Climate change exerts adverse effects on crop production. Plant researchers have therefore
focused on the identification of solutions that minimize the negative impacts of climate
change on crops. This Special Issue comprises 23 articles that celebrate the 2020
International Year of Plant Health, highlighting the processes, mechanisms, and traits
that will underpin future sustainability of crop quality and yield. These articles
critically evaluate recent advances in our understanding of climate change impacts
on plants, within the context of climate-smart agriculture.
Background concepts and considerations
Global agriculture is dependent on a relatively small number of crop species, which
have been bred to optimize productivity within a relatively narrow range of environmental
variations (Kukal and Irmak, 2018). Moreover, current food security has been achieved
through intensive industrial agriculture, in which large farms often grow the same
crops annually, using large amounts of pesticides and fertilizers that ultimately
deplete soils, pollute water, cause nutrient loss, decrease biodiversity, and contribute
to climate change. Most developed countries have had to embrace modern agricultural
technologies in order to achieve food security for increasing populations, as well
as to support agri-business and income generation. Although, the ecological consequences
of technocentric approaches to food production have been appreciated for many decades,
it is only relatively recently that the negative environmental impact of agriculture
has come into sharp focus. Agricultural emissions (carbon dioxide, methane, and nitrous
oxides) particularly linked to livestock production systems amount to about a quarter
of global greenhouse gas emissions. The significant climate footprint of food production
is similar to that of burning fossil fuels. There is growing agreement that current
agricultural practices are not sustainable, because they tend to squander valuable
resources and degrade the environment. These considerations have changed the ethos
of basic plant science research and direction of demand-led plant breeding to focus
more on mechanisms and processes that allow plants to be healthy and grow well on
limiting resources. Next-generation crop plants need to be water and nutrient use
efficient, and have sustainable yields over a wider range of environmental conditions.
On top of the uncertainty regarding the future environmental impact of agriculture
comes the looming threat to yield sustainability caused by climate change-induced
fluctuations in weather patterns (Porfirio et al., 2018). Predictions suggest that
on a global scale, an increase in land use of ~100 Mha with a tripling of international
trade is required by 2050 to meet the future crop demands of 9.8 billion people, without
causing any significant change in existing cropped land area (Pastor et al., 2019).
Extreme weather events cause enormous damage to crop production. Mitigation strategies
to combat the effects of such extreme events, which are destined to become much more
frequent, are required alongside the global drivers of agricultural production (Dhankher
and Foyer, 2018; Schewe et al., 2019; Nutan et al., 2020a). This Special Issue focuses
on the ways in which plant science is poised to meet these challenges and mitigate
the impact of climate change in accordance with the United Nations Sustainable Development
Goals. To celebrate the 2020 International Year of Plant Health, the contents of this
Special Issue consider how plant biology may be tailored to meet the challenges posed
by climate change. Together, these papers present and discuss the impacts of environmental
stresses on different crops, providing authoritative insights into the mechanisms,
processes, and genes/gene networks that will allow mitigation of negative effects.
We highlight how increases in current knowledge will provide effective solutions and
drive strategies for future plant improvement and breeding.
The concept that recent developments in plant science have the potential to address
the challenges facing agriculture and food production is widely accepted (Bailey-Serres
et al., 2019). There are now a range of strategies available for enhancing sustainable
crop production and resilience to climate change including high-throughput single
nucleotide polymorphism (SNP) genotyping, genomic selection and trait mapping. These
tools are essential not only for an in-depth understanding of trait variations but
also for the transformative engineering required to accelerate plant breeding efforts.
The ‘pyramid’ approach to introducing favourable alleles and genes combinations is
discussed in the review by Nutan et al. (2020a), which summarizes the physiological
and molecular architecture that determines rice yield traits under various environmental
(drought, salinity) stresses. This authoritative treatise suggests various gene combinations
that may be used to improve rice grain yields under optimal and stress conditions.
The importance of the accurate selection of key candidate structural and regulatory
genes underpinning selected traits for applications using gene editing tools is discussed
in the paper by Zafar et al. (2020). These authors present the opinion that the CRISPR/Cas9
system provides an efficient and practical solution to the production of improved
crop varieties with a greater sustainability of yield and hence better resilience
to climate change. However, much depends on producer adoption and favourable economic,
policy and other framework conditions to actually realize any of the pathways and
their benefits to crop production.
