Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Water Stress
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similar phenomenon has been observed in many other wild plants such as Citrullus
lanatus (Akashi et al., 2004), Lycopersicon pennellii (Mittova et al., 2002), Tortula
ruralis (Oliver et al., 2004), and Xerophyta viscose (Garwe et al., 2003).
During desiccation, the resurrection plant Craterostigma plantagineum
changes its main photosynthetic product from sugar 2-octulose to sucrose, increasing
the sucrose content to 40% of the dry weight of the plant body. Rehydration induces
normal photosynthesis within 24 hours (Ingram and Bartels, 1996). The CDT-1 gene
cloned from C. plantagineum by activation tagging was shown to be essential for
dehydration tolerance in this species, and the level of its transcript was shown to
increase in response to ABA (Furini et al., 1997). CDT-1 encodes a regulatory RNA or
short peptide involved in activation of the ABA signal transduction pathway. No
homologous gene has been found in the genome of Arabidopsis, suggesting a unique
signal transduction system for adaptation to severe dehydration in this plant.
Research with drought-tolerant plants has revealed how plants have adopted
a variety of signaling pathways that allow them to tolerate naturally dry environments.
Information from these studies is expected to indicate genes that would be useful in
future molecular breeding of crop plants.
5. CONCLUDING REMARKS
This chapter dealt with the basic responses of model plants to drought stress and the
systems adopted by wild plants to environmental drought. However, the mechanisms
for sensing environmental stimuli and transducing signals between organs and cells
require further elucidation even in model plants.
Based on their relationship with water, plants can be divided into three categories:
hydrophytes, mesophytes and xerophytes. Mesophytes survive in moist environments,
and include most experimental model plants. Xerophytes can further be divided
into two groups depending on their drought tolerance strategies. One group includes
plants able to tolerate and resurrect following desiccation into photosynthetically active
states in a short period of time, while the other is the desiccation-avoiding group.
Plants in the latter group possess an improved water-absorbing system or water-storing
organ, efficient water conduction within the plant body or a restricted transpiration
system, or all or a combination of these features. Moreover, plants in this group are
thought to have unique systems that allow stress tolerance, since they are able to
defend their photosynthetic apparatus and other fragile cellular components from composite
stresses in the harsh environments in which they are found. These plants are
often inconvenient but promising experimental materials, since they are truly able to
survive drought.