Sensing, Adapting and Thriving: How Fruit Crops Combat Abiotic Stresses.

BOOK REVIEW article Front. Plant Sci., 03 June 2016Sec. Plant Breeding Volume 7 - 2016 | https://doi.org/10.3389/fpls.2016.00798

Chapter 16 - Proline Protects Plants Against Abiotic Oxidative Stress: Biochemical and Molecular Mechanisms

Abiotic stresses such as drought, high salinity, and extreme temperatures are common adverse environmental conditions that significantly reduce the crop productivity. Plants have the capability to sense and adjust to abiotic stresses, although the degree of adaptability to specific stresses varies from species to species. The adaptability to environmental stresses is controlled by either simple or complex cascades of molecular networks. Transcription factors (TFs) play vital regulatory roles in abiotic stress responses in plants by interacting with cis-elements present in the promoter region of various abiotic stress responsive genes. The identification and molecular tailoring of novel TFs involved in environmental stress responses have the potential to overcome a number of important limitations encountered in the generation of transgenic crop plants with superior yield under stress conditions. This opens an excellent opportunity to develop stress tolerant crops in future. This review summarizes the role of various transcription factors in crop improvement through transgenic technology.

Climate change is a major global concern that can make agriculture even more risk-prone, especially in the developing world. Environmental stresses caused by climate change, such as drought, high salinity, low and high temperatures are predicted to become more severe and widespread. One of the most acute environmental stresses presently affecting agriculture is drought, which has pronounced adverse effects on the growth and development of crop plants. The effects of drought stress are expected to increase further with increases in climate change and a growing water crisis. Drought stress usually leads to reductions in crop yield, which can result from many drought-induced morphological, physiological, and metabolic changes that occur in plants. A key sign of drought stress at the molecular level is the accelerated production of reactive oxygen species (ROS) such as singlet oxygen (1O2), superoxide (O 2 •− ), hydrogen peroxide (H2O2), and hydroxyl radicals ( • OH). The excess production of ROS is common in many abiotic stresses, including drought stress, and results from impaired electron transport processes in the chloroplasts and mitochondria. One of the major causes of ROS production under drought stress is photorespiration, which accounts for more than 70 % of the total H2O2 produced. Plants have endogenous mechanisms for adapting to ROS production and are thought to respond to drought stress by strengthening these defense mechanisms. Therefore, enhancement of the functions of the naturally occurring antioxidant components (enzymatic and nonenzymatic) may be one strategy for reducing or preventing oxidative damage and improving the drought resistance of plants. In this chapter, we review the most recent reports on drought-induced responses in plants, focusing on the role of oxidative stress as well as on other possible mechanisms and examining how different components of the antioxidant defense system may confer tolerance to drought-induced oxidative stress. Transgenic approaches are one of the many tools available for modern crop improvement programs. Gene discovery and functional genomics projects have revealed multitudinous mechanisms and gene families that confer improved productivity and adaptation to drought stresses. The overall aim of genetically improving crops for drought resistance is to develop plants able to obtain water and use it to produce sufficient yields for human needs under drought conditions. Although advances have been made in developing crops that are genetically improved with traits such as herbicide and pesticide resistance, attempts to improve plant drought resistance have been hindered by the complexity of plant drought resistance mechanisms at the whole plant, cellular, metabolic, and genetic levels. Interactions between these mechanisms and the complex nature of drought itself adds another layer of intricacy to this problem. The transgenic plants developed via transformation of several different genes that leads to enhanced antioxidant content and consequently their role in the drought stress tolerance are taken into account.

