e-letter, Plant Physiol. Vol. 130, October, 2002
Plants Ectopically Expressing Arabidopsis CBF1 Show Enhanced Resistance to...
HOW TO DEFINE RESISTANCE TO WATER DEFICIT STRESS ?
In a recent article, Hsieh et al (2002a) report that “Tomato plants ectopically expressing Arabidopsis CBF1 show enhanced resistance to water deficit stress” (Plant Physiol 130: 618-626). Water was withheld from wild -type (WT) and transgenic (T1) plants. Plant growth and survival, leaf wilting, leaf and root water contents, maximal photochemical efficiency of photosystem II in the dark-adapted state (Fv / Fm), leaf conductance, leaf proline concentrations and catalase activities were measured after water had been withheld for various periods of time. T1 plants showed greater survival after prolonged soil drying and at any time point, T1 plants showed less leaf wilting and greater Fv / Fm. Does this make T1 plants “more resistant to water deficit stress” as the authors claim ?
Plant water deficit can be characterized by decreases in plant water content, turgor or total plant water potential (Kramer and Boyer 1995). The resistance of a particular plant parameter (eg Fv / Fm) to water deficit stress can be defined as the slope of the relationship between that parameter (dependent variable) and a measurement of water deficit such as leaf water content (independent variable). This slope would be smaller in a genotype that shows increased resistance to water deficit stress. When this analysis is applied to the data of Hsieh et al. (2002a), it becomes apparent that the Fv / Fm of T1 plants is actually less resistant to water stress. After 28 days of soil drying, T1 plants had an Fv / Fm of circa 0.4 with a leaf water content of 3.5-4.5 g water / g DW. In contrast, after 14 days of soil drying, WT plants had a similar Fv / Fm and a leaf water content between 1 and 3 g water / g DW. For a given change in leaf water content, the Fv / Fm of WT plants is more resistant to water stress.
Is this an issue of semantics ? If the authors had meant soil water deficit stress (even though soil water content data were not reported), would this alter the interpretation of resistance to water deficit stress ? In this case, the resistance of a particular plant parameter to soil water deficit can be defined as the slope of the relationship between that parameter (dependent variable) and soil water content (independent variable). Soil water depletion will depend on the transpiration rate of the plant, which will depend on the transpiring surface (leaf area) and the leaf conductance of which the stomatal conductance is the dominant term. WT plants seem to show increased leaf area (Fig. 2 – Hsieh et al. 2002a) and certainly show greater leaf conductance (Fig. 4 – Hsieh et al. 2002a) and thus might be expected to lose more water. It is therefore likely that the lower leaf water contents of WT plants (Fig. 3 – Hsieh et al. 2002a) correlate with lower soil water contents. In this case, for a given change in soil water content, again the Fv / Fm of WT plants is more resistant to water stress.
Hsieh et al. (2002a) were rightly concerned that the dwarf phenotype of T1 plants may have apparently increased resistance to water deficit stress due to less soil water depletion. To remedy this situation, foliar gibberellic acid sprays (GA3) were applied to reverse the dwarf phenotype by increasing internode length (Fig. 4 – Hsieh et al. 2002b) and probably increasing leaf area (Ross et al. 1993). GA3 application did not significantly alter either Fv / Fm or leaf water content in T1 or WT plants (Fig. 3 – Hsieh et al. 2002a). Thus in GA3-treated plants, for a given change in leaf water content, the Fv / Fm of WT plants is more resistant to water stress.
The greater survival and decreased leaf wilting of T1 plants after a similar period of water deprivation can be attributed not to altered resistance to water stress, but to an escape from water stress due to a more sensitive stomatal closure in response to soil drying. By closing their stomata earlier, T1 plants maintained their leaf water content, which is even more critical given that their photochemical apparatus is less resistant to water deficit. It should be emphasised that tomato plants ectopically expressing Arabidopsis CBF1 show enhanced stomatal closure, thus avoiding water deficit stress.
Plant genetic manipulation offers various physiological routes to alter plant drought stress responses. However, the response of photosynthetic capacity to water deficit is relatively conservative across genotypes (Kaiser 1987). Alternative approaches such as the promotion of root elongation in response to soil drying (Sharp and Davies 1985), or altering stomatal sensitivity to changes in soil water content may prove valuable ways of avoiding water deficit. In this context, the more sensitive stomatal closure of plants ectopically expressing Arabidopsis CBF1 may represent an important opportunity, especially if this attribute can be conferred without the attendant dwarf phenotype. Irrespective, further progress in this area will depend on carefully designed experiments comparing genotypes on an appropriate physiological basis such as plant or soil water status.
Hsieh TH, Lee JT, Charng YY, Chan MT (2002a) Tomato plants ectopically
expressing Arabidopsis CBF1 show enhanced resistance to water deficit stress.
Plant Physiol 130: 618-626 Hsieh TH, Lee JT, Yang PT,
Chiu LH, Charng YY, Chan MT (2002b) Heterology expression of the Arabidopsis
C-repeat/dehydration response element binding factor 1 gene confers elevated
tolerance to chilling and oxidative stresses in transgenic tomato. Plant Physiol 129: 1086-1094 Kaiser WM
(1987) Effects of water deficits on photosynthetic capacity. Physiol Plant 71: 142-149 Kramer PJ, Boyer JS (1995) Water
relations of plants and soils. Academic Press,
Ian C. Dodd Department