Frontiers in Plant Science recently collaborated with two other journals in putting together a number of articles related to the research topic “Mechanisms of abiotic stress responses and tolerance in plants: physiological, biochemical and molecular interventions”. As part of the topic a mini review was published reviewing what we know about abscisic acid signalling and abiotic stress tolerance, what further information we need to acquire and what we may be able to do with the knowledge for future food production.
Biological chemistry is mind-blowingly complex yet crucial to developing our understanding how organisms grow, reproduce, protect themselves and die, amongst others. How plants, stuck in one spot and forced to deal with whatever conditions may befall them, have the ability to respond to biotic and abiotic stress is important to know if we are going to be able to adapt crops to changing environmental conditions and improve food production security and efficiency.
Abscisic Acid (“ABA”) is one of a small number of phytohormones that play a significant role in how plants develop and grow. The amount of ABA produced by a plant is known to be affected by extracellular stresses and is considered important in assisting plants to adapt to and withstand abiotic stresses when they arise as well as being involved in such processes as seed development.
Abscisic Acid phytohormone. Credit: Wikipedia
ABA is produced in all parts of plants but accumulate in roots and terminal buds of growing plants, forming the basis of the hypotheses that they play a role in communication between the two ends of the plant during periods of stress. In the context of stress response, it plays a crucial role in assisting the plant in times of dehydration, thermal stress, strong UV absorption and uptake of heavy metals. The paper cites previous studies which have shown that mutant plants lacking ABA biosynthesis show decreased tolerance to fluctuations in their environments compared to the wild-type, while overexpressing plants have increased resilience.
Table 1 from article. ABA regulation of stress effects on plants.
Water deficiency in plants results in an osmotic stress in plant cells which cause cell desiccation and a resistance to water uptake. ABA levels rise in these conditions with one study showing that the levels of ABA under drought stress can be 40 times higher than when the plant isn’t stressed. Studies involving transgenic plants overexpressing ABA genes suggest that the hormone assists plants by maintaining membrane stability and causing changes in metabolite accumulation when water availability is low. Further, ABA is believed to be involved in stomatal closure during drought periods when water retention is of paramount importance to the plant. Studies which have applied ABA exogenously have also been shown to result in increased resistance to drought stress.
Drought stress can also result in the production of ethylene, a chemical which is commonly used in agricultural industries to bring on fruit ripening due to its effect of bringing on early senescence. However, early senescence in crops not yet at the stage of being useful for food production is problematic. Studies have shown that increased ABA production reduced ethylene production associated with this type of stress and others such as UV stress (see below).
Whilst ABA acts to protect the plant from water stress, it is generally agreed that the protective mechanisms initiated also result in decreased plant growth above that caused by the water stress itself. Plants subjected to drought stress with purposefully lowered ABA levels were demonstrated to have reduced growth. Further, plant shoot growth may be inhibited by ABA even when appropriately watered.
Heavy Metal Stress
Heavy metals such as cadmium, iron, mercury, copper and chromium, which are generated as a result of human activities and which can pollute agricultural land, can be taken up by plants leading to a toxicity caused by the reactivity of these metals. Reacting with cellular components results in energy loss, lowered photosynthetic capacity, reduced growth and early death.
Heavy metal absorbance results in increases in ABA levels. Testing of cadmium-tolerant and cadmium-susceptible rice cultivars has shown that the ABA level of the tolerant cultivar exceeded that of the susceptible one. Further, the application of exogenous ABA to the susceptible cultivar increased resistance to cadmium.
Chromium and copper can cause intracellular stress and resultant production of oxidants and free radicals. The uptake of these metals in plants has also been shown to result in ABA biosynthesis. Conversely, reduced intake of heavy metals leads to a reduction in the levels of ABA, indicating that ABA plays some role in the stress response.
Ultraviolet radiation absorbed by plants can result in the creation of reactive oxygen species within the plant, resulting in damage to cellular components which leads to retardation in plant development and growth.
Ethylene production increases with UV-B radiation exposure, causing early senescence in plant tissues. The presence of ABA in the plant tissue reduces ethylene production and thereby reduces the detrimental effects of the radiation exposure, a discussed above in relation to drought stress. In fact, research has demonstrated that a plant under one form of stress will also have an increased resistance to other forms of stress such as in the case drought stress and ultraviolet radiation stress.
The importance of Abscisic Acid
As can be taken from the examples provided, the production of ABA has been linked to the response of plants to such stresses as radiation, drought and heavy metal uptake. In some cases, research has linked stress tolerance to the presence of ABA and susceptibility to the lack of ABA present. However, the precise mechanism of action is not well understood.
The article also provides some background into the production of ABA and hints that, while we know many of the genes and proteins involved in its pathway to production we are still missing a complete understand of a number of steps and how the many branches of the ABA reaction link and/or work together.
This mini-review helpfully outlines some of the research surrounding ABA involvement in stress tolerance and the importance of understanding this if we are to enhance our crops.
It must be said that the review was in parts difficult to read and understand, perhaps due to to some issues with a translation to English, and that there was some unnecessary repetitiveness through the piece.
However, it is a good starting point for further research into abiotic stress tolerance and future food production.