Photosynthetic organisms have sought out the delicate balance between
efficient light harvesting under limited irradiance and regulated energy
dissipation under excess irradiance. One of the protective mechanisms is the
thermal energy dissipation through the xanthophyll cycle that may
transform harmlessly the excitation energy into heat and thereby prevent the
formation of damaging active oxygen species (AOS). Violaxanthin deepoxidase
(VDE) converts violaxanthin (V) to antheraxanthin (A) and
zeaxanthin (Z) defending the photosynthetic apparatus from excess of light.
Another important biological pathway is the chloroplast water-water cycle,
which is referred to the electrons from water generated in PSII reducing
atmospheric O2 to water in PSI. This mechanism is active in the scavenging
of AOS, when electron transport is slowed down by the over-reduction of
NADPH pool.
The control of the VDE gene and the variations of a set of physiological
parameters, such as chlorophyll florescence and AOS content, have been
investigated in response to excess of light and drought condition using
Arabidopsis thaliana and Arbutus unedo.. Pigment analysis showed an
unambiguous relationship between xanthophyll de-epoxidation state
((A+Z)/(V+A+Z)) and VDE mRNA amount in not-irrigated plants.
Unexpectedly, gene expression is higher during the night when
xanthophylls are mostly epoxidated and VDE activity is supposed to be
very low than during the day.
The importance of the water-water cycle in protecting the chloroplasts from
light stress has been examined through Arabidopsis plant with a suppressed
expression of the key enzyme of the cycle: the thylakoid-attached
copper/zinc superoxide dismutase. The analysis revealed changes in
transcript expression during leaf development consistent with a signalling
role of AOS in plant defence responses but no difference was found any in photosynthesis efficiency or in AOS concentration after short-term
exposure to excess of light.
Environmental stresses such as drought may render previously optimal light
levels excessive. In these circumstances the intrinsic regulations of
photosynthetic electron transport like xanthophyll and water-water cycles
might modify metabolism and gene expression in order to deal with
increasing AOS.
| Identifer | oai:union.ndltd.org:unibo.it/oai:amsdottorato.cib.unibo.it:518 |
| Date | 21 May 2007 |
| Creators | Zamboni, Alessandro <1978> |
| Contributors | Magnani, Federico |
| Publisher | Alma Mater Studiorum - Università di Bologna |
| Source Sets | Università di Bologna |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, PeerReviewed |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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