ABIOTIC STRESS AND PLANT BREEDING

Created on Thursday, 21 March 2013 Written by Hafiz Ghulam Muhu Din Ahmed*, Sajjad Akhtar and Syed Adeel Zafar Department of Plant Breeding and Genetics University of Agriculture, Faisalabad
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Abiotic stress is defined as the negative impact of non-living factors on the living organisms in a specific environment. The non-living variable must influence the environment beyond its normal range of variation to adversely affect the population performance or individual physiology of the organism in a significant way. Whereas a biotic stress would include such living disturbances as fungi or harmful insects, abiotic stress factors, or stressors, are naturally occurring, often intangible, factors such as intense sunlight or wind that may cause harm to the plants and animals in the area affected. Abiotic stress is essentially unavoidable. Abiotic stress affects animals, but plants are especially dependent on environmental factors, so it is particularly constraining. Abiotic stress is the most harmful factor concerning the growth and productivity of crops worldwide.Research has also shown that abiotic stressors are at their most harmful when they occur together, in combinations of abiotic stress factors.

Abiotic stress comes in many forms

The most common of the stressors are the easiest for people to identify, but there are many other, less recognizable abiotic stress factors which affect environments constantly. The most basic stressors include: high winds, extreme temperatures, drought, flood, and other natural disasters, such as tornados and wildfires. The lesser-known stressors generally occur on a smaller scale and so are less noticeable, but they include: poor edaphic conditions like rock content and pH, high radiation, compaction, contamination, and other, highly specific conditions like rapid rehydration during seed germination.

Abiotic stress In Plants

A plant’s first line of defense against abiotic stress is in its roots. If the soil holding the plant is healthy and biologically diverse, the plant will have a higher chance of surviving stressful conditions. Facilitation, or the positive interactions between different species of plants, is an intricate web of association in a natural environment. It is how plants work together. In areas of high stress, the level of facilitation is especially high as well. This could possibly be because the plants need a stronger network to survive in a harsher environment, so their interactions between species, such as cross-pollination or mutualistic actions, become more common to cope with the severity of their habitat. This facilitation will not go so far as to protect an entire species, however. For example, cold weather crops like rye, oats, wheat, and apples are expected to decline by about 15% in the next fifty years and strawberries will drop as much as 32% simply because of projected climate changes of a few degrees. Plants are extremely sensitive to such changes, and do not generally adapt quickly. Plants also adapt very differently from one another, even from a plant living in the same area. When a group of different plant species was prompted by a variety of different stress signals, such as drought or cold, each plant responded uniquely.

Plant Breeding

Because individual plants react so differently to similar abiotic stress factors, it can be difficult to breed a species for more than one resilient trait at a time, but that is precisely what plant breeders are looking to do. For example, rice has a high tolerance to flooded areas and soil salinity, but is sensitive to cold, so each trait must be isolated and magnified when looking to produce new plants with the best of all strengths. This idea has worked for rice. Rice grown created to be grown in cold weather now lives in Nepal and Bangladesh. One study has suggested the idea of exposing plants to stress factors to increase their resilience. This will actually activate a stress-response signal in the plant, so it will be more able to respond quickly and efficiently in case of a real abiotic stressor one important note with plant breeding is that when the new crops are being created, a person should keep in mind what crops are already living in the area. If those crops, and their traits, are ignored, then the new variety might actually be a step backward. Traditional values and practices should be allowed to filter into the planting of whatever newly resilient seed is developed. That way, the new crop will best fit the natural landscape, as the old one did, and will have adapted more quickly to whatever changing abiotic factors are present.

Plant breeders are starting to use knowledge gained from new approaches like functional genomics to investigate plants' responses to abiotic stresses, such as at the Australian Centre for Plant Functional Genomics.

Enhancing wheat field performance and response to abiotic stress with novel growth-regulatory alleles

The "Green Revolution" dwarfing (Rht) alleles that increase wheat yields under high input conditions are orthologues of the Arabidopsis GAI gene and encode mutant DELLA proteins. DELLAs are repressors of plant growth that are degraded in the presence of gibberellin (GA) whereas the gai/Rht mutants are insensitive to GA. Most UK wheat varieties carry the semi-dwarfing Rht2 (Rht-D1b) allele but variation in height between genotypes suggests that other loci play a role in determining stature. We aim to identify these loci through co-localisation of quantitative stature traits identified in UK wheat germplasm with genes in the GA-DELLA pathway.

Additionally, TILLING will be used to identify novel alleles of key genes from mutagenized populations of wheat. Based on functional analyses in vitro and performance in the field alleles will be selected for use in wheat breeding. There are reports of Rht mutations affecting the responses of wheat to stress, and a negative correlation between GA content/responsiveness and stress tolerance has been documented. Moreover, our recent work in Arabidopsis implicates the GA-DELLA pathway as a central regulator linking GA, abscisic acid and ethylene in a common stress- related network. It is timely to translate these key discoveries into crop improvement to enhance the tolerance of hexaploid wheat to environmental stresses without compromising productivity. To this end, we will take a knowledge-based approach to compare Arabidopsis and wheat DELLA-mediated stress responses. We will use available genetic stocks to determine whether existing, but relatively untested, Rht alleles affect tolerance to salt, drought and heat stress. Near-isogenic lines will be tested under controlled and field conditions to select alleles that will be taken forward by introgression into elite varieties. Novel alleles of GA-DELLA alleles identified by TILLING will also be assessed for effects on tolerance to drought, heat and other abiotic stresses.

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