The third category of stress preparedness involves long-term adaptations. They are set in motion by signaling pathways that respond to immediate changes following stress and lead to changes in development and growth. The classical explanation of a stress-mediated decline in photosynthesis followed by a retardation of plant growth has been based on metabolic correlations, i.e., assuming altered development as a consequence of impaired metabolism (147). Another interpretation suggests that dehydration and salinity stress initiate a sequence of events in which plants move toward a state of minimal metabolic activity that persists until the stress is relieved (148). In this view, Ca2+ and ABA signaling lead to adjustments in proton gradients across membranes and the associated K+ and water movements generate signals. Yet it seems probable that other, as yet unknown signaling pathways exist that actively slow or stop meristem activity, cell expansion, and growth. Suppression of growth may be an active process rather than a passive response to stress. Compared with normal developmental growth processes, stress-induced developmental changes could be indicators of tolerance mechanisms.
Apart from the cessation or delay of normal growth, stress reactions include a host of morphological changes. Often observed are a shift in the root-to-shoot ratio (149), changes in lignification, epidermis thickening and the development of trichomes, glands or wax additions to the cuticle, altered branching, different leaf shape and size, or stress-enhanced entry into flowering and seed production (14,150-156). Less obvious in their relevance to stress yet equally important will be mutations that alter meristematic activity by either fewer or more cell divisions or by changes in the elongation rate of meristematic cells or mutants in which the control of development by growth regulators (e.g., ABA) is altered.
Considering that mild abiotic stresses are generally accompanied by an acceleration of ontogeny, a mutational or transgenic approach to alter development in crop species can be imagined. Mutants in Arabidopsis and other species have been described that show such morphological and developmental changes, but to our knowledge these mutants have not been investigated from the perspective of stress tolerance. Another example, in a different category, is provided by the Arabidopsis or corn cer mutants, which are deficient in epidermal wax deposition (118,157). The plants desiccate and in extreme cases are not viable. A transgenic approach enhancing wax production has, to our knowledge, not been attempted. Similarly, we can imagine mutants with deeper roots, or roots with larger xylem capacity, or plants with large leaves early in the growing season and small leaves in hotter, dryer times later in the season. The realization of such concepts is not yet possible, but the completed sequence of the Arabidopsis genome, saturation mutagenesis in Arabidopsis and corn (158,159), and other ongoing high-throughput sequencing projects may soon provide the gene material to consider such work.
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