Breeding as a Tool for Adaptability to Climate Change

Introduction

Climate change is the change in the earth’s weather patterns that last for at least a few decades (Wikipedia, 2019). This variation is mainly as a result of “the greenhouse effect“ which leads to a generalized increase in temperatures of the earth (Western Cape Government, 2018). The sun emits energy in the form of radiation and some of this solar radiation is reflected by the earth’s atmosphere and surface. Part of this energy is as well absorbed by the earth surface thereby warming it. As the earth surface gets warmer, it later releases this energy in the form of infrared radiation. The emitted infrared radiation directed towards space is partially trapped by greenhouse gases and this results into a generalized warming of the earth’s atmosphere (Wikipedia, 2019). These greenhouse gases form a sort of gaseous layer around the earth acting as a shield. Greenhouse gases are generated mainly as a result of human industrial activities. some of these gases include; carbon dioxide, nitrous oxide, methane and fluorinated gases (Wikipedia, 2019).

General Consequences of Climate Change

In a whole, climate change has resulted into an Increase in the frequency and intensity of extreme weather events such as floods, droughts, and storm surges (Bunker, 2018). Some of the specific effects of climate change include:

• Higher average annual temperature.
• Higher maximum temperatures.
• Higher minimum temperatures.
• Unpredictable length and period of seasons.
• Reduced average rainfall.
• Rising sea levels
• Increased fire risks
• Change in the pests and disease fauna

Climate Change and Agriculture

Climate change has led to an average increase of CO2 concentrations and temperatures (>2°c on average) of the globe (Daryanto, Wang, & Jacinthe, 2016). These changes have negatively impacted various requirements of plants to grow and express their maximum yielding potentials. The remarkable impacts are the reduced availability of water because of lowered and unpredictable rainfall patterns as well as dryer soils due to increasing temperatures (Bunker, 2018). These changes have affected established patterns used by crop producers. Some known and exploited patterns such as distinct dry/ cold/rainy seasons, available soil water, pests and disease fauna and temperature ranges have been disrupted (Bunker, 2018). For now, planting a crop tends to be more of a gamble (blind man’s game). These disruptions have made crop production to become an unpredictable business thus increasing food insecurity (FAO, 2002).

A non-negligible consequence of climate change on agriculture is the change in pest and disease fauna. Climate change creates opportunities for pests to extend their habitats or to relocate. An example is the stink bug currently “eating” its way through in Eastern Europe. This invasive insect is responsible for huge damages on hazelnuts production in Abkhazia and in the USA (Wegner, 2019).

As all living organisms, plants also need water to survive and carry out their basic metabolic activities. For this reason, agriculture is highly dependent on water as main input for production. The FAO declares that most of the agriculture is either rain fed or irrigated. Though crop production by irrigation is more productive, rain fed agriculture remains the most predominant practice worldwide (FAO, 2002). Around 83 percent of cultivated land is rain fed and it is as well mentioned that this percentage goes up to about 95 percent in developing countries. The FAO justifies this predominance is as a result of the high input cost for the establishment of irrigation facilities. Climate alteration obviously has several consequences but one of the most impacting is drought. Drought is a prolonged period of abnormally low rainfall usually correlated with high temperatures, leading to a shortage of water (Chaudhary, 1984). This same correlation between water and temperature persists in plants eventually justifying the fact that adaptability to drought in plants is related to their adaptability to increasing temperatures (Pantaliao & Narcisco, 2016). Drought therefore decreases the yielding performance of a plant. The severity of drought will not only depend on the crop species concerned but also on the plant’s developmental stage at which the stress occurs (Daryanto, Wang, & Jacinthe, 2016). A field experiment was carried out to demonstrate the effect of drought on the productivity of 3 of the most consumed cereal crops. In this trial, when maize, wheat and rice plants were subjected to the same water shortage stress, Maize plants lost on average 39% of their potential yield while wheat and rice respectively lost around 27.7% and 25% (Zhang, et al., 2018). Due to these observed threatening changes, it is thus of ultimate importance to breed for crops which will be apple to maximize their yielding potentials in an unpredictably harsh environment.