Water use efficiency and drought tolerance
Water deficits pose a serious threat to crop productivity and food security in many
parts of the world due to poor or erratic rainfall and depletion of groundwater reserves
(Hussain et al., 2019). Improvements in crop productivity under conditions of limited
water availability are vital to meet global food demand (Balyan et al., 2017). Agricultural
crop production requires substantial amounts of water. For example, it has been calculated
that 2497 litres of water are required to produce 1 kg of rice (Fig. 1; Rahaman et
al., 2016). Therefore, the development of improved rice genotypes with increased water
use efficiency is essential without compromising yields (Shahane et al., 2019). Climate
change is predicted to increase the frequencies of droughts and floods, both of which
will be problematic for food production (Mar et al., 2018). In China, variations in
rainfall have already led to a water crisis and a severe decline in rice production
(Yao et al., 2017). Recent strategies such as growth enhancements or increases in
photosynthetic efficiency have the potential to increase intrinsic yields (Ambavaram
et al., 2014). The identification of new molecular markers and their effective utilization
in plant breeding will accelerate the production of improved crop cultivars that are
more tolerant to drought and other stresses.
Fig. 1.
Rice growing in the field. Rice cultivation requires large amounts of water. (Photo
courtesy of Rohit Joshi and Ashwani Pareek, India.)
Several manuscripts in this issue (Kunert and Vorster, 2020, Melandri et al., 2020a;
Nutan et al., 2020b
; Sulpice, 2020; Ye et al., 2020) highlight the physiological, molecular, and biochemical
responses of plants to drought stress. A metabolite profiling analysis of the flag
leaves of 292 indica rice accessions has led to the identification of new molecular
markers for drought tolerance and sensitivity in terms of grain yield (Melandri et
al., 2020a; Sulpice, 2020). The article by Melandri et al. (2020a) highlights the
central role of the ascorbate–glutathione cycle and of lipid peroxidation in mitigating
drought-induced yield losses. Dehydroascorbate reductase activity and malondialdehyde
levels were shown to be accurate biomarkers for drought tolerance. These markers have
potential use in breeding for improved rice grain yield stability under drought. An
association mapping and genetic dissection study of drought-induced canopy temperature
differences in rice is reported in another paper by Melandri et al. (2020b). Intriguingly,
these authors report that low canopy temperature is a useful indicator of access to
moisture during drought (Kaler et al., 2018). Ye et al. (2020) describe an important
strategy for soybean yield improvement. These authors have identified quantitative
trait loci (QTLs) regulated by slow canopy wilting (SW) in late maturing soybean genotypes
(Ye et al., 2020). The SW trait, which is associated with drought tolerance, involves
at least two distinct mechanisms: water use efficiency and conservation. Since drought
already causes ~40% reduction in soybean yields (Specht et al., 1999), the findings
reported by Ye et al. (2020) represent an important new direction of research (Kunert
and Vorster, 2020). These authors have also identified genetic resources for improving
drought tolerance in early maturity group soybeans.
Salinity stress
Salinity stress is an important yield-limiting factor that poses a significant threat
to agriculture worldwide. The identification of traits that underpin salt stress tolerance
is the prerequisite to develop improved cultivars (Akrami and Arzani, 2019). This
requires a better understanding of stress tolerance mechanisms, for example in halophytic
species such as Suaeda fruticosa (Fig. 2), which can be used as a model system for
studies on salinity tolerance (Flowers and Colmer, 2008). Suaeda fruticosa can not
only survive but also complete its life cycle in conditions of soil salinity of 65
dS m–1, pH of 10.5, and under little or no water (Wungrampha et al., 2019). Several
papers in this issue focus on improving salinity tolerance in crop plants (Hartley
et al., 2020; Joshi et al., 2020; Nongpiur et al., 2020; Nutan et al., 2020b
). Potential mechanisms for osmosensing in plants are expertly discussed by Nongpiur
et al. (2020), who highlight the roles of key proteins such as receptor-like kinases,
mechanosensitive calcium channels, phospholipase C, aquaporins, and membrane-bound
histidine kinases as osmosensors in stress perception. These osmosensors, which may
serve as master regulators of the osmotic stress response, are useful targets for
the development of osmotic stress-tolerant crops.
Fig. 2.
Suaeda fruticosa growing on saline sandy soil. (Photo courtesy of Wungrampha Silas
and Ashwani Pareek, India.)
