Uptake, transport and bioactivity of exogenously applied ABA and ABA analogues in white spruce and wheat seedlings




Kaul, Sonu

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There are significant differences between conifers and herbaceous species in their stomatal sensitivity to exogenously applied ABA. Experiments on white spruce (Picea glauca (Moench) Voss) and wheat (Triticum aestivum L. cv Katepwa) seedlings, whose roots were sealed in an aeroponic misting chamber, confirmed that 200-fold higher concentrations (2 x 10⁻³ M) of exogenously applied (±)ABA were required to close stomata in spruce than in wheat (10⁻5 M). I tested the hypothesis that this difference in response between species was because: (i) stomata are inherently more sensitive to ABA in wheat than in spruce; (ii) in wheat, ABA is taken up more efficiently by roots and more ABA is subsequently delivered to the shoots and (iii) a combination of (i) and (ii). Tritiated ABA was applied to plants over approximately 10 hours and their water uptake (transpiration rate, E) measured continuously. ABA uptake efficiency (UE) was calculated as the ratio of the scintillation count of root and shoot tissue extract to the product of the activity of the misting solution and total water uptake. Transport efficiency (TE) was calculated as the ratio of the shoot to the total tissue scintillation count. UE was almost twice as high in spruce (31.0 %) as in wheat (18.6 %). However, in spruce, virtually all of the ABA taken up remained in the roots (94.5 %). In contrast, in wheat, a much higher proportion of ABA taken up by the plant was delivered to the shoots (48.8 %). Thus TE was almost 9 times higher in wheat than spruce. Treatments such as increasing root temperature or the use of dimethyl sulphoxide as an organic solvent,brought about dramatic increases in UE in both species (in spruce, UE, in some cases, was almost 80%). However, in spruce this did not result in increased delivery of ABA to the shoots and TE declined. When the roots were excised from spruce seedlings, there was a 55-fold increase in the amount of ABA delivered to the shoots and a concomitant 20-fold increase in stomatal sensitivity to the application of ABA. Immunofluorescence labeling technique, used to localize ABA, showed that the cortical cells around the endodermis were the main site of exogenous ABA accumulation in sprace roots. In contrast, in wheat, the major portion o f the exogenous ABA was found inside the vascular tissue in the roots. I conclude that in spruce, the roots provide a major barrier to the transport of ABA to the shoots. However, differences in TE between wheat and spruce, while very large, do not fully account for differences in their stomatal response to exogenously applied ABA. Thus it is likely that wheat stomata are inherently mere sensitive to ABA than those of spruce. Experiments were also conducted on white spruce and wheat seedlings, to determine the uptake and transport from roots to shoots of (+)- and (—)-ABA enantiomers and their respective methyl ester derivatives. I tested the hypothesis that the higher biological activity, determined as their ability to affect gas exchange, of ABA enantiomers or specifically tailored analogues would be related to their being more efficiently incorporated into roots and subsequently transported to shoots. Tritiated ABA and MeABA enantiomers were applied, using an aeroponic root misting system, for 10 hours and seedling transpiration and photosynthesis rates monitored. Uptake efficiency (UE) and Transport efficiency (TE) were calculated as described earlier. In both species, (+)-ABA was more biologically active than (—)-ABA. However, differences in TE between the ABA enantiomers were significant only in wheat with the natural enantiomer having twice as high a TE as (-)-ABA. In spruce, the UE of the methyl ester enantiomers (~87 %) was almost twice as high as that of the respective ABA enantiomers. However, virtually all of the MeABA taken up remained in the roots with less than 2 % reaching the shoots. Thus, despite its higher transport across root membranes, MeABA, at all concentrations tested, had a lower biological activity than ABA and there was no correspondence between root uptake and bioactivity. Adding an isopropyl ester to the C-1 carbon of ABA brought about an increased bioactivity only in spruce where (±)- iPrABA induced stomatal closure at a 10-fold lower concentration (10⁻⁴ M), than (±)- ABA. I conclude that a much larger proportion of exogenously applied ABA is sequestered in spruce roots than in wheat. Thus it is likely that, in the former species, any increased biological activity of ABA analogues depends on how effectively they are transported from the roots to receptor sites in the shoots.



Spruce, Wheat