Breeding Goals

A breeding goal is the specification of the distinguishing quality or characteristic to be improved including the emphasis given to each (Chaudhary, 1984). It gives the direction towards which the breeder must improve the current plant population in order to achieve his objective. Climate change causes abiotic stress to the plant. This implies a non-biologic (environmental) cause with huge biological consequences generally expressed in the form of yield loss (Maazou, Tu, Qiu, & Liu, 2007). In order to understand the breeding goals for climate change, one must employ a systematic approach. This approach involves matching of the results of climate to the morphology, physiology and genetics of plants (Farrant, 2016). Adaptability is already a sort of broad objective on its own. In a general perspective, breeding for adaptability will be breeding for plants with reduced vulnerability to sudden change (Bunker, 2018). Because of the various components of climate change, adaptability by plants thereby includes several traits. Although adaptability to climate change includes a collection of characteristics, one of the most important will be resilience to drought (FAO, 2002).

Plants employ different mechanisms to face drought stress. These mechanisms can be differentiated into three main groups: escape, avoidance and tolerance (Basu, Ramegowda, Kumar, & Pereira, 2016). Resistance and tolerance are relatively similar. The main difference between avoidance and tolerance is that, unlike tolerance, the mechanism of avoidance refers to a situation where the plant tries as much as possible to maintain favorable tissue water content. While plants in the tolerance category can to a certain extend strive without maintaining any favorable cellular condition, the mechanism of drought avoidance usually includes reduced photosynthesis with increased root development. Drought escape is a situation whereby the plant completes its life cycle before the occurrence of stress (Basu, Ramegowda, Kumar, & Pereira, 2016). The disrupted and unpredictable seasonal patterns have rendered the mechanism of escape inefficient for both plants and crop producers.

Adaptability to climate change is thus a complex goal as it results from the additive effect of several other plant responses (Farrant, 2016). A plant ideotype will ideally be one which is of a tolerant type or at least one which combines a series of known adaptive characteristics (Pantaliao & Narcisco, 2016). This implies that the plant would be able, in situations of drought to yield at an acceptable level. The breeding of crops to achieve this goal must there by consider these numerous other morphological and physiological characteristics.

The main objective while breeding for adaptability to climate change is to find plants that show a positive response with respect to both yield and the trait of adaptability. Because of the complexity of the trait, this can be further subdivided into “sub goals”. Some of the traits that must be taken into consideration when breeding for adaptability to climate change include:

Short Pollen Shedding to Flowering Interval:

Shorter pollen shedding to flowering interval can serve as an indicator for drought resistance because plants are particularly sensitive in the period before and during fertilization (Zhang, et al., 2018). During drought stress plants usually close their stomata to avoid water loss. This closure eventually reduces rate of Photosynthesis because Co2 is no more taken up by the plant. Reduced photosynthesis implies malnourishment of the plant leading to slower overall developmental growth. Slow development cause reduction in pollen viability but also increases the time laps between anther maturities to female maturity (Bolaños and Edmeades, 1993). The longer this delay would be, the higher shall be the chances of fertilization period mismatch. This is to say that the weak pollen might be shed at a time window when the female inflorescence is not yet mature, thus low pollination and low yield (Bolaños and Edmeades, 1993).

Smaller Male Inflorescence Seizes:

With reference to maize, it has been demonstrated that tassel and ear seize/ dry weights share a negative correlation. This correlation shows that reduced tassel sizes is because of an improved photosynthate partitioning toward the ear (Monneveux et al., 2008). To push the correlation a little bit further, in an experiment, Monneveux et al in 2008 showed that detasselling can lead to an important increase of yield under drought conditions thus comforting the fact that tassel seize, and weight could be used as trait for selecting drought tolerant genotypes.

