Uptake, transport and bioactivity of exogenously applied ABA and ABA analogues in white spruce and wheat seedlings
Date
2017-09-27
Authors
Kaul, Sonu
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Abstract
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.
Description
Keywords
Spruce, Wheat