Understanding the Impact of Global Climate Change on Abiotic Stress in Plants and the Supportive Role of PGPR

Abstract

Plants form the fundamental trophic level of almost all the food chains, and as such are the most significant biotic component of our ecosystems. However, there is a rising threat on the growth and well-being of these organisms due to variations in climatic conditions. Climate change conditions pose threat to plants by exposing them to various abiotic stresses, such as salinity, drought and UV-B radiation, eventually leading to oxidative stress in plant cells. Plants can put up their defence against such stressors using a number of strategies namely, adaptation, avoidance and tolerance. The action of antioxidant molecules and enzymes play a pivotal role in fighting the oxidative stress and its key player, reactive oxygen species (ROS). Plants can also develop an epigenetic memory of the stress, by modulating the expression of genes involved in stress tolerance via the epigenetic code. With the rise in environmental challenges due to climate change in recent times, it is also important to underline the helpful role played by plant growth-promoting rhizobacteria (PGPR) in building more stress-resilient plants, and the diverse array of plant genera with which these PGPR can associate.

1. Introduction

Presently, there is a rising concern regarding the extreme changes in climatic conditions and their related impact on the living (biotic) and non-living (abiotic) components of our ecosystems. The recent cases of flooding, drought, cyclones and hurricanes have caused an alarm and called for us to review the way in which we understand and interact with our biosphere. These situations arise not only due to anthropogenic activities but also due to natural phenomenon. A vital factor that contributes to climatic fluctuation is upsurge and accumulation of greenhouse gases which eventually cause a rise in global temperature [1, 2, 3, 4, 5]. There has been a significant increase in the amount of research pertaining to the mechanisms for and development of abiotic stress tolerance in plants over the past couple of decades. This is because plants are the beings most readily and deeply affected by climate change issues due to their immobile nature.

Under environmental stress conditions, arising from temperature extremes, fluctuations in rainfall and wind patterns, heat, salinity, pH variations, drought, electromagnetic radiation, etc., plants show numerous symptoms of stress-induced phytotoxicity. This ranges from modifications in metabolic and physiological activities to depletion in overall productivity. Consequently, this becomes a raging concern in case of crop plants and other plant beings that are crucial for providing ecosystem services (such as pulpwood, timber, ecotourism and natural habitat for wildlife). Due to loss in growth and productivity, there is an issue of global food shortage and also the deeply worrying aspect of oxygen depletion [6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Therefore, among all the effects of the current trend of climate change on our planet, the ones on plant systems are of utmost importance.

In the backdrop of climate change, some abiotic elements of our biosphere are more susceptible to variations than the others. These include drought, salinity and UV-B. These factors influence plant life forms in solitary or in combined ways, whereby morphological, physiological and biochemical attributes of plants are effectively altered. Simultaneously, there are revelled modifications in the epigenetic codes of the plant genome, leading to what is known as chromatin-based ‘epigenetic memory’. This memory may help the plant in future when it is exposed to similar kinds of stress, wherein there is elevation of plant defence activities. Plants can also respond to environmental stressors by adapting and acclimatising using various strategies. However, the climate-influenced rise in abiotic stressors continues to hamper plant growth and productivity on a larger scale and also draw in the biotic or pathogenic stress challenge into the picture.

As such, we have discussed in the present chapter some key issues related to climate change and its impact, such as the nature and origin of climate variability, its expected trend in near future, how the climate change conditions affect the sessile plant beings, the strategies adapted by plants to overcome the stress created due to the increasingly challenging environment, and the role of epigenetic mechanisms in helping the plants adapt and acclimatise better to these conditions. Moreover, the role of friendly bacteria also termed as ‘plant growth-promoting rhizo-bacteria’ in alleviating the negative effects of climate change on plants has also been delineated. With a clear understanding of the issue at hand, we can equip ourselves better to face the testing times that lay ahead of us.