Understanding genetic variations in plant responses to salinity stress and associated
traits is critical for improving plant adaptation to saline conditions. Hartley et
al. (2020) report the findings of a genome-wide association study (GWAS) on diverse
rice genotypes that has led to the identification of three QTLs associated with potassium
use efficiency (KUE). The requirement for potassium in multiple plant processes and
hence its central importance in attaining high crop yields and ecosystem stability
is expertly discussed in the review by Srivastava et al., (2020). In their comprehensive
consideration of this topic, these authors discuss the complexity of the signalling
network that involves reactive oxygen species (ROS), calcium and phytohormones for
the sensing of potassium deficiency in plants. Since relatively little soil potassium
is available in forms that are accessible to plants, the authors consider the possibilities
for the successful application of genetic approaches using potassium transporters
to increase plant KUE and so achieve sustainable food production. Like potassium,
sodium is an essential plant nutrient. It is not surprising therefore that a sodium
transporter gene called OsHKT2;1 was shown to be a key player in sodium/potassium
interactions underpinning KUE (Hartley et al., 2020). Furthermore, overexpression
of OsGATA8 localized in the Saltol QTL region in Arabidopsis and rice imparted tolerance
to salinity and drought, together with improved yield (Nutan et al., 2020b
). Moreover, this study provides evidence that OsGATA8 regulates the expression of
critical genes involved in stress tolerance, including those encoding ROS scavenging
and chlorophyll biosynthesis proteins. A further important study reported in this
issue characterized marker-free transgenic rice lines overproducing trehalose. This
was achieved by overexpression of genes associated with trehalose-6-phosphate synthase/phosphatase-mediated
regulation of sugar metabolism. Remarkably, this metabolic engineering led to improved
yield potential even under saline–alkaline conditions (Joshi et al., 2020). A metabolic
profiling of resulting rice lines showed that overproduction of trehalose in leaves
differently modulates other metabolic switches leading to significant changes in the
levels of sugars, amino acids, and organic acids in transgenic plants under control
and stress conditions.
High temperature stress
Climate change-led increases in local and global temperatures pose a significant threat
to plant growth and crop production (Priya et al., 2019). The Intergovernmental Panel
on Climate Change reported that if current rates of global warming continue, global
temperatures will continue to increase by a further 1.5 °C between 2030 and 2052 (Intergovernmental
Panel on Climate Change, 2018). Heat stress can impair all stages of plant growth
from germination to reproduction, limiting the productivity of major staple food crops
(Hussain et al., 2019). For example, heat stress has a negative impact on wheat yields.
A 4–6% reduction in average global yields of wheat is predicted for each 1 °C increase
in global mean temperature (Asseng et al., 2015). Current concepts concerning heat
stress effects on source–sink relationships and metabolome dynamics in wheat is competently
reviewed by Abdelrahman et al. (2020), who place emphasis on the temperature susceptibility
of the reproductive and grain-filling stages, and discuss the selection and development
of germplasm that can maintain high yields under heat stress. The plant reproductive
organs and processes leading to seed set are extremely vulnerable to increasing temperatures.
Our current knowledge and understanding of the molecular mechanisms that contribute
to this temperature sensitivity are ably discussed by Lohani et al. (2020), who summarize
the regulation of male and female reproductive organ development and fertilization,
together with heat-induced abnormalities at flowering. This review highlights the
high-temperature-sensitive stage-specific bottlenecks in sexual reproduction.
The importance of genetic mechanisms in the heat stress responses of crop plants is
described in the review by Janni et al. (2020), who evaluate the potential roles of
different processes in increasing crop resilience and productivity. A metabolite profiling
analysis of winter wheat genotypes revealed a significant increase in sugars, sugar‐alcohols,
and phosphate in the more temperature-tolerant genotypes (Impa et al., 2019). Carbon
loss caused by high night‐time temperatures led to a significant reduction in winter
wheat yields (Impa et al., 2019). The study by Sharma et al. (2020) demonstrates that
plant growth regulators (PGRs) can afford protection against high-temperature stress
(HTS). These authors report that PGR-treated plants were more resilient to heat stress
in terms of less damage to membranes, improved photosynthesis and leaf water status,
and carbon allocation than the untreated HTS controls (Sharma et al., 2020).
Soil health
Soil health and fertility are not only important to sustainable agriculture but they
are also key considerations in poverty alleviation and the improvement of livelihoods
of resource-poor farmers (Heger et al., 2018). Several manuscripts in this issue focus
on different aspects of climatic change impacts on soil fertility (Jiang et al., 2020;
Middendorf et al., 2020; Stewart et al., 2020). The review by Stewart et al. (2020)
provides a comprehensive survey of the barriers to crop productivity and improving
soil fertility in sub-Saharan Africa, providing evidence-based recommendations. The
holistic solutions described by Stewart et al. (2020) cover socio-economic considerations,
farming system approaches, and soil management strategies using inorganic and organic
sources of nutrients, leading to highly recommended solutions to current soil fertility
issues in sub-Saharan Africa. The evidence-based process and methodology for prioritizing
recommendations will be extremely useful for future action plans, investments, and
strategies deployed in sub-Saharan Africa as well as other parts of the developing
world. The review by Middendorf et al. (2020) describes climate change effects on
global crop productivity, stressing the need for an interdisciplinary and multinational
initiative to develop better models for determining research priorities for climate-resilient
agriculture. These authors highlight the importance of participatory approaches that
provide a variety of perspectives in order to gain insights into critical issues such
as defining and understanding sustainable intensification, climate-smart agriculture,
and soil fertility prioritization in sub-Saharan Africa.