The fruit crop industry is continuously seeking for new technologies able to improve the overall sustainability of the production systems together with an enhancement of quality and safety of the products. Over the last 10 years, plant biostimulants have emerged as new and promising production tool able to increase the use efficiency of other agronomical inputs (i.e. irrigation water and fertilizers), to promote crop tolerance toward different abiotic stressors (i.e. drought, salinity, extreme temperature and radiation) and to enhance the final quality of food products. Despite the considerable amount of researches that have been conducted to elucidate the mode of action of plant biostimulants and their agronomical performances, the number of studies focusing on fruit crops is relatively limited and the current knowledge on the interaction of these substances with the fruit tree growth and physiology is still largely incomplete. This review provides a complete scientific overview of the available literature on the experimental application and the physiological mechanisms of plant biostimulants on the most relevant fruit crops, including berry crops, grapevine, olive, citrus fruits, pome fruits, stone fruits and other fruit species. Following the functional claims currently used for the biostimulants definition at European level, the review is articulated in sections focusing on i) the evidences of the biostimulants implication for fruit tree resistance toward abiotic stresses (drought, salinity and thermal extremes); ii) the evidences of the biostimulants capacity to enhance nutrients use efficiency in fruit crops; iii) the evidences of the biostimulants effects on tree yield and final fruit quality. Overall, the potentiality of plant biostimulants to be successfully integrated within the current management of modern orchards and vineyards emerges as clear. There are anyway aspects typically linked to the perennial nature of fruit trees (i.e. the carry-over effect of previous seasons on the tree internal reserves) or the different environmental conditions imposed by the open-field cultivation systems that need to be further investigated and that represent the future challenges for the research applied on the use of plant biostimulants for fruit crops.

Managing reactive oxygen species—Some learnings from high altitude extremophytes

Divergent pathways, convergent outcomes: Mepiquat chloride and melatonin in crop stress tolerance from GA–DELLA and PMTR1–ROS signaling to yield stability

BackgroundSalt stress is a serious abiotic stress that caused crop growth inhibition and yield decline. Previous studies have reported on the the synthesis of gamma-aminobutyric acid (GABA) and its relationship with plant resistance under various abiotic stress. However, the relationship between exogenous GABA alleviating plant salt stress damage and ion flux, amino acid synthesis, and key enzyme expression remains largely unclear. We investigated plant growth, Na+ transportation and accumulation, reactive oxygen species (ROS) metabolism and evaluated the effect of GABA on amino acids, especially SlGADs gene expression and the endogenous GABA content of tomato (Solanum lycopersicum L.) seedlings treated with or without 5 mmol·L− 1 GABA under 175 mmol·L− 1 NaCl stress.ResultsExogenous application of GABA significantly reduced the salt damage index and increased plant height, chlorophyll content and the dry and fresh weights of tomato plants exposed to NaCl stress. GABA significantly reduced Na+ accumulation in leaves and roots by preventing Na+ influx in roots and transportation to leaves. The transcriptional expression of SlGAD1–3 genes were induced by NaCl stress especially with GABA application. Among them, SlGAD1 expression was the most sensitive and contributed the most to the increase in glutamate decarboxylase (GAD) activity induced by NaCl and GABA application; Exogenous GABA increased GAD activity and amino acid contents in tomato leaves compared with the levels under NaCl stress alone, especially the levels of endogenous GABA, proline, glutamate and eight other amino acids. These results indicated that SlGADs transcriptional expression played an important role in tomato plant resistance to NaCl stress with GABA application by enhancing GAD activity and amino acid contents. GABA significantly alleviated the active oxygen-related injury of leaves under NaCl stress by increasing the activities of antioxidant enzymes and decreasing the contents of active oxygen species and malondialdehyde.ConclusionExogenous GABA had a positive effect on the resistance of tomato seedlings to salt stress, which was closely associated with reducing Na+ flux from root to leaves, increasing amino acid content and strengthening antioxidant metabolism. Endogenous GABA content was induced by salt and exogenous GABA at both the transcriptional and metabolic levels.