2. Causal factors of climate change and forecast

The various physical processes in our environment at a local scale combined with responses to global climatic phenomena at a large scale form the basis of ‘regional climates’. Notable examples of global-scale climatic phenomena and their respective variability in present times include Monsoon Systems, Tropical Phenomena (including the popular El Nino Southern Oscillation), Cyclones, Blocking, Atlantic Multi-decadal Oscillation, Pacific South American Pattern, etc [1]. These phenomena are largely considered relevant to climate dynamics because of perceived or proven confidence that they can influence regional climate and are likely to change over time. Various physical parameters are taken into account while understanding the dynamics of regional climate, such as overall transfer of heat and moisture, and their momentum into a region [1]. According to recent updates, there continues to be a debate around the causality of natural and anthropogenic factors towards influencing climate change and also the impact of each of the causal factors. One of the latest findings has suggested that both natural and anthropogenic factors are responsible for temperature changes, contrary to what one might think, that the anthropogenic factors majorly contribute to the global rise in temperature. In addition to this, the study also indicates that there is a connection between the rise in temperature and concentration of greenhouse gases [2]. The current trend of temperature variability is arguably accredited to both natural and anthropogenic factors. For instance, sharp deviation from the present greenhouse warming trend has been noted in regions with variable volcanic activities—a period of unusually heavy activity is followed by strong cooling, while a period of low activity is accompanied with greater warming [3]. However, it is still supported by several studies that by limiting the timescale of climate analyses to more recent times, we can see that anthropogenic factors have been the major contributors to the greenhouse global warming trend. Additionally, the anthropogenic aerosols as well as greenhouse gases have influenced climate change through influencing regional temperatures and long-term changes in monsoons [3].

Despite the need to understand and precisely delineate the underlying causes of the changes in climatic conditions (such as temperature change and precipitation variability), we also need to keep an eye on the current trend of climate variability. Keeping in mind the Indian subcontinent, we have presented here the status of and predictions for climate change in South East Asia. It has been reported that S.E. Asia has observed a rise in temperature at the rate of 0.14–0.20°C per decade since the 1960s [4]. This is also accompanied by an increase in number of hotter days and warmer nights, with a simultaneous decrease in cooler weather [5, 6]. The trend for incidence of heavy and light rain episodes is positive, while that of moderate rain episodes is negative [7]. The annual rainfall on total wet day has increased at an average of 22 mm per decade, while that on extreme rain days has increased at an average of 10 mm per decade [6, 8]. It is indicated that warming is expected to persist, with extensive variation on regional basis [1]. Also, there is likely to be moderate increase in precipitation, with the exception of the part of Indonesia near to the southeast Indian Ocean. In parts of terrain, variation in precipitation is most likely to be strong [1].

3. Impact of climate change from ecological perspective

For quite a few decades now, the impact of global climate change on regional climates and the associated effects on regional as well as local ecosystems have been a matter of extensive discussion. Changes in climatic conditions in different regions across the globe have adversely affected agricultural productivity, food security, various ecosystem services, and overall composition as well as quality of flora and fauna [9, 10, 11]. Some of the major outcomes of climate changes have been temperature extremes and uncertainty or unevenness in rainfall patterns, which eventually pose a threat to agricultural crops [12, 13, 14]. Variability in temperatures and precipitation has also been found to influence cropping patterns, crop yields, and phenology, i.e. leaf development, anthesis, asynchrony between anthesis and pollinators, increased respiration, decrease in pollen germination, shorter grain filling period, and lesser biomass production [12, 15, 16].

Greenhouse gas emission and/or concentration pose a threat to the flexibility and adaptability of natural ecosystems, through influencing climate change as well as ocean acidification [17]. The recent 2018 International Panel on Climate Change (IPCC) Special Report on 1.5°C alerts that drastic climate change impacts will ensue if the planet is allowed to warm beyond 1.5°C, and such impacts include drought, flood, heat waves and sea-level rise [17, 18]. Such effects would not only harm man-kind and the lifestyle we are presently accustomed to but also the natural biodiversity in general. The previously agreed upon temperature target was 2°C; however, the half-degree variation was considered vital to avert the risk of Arctic and Coral Reef Ecosystems’ degradation [17, 18]. A vital lesson to learn from this Special Report is that there is an estimated 12 years of time to reduce the net carbon emissions by half in order to avert the severe impacts mentioned earlier; however, achievement of this target would still potentially result in continued global warming as well as the associated impacts [17].

Among the most notable effects of global climate change on ecology and biosphere, salinity is number one. Several researchers have noted that one of the main reasons behind the rising levels of soil salinity across the globe is global climate change and its associated impacts, such as increasing temperature, lower precipitation, higher evapotranspiration, consequent aridisation of susceptible regions and rise in sea levels [19, 20]. Although salinity or sodicity in soil mostly originates due to natural factors such as weathering and there is some amount of it always existing in soils, the influence of climate change conditions is also substantial, whereby the amount of salinity exceeds beyond the threshold.