Endophytes and microbial symbionts such as bacteria, fungi, or yeast can provide substantial
benefits for plant growth and development, particularly under conditions of environmental
stress. Symbiotic endophytes can also facilitate better CO2 diffusion in rice leaves
grown under elevated atmospheric CO2 conditions. This strategy alleviated the drought
stress-induced inhibition of photosynthesis and improved water use efficiency in rice
(Rho et al., 2020).
Biochars are carbon-rich materials that can be added to soils to greatly enhance the
moisture content (Burrell et al., 2016), soil organic carbon content (Luo et al.,
2016), and nutrient retention capacity (Peng et al., 2011) to increase sustainable
crop production. The review by Jiang et al. (2020) summarizes the biochar modification
approaches (physical, chemical, and biochar-based organic composites) to soil remediation
and discusses the potential role of biochar in sustainable crop production and soil
resilience, including the degradation of soil organic matter, the improvement of soil
quality, and reductions in greenhouse gas emissions.
Nanotechnology and plant health
Phytopathogens cause estimated crop losses of up to 20–30% annually (Kashyap et al.,
2017). Changing climatic conditions, particularly global warming, are considered to
favour pathogen expansion and increase the aggressiveness of infestation (Bebber et
al., 2013). Moreover, some authors consider that elevated atmospheric CO2 levels may
increase the susceptibility of host plants (Juroszek and von Tiedemann, 2011). The
global use and demand of pesticides and synthetic toxic agrochemicals may therefore
increase (FAOSTAT, 2019). The use of engineered nanomaterials (NMs) in agriculture
is rapidly increasing, with applications ranging from nanofertilizers to nanopesticide/insecticides.
NMs, due to their unique properties, offer a promising alternative as a less toxic
and sustainable product in plant disease management (Elmer and White, 2018). The review
by Fu et al. (2020) describes the potential applications of NMs in crop disease management,
together with the benefits for crop adaptation measures.
Oxidative stress and cellular antioxidants
Respiratory pathways are crucially important determinants of plant defences against
abiotic stresses and climate change. The importance of the alternative oxidase (AOX)
in preventing the respiratory production of ROS and reactive nitrogen species (RNS)
is discussed in the review by Florez-Sarasa et al. (2020). These authors consider
how the AOX pathway enables plants to deal with gaseous pollutants such as elevated
carbon dioxide (CO2), nitrogen oxides (NOx), and ozone (O3).
Glutathione (GSH) is the key redox molecule in all living cells and is directly involved
in the maintenance of intracellular redox homeostasis. GSH is involved in numerous
cellular processes such as detoxification of heavy metals and metalloids (Dhankher
et al., 2002; Paulose et al., 2013). The study described by García-Quirós et al. (2020)
highlights the importance of the GSH pool in the development of reproductive tissues
and in pollen tube growth in Arabidopsis thaliana. Reporter lines expressing the redox-sensitive
green fluorescent protein (roGFP2) were used to measure the glutathione redox state
of cells in each part of the flower. This study reveals that the ungerminated pollen
resides in a highly oxidized state that is commensurate with quiescence. Moreover,
analysis of the glutathione-deficient cad2-1 (cad2-1/roGFP2) mutants revealed that
the pollen achieves high GSH/GSSG ratios upon germination and that this highly reduced
state is required to sustain pollen tube growth (García-Quirós et al., 2020).
The compartmentation of proteins is a fundamental concept in cellular regulation,
but it has long been known that the location of many proteins is not totally fixed
and that some proteins can relocate between different intracellular compartments,
particularly between the cytosol and nucleus. The proteins and processes involved
in redox-dependent intercompartmental switching is expertly discussed in the review
by Foyer et al. (2020). These authors provide an insightful overview of the topic
that highlights new advances in our current understanding of protein movement and
relocation, considering that redox-dependent processes could underpin protein relocation
between different cellular compartments in response to metabolic or environmental
triggers. These authors describe how redox post-translational modifications (PTMs)
can control the compartmentation of many proteins, including antioxidant and/or redox-associated
enzymes.
Finally, our current knowledge of the impacts of climate and related stresses on physiological
processes in woody tree species and vine crops remains poor. The study reported by
Signorelli et al. (2020), which uses a combination of micro-computed tomography, histology,
and oxygen microsensors to study grapevine buds is an elegant example of contemporary
physiological approaches to tackle this problem. Signorelli et al. (2020) demonstrate
that the apoplastic pore size of the grapevine latent bud is highly regulated, and
that this probably forms an important feature of the seasonal behaviour and resilience
of the species.