High Chlorophyll Content:

Chlorophyll is the pigment in plants which is responsible for their green color. The ability of plants to stay green and to eventually utilize solar radiation for photosynthesis is determined by their chlorophyll content (Araus et al., 2008). This molecule has an optimum absorption range of 650nm (reflectance signature). Chlorophyll content of a plant can be measured in the lab via samples or on the field using an SPAD meter. This instrument calculates the ratio of absorbance at 650 and 940nm which are respectively Chlorophyll maximal absorbance and non-absorbance wave lengths. Under stress, the optimization of a plant’s rate of photosynthesis is hence of great importance as greater chlorophyll concentrations would lead to a gain in grain yield (Bolaños & Edmeades, 1993))

Spectral Reflectance:

This is the measure of the reflectance signature of leaves on plants. Leaf pigments such as chlorophyll strongly absorb light in the photosynthetic active radiation length (400-700nm) but not in the near infrared region (NIR ~700nm- 1200nm) (Araus, et al., 2008). This implies that reduced reflectance of the photosynthetic active radiation but not of near infrared radiation would be a good characteristic. Weber et al. (2012) reported, that spectral reflectance measures on leaf and canopy level could explain at maximum 40% of grain yield under drought and well-watered conditions.

Stomatal Conductance:

When temperatures are high or in situations of drought, the stomata would close to avoid water lose though this response is at the same time harmful to the plants (Basu, Ramegowda, Kumar, & Pereira, 2016). Leaf conductance is determined by the degree at which stomata are open which subsequently depends on the water status and transpiration demand of the plant (Araus et al., 2008). As the stomata influences the inflow and outflow of CO2, the closing of stomata would lead to reduced photosynthesis and eventually growth of the plant. Stomatal conductance is hence a characteristic which will be observed on tolerant plants which should have found the optimal balance between transpiration and stomatal conductance (Araus et al., 2008). This parameter often measured using a porometer on leaf surfaces. It measures the wideness of the stomatal opening during drought conditions and allow for calculations to determine the rate of gaseous inflow into the plants leaves.

Low Leaf Canopy Temperature

Leaf canopy temperature can be measured using an infrared thermometer. When the stomata close due to drought, the cooling effect of transpiration is reduced leading to leaf and canopy temperature increase (Selmani & Wassom, 1993). Selmani and Wassom, 1993 reported a negative correlation between grain yield and canopy temperature under water-stressed conditions, thus showing that plants which could maintain low canopy temperatures had higher yields.

Root Structure:

Plants have various root types which are mainly taproots and adventitious roots. The preferable type of root system will be dependent on the soil type and usually crop cultivation methods. In general, plants with deeper roots will be preferred because they will be able to look for water in the deeper soil (Araus, 2002). This is of critical importance for example in rice cultivation because the plants are usually grown in well flooded soils. So, a plant with deeper root system has greater chances of better yielding in harsh climatic conditions (Zhang, et al., 2018).

Reduced Leaf Rolling and Osmotic Adjustment:

Leaf rolling is a characteristic effect to drought and heat stress. Leaf rolling occurs as a result of poor leaf cells osmotic adjustment (Baret, et al., 2018). A plant with good osmotic adjustment would be one which, in case of drought would be able to actively accumulate solutes in its cells to force water retention. The accumulation limits turgor loss and avoids possible cell damage due to shrinkage. Under high temperatures and drought, rise in internal leaf temperatures leads to a reduction in leaf water content and eventually shrinkage of cells due to reduced turgor pressure (Baret, et al., 2018). The reduced turgor pressure is mainly due to poor osmotic adjustment by the plant cells. Leaf rolling is thus an indicator of plant water status and may be useful to identify plants with inefficiencies in water uptake or turgor pressure maintenance (Edmeades et al., 1999). Leaf rolling hence has a negative impact on yield as it leads to cell damage and reduces the quantity of light intercepted by the canopy(leaves). Leaf rolling can be measured by visual scoring on scale ranging from 1 to 5, where 1 is low and 5 being severe leaf rolling (Basu, Ramegowda, Kumar, & Pereira, 2016).
Apart from the above mentioned characteristics, other traits such as root system efficiency, plant water structure rather than just content, death leaf scores are also often recorded.

