Plants rely on diverse
strategies to regulate water loss during drought. The phytohormone abscisic
acid (ABA) is a critical mediator of stomatal closure during water stress in
seed plants. Studies in conifers identified diverging strategies in long-term
drought of ABA-mediated dynamics, particularly a peaking type dynamic during
long term drought in some conifers. Few studies have reported this dynamic in angiosperms,
and no study has revealed the mechanism driving declines in ABA levels as
drought progresses in peaking type species. To understand peaking type
dynamics, we exposed the model peaking type gymnosperm species <i>Callitris
rhomboidea</i> and the highly drought resistant evergreen angiosperm <i>Umbellularia
californica</i> to controlled long-term drought. We measured leaf water
potentials (Ψ<sub>l</sub>), stomatal conductance, ABA and the ABA catabolite
phaseic acid (PA) levels in potted plants during a prolonged but non-fatal
drought. We aimed to determine which of three potential drivers of peaking type
dynamic were responsible for this response: (1) increased catabolism of ABA into
PA at a threshold Ψ<sub>l</sub> , (2)
ABA export from the leaf is enhanced under drought, and (3) ABA biosynthesis
ceases at a threshold Ψ<sub>l</sub>. During long term drought, the evergreen angiosperm
species <i>U. californica</i> demonstrated peaking type ABA dynamics like
gymnosperms. In both species, PA levels did not increase significantly, in
fact, PA levels tracked ABA levels, suggesting that ABA catabolism to PA may be
a function of ABA levels. Girdling experiments to determine whether export from
the leaf drove declines in ABA levels demonstrated that of the majority of ABA
was likely converted to ABA glucose ester (ABA-GE), an inactive storage form of
ABA, and exported from shoots during drought. Finally, by rapidly dehydrating
branched collected at different timepoints during long-term drought we were
able to determine that ABA biosynthesis is completely down regulated in leaves
that have been dehydrated beyond leaf turgor loss point. The decline in ABA
levels in peaking type species appears conserved across seed plants and is
mediated by high export rates in the form of ABA-GE. Future work should assess
a more diverse selection of species as well as study long-term drought in less
tolerant species to test whether ABA biosynthesis is deactivated in all species
once Ψ<sub>l </sub>declines
below turgor loss point.
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/17159285 |
| Date | 19 December 2021 |
| Creators | Joel Abdel Mercado Reyes (11824124) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/CHARACTERIZATION_OF_THE_ABA_PEAKING_TYPE_DYNAMIC_DURING_LONG_TERM_DROUGHT/17159285 |
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