Plants, unlike animals, are anchored to one place and, therefore, forced to sustain any environmental condition present. Unfavourable environmental conditions include abiotic (extreme temperatures, water deficits, floods, salinity, light intensities) and biotic (pests, viral, bacterial and fungal diseases) stress factors. Both types of stresses induce the production of reactive oxygen species (ROS), which damage macromolecules such as proteins, lipids, nucleic acids and cell structures like membranes. The effect of each stress factor depends on its intensity. When the stress is severe and the production of ROS is high, it might result to plant death. To avoid such event, plants have developed advanced physiological and chemical defence mechanisms of stress avoidance and/or tolerance, which allow growth only when the environmental conditions are optimum for each species, like in the case of seed dormancy. Plants have also evolved specific enzymatic defence mechanisms, including enzymes like catalase, peroxidase, super oxide dismutase and glutathione transferases. These defence mechanisms help plants either to avoid adverse environmental conditions or to combat their negative effects. A major defence mechanism involves the action of antioxidant enzymes. Glutathione transferases (GSTs) are antioxidant enzymes of great importance for the detoxification of plants from toxic compounds. GSTs have also important involvement in plant stress tolerance against biotic and abiotic stress tolerance like extreme heat, cold, salt and herbicides.

Tomato (Solanum lycopersicum L.) is a major vegetable crop worldwide, however, its production is adversely affected by a wide range of abiotic and biotic stresses at various stages of its cultivation. Abiotic stresses, such as drought, salinity, flooding, extreme temperatures and heavy metals can cause morphological, physiological and biochemical changes in the plant, which ultimately affect crop growth, yield and quality. Among various mitigation strategies, grafting technology has emerged as an effective and alternative tool to slow breeding programs to alleviate various biotic and abiotic stresses. Various studies indicated that increased osmolytes accumulation, synthesis of antioxidant enzymes, improved photosynthesis and vigorous root architecture in grafted tomatoes confer tolerance towards abiotic stresses, besides improving plant growth and fruit yields. Moreover, tomato grafting on suitable rootstocks has also a positive effect on the nutritional qualities of fruits, such as, lycopene, β-carotene, ascorbic acid, proteins, etc.). In this review, we have tried to summarize the adverse effects of abiotic stresses in tomatoes and include all possible recent references related to the response of grafted tomatoes using different intra-and/ or inter-specific rootstocks on improving plant growth, yield and quality and alleviation in various abiotic stresses.

11 - Regulation of transporters through different exogenously applied chemicals during environmental stress

The environment entails set of relationships between biotic and abiotic modules that are impeccably well adjusted by various natural processes. Being static, land floras are wide open to an array of harsh environmental settings particularly abiotic stresses (salinity, drought, temperature fluctuations, heavy metal, ozone, ultraviolet radiation, as well as nutrient deficits). Oxidative stress promulgated by the environmental stressors has been considered as a serious constraint that impacts the development, growth, and reproduction in plants. Due to upsurged and alarming setup of global warning, the frequency, duration, and intensity of hostile environmental conditions are projected to be enhanced that will ultimately have a deleterious bearing on yield and world food production. Plants subjected to contrary environmental factors promote oxidative stress through reactive oxygen species (ROS) generation that activates signaling pathways impacting physiological, biochemical, and molecular processes. In order to maintain a proper physiological level in plants, ROS is crucial which in turn heavily relies on enzymatic and nonenzymatic antioxidant operations. So, reinforcing the understanding and comprehension of oxidative stress and antioxidant system in plants can aid in improving its tolerance toward punitive environment. In this book chapter, contemporary outcomes on the metabolism of ROS and anti-oxidative defense of plants will be elucidated in addition to antioxidant regulation under adverse environmental factors.KeywordsCombined abiotic stressSalinityDroughtNutrient deficienciesUV radiationHeat stressCold stressAntioxidative modulation