Breeding Processes, Schemes, Techniques

Breeding for climate change and particularly drought tolerance is a difficult task as this can affect the crop at any stage of development. Conventional breeding has improved the drought tolerance capacity of plant populations mainly by exploiting genetic diversity as well foreign plants, using drought environments for testing, identifying and screening for traits and accurate phenotyping. Some of the schemes used in breeding for adaptability to climate change include:

Recurrent mass selection:

This is a classical technique used by breeders to develop High Valued end plant populations (Chaudhary, 1984). This method involves the cross pollination of two or more useful plant populations in order to create a base population. As a result of the cross, the newly formed base population is characterized by high level of heterozygosity and heterogeneity (Brown & Caligari, 2014). The new population serves as starting ground from which the breeder can select best performing individuals. Many plants are then grown from this base population and a subset of desirable phenotypes are harvested as individual plants. The seeds from these selected plants are then grown, allowed to cross pollinate to thus form the next generation of improved plants. This process is repeated several times to increase the frequency of good Phenotype thus the term recurrent selection. Recurrent selection should eventually result into the creation of a superior population cultivar because the frequency of favorable individuals is expected to have been increased (Chaudhary, 1984). This scheme can be improved through multilocational drought stress environment growing, marker assisted selection, genome wide selection or via male gametic control with use of remnant seeds (Brown & Caligari, 2014). This methodology was used the CIMMYT to develop lowland drought tolerant populations (DTP1 & DTP2) from a base population made of 25 drought tolerant plant sources collected worldwide.

Backcrossing:

Backcrossing may be deliberately employed in plants to transfer a desirable trait to another plant of preferable genetic background (Chaudhary, 1984). This form of breeding scheme usually involves a cross between an elite cultivar and a donor plant having the desired trait. The resulting seeds are grown then crossed with plants from the elite cultivar. Seeds from desirable phenotypes are harvested then grown back. This process of mating to the elite parent is repeated for several generations till the desired plants are obtained after several generations (Brown & Caligari, 2014). This scheme is common when breeding for disease and pest resistance whereby resulting backcrosses are subsequently screened and tested the resistance of interest (Brown & Caligari, 2014). The efficiency of this scheme can as well be improved through multilocational testing, MAS, GWS.

Transgenic and Mutation Breeding:

Drought stress alone triggers the expression of about 163 expression sequence tags (ESTs) which might or not correspond to individual genes expressed under drought conditions (Zhang, et al., 2018). Reduction in the price of biotechnological technology has led into an increase utilization of these techniques in breeding. Methods such as CRISPR Cas9, mutation and bacterial transformation, viral transduction have allowed breeders to literally identify and transfer desirable genes to desirable phenotypes (Farrant, 2016). Regarding diseases and change in pest fauna, A typical example would be the transgenic Bt maize which is a maize plant modified to produce insecticidal proteins. (Agrios, 2005)

Barriers to Improvement

There currently exist no per se breeding scheme for “adaptability” to climate change. Breeding for climate change is probably one of the most difficult challenges humankind will have to face (Farrant, 2016). This difficulty is not only due to the unpredictability of the climate but also because of the complexity of the traits in “adaptability” and the difficulty of measure of these traits.

Breeding for adaptability to climate change faces several barriers ranging from the proper understanding of the traits through its genetic aspect to ethical issues. One of the barriers related to breeding for adaptability to climate change is related to genetics. The genetic barrier in the achievement of the goal arises as result of the already alarming and increasing loss of genetic resources (Willian, 1991). Human activities such as breeding and other ecosystem damaging activities have led to the narrowing of plant genetic diversity (Willian, 1991). This negative dynamic slows down breeding objectives as breeders aren’t anymore able to screen and exploit genetic resources as could have been available before.

