combination, are largely unknown. As an allohexaploid, wheat consists of three closely related subgenomes (A, B, and D), and was reported to show improved tolerance to stress conditions compared to tetraploid. But so far very little is known about how wheat coordinates the expression of homeologous genes to cope with various environmental constraints on the whole-genome level. To explore the transcriptional response of wheat to the individual and combined stress, we performed high-throughput transcriptome sequencing of seedlings under normal condition and subjected to drought stress (DS), heat stress (HS) and their combination (HD) for 1 h and 6 h, and presented global gene expression reprograms in response to these three stresses. Gene Ontology (GO) enrichment analysis of DS, HS and HD responsive genes revealed an overlap and complexity of functional pathways between each other. Moreover, 4,375 wheat transcription factors were identified on a whole-genome scale based on the released scaffold information by IWGSC, and 1,328 were responsive to stress treatments. Then, the regulatory network analysis of HSFs and DREBs implicated they were both involved in the regulation of DS, HS and HD response and indicated a cross-talk between heat and drought stress. Finally, approximately 68.4 % of homeologous genes were found to exhibit expression partitioning in response to DS, HS or HD, which was further confirmed by using quantitative RT-PCR and Nullisomic-Tetrasomic lines. A large proportion of wheat homeologs exhibited expression partitioning under normal and abiotic stresses, which possibly contributes to the wide adaptability and distribution of hexaploid wheat in response to various environmental constraints.

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Temporal transcriptome profiling reveals expression partitioning of homeologous genes contributing to heat and drought acclimation in wheat (Triticum aestivum L.)

Sweet sorghum is a promising target for biofuel production. It is a C4 crop with low input requirements and accumulates high levels of sugars in its stalks. However, large-scale planting on marginal lands would require improved varieties with optimized biofuel-related traits and tolerance to biotic and abiotic stresses. Considering this, many studies have been carried out to generate genetic and genomic resources for sweet sorghum. In this review, we discuss various attributes of sweet sorghum that make it an ideal candidate for biofuel feedstock, and provide an overview of genetic diversity, tools, and resources available for engineering and/or marker-assisting breeding of sweet sorghum. Finally, the progress made so far, in identification of genes/quantitative trait loci (QTLs) important for agronomic traits and ongoing molecular breeding efforts to generate improved varieties, has been discussed.

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Sweet sorghum as biofuel feedstock: recent advances and available resources

Abiotic stresses which include drought and heat are amongst the main limiting factors for plant growth and crop productivity. In the field, these stress types are rarely presented individually and plants are often subjected to a combination of stress types. Sorghum bicolor is a cereal crop which is grown in arid and semi-arid regions and is particularly well adapted to the hot and dry conditions in which it originates and is now grown as a crop. In order to better understand the mechanisms underlying combined stress tolerance in this important crop, we have used microarrays to investigate the transcriptional response of Sorghum subjected to heat and drought stresses imposed both individually and in combination. Microarrays consisting of 28585 gene probes identified gene expression changes equating to ~4% and 18% of genes on the chip following drought and heat stresses respectively. In response to combined stress ~20% of probes were differentially expressed. Whilst many of these transcript changes were in common with those changed in response to heat or drought alone, the levels of 2043 specific transcripts (representing 7% of all gene probes) were found to only be changed following the combined stress treatment. Ontological analysis of these ‘unique’ transcripts identified a potential role for specific transcription factors including MYB78 and ATAF1, chaperones including unique heat shock proteins (HSPs) and metabolic pathways including polyamine biosynthesis in the Sorghum combined stress response. These results show evidence for both cross-talk and specificity in the Sorghum response to combined heat and drought stress. It is clear that some aspects of the combined stress response are unique compared to those of individual stresses. A functional characterization of the genes and pathways identified here could lead to new targets for the enhancement of plant stress tolerance, which will be particularly important in the face of climate change and the increasing prevalence of these abiotic stress types.

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Transcriptomic analysis of Sorghum bicolor responding to combined heat and drought stress

Sorghum bicolor is an important cereal crop grown on the arid and semi-arid regions of >98 different countries. These regions are such that this crop is often subjected to low water conditions, which can compromise yields. Stay-green sorghum plants are able to retain green leaf area for longer under drought conditions and as such have higher yields than their senescent counterparts. However, the molecular and physiological basis of this drought tolerance is yet to be fully understood. Here, a transcriptomic approach was used to compare gene expression between stay-green (B35) and senescent (R16) sorghum varieties. Ontological analysis of the differentially expressed transcripts identified an enrichment of genes involved with the 'response to osmotic stress' Gene Ontology (GO) category. In particular, delta1-pyrroline-5-carboxylate synthase 2 (P5CS2) was highly expressed in the stay-green line compared with the senescent line, and this high expression was correlated with higher proline levels. Comparisons of the differentially expressed genes with those that lie in known stay-green qualitative trait loci (QTLs) revealed that P5CS2 lies within the Stg1 QTL. Polymorphisms in known cis-elements were identified in the putative promoter region of P5CS2 and these could be responsible for the differences in the expression of this gene. This study provides greater insight into the stay-green trait in sorghum. This will be greatly beneficial not only to improve our understanding of drought tolerance mechanisms in sorghum, but also to facilitate the improvement of future sorghum cultivars by marker-assisted selection (MAS).

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Stephanie M. Johnson

Papers

Transcriptomic analysis of Sorghum bicolor responding to combined heat and drought stress

Transcriptomic analysis comparing stay-green and senescent Sorghum bicolor lines identifies a role for proline biosynthesis in the stay-green trait