An important stress factor for living organisms, in particular plant life forms, is water-deficit or drought. Drought or drought-like situation arises for plants when there is inadequate water supply near the roots. This is caused by several factors such as natural climatic or geographical conditions, irregular rainfall pattern, high environmental temperature, high light intensity, spells of dry wind, water-retaining capacity of soil and water-deficit due to high transpiration rate [21, 22]. Although agricultural drought is not a big threat in itself, since it is a common natural phenomenon, and is often preceded by meteorological drought, it can still be seen as a rising abiotic stressor for plants due to the wasteful and careless anthropogenic practices. The alarming rise in greenhouse gas concentration in environment and the subsequent global warming (due to the tendency of these gases to be well mixed in atmosphere) has led to an upsurge in soil and surface water temperatures, leading to drought-like conditions. Over the past two centuries, the concentrations of carbon dioxide and methane have increased to 30% and 150%, respectively, and have thus influenced climate change through global warming and alterations in rainfall pattern [22, 23].

A direct impact of global change in climatic conditions of temperature and accumulation of greenhouse gases is thinning of the protective ozone layer of Earth’s stratosphere, a phenomenon that is being studied for almost three decades now [24]. An immediate concern arising from this observation was the impact of solar UV radiation on animal as well as plant life forms. Both UV-A and UV-B are potentially harmful to biological molecules and cellular systems, and it has been noted that the interaction of UV-B with several other climate change factors (such as temperature, drought/precipitation and greenhouse gas like CO₂) can further complicate its effect, as depicted in Figure 1 [24].

Figure 1.

Schematic overview of how UV-B affects life forms at different levels, alone as well as through its interaction with various climate change factors (derived from Caldwell et al. [24]).

It is thus understandable that climate change is leading to enhanced abiotic stress for organisms, in particular, the sessile plant beings. The ongoing section shall throw light on how the abiotic stress factors catalyse their detrimental effects on plants and the strategies employed by plants to tackle these effects.

4. The challenge of abiotic stress for plants: Harms and defence strategies

As discussed in the preceding section, salinity is one of the significant stressors for plants and is currently being elevated by climate change conditions. It is crucial to discuss the impact of salinity stress because according some reports, and it is estimated that salt-affected land is leading to a loss of approximately 12 billion USD annually, and future predictions for agricultural production highlights the significance of working efficiently under high saline conditions [25]. Some of the principal ways by which salinity manifests its adverse effects on the physiology and biochemistry of plant systems are as follows:

1. Heightened Na⁺ accumulation in plant cells causes efflux of K⁺ and Ca⁺ ions, which eventually leads to imbalance in cellular homeostasis, nutrient deficiency, oxidative stress, growth retardation and cell death [19]. Additionally, higher concentration of Na⁺ ions in soil may lead to reduced uptake of K⁺ and Ca⁺ ions by the plants, further causing hindrance to proper cellular functioning and enzymatic activities [19].

2. Impairing photosynthesis through several means such as stomatal closure, decline in primary and accessory photosynthetic pigments concentrations and damage to chloroplast ultrastructure [19, 26]. Additionally, reduction in photosynthetic pigment concentrations and photosynthetic efficacy together under increasing saline stress are attributed to loss of photosynthetic-membrane integrity, destruction of proteins and enzymes in photosynthetic pathway, dehydration of cell membrane leading to reduction in CO₂ permeability, enhanced senescence, alteration in enzymatic activities due to morphed cytosolic integrity and negative feedback by reduced sink activity [26].

Apart from salinity, abiotic stressors such as drought and UV-B radiation are also currently on the rise due to climate change conditions. Some of the principal effects of drought and excess UV-B light on plant systems are as follows:

1. Water-deficit conditions lead to loss of turgor in plant cells, reduction in plant water potential, disruption of enzymatic activities and reduced energy supply from photosynthesis. These factors eventually affect vital physiological processes such as cell division, elongation and differentiation, thereby arresting plant growth and development [27, 28, 29, 30, 31].

2. Drought stress can also induce changes in morphology and anatomy of plants, i.e. reduced leaf size, lower aperture in and reduced number of stomata, thickened cell wall, cutinisation of leaf surface, enhancement in conductive system (viz. large vessels), etc. [21]. Additionally, the total biomass of plant greatly reduced under drought conditions, with an increase in root-to-shoot ratio [21].

3. Photosynthetic efficacy is highly affected under water-deficit stress due to some notable reasons: decrease in chlorophyll pigment concentration, reduced leaf surface (due to arrested growth and development), disruption in activity and/or concentration of enzymes like RuBisCO, PEP carboxylase, fructose-1,6 bisphosphatase, sucrose phosphate synthase as a result of reduced water potential, and decline in the efficacy of both the cyclic and non-cyclic types of electron transport in the photosynthetic light reactions [32, 33, 34].

4. Impact of drought stress is also seen on plant cell membranes, whereby the association of lipids with proteins, activity of bound enzymes and transport capacity all are hampered [21].

