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36.5.1: Response to Water Stress

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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.

    Structural formula of abscisic acid. It has a carbon ring with a long carbon chain supporting a carboxyl group (COOH).
    Figure \(\PageIndex{1}\): Chemical structure of the plant hormone abscisic acid (ABA). Image by Socrtwo (public domain).
    The chemical structure of the carotenoid zeaxanthin. Two carbon rings are attached by a long carbon chain.
    Figure \(\PageIndex{2}\): The carotenoid zeaxanthin absorbs blue light and reflecting orange and red light. It is a precursor to abscisic acid. Image by Shaddack (public domain).

    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 \(\PageIndex{3}\)).

    The viviparous red mangrove (Rhizophora mangle), demonstrated, elongated seedlings attached to mother plant.
    Figure \(\PageIndex{3}\): The elongated seedlings of the red mangrove, Rhizophora mangle, are still attached to the parent plant, illustrating viviparous germination. Image by M.elle F. Legendre (public domain).

    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.

    Stomata open (left) in response to light, carbon dioxide, and auxin. Drought, signaled by ABA, causes them to close.
    Figure \(\PageIndex{4}\): Several factors mediate the opening and closure of stomata, which are surrounded by kidney-shaped guard cells. Stomatal gas exchange occurs through the stomata. Darkness, drought, cold, pathogens, and abscisic acid (ABA) mediate stomatal closure. Specifically, drought causes the releases of ABA, which signals stomata closure. Stomatal opening is triggered by light, carbon dioxide (CO2), and auxin. Image by June Kwak, Pascal Mäser (public domain).

    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).

    Attributions

    Curated and authored by Melissa Ha from the following sources:

    This page titled 36.5.1: Response to Water Stress is shared under a CC BY license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers (ASCCC Open Educational Resources Initiative) .