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16.4: Abscisic Acid - Biology

16.4: Abscisic Acid - Biology



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Learning Objectives

  • Identify the locations of synthesis, transport, and actions of abscisic acid.
  • Describe how ABA interacts with other plant hormones.

The plant hormone abscisic acid (ABA) was was once thought to be responsible for abscission; however, this is now known to be incorrect. Instead, ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Unlike animals, plants cannot flee from potentially harmful conditions like drought, freezing, exposure to salt water or salinated soil, and ABA plays in mediating adaptations of the plant to stress. Abscisic acid (figure (PageIndex{1})) resembles the carotenoid zeaxanthin (figure (PageIndex{2})), from which it is ultimately synthesized. It is produced in mature leaves and roots and transported through the vascular tissue.

Maintaining Dormancy

Seed Maturation and Inhibition of Germination

Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after flowering and seed formation is complete. Abscisic acid is essential for seed maturation and also enforces a period of seed dormancy, by blocking germination and promoting the synthesis of storage proteins. It is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. As discussed in the Environmental Responses chapter, other environmental cues such as exposure to a cold period, light, or water are often also needed to for germination to occur.

Interestingly, mangrove species with viviparous germination, meaning that seeds germinate while still attached to the parent plant have reduced levels of ABA during embryo formation, providing further evidence of ABA's role in maintain seed dormancy (Farnsworth and Farrant 1998, Am J. Bot.). These mangroves are adapted to drop germinated seeds into surrounding water to be dispersed (figure (PageIndex{3})).

Bud Dormancy

Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter. Abscisic acid in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted.

Response to Water Stress

Stomatal Closure

Abscisic acid also regulates the short-term drought response. Recall that stomata are pores in the leaf and are surrounded by a pair of guard cells. Much of the water taken up by a plant is lost as water vapor exists stomata. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss. Note that stomatal closure also prevents exchange of oxygen and carbon dioxide, which is necessary for efficient photosynthesis (see Photorespiration and Phytosynthetic Pathways). The response to abscisic acid occurs even if blue light is present; that is, signaling from drought via ABA overrides the signaling from blue light to open stomata. See Transport for more details about stomatal opening and closure.

Cellular Protection from Dehydration

Abscisic acid turns on the expression of genes encoding proteins that protect cells - in seeds as well as in vegetative tissues - from damage when they become dehydrated.

Interactions with Other Hormones

At a cellular level, abscisic acid inhibits both cell division and cell expansion. It often opposes the growth-inducing effects of auxin and gibberellic acid. For example, abscisic acid prevents stem elongation probably by its inhibitory effect on gibberellic acid. In maintaining apical dominance, however, ABA synergizes with auxin. Abscisic acid moves up from the roots to the stem (opposite the flow of auxin) and suppresses the development of axillary buds. The result is inhibition of branching (maintaining apical dominance).


Abscisic Acid

7.2.3. The Oxidative Pathway for ABA Catabolism

The conversion of ABA to PA occurs via an intermediate, 8′-OH-ABA, which is unstable and rearranges to PA. The reaction catalyzed by ABA-8′-hydroxylase, which is expressed at high rates in various tissues recovering from water stress, is a major regulatory step in ABA homeostasis. Maize cells in suspension culture are a good system to study ABA metabolism. These cells do not have endogenous PA, but, as mentioned earlier, they synthesize PA on the addition of ABA and do not accumulate αβα-GE moreover, PA can be quantitated readily because it diffuses out in the culture medium and samples can be collected for analysis. Using such a system, it has been shown that PA is synthesized rapidly on the addition of ABA and that such synthesis is blocked if cells are pretreated with transcription or protein synthesis inhibitors, cordycepin or cycloheximide, respectively. These data suggest that the gene encoding ABA-8′-hydroxylase is induced de novo and the inducer is ABA. These observations explain the accumulation of PA in water-stressed leaves referred to in the previous subsection. They also provide a parallel situation to that for cytokinin oxidase, the principal enzyme responsible for the catabolic breakdown of endogenous zeatin and isopentenyl adenine, which is also induced by its substrate (see Chapter 8 ).

ABA-8′-hydroxylase is a cytochrome P450 monooxygenase. It should be recalled that enzymes in stage 2 of GA biosynthesis are also cytochrome 450 monooxygenases. Cytochrome P450 monooxygenases are a large superfamily of enzymes, which are involved in many metabolic pathways in plants. They also play a significant role in the detoxification of allelopathic substances and herbicides (see Chapter 6 ). They are integral membrane proteins of the endoplasmic reticulum that use molecular oxygen, require NADPH and NADPH-dependent cytochrome 450 reductase for activity, and are inhibited by CO. This inhibition is reversible by blue light. They are also inhibited, nonspecifically, by tetcyclasis and triazole-type growth retardants. In the case of ABA-8′-hydroxylase, the use of tetcyclasis results in inhibition of PA synthesis while ABA and αβα-GE accumulate. Attempts to purify and clone the gene for ABA- 8′-hydroxylase are in progress.

Synthetic ABA analogs, which are altered in the 8′-OH and thus are immune to attack by ABA-8′-hydroxylase, are much more stable in plant tissues under stress conditions. They may also show a higher level of biological activity than ABA in bioassays.

The fates of compounds such as αβα-GE, DPA, and DPA-GS, which seem to be the end products of ABA metabolism, as also that of C25 apoaldehydes released from oxidative cleavage of C40 epoxy-xanthophylls, are not known. Most likely, they are broken down by various lipoxygenases and peroxidases in the cell.


INTRODUCTION

Plants, as sessile organisms, need to adapt to changes in their environment to survive, develop and propagate. As source of energy, light is one of the most important environmental factors that determine plant productivity. Solar energy is used in photosynthesis to produce the carbohydrates driving plant metabolism and producing the building blocks supporting plant growth. Carbon dioxide enters the leaf through the stomata, which are also the main sites of water loss. A careful balance has to be found in controlling stomatal aperture to limit water loss while maintaining photosynthetic carbon production. As a result, the regulatory pathways that respond to light conditions, water availability and the rate of carbohydrate production and use are intimately linked. Signals derived from light, carbohydrates and the water stress-associated plant hormone abscisic acid (ABA), controlling stomatal closure and storage metabolism, are integrated to achieve this balanced outcome. Consequently, it is not always clear to what extent signalling components and regulators are specific to one response pathway or are shared between pathways. Increasingly, these interactions are referred to as ‘crosstalk’ between signalling pathways or are defined as signalling networks ( Gibson 2000 Brocard-Gifford, Lynch & Finkelstein 2003 ). A substantial part of the output of these signalling networks are changes in gene expression which can increasingly be understood in terms of the various signalling pathways coming together at the promoters of individual genes. This review describes interactions between these three signalling pathways and how they control gene expression at a molecular level. Its main focus is on the interaction between the response pathways mediated by sugar and the plant hormone ABA. Sugar sensing and signalling is more generally discussed in several excellent reviews ( Smeekens 2000 Rolland, Moore & Sheen 2002 Rook & Bevan 2003 ).


Watch the video: 2ND YEAR BIOLOGY CHAPTER NO 17 LEC#4 PLANTS HORMONES CYTOKININS, ABSCISIC ACID, ETHENE (August 2022).