5. UV-B radiation influences its adverse effects on plant systems by targeting biomolecules and metabolic pathways. For instance, impairment of photosynthetic electron transport chain and increased activity of membrane localised NADPH oxidases and peroxidises both eventually lead to an overproduction of reactive oxygen species (ROS) which is the main effector molecule of oxidative stress. additionally, and UV-B stress is found to be associated with impaired pathogen resistance and alteration in the antioxidant machinery (i.e. pathways of glutathione, phenylpropanoids, cinnamates, flavonoid, respectively, and pyridoxine biosynthesis pathways) [35, 36, 37]

In addition to the points mentioned above, all the abiotic stress factors discussed herein are also potential contributors to the rise in cellular concentrations of ROS. As rightly pointed out, all kinds of stress eventually lead to a rise in ROS concentrations beyond their threshold value, thereby manifesting symptoms of oxidative stress [38]. The same has been depicted in Figure 2. Moreover, climate change conditions have also witnessed a rise in biotic stress for plants, in particular for the agronomically important plants, for instance, heightened pathogenic and pest stress, and weed stress [39].

Figure 2.

Schematic overview of oxidative stress effects induced due to ROS overproduction (Taken from Dutta et al. [39]).

Even though there is an upsurge of stressing conditions for plants, these immobile yet versatile organisms prove their resilience by strategically responding to the environmental stressors. Some of the major defence strategies employed by plants against the abiotic stress factors have been depicted in Figure 3.

Figure 3.

Schematic representation of some major defence strategies employed by plants in response to various abiotic stresses. The strategies are categorised as ‘escape/adaptation’, ‘avoidance’ and ‘tolerance’, respectively. Colour codes—Blue: Drought stress; Red—Salinity stress; Pink—UV-B stress; Yellow-Common to different kinds of stress. Symbols—‘↑’ indicates ‘increase in’; ‘↓’ indicates ‘decrease in’; Bold Arrows indicate the part of plant affected by each type of abiotic stress, i.e., leaf, stem, and root, respectively. (Extracted from Müller-Xing et al. [40], Salehi-Lisar et al. [21], Kamran et al. [19], Podolec et al. [41]) (Figure is slightly modified from the literature for simplicity).

5. Climate change conditions leave their imprints in plants through abiotic stress: the epigenetic effect

It is clear to us that the sessile plant beings are capable of displaying various defence strategies in response to stress factors. In nature, the plants are almost always exposed to more than one kind of stress factors, implying that response strategies are also versatile. The diverse array of signalling pathways and molecules that are involved in environmental stress defence result from reprogramming of gene expression patterns, which in turn are beautifully regulated. From research spanning the last two decades, there is growing evidence that an epigenetic regulation of gene expression also takes place under different kinds of abiotic stress [40, 41, 42, 43]. Following the consensus, we are using the definition of ‘epigenetics’ as the changes in gene expression activity due to alterations that are outside of the DNA sequence of the gene, and that these changes may be meiotically or mitotically inheritable but are largely displaying the non-Mendelian feature of reversibility [40, 42, 44].

The epigenetic response of plant to different environmental stimuli is essential not only for abiotic stress tolerance but also for various other essential processes such as leaf development, floral transition and bud dormancy [42]. In case of environmental stress, it has been noted that the marks of epigenetic changes display stability or transgenerational inheritance, leading to what is popularly known as ‘epigenetic memory’ [40, 42]. This also becomes of source of ‘phenotypic plasticity’ (represented as a simplified equation below).

$$\mathit{\text{NATURAL GENETIC VARIATION}}+\mathit{\text{EPIGENETIC VARIATION}}=\mathit{\text{PHENOTYPIC}}\&\mathit{\text{FUNCTIONAL DIVERSITY}}\mathit{\text{(origin of ‘phenotypic plasticity’)}}$$

However, not all epigenetic marks are stable; some of them are transient, i.e. in case of DNA repair and/or cell cycle phases. To delineate the epigenetic mechanisms of regulating gene expression, it is important to understand how the epigenetic changes work. Briefly, the genomic DNA in eukaryotes is packaged into organised structure called the chromatin. The chromatin is composed of basic units called nucleosome (as shown below):

The targets for epigenetic regulation are as follows:

1. DNA methylation, wherein the 5th C of cytosine residues of the DNA backbone get attached with methyl group.

2. Histone modification, wherein the histone proteins are subject to post-translational modifications such as acetylation, methylation, phosphorylation, sumoylation and ubiuitination, mostly at the N-terminal region of the core complex. Largely, the lysine residues are targets of histone modifications.