Another side of the difficulty is our limited understanding of the whole system of plant by climate change interaction (Farrant, 2016). The plant genome is a seal in the mystery around this interaction and at the same time a key. The study of uncultivated crop species has led to the discovery of potentially useful plants genes that could help solve climate change issues and at the same time compensate for the lack of genetic diversity (Chaudhary, 1984). One of the promising solutions are resurrection plants. These plants can lose 95 percent of their cellular water, remain in a dry, dead-like state for months to years, but when watered, they green up and start growing again (Farrant, 2016). Desiccation tolerance is a complex phenomenon involving both “switching on” and “switching off” many genes. Research has shown that there is considerable similarity in the mechanisms of desiccation tolerance in seeds and resurrection plants. These research work has also shown that resurrection plants use the same set of genes involved in seed desiccation tolerance (Jill Farrant et al). This therefore implies that cultivated plants do have these genes but can only activate them in seeds. Jill farrant’s group task now is to understand the environmental and cellular signals that switch on and off this swift of genes in resurrection plants, to mimic the process in crops. They are essentially, looking for better performance in crops during drought, not resurrection. Several other researches on economic and non-economic plants slowly enables breeders and scientist to evolve and close the gap present in their understanding of adabtabilty.

“If your question is, I’m I going to put resurrection plant genes into crops, your answer is yes.” Jill Farrant tedex speech (2016). A none negligible obstacle to breeding for adaptability to climate change is ethics related. Great advances in plant breeding were as a result of the utilization of biotechnological tools. This advance also brought with it a lot of skepticism, health risks and numerous scandals associated with the use of these new technologies. A lot therefore must be done in order to fix back the general public’s perception regarding these technological tools. The general public’s reaction can be characterized as being to a certain extend a bit excessive because people will be more prone to the use of biotechnology for health issues than with their food even though these are technically the same (Farrant, 2016). There is thus a crying lack of scientific communication. An example would be the concept of GMO and what the general public really understands about it. Most of the crops that we consume today; wheat, rice and maize, are highly genetically modified from their ancestors, but are not considered as GMO because they had been produced by conventional breeding (Farrant, 2016). Plant such as bread wheat which we consume today are as result of combination of genomes (Chaudhary, 1984). This was achieved through conventional breeding, but nowadays, if the process was redone using any technological method involving directing genome transfer, the plants won’t be as welcomed as those produced by conventional breeding. This simple example therefore shows the long path through which scientist must guide the public’s perception. This is therefore an obstacle to plant breeding because the general public is represented by governmental institutions and law makers. If the public can’t be convinced, governing institutions won’t be either and as a result scientific progress would be tampered. Particularly adaptability to climate change, the loss of genetic diversity in concerned plant species has encouraged breeders and scientist to mine for genes at higher hierarchy in the phylogeny of crops (Willian, 1991). This search has made them discover and study potentially useful genes in plants such as resurrection plants which could be directly introgressed into current plant populations. Without proper scientific communication and changing of laws, these innovations will therefore never be applicable outside of a lab (Farrant, 2016).

Conclusions

Even though all parts of the globe don’t experience the same consequences nor do face it at the same intensity, climate change is a real issue. Climate change leads to a large panel of consequences but the most remarkable of them in drought. Breeding for adaptability to climate change and particularly for drought can consequently be summarized as breeding plants for more yield per drop (FAO, 2002). The breeding task is particularly difficult because of the inexistence of a per se breeding scheme, narrowed current existing genetic diversity, ethical barriers to some scientific approaches, unpredictability of the climate and the complexity of the gross trait “adaptability” as this entails several other sub traits. Nevertheless, even with that much constrains, the degree of climate resilience in current plant populations are continuously being improved on via conventional schemes and the progressive improvement of genetic tools and biotechnological methods tend to be promising better results.