PlantDefenses213 - Plant Stress and Defense Mechanisms - 1...

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Plant Stress and Defense Mechanisms - 1 Plant Responses to Environmental Stress Plants have a number of mechanisms to cope with stresses in their environment, which include such physical conditions as water (too much as well as drought), temperature (hot and cold), saline soils and oxygen deprivation, as well coping with biotic stresses such as predators and pathogens. Physical Stresses in the Environment Water We have discussed previously how plants respond to potential dehydration, and in some cases, excess water, through a number of modifications, such as leaf drop, xeromorphic leaf structure, leaf or stem succulence, or sometimes for roots, deep taproots to find the water table. As mentioned in our discussion of chemical growth regulators, Abscisic acid monitors water condition in cells and leads to stomatal closure to minimize immediate water loss. Abscisic Acid and LEA Proteins Abscisic acid also initiates gene transcription for additional water conservation measures on the part of the plant. Of particular interest is the synthesis of LEA proteins (late embryogenesis proteins). LEA proteins occur naturally in maturing seeds as they desiccate for dormancy. The LEA proteins help to stabilize the membranes and other proteins of the dehydrated cells, but LEA genes can also help plants grow better during drought. Transgenic rice plants with the LEA gene are more drought tolerant than rice plants lacking LEA expression in shoot growth. The HARDY gene in Arabidopsis also promotes more root growth and thicker drought resistant leaves to improve water efficiency. The HARDY gene has also been incorporated into rice with success.
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Plant Stress and Defense Mechanisms - 2 Oxygen We mentioned in our root structures section the specialized roots, pneumatophores, that many swamp trees have to obtain oxygen in waterlogged soils. Some plants, such as cattails, have large air spaces in their stems and leaves so that oxygen can more readily diffuse towards roots. An in some experimental situations, plants respond to water-logged substrate with development of air spaces within the stem tissue. Unfortunately, many house plants die of suffocation when pots are set in saucers and water is allowed to sit in the saucer, causing the soil to become waterlogged. Such plants cannot develop adequate aeration. Heat Plants have poor heat-regulating mechanisms. Their metabolism, in particular, can be seriously impacted by hot temperatures. Although transpiration cools the plant through evaporation, when it’s hot the plant is most likely to have a water deficit, closing stomata, and that shuts down transpiration. Plants synthesize a class of proteins when temperatures are high, and these proteins may function to protect enzymes that would be denatured by the excess heat. These proteins are called heat-shock proteins. Cold
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PlantDefenses213 - Plant Stress and Defense Mechanisms - 1...

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