Global warming is a major threat for agriculture and food safety and in many cases the negative effects are already apparent. The current challenge of basic and applied plant science is to decipher the molecular mechanisms of heat stress response (HSR) and thermotolerance in detail and use this information to identify genotypes that will withstand unfavorable environmental conditions. Nowadays X-omics approaches complement the findings of previous targeted studies and highlight the complexity of HSR mechanisms giving information for so far unrecognized genes, proteins and metabolites as potential key players of thermotolerance. Even more, roles of epigenetic mechanisms and the involvement of small RNAs in thermotolerance are currently emerging and thus open new directions of yet unexplored areas of plant HSR. In parallel it is emerging that although the whole plant is vulnerable to heat, specific organs are particularly sensitive to elevated temperatures. This has redirected research from the vegetative to generative tissues. The sexual reproduction phase is considered as the most sensitive to heat and specifically pollen exhibits the highest sensitivity and frequently an elevation of the temperature just a few degrees above the optimum during pollen development can have detrimental effects for crop production. Compared to our knowledge on HSR of vegetative tissues, the information on pollen is still scarce. Nowadays, several techniques for high-throughput X-omics approaches provide major tools to explore the principles of pollen HSR and thermotolerance mechanisms in specific genotypes. The collection of such information will provide an excellent support for improvement of breeding programs to facilitate the development of tolerant cultivars. The review aims at describing the current knowledge of thermotolerance mechanisms and the technical advances which will foster new insights into this process.

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Perspectives on deciphering mechanisms underlying plant heat stress response and thermotolerance

Summary
Ralstonia solanacearum is a soil-borne bacterium causing the widespread disease known as bacterial wilt. Ralstonia solanacearum is also the causal agent of Moko disease of banana and brown rot of potato. Since the last R. solanacearum pathogen profile was published 10 years ago, studies concerning this plant pathogen have taken a genomic and post-genomic direction. This was pioneered by the first sequenced and annotated genome for a major plant bacterial pathogen and followed by many more genomes in subsequent years. All molecular features studied now have a genomic flavour. In the future, this will help in connecting the classical field of pathology and diversity studies with the gene content of specific strains. In this review, we summarize the recent research on this bacterial pathogen, including strain classification, host range, pathogenicity determinants, regulation of virulence genes, type III effector repertoire, effector-triggered immunity, plant signalling in response to R. solanacearum, as well as a review of different new pathosystems.

Taxonomy
Bacteria; Proteobacteria; β subdivision; Ralstonia group; genus Ralstonia.

Disease symptoms
Ralstonia solanacearum is the agent of bacterial wilt of plants, characterized by a sudden wilt of the whole plant. Typically, stem cross-sections will ooze a slimy bacterial exudate. In the case of Moko disease of banana and brown rot of potato, there is also visible bacterial colonization of banana fruit and potato tuber.

Disease control
As a soil-borne pathogen, infected fields can rarely be reused, even after rotation with nonhost plants. The disease is controlled by the use of resistant and tolerant plant cultivars. The prevention of spread of the disease has been achieved, in some instances, by the application of strict prophylactic sanitation practices.

Useful websites
Stock centre: International Centre for Microbial Resources—French Collection for Plant-associated Bacteria CIRM-CFBP, IRHS UMR 1345 INRA-ACO-UA, 42 rue Georges Morel, 49070 Beaucouze Cedex, France, http://www.angers-nantes.inra.fr/cfbp/. Ralstonia Genome browser: https://iant.toulouse.inra.fr/R.solanacearum. GMI1000 insertion mutant library: https://iant.toulouse.inra.fr/R.solanacearumGMI1000/GenomicResources. MaGe Genome Browser: https://www.genoscope.cns.fr/agc/microscope/mage/viewer.php?

http://diposit.ub.edu/dspace/bitstream/2445/65116/1/634668.pdf

Ralstonia solanacearum, a widespread bacterial plant pathogen in the post-genomic era

Generating high temperature tolerant transgenic plants: Achievements and challenges.

Increasing severity of high temperature worldwide presents an alarming threat to the humankind. As evident by massive yield losses in various food crops, the escalating adverse impacts of heat stress (HS) are putting the global food as well as nutritional security at great risk. Intrinsically, plants respond to high temperature stress by triggering a cascade of events and adapt by switching on numerous stress-responsive genes. However, the complex and poorly understood mechanism of heat tolerance (HT), limited access to the precise phenotyping techniques, and above all, the substantial G × E effects offer major bottlenecks to the progress of breeding for improving HT. Therefore, focus should be given to assess the crop diversity, and targeting the adaptive/morpho-physiological traits while making selections. Equally important is the rapid and precise introgression of the HT-related gene(s)/QTLs to the heat-susceptible cultivars to recover the genotypes with enhanced HT. Therefore, the progressive tailoring of the heat-tolerant genotypes demands a rational integration of molecular breeding, functional genomics and transgenic technologies reinforced with the next-generation phenomics facilities.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pbr.12217