3. Incorporation of histone variants.

These mechanisms, also sometimes known as the ‘epigenetic code’ directly regulate the activity of a gene by influencing the arrangement of nuleosome and consequently the compactness of chromatin. If the chromatin is tightly packed (‘heterochromatin’), it is less accessible for expression, and if the chromatin is loosely packed (‘euchromatin’), it is readily accessible by RNA Pol II for carrying out gene transcription. It has been indicated that under abiotic stress, plants can display three different kinds of epigenetic memories (Figure 4) [40, 45].

Figure 4.

Different types of epigenetic memories (Extracted from Müller-Xing et al. [40]).

There is growing evidence that plants from diverse genera, including forest trees, respond to abiotic stresses of drought, salinity and UV through epigenetic modulation of gene expression [40, 46]. Additionally, these epigenetic imprints are also linked with adaptation, acclimation as well as acclimatisation under stressful conditions.

6. Plant growth-promoting rhizobacteria as a vital tool under abiotic stress

Micro-organisms are an integral part of all ecosystems. They perform varied functions that are not only vital to their own survival but are also beneficial to different other life forms, including plants. Be it the free-living or symbiotic N₂ fixing bacteria, or the mycorrrhizal associations of fungi and plants, micro-organisms are famous for their service to the most fundamental trophic level of all food chains, i.e. plants, and consequently to the entire planet as well. For several decades now, the environmental significance of bacteria has been a matter of utmost interest, and as such scientists from all over the world have kept a close watch on the developments in this field. Bacteria are exploited to amend environmental problems arising from both natural and anthropogenic challenges, and this includes environmental pollution with toxic heavy metals or pesticides, and abiotic stress factors such as salinity or drought. Bacteria are also utilised to assist the sessile plant beings in overcoming such environmental challenges, and one of the versatile number of tools with which they help their plant hosts is collectively termed as ‘plant growth promoting properties’. Some essential plant growth-promoting properties of bacteria include N₂ fixation, phosphate or potassium or other mineral solubilisation (through organic acid production), siderophore production, auxin (indole acetic acid) production and ACC deaminase activity [47, 48, 49]. If these bacteria can colonise in the rhizosphere of the plant, they form a group called ‘plant growth-promoting rhizobacteria’ (PGPR in short). The PGPR can also assist the plants in enhancing their antioxidant defence machinery and adapt better to the growing stress [50, 51]. In recent times, the role of PGPR in assisting plants under various abiotic stresses is being extensively explored. For instance, the efficient role of PGPR of Bacillus genus in salt tolerance in tall fescue, alleviating drought stress in maize and wheat through PGPR of genera Bacillus and Enterobacter, using PGPR Bacillus subtilis to mitigate drought stress in potatoes by suppressing oxidative stress in the plant and enhancing antioxidative enzymes, improvement in essential oil production by the medicinally important rosemary plant under salinity stress through treatment with PGPR Pseudomonas fluorescens, and many more [52, 53, 54, 55]. Moreover, given the fact that bacterial species have high potential for adaptation and plasticity in metabolism, their applicability can be dynamic. The utility of PGPR thus covers a wide range of plants, from forest trees, grassland, agricultural crops to medicinally as well as aesthetically important plants, making them well applicable for cultivating more stress-resilient plants in the current backdrop of climate change.

7. Conclusion

The present trend of climate change and the resulting variation in temperature and precipitation indicate a global rise in abiotic stress factors for plants, mainly drought, salinity and UV radiation. These factors affect the plant in an integrated way, eventually leading a loss in the plant productivity. This becomes an alarming concern from agricultural as well as ecological perspective. The versatile plant beings have a number of mechanisms to articulate their defence against these stress-inducing factors, which are controlled genetically via a well coordinated cascade of signalling events. Aside from this, there is also interplay of these mechanisms with epigenetic memory, which makes the plant more resilient and better adapted to climate change conditions. However, even though plants have devised strategies of their own, it is still desirable to provide them with added assistance, given the fact that plants in the open are often exposed to multiple threats or stressors simultaneously. In this regard, the role of PGPR is imperative, because not only are they a source of beneficial activities that aid in the overall growth of plants but also a decisive tool for boosting the plants’ defence mechanism against ROS and abiotic stress. Thus, the impact of climate change on plant life is manifolds, and we need to address it more resolutely with an understanding that friendly micro-organisms also play a vital role in this battle.

Acknowledgments

The authors like to extend their gratitude to Rev. Fr. Dr. Dominic Savio, S.J., principal of St. Xavier’s College (Kolkata), for his continuous support. We would also like to thank the editors and the entire team of IntechOpen for this wonderful opportunity.

Conflict of interest

None declared.