Abiotic stress management remains under scrutiny because of the unpredictable nature of climate, which undergoes abrupt alterations. Population pressure, loss of cultivable lands, environmental pollution and other anthropogenic disturbances add to the problem and grossly hinder ongoing management strategies. This has driven increasing effort to find better performing, eco-friendly and reliable alternatives that can contribute to sustainable agricultural practices to manage abiotic stress. Nanotechnology and its implementation in agriculture have emerged as a promising option to cater to the problem of abiotic stress. Induction of reactive oxygen species (ROS) is an inevitable phenomenon linked to stress. Nanoparticles (NPs) perform dual actions in regulating ROS biology. The bidirectional roles of NPs in modulating ROS generation and/or ROS detoxification is tightly coupled within the hormetic boundaries. Nonetheless, how these NPs control the ROS metabolism within hormetic limits demands extensive investigation. This review focuses on the details of ROS metabolism under normal versus stressed conditions. It shall elaborate on the types, modes and process of uptake and translocation of NPs. The molecular dissection of the role of NPs in controlling transcriptomic expressions and modulating molecular crosstalks with other growth regulators, ions, reactive nitrogen species and other signalling molecules shall also be detailed. Throughout, this review aims to summarise the potential roles and regulation of NPs and consider how they can be used for green synthesis within a sustainable agricultural industry.

Sugarcane (Saccharum species hybrids) is a long duration, high-water-requiring agroeconomic crop cultivated in various climatic conditions. Being a long duration crop, it faces vagaries of climate all the year-round affecting plant growth and development, synthesis of sugar, its accumulation and recovery, and ratooning ability. Climate change brings an increase in greenhouse gases (GHGs), temperature, extreme events, drought, heavy rainfall, western disturbance, changes in the level of the sea, etc. and culminates into multiple abiotic stresses affecting soil health, growth, and development of cane and its chemical composition, ripening, and availability of the seed cane. Besides, it aggravates some other abiotic and biotic stresses augmenting the losses further. But, sugarcane is relatively more resilient to abiotic stresses probably due to some natural endowments like a good deal of compensatory ability, C4 photosynthesis, higher-temperature optima for most of the growth processes except sugar accumulation, higher water-use efficiency, and genetic components from Saccharum spontaneum. All these characteristics impart tolerance in sugarcane to various abiotic stresses and carbon sequestration as phytoliths, etc. which helps reduce the damaging impact of various abiotic stresses. Further, the development of climate-resilient sugarcane varieties and technological interventions for stress management may mitigate the impact of climate change-induced multiple abiotic stresses and sustain sugarcane and sugar productivity. Physiological interventions like inducing drought hardiness, deeper root system, reduction of heat load by trash mulching, increasing the age of the crop at the advent of moisture stress, organic matter amendment in the soil, nutrient management, managing rhizospheric salinity/alkalinity, etc. also contribute to elevate its stress tolerance. A number of genes, molecular markers, and miRNAs related with abiotic stress response also contribute to the resilience of sugarcane to abiotic stresses.