Heat stress in crop plants: its nature, impacts and integrated breeding strategies to improve heat tolerance

The sedentary life of plants has forced them to live in an environment that is characterized by the presence of numerous challenges in terms of biotic and abiotic stresses. Phytohormones play essential roles in mediating plant physiology and alleviating various environmental perturbations. Jasmonates are a group of oxylipin compounds occurring ubiquitously in the plant kingdom that play pivotal roles in response to developmental and environmental cues. Jasmonates (JAs) have been shown to participate in unison with key factors of other signal transduction pathway, including those involved in response to abiotic stress. Recent findings have furnished large body of information suggesting the role of jasmonates in cold and heat stress. JAs have been shown to regulate C-repeat binding factor (CBF) pathway during cold stress. The interaction between the integrants of JA signaling and components of CBF pathway demonstrates a complex relationship between the two. JAs have also been shown to counteract chilling stress by inducing ROS avoidance enzymes. In addition, several lines of evidence suggest the positive regulation of thermotolerance by JA. The present review provides insights into biosynthesis, signal transduction pathway of jasmonic acid and their role in response to temperature stress.

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Jasmonates: Emerging Players in Controlling Temperature Stress Tolerance

WRKY proteins form a large family of plant transcription factors implicated in the modulation of numerous biological processes, such as growth, development and responses to various environmental stresses. However, the roles of the majority WRKY family members, especially in non-model plants, remain poorly understood. We identified CaWRKY40 from pepper. Transient expression in onion epidermal cells showed that CaWRKY40 can be targeted to nuclei and activates expression of a W-box-containing reporter gene. CaWRKY40 transcripts are induced in pepper by Ralstonia solanacearum and heat shock. To assess roles of CaWRKY40 in plant stress responses we performed gain- and loss-of-function experiments. Overexpression of CaWRKY40 enhanced resistance to R. solanacearum and tolerance to heat shock in tobacco. In contrast, silencing of CaWRKY40 enhanced susceptibility to R. solanacearum and impaired thermotolerance in pepper. Consistent with its role in multiple stress responses, we found CaWRKY40 transcripts to be induced by signalling mechanisms mediated by the stress hormones salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). Overexpression of CaWRKY40 in tobacco modified the expression of hypersensitive response (HR)-associated and pathogenesis-related genes. Collectively, our results suggest that CaWRKY40 orthologs are regulated by SA, JA and ET signalling and coordinate responses to R. solanacearum attacks and heat stress in pepper and tobacco.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pce.12011

CaWRKY40, a WRKY protein of pepper, plays an important role in the regulation of tolerance to heat stress and resistance to Ralstonia solanacearum infection.

The invention provides application of a chilli DN (dominant negative) mutant to the bacterial wilt gene engineering of tobacco. The chilli DN mutant is a DN-CaROP1 mutant. A gene of the mutant at least comprises a nucleotide sequence shown as SEQ ID No.1. A construction method comprises a step of fusing the chilli DN-CaROP1 mutant with a core sequence of a CaMV35S promoter to construct a DN-CaROP1 over-expression carrier. Compared with wild tobacco, by over-expression of the chilli DN-CaROP1 mutant in tobacco, the bacterial wilt resistance capability of the transgenic tobacco is obviously improved. The mutant has very important application value in the bacterial wilt gene engineering of plants.

Application of chilli DN (dominant negative) mutant to bacterial wilt resistance gene engineering of tobacco

The invention provides application of a capsicum DN (dominant negative) mutant in tobacco high-temperature-resistant gene engineering. The capsicum DN mutant is a DN-CaROP1 mutant. The gene of the DN-CaROP1 mutant at least contains a nucleotide sequence disclosed as SEQ ID No.1. The capsicum DN-CaROP1 mutant is fused with a CaMV35S promoter core sequence to construct an overexpression vector of DN-CaROP1. Compared with the wild tobacco, the overexpression of the capsicum DN-CaROP1 mutant in tobacco obviously enhances the high-temperature-resistant capacity of the transgenic tobacco, and the gene has a very important application value in plant high-temperature-resistant gene engineering.

Yansheng Chen

Papers

CaWRKY40, a WRKY protein of pepper, plays an important role in the regulation of tolerance to heat stress and resistance to Ralstonia solanacearum infection.

Application of chilli DN (dominant negative) mutant to bacterial wilt resistance gene engineering of tobacco

Application of capsicum DN (dominant negative) mutant in tobacco high-temperature-resistant gene engineering