In their habitats, plants are increasingly subjected to a wide spectrum of biotic as well as abiotic stresses, which limit crop productivity. However, plants have an ability to sense the stress environment and activate appropriate responses to protect themselves. Understanding signalling events leading to plant defence against environmental challenges can lead to effective strategies to improve plant stress tolerance. Reactive oxygen species (ROS) produced under a variety of different stress conditions are known to play important signalling roles during adaptation of plants to biotic and abiotic challenges. Nevertheless, a major gap exists in our understanding about how ROS induce large-scale and coordinated expression from many genes. In addition, very little is known about the upstream signalling components which perceive and transduce ROS signals within the cell. Even though many genes including transcription factors (TFs) are known to be ROS-induced transcriptionally, only very few of these TFs have been functionally characterised. This PhD study had three main objectives. The first was to identify transcriptional regulators that modulate the early signalling events in the ROS signal transduction pathway. After screening for early and highly oxidative stress-induced TFs in Arabidopsis thaliana, the Ethylene Response Factor 6 (ERF6) gene was identified and chosen for further functional characterisation. The second aim was to decipher the upstream signalling events through which ROS are produced, perceived, and delivered for the ROS-mediated induction of ERF6. And the third objective was to identify and analyse potential downstream targets of ERF6 during ROS signalling. To fulfil the first aim of this study, the expression of ERF6 under different stresses has been investigated. Wild-type Arabidopsis plants were treated with the ROS-generating herbicide paraquat, H2O2 or inoculated with the bacterial pathogen Pseudomonas syringae and the fungal pathogen Fusarium oxysporum. In addition, Arabidopsis plants were subjected to water stress, heat shock, salt stress and cold treatment, all of which are known to induce ROS production. The results showed that ERF6 is strongly induced by oxidative stress imposed by paraquat and H2O2. The common finding that almost all types of stresses produce ROS in plants raised the possibility that ERF6 is very likely to be induced by ROS-inducing environmental challenges as well. However, the results presented here showed that ERF6 is repressed by drought and heat, but induced by cold. ERF6 is also induced by the biotrophic bacterial pathogen Pseudomonas syringae (DC3000) but not by the hemibiotrophic fungal pathogen Fusarium oxysporum. In addition, the treatment of Arabidopsis plants with the defence signalling compound salicylic acid (SA) led to the induction of ERF6 while treatment with the abiotic stress-related phytohormone abscisic acid (ABA) led to repression of ERF6. To achieve the second objective of this study, the dependence of ERF6 expression on signalling intermediates such as NADPH oxidases, G proteins and calcium ions was investigated using Arabidopsis mutants defective for each of these components. Firstly, comparing the expression of ERF6 and the NADPH oxidase D encoding gene RbohD under different stresses showed that the expression patterns of both genes were very similar, i.e. RbohD is induced by paraquat, H2O2, Pseudomonas and SA but repressed by water stress, heat and ABA. Additional results showed that ERF6 required RbohD for full induction by oxidative stress as ROS-dependent expression of ERF6 was compromised in the rbohD mutant. Also, this finding suggests that, via the differential expression of NADPH oxidases, ERF6 is induced by actively ROS-generating stresses such as pathogen infection, but repressed by passively-ROS generating abiotic stresses as a consequence of cell damage. In addition, our data suggest that NADPH oxidase D could represent a point at which the control of abiotic and biotic stress responses by ROS diverges. In addition to RbohD, the results show that G protein a (Ga) and calcium are required for full induction of ERF6 by H2O2. For the third objective of this project, the erf6 knockout mutant was used to investigate the function of ERF6 in modulating plant oxidative stress responses. Results presented here suggest that ERF6 is one of the important signalling intermediates that are required to maintain ROS levels under control. Compared to wild-type, erf6 mutant plants showed a growth reduction, inefficient photosynthesis and high levels of H2O2. Out of fifty potential ERF6 target genes tested, MDAR3, encoding an enzyme (monodehydroascorbate reductase) involved in ROS scavenging, was found to have reduced ROS-responsiveness in erf6 mutant plants. This indicates that ERF6 might control a subset of genes, including MDAR3, encoding proteins involved in controlling ROS levels. Therefore, when ERF6 is knocked out, the ROS controlling machinery is impaired, leading to accumulation of H2O2 in erf6 plants. Interestingly, the knockout mdar3 mutant plants did not show the reduced growth phenotype of erf6 plants. This indicates the possibility that, in addition to MDAR3, other ROS-related genes may have compromised expression in erf6 plants. Moreover, under high light, erf6 mutant plants showed high levels of anthocyanin pigments. This was accompanied with high expression levels of the anthocyanin biosynthesis gene chalcone synthase (CHS), inferring that erf6 plants accumulate anthocyanin as an antioxidant to cope with the high levels of ROS in mutant plants. In addition, this high level of anthocyanin may be contributing to the reduced growth phenotype of erf6 plants owing to growth regulating properties of flavonoids. Collectively, the results presented in this PhD project suggest that ERF6 is one of the genes that establish the specific responses to ROS produced by different stresses. ERF6 TF is required to maintain ROS under control by upregulating genes required for ROS-scavenging. Overall, these results provide new insights into better understanding of the ROS signalling in plants and may help with the development of new stress protection strategies in plants.